uu.seUppsala University Publications
Change search
Refine search result
3456789 251 - 300 of 587
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 251.
    Kim, Duck Young
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Mao, Ho-Kwang
    Kang, Tae W.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    General trend for pressurized superconducting hydrogen-dense materials2010In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, no 7, p. 2793-2796Article in journal (Refereed)
    Abstract [en]

    The long-standing prediction that hydrogen can assume a metallic state under high pressure, combined with arguments put forward more recently that this state might even be superconducting up to high temperatures, continues to spur tremendous research activities toward the experimental realization of metallic hydrogen. These efforts have however so far been impeded by the enormous challenges associated with the exceedingly large required pressure. Hydrogen-dense materials, of the MH4 form ( where M can be, e. g., Si, Ge, or Sn) or of the MH3 form ( with M being, e. g., Al, Sc, Y, or La), allow for the rather exciting opportunity to carry out a proxy study of metallic hydrogen and associated high-temperature superconductivity at pressures within the reach of current techniques. At least one experimental report indicates that a superconducting state might have been observed already in SiH4, and several theoretical studies have predicted superconductivity in pressurized hydrogen-rich materials; however, no systematic dependence on the applied pressure has yet been identified so far. In the present work, we have used first-principles methods in an attempt to predict the superconducting critical temperature (T-c) as a function of pressure ( P) for three metal-hydride systems of the MH3 form, namely ScH3, YH3, and LaH3. By comparing the obtained results, we are able to point out a general trend in the T-c-dependence on P. These gained insights presented here are likely to stimulate further theoretical studies of metallic phases of hydrogen-dense materials and should lead to new experimental investigations of their superconducting properties.

  • 252.
    Kim, Duck Young
    et al.
    Theory of Condensed Matter Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
    Scheicher, Ralph H.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pickard, Chris J.
    Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
    Needs, R. J.
    Theory of Condensed Matter Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Predicted Formation of Superconducting Platinum-Hydride Crystals under Pressure in the Presence of Molecular Hydrogen2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 11, p. 117002-Article in journal (Refereed)
    Abstract [en]

    Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt) is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5 GPa. PtH transforms to a hexagonal close-packed or face-centered cubic (fcc) structure between 70 and 80 GPa. Linear response calculations indicate that PtH is a superconductor at these pressures with a critical temperature of about 10–25 K. These findings help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials. We show that the formation of fcc noble metal hydrides under pressure is common and examine the possibility of superconductivity in these materials.

  • 253. Kim, Duck Young
    et al.
    Srepusharawoot, Pornjuk
    Pickard, Chris J.
    Needs, Richard J.
    Bovornratanaraks, Thiti
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pinsook, Udomsilp
    Phase stability and superconductivity of strontium under pressure2012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 101, no 5, p. 052604-Article in journal (Refereed)
    Abstract [en]

    We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.

  • 254.
    Klaa, Kaltoum
    et al.
    Badji Mokhtar Univ, Dept Phys, LNCTS Lab, Annaba, Algeria;Badji Mokhtar Univ, Dept Phys, LEREC Lab, Annaba, Algeria.
    Labidi, Salima
    Badji Mokhtar Univ, Dept Phys, LNCTS Lab, Annaba, Algeria.
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Labidi, Malika
    Badji Mokhtar Univ, Dept Phys, LNCTS Lab, Annaba, Algeria.
    Amara, Abdelaziz
    Badji Mokhtar Univ, Dept Phys, LEREC Lab, Annaba, Algeria.
    Bououdina, Mohamed
    Univ Bahrain, Coll Sci, Dept Phys, POB Box 32038, Zallaq, Bahrain.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat, S-10044 Stockholm, Sweden.
    Composition dependent tuning of electronic and magnetic properties in transition metal substituted Rock-salt MgO2019In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 475, p. 44-53Article in journal (Refereed)
    Abstract [en]

    Full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT) is used to investigate the structural, electronic and magnetic properties of Fe and Ni (3d transition metal) substituted Rock-salt wide band gap insulator Mg1-xMxO (M = Fe, Ni). We have performed spin polarized calculations throughout this work with generalized gradient approximation (GGA) type exchange correlation functional. Additionally, the electronic structures and density of states are computed using modified Becke-Johnson (mBJ) potential based approximation with the inclusion of coulomb energy (U = 7 eV). Based on the Vegard's law and structural optimization, the lattice parameter and bulk modulus are found to be in good agreement with experimental values. Moreover, the analysis of electronic band structures reveals an insulating character for Ni substituted MgO while semiconducting and half-metallic character for Fe substituted case. It has been found that the p-d super-exchange interaction provides a ferromagnetic character due to the 3d transition metal impurities and oxygen atom. The observed p-d hybridization at the top of the valence band edge in this investigations could be useful for magneto-optic and spintronic applications.

  • 255. Kollár, J.
    et al.
    Vitos, Levente
    Osorio-Guillén, J. M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Calculation of surface stress for fcc transition metals2003In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 68, no 24, p. 245417-Article in journal (Refereed)
    Abstract [en]

    Using the density functional theory, formulated within the framework of the exact muffin-tin orbitals method, we have calculated the surface stress for the (111) free surfaces of the fcc 4d and 5d transition metals. Good agreement is obtained with the available ab initio data for Pd, Ir, and Au, while for Pt we predict a surface stress, which is about 33% lower compared to former theoretical results. The present surface stress values for the 4d and 5d fcc metals show the typical trend characteristic for the cohesive or surface energies of d series.

  • 256. Kong, Chang Sun
    et al.
    Luo, Wei
    Arapan, Sergiu
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Villars, Pierre
    Iwata, Shuichi
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Rajan, Krishna
    Information-Theoretic Approach for the Discovery of Design Rules for Crystal Chemistry2012In: Journal of Chemical Information and Modeling, ISSN 1549-9596, Vol. 52, no 7, p. 1812-1820Article in journal (Refereed)
    Abstract [en]

    In this work, it is shown that for the first time that, using information-entropy-based methods, one can quantitatively explore the relative impact of a wide multidimensional array of electronic and chemical bonding parameters on the structural stability of intermetallic compounds. Using an inorganic AB(2) compound database as a template data platform, the evolution of design rules for crystal chemistry based on an information-theoretic partitioning classifier for a high-dimensional manifold of crystal chemistry descriptors is monitored. An application of this data-mining approach to establish chemical and structural design rules for crystal chemistry is demonstrated by showing that, when coupled with first-principles calculations, statistical inference methods can serve as a tool for significantly accelerating the prediction of unknown crystal structures.

  • 257. Kong, P. P.
    et al.
    Sun, F.
    Xing, L. Y.
    Zhu, J.
    Zhang, S. J.
    Li, W. M.
    Liu, Q. Q.
    Wang, X. C.
    Feng, S. M.
    Yu, X. H.
    Zhu, J. L.
    Yu, R. C.
    Yang, W. G.
    Shen, G. Y.
    Zhao, Y. S.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mao, H. K.
    Jin, C. Q.
    Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se32014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, p. 6679-Article in journal (Refereed)
    Abstract [en]

    Recently, A(2)B(3) type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at similar to 3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (T-C) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of T-C slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

  • 258. Kotmool, Komsilp
    et al.
    Bovornratanaraks, Thiti
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    High-Pressure Phase Transition of ZnO Nanorods Using Density Functional Theory2014In: Integrated Ferroelectrics, ISSN 1058-4587, E-ISSN 1607-8489, Vol. 156, no 1, p. 122-128Article in journal (Refereed)
    Abstract [en]

    We constructed ZnO nanorods combining energy of surface and core atoms within B4 and B1 phases. Their morphologies were directly built up based on previous experiments and transformation path. ZnO nanorods were varied in diameter and length. By using density functional theory with GGA exchange-correlation functional, the surfaces were cleaved from the optimized crystal structures at various different pressures in both B4 and B1 phases. A slab model with a fixed top atomic layer was employed to achieve the surfaces under high pressure. The finding shows transition pressure increases with decreasing diameter and as well as increasing length of nanorods, which is in good agreement with previous experiments.

  • 259. Kotmool, Komsilp
    et al.
    Bovornratanaraks, Thiti
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The effect of morphology and confinement on the high-pressure phase transition in ZnO nanostructure2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 11, article id 114309Article in journal (Refereed)
    Abstract [en]

    The transition pressure (Pt) of the B4-to-B1 phase transformation of zinc oxide nanoparticle (n-ZnO) structures was investigated in terms of their size and morphology. Nanorods, nanopencils, nanopyramids, nanowires, and nanotubes of the B4 phase in various sizes were directly built up by accounting for the atomic basis of the core and surface regions. The previously proposed transformation path was performed for constructing shapes and sizes compatible with B1 phases. Using systematic density functional theory, the surfaces were cleaved from the optimized crystal structures at different pressures in both the B4 and B1 phases. A method for calculating the surface energy at different pressures is proposed using an asymmetric slab model. Using the proposed model, the transition pressure of n-ZnO structures was found to significantly depend on their morphology and size, which is in good agreement with the available experimental reports.  

  • 260.
    Kotmool, Komsilp
    et al.
    Mahidol Wittayanusorn Sch, Dept Phys, Nakhon Pathom 73170, Thailand.;Thailand Ctr Excellence Phys, Commiss Higher Educ, Bangkok 10400, Thailand..
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bovornratanaraks, Thiti
    Thailand Ctr Excellence Phys, Commiss Higher Educ, Bangkok 10400, Thailand.;Chulalongkorn Univ, Fac Sci, Dept Phys, ECPRL, Bangkok 10330, Thailand..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..
    Role of relativity in high-pressure phase transitions of thallium2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 42983Article in journal (Refereed)
    Abstract [en]

    We demonstrate the relativistic effects in high-pressure phase transitions of heavy element thallium. The known first phase transition from h. c. p. to f. c. c. is initially investigated by various relativistic levels and exchange-correlation functionals as implemented in FPLO method, as well as scalar relativistic scheme within PAW formalism. The electronic structure calculations are interpreted from the perspective of energetic stability and electronic density of states. The full relativistic scheme (FR) within L(S) DA performs to be the scheme that resembles mostly with experimental results with a transition pressure of 3 GPa. The s-p hybridization and the valence-core overlapping of 6s and 5d states are the primary reasons behind the f. c. c. phase occurrence. A recent proposed phase, i. e., a body-centered tetragonal (b. c. t.) phase, is confirmed with a small distortion from the f. c. c. phase. We have also predicted a reversible b. c. t. -> f. c. c. phase transition at 800 GPa. This finding has been suggested that almost all the III-A elements (Ga, In and Tl) exhibit the b. c. t. -> f. c. c. phase transition at extremely high pressure.

  • 261. Kotmool, Komsilp
    et al.
    Kaewmaraya, Thanayut
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Anversa, Jonas
    Bovornratanaraks, Thiti
    Luo, Wei
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Gou, Huiyang
    Piquini, Paulo Cesar
    Kang, Tae Won
    Mao, Ho-Kwang
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Revealing an unusual transparent phase of superhard iron tetraboride under high pressure2014In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 111, no 48, p. 17050-17053Article in journal (Refereed)
    Abstract [en]

    First principles-based electronic structure calculations of super-hard iron tetraboride (FeB4) under high pressure have been undertaken in this study. Starting with a "conventional" superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with Pnnm and I4(1)/acd space groups, respectively. In this work, the electron-phonon coupling-derived superconducting T-c has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.

  • 262.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Belonoshko, A. B.
    Ab initio and classical molecular dynamics of neon melting at high pressure2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 21, p. 214108-Article in journal (Refereed)
    Abstract [en]

    First-principles and classical molecular dynamics calculations have been performed to study the high-pressure melting curve of Ne. In the low temperature region, simulations with solid and liquid in coexistence (two-phase) with a Lennard-Jones interatomic potential well reproduce experimental findings. As anticipated, there is a melting temperature overestimation when heating a crystal (one-phase) compared to the two-phase results. Furthermore, there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions: at 150GPa , the calculations in this work show a melting temperature approximately 1000K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This could also imply an agreement between a two-phase ab initio and classical two-phase melting curve. Therefore, considering the documented accuracy of the coexistence method, the classical two-phase melting in this work could well indicate the most probable melting behavior. In conjunction with recent theoretical results for Xe, no significant melting slope decrease was observed for Ne in this study.

  • 263.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Belonoshko, A. B.
    Johansson, B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Study of the high-pressure helium phase diagram using molecular dynamics2007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 1, p. 016206-Article in journal (Refereed)
    Abstract [en]

    The rich occurrence of helium and hydrogen in space makes their properties highly interesting. By means of molecular dynamics ( MD), we have examined two interatomic potentials for He-4. Both potentials are demonstrated to reproduce high-pressure solid and liquid equation of state (EOS) data. The EOS, solid - solid transitions and melting at high pressures ( P) were studied using a two-phase method. The Buckingham potential shows a good agreement with theoretical and experimental EOS, but does not reproduce experimental melting data. The Aziz potential shows a perfect match with theoretical melting data. We conclude that there is a stable body-centred-cubic (bcc) phase for He-4 at temperatures ( T) above 340 K and pressures above 22 GPa for the Buckingham potential, whereas no bcc phase is found for the Aziz potential in the applied PT range.

  • 264.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Pinsook, U.
    Melting of Na at high pressure from ab initio calculations2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 13, p. 132101-Article in journal (Refereed)
    Abstract [en]

    The alkali metals continue to attract attention due to their enigmatic melting behavior at high pressure. In this study, ab initio molecular dynamics (AIMD) calculations for Na using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy were performed. The melting temperature (T-m) results from the AIMD at extreme conditions confirm the recent findings of a T-m maximum and show a decrease of similar to 200 K between 45 and 120 GPa. Furthermore, from elastic constant calculations using the exact muffin-tin orbitals method, a Lindemann model curve was calculated. The model reproduces the trend of the experimental results of Gregoryanz [Phys. Rev. Lett. 94, 185502 (2005)] who reported an extraordinarily big melting temperature drop of similar to 700 K between 31 and 118 GPa.

  • 265.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Belonoshko, A. B.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Molecular dynamics calculation of liquid iron properties and adiabatic temperature gradient in the Earth's outer core2007In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 168, no 2, p. 890-894Article in journal (Refereed)
    Abstract [en]

    The knowledge of the temperature radial distribution in the Earth's core is important to understand the heat balance and conditions in the Earth's interior. Molecular dynamics (MD) simulations were applied to study the properties of liquid iron under the pressure-temperature conditions of the Earth's outer core. It is shown that the model used for the MD simulations can reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. Applying this model for higher pressures, the calculated densities and diffusion parameters agree well with the results of first-principles. The MD calculations indicate that a reasonable estimate of the adiabatic temperature profile in the Earth's outer core could be evaluated.

  • 266.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Belonoshko, A. B.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Molecular dynamics study of liquid iron under high pressure and high temperature2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 22, article id 224113Article in journal (Refereed)
    Abstract [en]

    Unlike ab initio calculations, classical potentials in molecular dynamics (MD) allow the treatment of very large atomic systems. However, the quality of the potentials has to be carefully assessed, making the evaluation of the models important. By means of MD simulations, we have studied the properties of liquid iron (Fe) with an embedded-atom method (EAM) under extreme pressure-temperature conditions, to resemble the conditions found in the Earth's outer core. The model used for the MD simulations is demonstrated to reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. The calculations of densities and diffusion parameters at the various temperatures and pressures agree well with first-principles calculations. The results indicate the model's validity for the physical description of liquid iron at extreme conditions.

  • 267.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Bringa, E. M.
    Ivanov, D. S.
    Hawreliak, J.
    McNaney, J.
    Higginbotham, A.
    Zhigilei, L. V.
    Belonoshko, A. B.
    Remington, B. A.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Simulation of shock-induced melting of Ni using molecular dynamics coupled to a two-temperature model2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 1, p. 012101-Article in journal (Refereed)
    Abstract [en]

    Using nonequilibrium molecular dynamics (MD) simulations we study shock-induced melting in Ni with an embedded atom method (EAM). Dynamic melting is probed by the pair correlation function, and we find a melting lattice temperature of T-melt=6400 +/- 300 K for a melting pressure of P-melt=275 +/- 10 GPa. When a combined MD+TTM (two-temperature model) approach is used to include electronic heat conduction and electron-phonon coupling, P-melt and T-melt change. For a given pressure, the temperature behind the shock decreases due to electronic heat diffusion into the cold, unshocked material. This cooling of the material behind the shock slightly increases the melting pressure compared to simulations without electronic heat conduction and electron-phonon coupling. The decrease in the temperature behind the shock front is enhanced if the electron-phonon coupling is artificially made larger. We also explore the feasibility of using x-ray diffraction to detect melting.

  • 268.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ma, Y.
    Oganov, A. R.
    Souvatzis, Petros
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Elasticity study of the superconducting metals V, Nb, Ta, Mo and W at high pressure2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Physical Review B, Vol. 77, no 21, p. 214101-Article in journal (Refereed)
    Abstract [en]

    First-principles calculations have been performed for V, Nb, Ta, Mo, and W. The recently discovered bcc -> , rhombohedral transition for vanadium [Phys. Rev. Lett. 98, 085502 (2007)] was confirmed as the mechanical instability of c(44) was found at P=80 GPa. Furthermore, the c(11), c(12), and c(44) constants for the group-V elements showed erratic behaviors whereas the constants for the group-VI elements were monotonically increasing with pressure. The metals were analyzed with Fermi surface calculations, showing shrinking nesting vectors with pressure for V, Nb, and Ta but were not seen for Mo and W. From electronic topological transition contributions, a critical energy closely situated to the Fermi level for vanadium could be the reason why the elastic constants of V and Nb were difficult to reproduce at ambient pressure.

  • 269.
    Koči, Love
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ab initio calculations of the elastic properties of ferropericlase Mg(1-x)Fe(x)O (x<=0.25)2007In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 164, no 3-4, p. 177-185Article in journal (Refereed)
    Abstract [en]

    Ferropericlase Mg1-xFexO is believed to be the second most abundant mineral in the Earth's mantle. Therefore, the electronic and elastic properties of ferropericlase are important for the understanding of the Earth's interior. Ab initio total energy calculations have been performed for Fe concentrations x ≤ 0.25. The equation of state (EOS) clearly shows a volume expansion as a function of Fe concentration, consistent with experimental data. Magnetic moment calculations as a function of pressure show a high-spin to low-spin transition of Fe2+, and the theoretical transition pressure increases with iron composition. At ambient pressure, we have found that the shear constant C44 reproduces well the experimental data as a function of Fe concentration. The MgO and Mg0.9Fe0.1O minerals show an increasing C44 with pressure, whereas the Δ C44 / Δ P is slightly negative after 26 GPa for Mg0.8Fe0.2O. The C44 softening could be related to the transition from the cubic to a rhombohedrally distorted phase, recently found by experiment.

  • 270. Krisch, Michael
    et al.
    Farber, D. L.
    Xu, R.
    Antonangeli, Daniele
    Aracne, C. M.
    Beraud, Alexandre
    Chiang, Tai-Chang
    Zarestky, J.
    Kim, Duck Young
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Isaev, Eyvaz I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Phonons of the anomalous element cerium2011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 23, p. 9342-9345Article in journal (Refereed)
    Abstract [en]

    Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural gamma-alpha transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the gamma -> alpha transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Gruneisen parameters, particularly around the X point. We derive a vibrational entropy change Delta S-vib(gamma-alpha) approximate to (0.33 +/- 0.03)k(B), illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure.

  • 271. Kulkarni, Shrinivas R.
    et al.
    Vennila, R. Selva
    Phatak, Nishad A.
    Saxena, S. K.
    Zha, C. S.
    El-Raghy, T.
    Barsoum, M. W.
    Luo, Wei
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Study of Ti2SC under compression up to 47 GPa2008In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 448, no 1-2, p. L1-L4Article in journal (Refereed)
    Abstract [en]

    The pressure dependence of the lattice parameters of the ternary layered carbide, Ti2SC, was measured by using synchrotron radiation X-ray diffraction and a diamond anvil cell setup. The experiment was conducted at room temperature and no phase transformation was observed up to the maximum pressure of 47 GPa. The a and c lattice parameters at room condition are 3.216 (A) over circle and 11.22 (A) over circle, respectively. The bulk modulus, calculated using the Birch-Murnaghan equation of state, is 191 +/- 3 GPa, with a pressure derivative of 4.0 +/- 0.3 and that obtained by our ab initio calculations is 183 GPa, with a pressure derivative of 4.1. L Like the majority of the ternary layered carbides (MAX phases), compressibility along the c-axis was higher than that along the a-axis.

  • 272.
    Kumar, Sunil
    et al.
    Dongguk Univ Seoul, Nano Informat Technol Acad, Seoul 100715, South Korea.
    Magotra, Verjesh Kumar
    Dongguk Univ Seoul, Nano Informat Technol Acad, Seoul 100715, South Korea.
    Jeon, H. C.
    Dongguk Univ Seoul, Nano Informat Technol Acad, Seoul 100715, South Korea.
    Kang, T. W.
    Dongguk Univ Seoul, Nano Informat Technol Acad, Seoul 100715, South Korea.
    Inamdar, Akbar I.
    Dongguk Univ, Div Phys & Semicond Sci, Seoul 100715, South Korea.
    Aqueel, Abu Talha
    Dongguk Univ, Div Phys & Semicond Sci, Seoul 100715, South Korea.
    Im, Hyunsik
    Dongguk Univ, Div Phys & Semicond Sci, Seoul 100715, South Korea.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multifunctional ammonium fuel cell using compost as a novel electro-catalyst2018In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 402, p. 221-228Article in journal (Refereed)
    Abstract [en]

    Due to acute ammonium toxicity, it is always desirable to find a cheaper and abundant electro-catalyst other than platinum, iridium oxide, boron diamond etc with a high selectivity and negligible de-activation for its oxidation. Also ammonium is not known for electricity generation except biological nitrification process. So this paper elucidates the studies of compost as a novel electro-catalyst in a ammonium fuel cell configuration. These studies are done by varying type of electrodes & compost as well as ammonium concentration. Bi-polar cyclic voltammetry, electrochemical impedance spectroscopy, temperature dependence, cyclic stability and chronoamperometry techniques are used to study compost. Cow dung based compost is found to show the best electro-catalytic activity. IV measurements are conducted to study power generation in tune with the electro-catalytic activity. Finally, polarization and sustainability measurements are done on a comparatively larger fuel cell to check the size scalability. The results shows that the maximum power density is 108 mW/m(2) and this multifunctional device can be fueled after every 12 h for continuous operation and with negligible de-activation of electro-catalyst. These studies opens a window for doing further advanced research in compost triggered electro-catalysis to make multifunctional fuel cell devices for solving environmental and energy issues together.

  • 273.
    Kwon, S. K.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Nabi, Z.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Kádas, Krisztina
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Kollar, J.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Surface Energy and Stress Release by Layer Relaxation2005In: Physical Review B, Vol. 72, p. 235423-Article in journal (Refereed)
    Abstract [en]

    The influence of carbon impurities on the properties of iron phases (bcc, hcp, dhcp, fcc) has been studied using the first-principles projector augmented-wave (PAW) method for a wide pressure range. It is shown that the presence of ~6 at. % of interstitial carbon has a little effect on the calculated structural sequence of the iron phases under high pressure. The bcc → hcp transition both for pure iron and iron containing carbon takes place around 9 GPa. According to the enthalpies comparison, the solubility of carbon into the iron solid is decreased by high pressure. The coexistence of iron carbide (Fe3C) + pure hcp Fe is most stable phase at high pressure compared with other phases. Based on the analysis of the pressure-density dependences for Fe3C and hcp Fe, we suggest that there might be some fraction of iron carbide present in the core.

  • 274.
    Kádas, Krisztina
    et al.
    Applied Materials Physics, Department of Materials Science and Engineering, The Royal Institute of Technology, Stockholm.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thermo-physical properties of body-centered cubic iron-magnesium alloys under extreme conditions2011In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 151, no 3, p. 203-207Article in journal (Refereed)
    Abstract [en]

    Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron-magnesium alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi-Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus (C’). Magnesium alloying significantly increases C’ of Fe, and bcc Fe-Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe-Mg alloy with 5% Mg agree well with those of the Earth’s inner core as provided by seismic observations.

  • 275. Kádas, Krisztina
    et al.
    Nabi, Z.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Kwon, S. K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Kollar, J.
    Surface relaxation and surface stress of $4d$ transition metals2006In: Surface Science, Vol. 600, p. 395-402Article in journal (Refereed)
  • 276.
    Kádas, Krisztina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Elastic properties of iron-rich hcp Fe-Mg alloys up to Earth's core pressures2008In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 271, no 1-4, p. 221-225Article in journal (Refereed)
    Abstract [en]

    Using density functional theory formulated within the framework of the exact muffin-tin orbital method, we investigate the elastic properties of hexagonal closed-packed Fe-Mg alloys, containing 5 and 10 at.% Mg, up to pressures of the Earth's inner core. We demonstrate the effect of Mg alloying on the hexagonal axial ratio, elastic constants, density and sound wave velocities. We find that 10% Mg alloying decreases the shear modulus of iron by 23% and reduces the transverse sound velocity, nu(s) by 12% at core pressures. Although it is debated whether or not Mg can partition into the core, our results support Mg as a candidate light element in the core.

  • 277.
    Kádas, Krisztina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory.
    Theoretical evidence of a superconducting transition in doped silicon and germanium driven by a variation of chemical composition2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 5, p. 052505-Article in journal (Refereed)
    Abstract [en]

    We present the first theoretical evidence of chemical composition driven superconductivity in acceptor-doped silicon and germanium, using density functional theory. We examine the concentration dependence of T-c in B-doped Si and Ge and predict that B-doped Ge is a superconductor with a slightly higher T-c than B-doped Si. We show that there is a critical concentration above which B-doped Si and Ge become superconducting and estimate it to be similar to 2.6% in Si:B and similar to 2.2% in Ge:B. Considering the Al-doped Si and Ge, we point out the decisive role of the chemical element in the hole-doping scenario.

  • 278.
    Kádas, Krisztina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Johansson, Borje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Kollar, Janos
    Temperature dependent elastic properties of alpha-beryllium from first principles2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 23, p. 235109-Article in journal (Refereed)
    Abstract [en]

    Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the temperature dependence of the structural parameters and the elastic properties of the hexagonal closed-packed phase of Be (alpha-Be). We find that the elastic constants follow a normal behavior with temperature: decrease with increasing temperature with a slightly increasing slope. Up to the melting point, the monocrystalline elastic constants decrease by an average of 16% and the polycrystalline elastic constants by 10%. These trends contradict the large temperature factor observed in high-temperature direct pulse ultrasonic experiments. At the same time, the low-temperature pulse echo measurements confirm the present theoretical findings. Our results call for further accurate experimental studies on the elastic properties of alpha-Be at high temperatures.

  • 279.
    Kádas, Krisztina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Stability of body-centered cubic iron-magnesium alloys in the Earth's inner core2009In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 106, no 37, p. 15560-15562Article in journal (Refereed)
    Abstract [en]

    The composition and the structure of the Earth's solid inner core are still unknown. Iron is accepted to be the main component of the core. Lately, the body-centered cubic (bcc) phase of iron was suggested to be present in the inner core, although its stability at core conditions is still in discussion. The higher density of pure iron compared with that of the Earth's core indicates the presence of light element(s) in this region, which could be responsible for the stability of the bcc phase. However, so far, none of the proposed composition models were in full agreement with seismic observations. The solubility of magnesium in hexagonal Fe has been found to increase significantly with increasing pressure, suggesting that Mg can also be an important element in the core. Here, we report a first-principles density functional study of bcc Fe-Mg alloys at core pressures and temperatures. We show that at core conditions, 5-10 atomic percent Mg stabilizes the bcc Fe both dynamically and thermodynamically. Our calculated density, elastic moduli, and sound velocities of bcc Fe-Mg alloys are consistent with those obtained from seismology, indicating that the bcc-structured Fe-Mg alloy is a possible model for the Earth's inner core.

  • 280.
    Lai, Qiwen
    et al.
    Univ New S Wales, Sch Chem Engn, MERLin Grp, Sydney, NSW 2052, Australia..
    Paskevicius, Mark
    Aarhus Univ, Dept Chem, DK-8000 Aarhus, Denmark.;Aarhus Univ, iNANO, DK-8000 Aarhus, Denmark.;Curtin Univ, Dept Phys Astron & Med Radiat Sci, Bentley, WA 6102, Australia..
    Sheppard, Drew A.
    Curtin Univ, Dept Phys Astron & Med Radiat Sci, Bentley, WA 6102, Australia..
    Buckley, Craig E.
    Curtin Univ, Dept Phys Astron & Med Radiat Sci, Bentley, WA 6102, Australia..
    Thornton, Aaron W.
    CSIRO, Clayton, Vic 3169, Australia..
    Hill, Matthew R.
    CSIRO, Clayton, Vic 3169, Australia..
    Gu, Qinfen
    Australian Synchrotron, Clayton, Vic 3168, Australia..
    Mao, Jianfeng
    Univ Wollongong, Inst Superconducting & Elect Mat, Wollongong, NSW 2500, Australia..
    Huang, Zhenguo
    Univ Wollongong, Inst Superconducting & Elect Mat, Wollongong, NSW 2500, Australia..
    Liu, Hua Kun
    Univ Wollongong, Inst Superconducting & Elect Mat, Wollongong, NSW 2500, Australia..
    Guo, Zaiping
    Univ Wollongong, Inst Superconducting & Elect Mat, Wollongong, NSW 2500, Australia..
    Banerjee, Amitava
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chakraborty, Sudip
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Aguey-Zinsou, Kondo-Francois
    Univ New S Wales, Sch Chem Engn, MERLin Grp, Sydney, NSW 2052, Australia..
    Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art2015In: ChemSusChem, ISSN 1864-5631, E-ISSN 1864-564X, Vol. 8, no 17, p. 2789-2825Article in journal (Refereed)
    Abstract [en]

    One of the limitations to the widespread use of hydrogen as an energy carrier is its storage in a safe and compact form. Herein, recent developments in effective high-capacity hydrogen storage materials are reviewed, with a special emphasis on light compounds, including those based on organic porous structures, boron, nitrogen, and aluminum. These elements and their related compounds hold the promise of high, reversible, and practical hydrogen storage capacity for mobile applications, including vehicles and portable power equipment, but also for the large scale and distributed storage of energy for stationary applications. Current understanding of the fundamental principles that govern the interaction of hydrogen with these light compounds is summarized, as well as basic strategies to meet practical targets of hydrogen uptake and release. The limitation of these strategies and current understanding is also discussed and new directions proposed.

  • 281.
    Larsson, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Liivat, Anti
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Thomas, John O.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Structural and electrochemical aspects of Mn substitution into Li2FeSiO4 from DFT calculations2010In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 47, no 3, p. 678-684Article in journal (Refereed)
    Abstract [en]

    DFT calculations are presented which probe the effect of low-concentration Mn substitution of the Fe-sites in Li2FeSiO4: the promising new and potentially cheap cathode material for upscaled Li-ion battery applications. The LixFe0.875Mn0.125SiO4 System investigated could be achieved by replacing 12.5% of the Fe-sites in 2 x 2 x 1 and 2 x 2 x 2 supercells by Mn ions. The evolution of Bader charges and partial densities of states (DOS) have been followed under a stepwise delithiation process. A clear structural distortion is seen to occur at the Mn-site on delithiation, suggesting possible structural instability. Oxidation of Mn beyond 3+ is calculated to occur at potentials in excess of 4.7 V, implying that oxidation of well separated (>10 angstrom) low-concentration Mn ions to Mn4+ is energetically unfavourable in the LixFe0.875Mn0.125SiO4 structure. This, together with previous DFT results for higher levels of Mn substitution into Li2FeSiO4, indicates that capacity increase in Li2Fe1 (-) yMnySiO4 through a > 1 electron redox reaction may not be so readily attainable in practice, either for high or low Mn concentrations.

  • 282.
    Larsson, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Araujo, C. Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Larsson, J. Andrea
    Jena, Puru
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Role of catalysts in dehydrogenation of MgH2 nanoclusters2008In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 105, no 24, p. 8227-8231Article in journal (Refereed)
    Abstract [en]

    A fundamental understanding of the role of catalysts in dehydrogenation of MgH2 nanoclusters is provided by carrying out first-principles calculations based on density functional theory. It is shown that the transition metal atoms Ti, V, Fe, and Ni not only lower desorption energies significantly but also continue to attract at least four hydrogen atoms even when the total hydrogen content of the cluster decreases. In particular, Fe is found to migrate from the surface sites to the interior sites during the dehydrogenation process, releasing more hydrogen as it diffuses This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2. which is essential for the use of this material for hydrogen storage in fuel-cell applications.

  • 283.
    Larsson, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Araújo, C. Moysés
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Larsson, J. Andreas
    Jena, Puru
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Role of Catalyst in Dehydrogenation of MgH2 Nanoclusters2008In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 105, no 24, p. 8227-8231Article in journal (Refereed)
  • 284.
    Larsson, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Larsson, J. Andreas
    Tyndall National Institute.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ding, Feng
    Fysiska Institutionen, Göteborgs Universitet.
    Yakubson, Boris I.
    ME&MS Department, Rice University, Houston, Texas.
    Duan, Haiming
    Fysiska Institutionen, Göteborgs Universitet.
    Rosén, Arne
    Fysiska Institutionen, Göteborgs Universitet.
    Bolton, Kim
    School of Engineering, University College of Borås.
    Calculating carbon nanotube–catalyst adhesion strengths2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 11, p. 115419-Article in journal (Refereed)
    Abstract [en]

    Density-functional theory is used to assess the validity of modeling metal clusters as single atoms or rings of atoms when determining adhesion strengths between clusters and single-walled carbon nanotubes (SWNTs). Representing a cluster by a single atom or ring gives the correct trends in SWNT-cluster adhesion strengths (Fe ≈ Co > Ni), but the single-atom model yields incorrect minimum-energy structures for all three metals. We have found that this is because of directional bonding between the SWNT end and the metal cluster, which is captured in the ring model but not by the single atom. Hence, pairwise potential models that do not describe directional bonding correctly, and which are commonly used to study these systems, are expected to give incorrect minimum-energy structures.

  • 285. Lebegue, Sebastien
    et al.
    Araujo, Carlos Moyses
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kim, Duck Young
    Ramzan, Muhammad
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mao, Ho-kwang
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Semimetallic dense hydrogen above 260 GPa2012In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 109, no 25, p. 9766-9769Article in journal (Refereed)
    Abstract [en]

    Being the lightest and the most abundant element in the universe, hydrogen is fascinating to physicists. In particular, the conditions of its metallization associated with a possible superconducting state at high temperature have been a matter of much debate in the scientific community, and progress in this field is strongly correlated with the advancements in theoretical methods and experimental techniques. Recently, the existence of hydrogen in a metallic state was reported experimentally at room temperature under a pressure of 260-270 GPa, but was shortly after that disputed in the light of more experiments, finding either a semimetal or a transition to an other phase. With the aim to reconcile the different interpretations proposed, we propose by combining several computational techniques, such as density functional theory and the GW approximation, that phase III at ambient temperature of hydrogen is the Cmca-12 phase, which becomes a semimetal at 260 GPa. From phonon calculations, we demonstrate it to be dynamically stable; calculated electron-phonon coupling is rather weak and therefore this phase is not expected to be a high-temperature superconductor.

  • 286.
    Lebègue, Sébastien
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Araújo, C. Moysés
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Arnaud, B.
    Alouani, M.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Quasiparticle and optical properties of BeH22007In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 3, p. 036223-Article in journal (Refereed)
    Abstract [en]

    The quasiparticle and optical properties of BeH2 are computed by means of the all-electron GW approximation in conjunction with the projector augmented wave (PAW) method. The GW approximation, through the calculation of the self-energy and the optical dielectric function in the random phase approximation, shows that BeH2 is a large band gap insulator. The results are discussed in view of future experiments.

  • 287. Lee, J. -Y
    et al.
    Punkkinen, M. P. J.
    Univ Turku, Dept Phys & Astron, FI-20014 Turku, Finland.
    Schönecker, S.
    KTH Royal Inst Technol, Appl Mat Phys, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden.
    Nabi, Z.
    Univ Djillali Liabes Sidi Bel Abbes, Lab Catalysis & React Syst, Sidi Bel Abbes 22000, Algeria.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Wigner Res Ctr Phys, Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary.
    Zolyomi, V.
    Wigner Res Ctr Phys, Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary;Univ Manchester, Natl Graphene Inst, Manchester M13 9PL, Lancs, England.
    Koo, Y. M.
    Pohang Univ Sci & Technol, Grad Inst Ferrous Technol, Pohang 37673, South Korea.
    Hu, Q. -M
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Appl Mat Phys, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Appl Mat Phys, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden.
    Kollar, J.
    Wigner Res Ctr Phys, Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Appl Mat Phys, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden;Wigner Res Ctr Phys, Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary.
    Kwon, S. K.
    Pohang Univ Sci & Technol, Grad Inst Ferrous Technol, Pohang 37673, South Korea.
    The surface energy and stress of metals2018In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 674, p. 51-68Article in journal (Refereed)
    Abstract [en]

    We investigated surface properties of metals by performing first-principles calculations. A systematic database was established for the surface relaxation, surface energy (gamma), and surface stress (tau) for metallic elements in the periodic table. The surfaces were modeled by multi-layered slab structures along the direction of low-index surfaces. The surface energy gamma of simple metals decreases as the atomic number increases in a given group, while the surface stress tau has its minimum in the middle. The transition metal series show parabolic trends for both gamma and tau with a dip in the middle. The dip occurs at half-band filling due to a long-range Friedel oscillation of the surface charge density, which induces a strong stability to the Peierls-like transition. In addition, due to magnetic effects, the dips in the 3d metal series are shallower and deeper for gamma and tau respectively, than those of the 4d and 5d metals. The surface stress of the transition metals is typically positive, only Cr and Mn have a negative tau for the (100) surface facet, indicating that they are under compression. The light actinides have an increasing gamma trend according to the atomic number. The present work provides a useful and consistent database for the theoretical modelling of surface phenomena.

  • 288. Lee, Jae-chul
    et al.
    Lee, Ji-eun
    Lee, Ju-won
    Lee, Jae-choon
    Subramaniam, N. G.
    Kang, Tae-won
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Se concentration dependent band gap engineering in ZnO1-xSex thin film for optoelectronic applications2014In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 585, p. 94-97Article in journal (Refereed)
    Abstract [en]

    ZnO1-xSex films with various selenium concentrations are deposited on the sapphire substrate (0001) by pulsed laser deposition technique. Structural properties of the thin films studied by X-ray diffraction (XRD) and chemical bonding studied by X-ray photoelectron spectroscopy (XPS) reveals that Se is substituted in O site during the growth of ZnO1-xSex films. Optical properties are analyzed by UV-Visible spectrometer. From the plot for (alpha h upsilon)(2) vs photon energy, it is inferred that the band gap energy of ZnO1-xSex gradually reduces to 2.85 eV with increasing Se concentration.

  • 289.
    Lee, Juwon
    et al.
    Dongguk Univ, QSRC, Seoul 100715, South Korea..
    Subramaniam, Nagarajan Ganapathi
    Dongguk Univ, QSRC, Seoul 100715, South Korea.;Dongguk Univ, Nano Informat Technol Acad, Seoul 100715, South Korea..
    Kowalik, Iwona Agnieszka
    Polish Acad Sci, Inst Phys, PL-02668 Warsaw, Poland..
    Nisar, Jawad
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Lee, Jaechul
    Dongguk Univ, QSRC, Seoul 100715, South Korea..
    Kwon, Younghae
    Dongguk Univ, QSRC, Seoul 100715, South Korea..
    Lee, Jaechoon
    Dongguk Univ, QSRC, Seoul 100715, South Korea..
    Kang, Taewon
    Dongguk Univ, QSRC, Seoul 100715, South Korea.;Dongguk Univ, Nano Informat Technol Acad, Seoul 100715, South Korea.;Hindustan Univ, Clean Energy & Nano Convergence Ctr CENCON, Madras 603103, Tamil Nadu, India..
    Peng, Xiangyang
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Arvanitis, Dimitri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Towards a new class of heavy ion doped magnetic semiconductors for room temperature applications2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 17053Article in journal (Refereed)
    Abstract [en]

    The article presents, using Bi doped ZnO, an example of a heavy ion doped oxide semiconductor, highlighting a novel p-symmetry interaction of the electronic states to stabilize ferromagnetism. The study includes both ab initio theory and experiments, which yield clear evidence for above room temperature ferromagnetism. ZnBixO1-x thin films are grown using the pulsed laser deposition technique. The room temperature ferromagnetism finds its origin in the holes introduced by the Bi doping and the p-p coupling between Bi and the host atoms. A sizeable magnetic moment is measured by means of x-ray magnetic circular dichroism at the O K-edge, probing directly the spin polarization of the O(2p) states. This result is in agreement with the theoretical predictions and inductive magnetometry measurements. Ab initio calculations of the electronic and magnetic structure of ZnBixO1-x at various doping levels allow to trace the origin of the ferromagnetic character of this material. It appears, that the spin-orbit energy of the heavy ion Bi stabilizes the ferromagnetic phase. Thus, ZnBixO1-x doped with a heavy non-ferromagnetic element, such as Bi, is a credible example of a candidate material for a new class of compounds for spintronics applications, based on the spin polarization of the p states.

  • 290.
    Leifer, Klaus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Blom, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Jafri, Hassan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Löfås, Henrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grigoriev, Anton
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Wallner, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
    Ottosson, Henrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Use of a nanoelectrode nanoparticle bridge platform in molecular electronics2010In: ElecMol’10, 5th International Meeting on Molecular Electronics, Grenoble, France, December 6-10, 2010, 2010, p. 116-116Conference paper (Refereed)
  • 291.
    Leifer, Klaus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Blom, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Jafri, S.Hassan M.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Welch, Ken
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Strömme, Maria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
    Löfås, Henrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Grigoriev, Anton
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    FIB Fabrication and use of high resistance nanogaps for application in molecular electronics2010In: 17th International Microscopy Congress, IMC17, Brazil, 2010Conference paper (Refereed)
  • 292.
    Leifer, Klaus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Jafri, S.Hassan M.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Löfås, Henrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Agustsson, J.
    University Basel, Department of Physics, CH-4056 Basel, Switzerland.
    Blom, Tobias
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics.
    Grigoriev, Anton
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Fransson, J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Wallner, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Biochemistry and Organic Chemistry.
    Calame, M.
    University Basel, Department of Physics, CH-4056 Basel, Switzerland.
    Vibrational signatures in inelastic tunneling spectroscopy from short molecule-nanoparticle chains trapped in versatile nanoelectrodesManuscript (preprint) (Other academic)
  • 293. Li, Bing
    et al.
    Ding, Yang
    Kim, Duck Young
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Zou, Guangtian
    Mao, Ho-Kwang
    Rhodium dihydride (RhH(2)) with high volumetric hydrogen density2011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 46, p. 18618-18621Article in journal (Refereed)
    Abstract [en]

    Materials with very high hydrogen density have attracted considerable interest due to a range of motivations, including the search for chemically precompressed metallic hydrogen and hydrogen storage applications. Using high-pressure synchrotron X-ray diffraction technique and theoretical calculations, we have discovered a new rhodium dihydride (RhH(2)) with high volumetric hydrogen density (163.7 g/L). Compressing rhodium in fluid hydrogen at ambient temperature, the fcc rhodium metal absorbs hydrogen and expands unit-cell volume by two discrete steps to form NaCl-typed fcc rhodium monohydride at 4 GPa and fluorite-typed fcc RhH(2) at 8 GPa. RhH(2) is the first dihydride discovered in the platinum group metals under high pressure. Our low-temperature experiments show that RhH(2) is recoverable after releasing pressure cryogenically to 1 bar and is capable of retaining hydrogen up to 150 K for minutes and 77 K for an indefinite length of time.

  • 294.
    Li, Jibiao
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Unsaturated surface in CO saturation2017In: Surface and Interface Analysis, ISSN 0142-2421, E-ISSN 1096-9918, Vol. 49, no 9, p. 892-897Article in journal (Refereed)
    Abstract [en]

    In this paper, density functional theory calculations are used to address the puzzle of CO saturation on Fe(100). Through a comprehensive modeling of possible local configurations, we would be able to figure out a structural model for the CO-saturated Fe(100). The structural model is featured by forming a local structural motif of coupled on-top and hollow CO molecules. In this model, the on-top-bonded CO molecules pertain to critical couplings in the next neighbor spacing, which exclude all couplings below and above the next neighbor spacing. Rather than forming tilted on-top CO on the surface, the critical couplings align the on-top CO molecules in the perpendicular orientation. The new model with the coverage of 0.5ML yields even better X-ray emission and absorption spectra calculated by using density functional theory and shows consistent agreement with all previous experiments.

  • 295.
    Li, Jibiao
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Yangtze Normal Univ, Chongqing Key Lab Extraordinary Bond Engn & Adv M, Chongqing, Peoples R China; Stockholm Univ, AlbaNova Univ Ctr, Dept Phys, Stockholm, Sweden.
    He, Xin
    Sichuan Univ Arts & Sci, Sch Intelligent Mfg, Dazhou, Peoples R China.
    Peng, Cheng
    Yangtze Normal Univ, Chongqing Key Lab Extraordinary Bond Engn & Adv M, Chongqing, Peoples R China.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chemical Bonding of Unique CO on Fe(100)2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 16, p. 9062-9074Article in journal (Refereed)
    Abstract [en]

    At low coverage, CO molecules are known to preferentially occupy the hollow sites of Fe(100) with considerably inclined molecular orientations. This CO configuration serves as the precursor state of CO dissociation, which is particularly important in several important catalytic reactions. In this study, we present a unique bonding picture of the precursor state from the spin, charge, and orbital perspectives. From the spin and orbital views, we show the antiferromagenetic nature of the adsorbate–metal coupling, where 2π magnetism prevails with a dominant spin-down channel. However, contrasting tendencies are found for the two 1π orbitals in two orthogonal directions: the 1π orbital in the vertical plane loses its symmetry, whereas the other 1π orbital remains intact with a preserved symmetry. The 1π symmetry in the vertical plane favors the 1π → 2π* excitation mechanism owing to the partial opening up of the 1π symmetry. In the charge perspective, we have identified a charge transfer mechanism involving the local structural IFeC–C–O motif, in which the surface slightly charges the adsorbate with additional partial electrons located at the surface Fe atoms bonded to the carbon end, whereas the charges of the metallic atoms beneath the IFeC–C–O motif are found to be depleted. In both the adsorbate and metal sides, the depletion of s electrons serves as a good measure of orbital repulsion and delocalization. Interestingly, the carbon and oxygen ends exhibit contrasting electron affinity with the metal surface: the carbon end is attractive, whereas the oxygen end is repulsive in terms of the contrasting charge rearrangement in the bonded metallic atoms.

  • 296.
    Li, Mingkai
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Hubei Univ, Wuhan, Peoples R China.
    Long, Debing
    Hubei Univ, Wuhan, Peoples R China.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    He, Yunbin
    Hubei Univ, Wuhan, Peoples R China.
    Magnetic order and phase diagram of magnetic alloy system: MgxNi1-xO alloy2017In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 254, no 9, article id 1700085Article in journal (Refereed)
    Abstract [en]

    The antiferromagnetic phase diagram of MgxNi1-xO alloy was reexamined theoretically by multicomponent cluster expansion method. It predicts antiferromagnetic state in the alloy with high Mg content, instead of a mixture of antiferromagnetic and paramagnetic states, which was deduced previously from neutron diffraction results. The theoretically found intermediate antiferromagnetic ground states at Mg contents of x(Mg) = 0.25 and 0.75 exhibit very low neutron diffraction intensities, which leads to confusion with paramagnetic state. Our approach here reduced remarkably the computation time in searching the intermediate ground states as well as calculating the magnetic phase diagram.

  • 297.
    Li, S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Barsoum, M. W.
    Jena, P.
    Johansson, B.
    Optical properties of Ti3SiC2 and Ti4AlN32008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 22, p. 221907-Article in journal (Refereed)
    Abstract [en]

    The dielectric functions of the MAX phases, Ti3SiC2 and Ti4AlN3, have been determined from first principles calculations. We compared the dielectric functions and the reflectivity spectra of Ti3SiC2 and Ti4AlN3 with those of TiC and TiN. The optical spectra were analyzed by means of the electronic structure, which provides theoretical understanding of the conduction mechanism of these two phases. We found that Ti4AlN3 can be used to avoid solar heating and also increase the radiative cooling due to the increased thermal emittance as compared to TiN. Ti4AlN3 can therefore be a candidate coating material for temperature control of space vehicles.

  • 298.
    Li, S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Ahuja, Rajeev
    Wang, Y.
    Johansson, Börje
    Crystallographic structures of PbWO42003In: High Pressure Research, Vol. 23, no 3, p. 343-Article in journal (Refereed)
  • 299. Li, S.
    et al.
    Jena, P.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Dehydrogenation Mechanism in Catalyst-activated MgH22006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 13, p. 132106-Article in journal (Refereed)
    Abstract [en]

    A small amount of Nb2O5 catalyst is known to substantially improve the desorption thermodynamics and kinetics of MgH2. Using density functional theory in combination with ab initio molecular dynamics simulation, we provide theoretical understanding of the mechanism of dehydrogenation in Nb doped MgH2. We show that the substitution of Nb at the Mg site followed by the clustering of H around Nb is a likely pathway for hydrogen desorption. We also find that dehydrogenation from the vicinity of Mg vacancies is exothermic. However, the vacancies are not likely to play a significant role in hydrogen desorption due to their high formation energy (3.87 eV).

  • 300.
    Li, S.
    et al.
    Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, USA.
    Jena, P.
    Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, USA.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Effect of Ti and metal vacancies on the electronic structure, stability, and dehydrogenation of Na3AlH6: Supercell band-structure formalism and gradient-corrected density-functional theory2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 21, article id 214107Article in journal (Refereed)
    Abstract [en]

    Electronic and structural properties of sodium-aluminum hexahydride (Na3AlH6) formed during the decomposition reaction of sodium alanate (NaAlH4) and the effects of Ti catalyst are studied using supercell approach and density-functional theory. The preferred site of Ti has been determined by substituting it at both the Na and Al sites and comparing the respective formation energies. The least unfavorable site for Ti is found to be the Al site. To examine the role of Ti substitution on the desorption of hydrogen, the energy cost to remove a H atom from the vicinity of Ti was calculated and compared with that from the pure Na3AlH6 The improvement in dehydrogenation of Na3AlH6 was found to be due to the weakening the Al-H bond caused by Ti substitution. We also studied the role of metal vacancies on hydrogen desorption. Although this desorption was exothermic, the energies to create these vacancies are high.

3456789 251 - 300 of 587
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf