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First-Principles Studies of Materials Properties: Pressure-Induced Phase Transitions & Functional Materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Division of Materials Theory)
2015 (English)Doctoral thesis, comprehensive summary (Other academic) [Artistic work]
Abstract [en]

This thesis presents the first-principles studies of materials properties within the framework of the density functional theory (DFT). The thesis constitutes three main parts, i. e., pressure-induced phase transitions in solids, data-storage and clean-energy materials.

The first part focuses on the predictions of crystal structures and the determinations of electronic properties of Xe-H2, FeB4 and Co3O4. Pressurizing Xe-H2 compound yields the formation of H-rich Xe(H2)8, which can exhibit a metallic feature at comparatively lower pressure than pure hydrogen. Hard superconducting FeB4 gets transformed into a novel transparent phase under pressure owing to the enhanced overlap of atomic cores. Spinel Co3O4 undergoes the phase transition from a cubic to a monoclinic because of the charge transfer between cations via the increased 3d-3d interactions.

The second part involves the study of structural and electronic properties of phase-change memory materials (PCMs), i. e., Ge2Sb2Te5 (GST) and Ga-doped In2O3. Van der Waals (vdW) interaction must be considered to obtain accurate crystal structure of layered GST. For Ga-doped In2O3 (GIO), the local structure of amorphous GIO is found to resemble that of amorphous In2O3, except the vicinity of doping atoms. The electronic property of a-GIO is metallic, which considerably differs from the semiconducting feature of the crystalline GIO. This emphasizes the contrast in the conductivity of the crystalline and amorphous upon phase switching of GIO.

The third part associates with the search for clean-energy materials, viz., hydrogen production, hydrogen storage and green Mg-ion batteries. For hydrogen production, the role of intrinsic point defects to water adsorption on ZnO(10-10) surface is investigated. The findings show that the Zn and O defect-sites are energetically not favorable for the water adsorption and dissociation. For the purpose of storing hydrogen in a solid phase, silicene, doped by alkaline and alkaline earth metals, is investigated. We find that Li-doped and Na-doped silicene can attain the superior storage capacity. For cathode material of Mg-ion batteries, Mg2Mo6S8, the diffusivity of Mg ions occurs through an available channel in the bulk with the onset temperature of 200 K.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2015. , 69 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1254
Keyword [en]
Density functional theory, Pressure-induced phase transitions, Ab-initio molecular dynamic, hybrid functional, Ab-initio random structure searching, Phase change material
National Category
Physical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-251343ISBN: 978-91-554-9247-2 (print)OAI: oai:DiVA.org:uu-251343DiVA: diva2:805577
Public defence
2015-06-05, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2015-05-12 Created: 2015-04-15 Last updated: 2015-07-07
List of papers
1. Theoretical investigation of xenon-hydrogen solids under pressure using ab initio DFT and GW calculations
Open this publication in new window or tab >>Theoretical investigation of xenon-hydrogen solids under pressure using ab initio DFT and GW calculations
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, 092101- p.Article in journal (Refereed) Published
Abstract [en]

We have calculated crystal structures and electronic properties of Xe-H(2) compounds under high pressures using first-principles density functional theory calculations and ab-initio random structure searching. We present results for the equation of state, Xe-Xe separations, and the electronic charge transfer between the Xe and H atoms. Our results are broadly consistent with experimental results by M. Somayazulu et al. [Nature Chem. 2, 50 (2010)]. We have in addition calculated the metallization pressure within the GW approximation, finding it to be around 250 GPa, which is close to the maximum pressure reached in the experiment.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-158866 (URN)10.1103/PhysRevB.84.092101 (DOI)000294402200001 ()
Available from: 2011-09-20 Created: 2011-09-19 Last updated: 2017-12-08Bibliographically approved
2. Revealing an unusual transparent phase of superhard iron tetraboride under high pressure
Open this publication in new window or tab >>Revealing an unusual transparent phase of superhard iron tetraboride under high pressure
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2014 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 111, no 48, 17050-17053 p.Article in journal (Refereed) Published
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.

Keyword
metal-semiconductor phase transition, superhard material, first principle study, high pressure, superconductivity
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-240205 (URN)10.1073/pnas.1419244111 (DOI)000345920800029 ()25404295 (PubMedID)
Available from: 2015-01-07 Created: 2015-01-06 Last updated: 2017-12-05Bibliographically approved
3. A new layered monoclinic phase of Co3O4 at high pressure
Open this publication in new window or tab >>A new layered monoclinic phase of Co3O4 at high pressure
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-251335 (URN)
Available from: 2015-04-15 Created: 2015-04-15 Last updated: 2015-05-12
4. Hybrid density functional study of electronic and optical properties of phase change memory material: Ge2Sb2Te5
Open this publication in new window or tab >>Hybrid density functional study of electronic and optical properties of phase change memory material: Ge2Sb2Te5
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 3, 033510- p.Article in journal (Refereed) Published
Abstract [en]

In this article, we use hybrid density functional (HSE06) to study the crystal and electronic structures and optical properties of well known phase change memory material Ge2Sb2Te5. We calculate the structural parameters, band gaps, and dielectric functions of three stable structures of this material. We also analyze the electron charge distribution using the Bader's theory of charge analysis. We find that hybrid density functional slightly overestimates the value of "c" parameter. However, overall, our results calculated with the use of hybrid density functional (HSE06) are very close to available experimental values than calculated with the use of Perdew Burke-Ernzerhof functional. Specifically, the electronic band gap values of this material calculated with HSE06 are in good agreement with the available experimental data in the literature. Furthermore, we perform the charge analysis and find that naive ionic model fails to explain the charge distribution between the constituent atoms, showing the complex nature of this compound.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-195377 (URN)10.1063/1.4775715 (DOI)000313670600020 ()
Available from: 2013-02-25 Created: 2013-02-25 Last updated: 2017-12-06Bibliographically approved
5. Structural and Vibrational Properties of Layered Data Storage Material: Ge2Sb2Te5
Open this publication in new window or tab >>Structural and Vibrational Properties of Layered Data Storage Material: Ge2Sb2Te5
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2013 (English)In: SCI ADV MATER, ISSN 1947-2935, Vol. 5, no 10, 1493-1497 p.Article in journal (Refereed) Published
Abstract [en]

Ge2Sb2Te5 is a prototype material for phase-change memory, while its stable layered phase was recently predicted to be a topological insulator. In this work, we show that standard density functional theory (DFT) calculations provide much larger Te-Te bond length and lattice parameters for layered Ge2Sb2Te5 compared with experimental results. However, by considering van der Waals interactions in DFT calculations, we obtained correct structure information and lattice dynamics properties. It is clear that the discrepancy results from the neglecting the van der Waals interaction between directly weak bonded adjacent Te atoms.

Keyword
Chalcogenide, Density Functional Theory Calculations, Weak Bond, Layered Structure
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-208353 (URN)10.1166/sam.2013.1610 (DOI)000323356200021 ()
Available from: 2013-10-01 Created: 2013-09-30 Last updated: 2015-05-12Bibliographically approved
6. Molecular dynamics study of amorphous Ga-doped In2O3: a promising material for phase change memory devices
Open this publication in new window or tab >>Molecular dynamics study of amorphous Ga-doped In2O3: a promising material for phase change memory devices
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 7, 072113Article in journal (Refereed) Published
Abstract [en]

In this paper, we employ the ab-initio molecular dynamics simulations, within the framework of density functional theory, to construct and characterize the amorphous structure of gallium-doped indium oxide. The electronic properties of amorphous and crystalline structures of Ga:In2O3 are investigated and compared by calculating the density of states, radial distribution function, bond angle distribution, and Bader charge analysis. We observe the band gap closure in amorphous structure, which corresponds to semiconductor to metallic transition in this material on amorphization. Our calculated results show the same characteristics of Ga:In2O3 with other phase change memory (PCM) materials available in literature and suggest it a promising candidate for PCM devices.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-204964 (URN)10.1063/1.4818788 (DOI)000323769000044 ()
Funder
Swedish Research Council
Available from: 2013-08-13 Created: 2013-08-13 Last updated: 2017-12-06Bibliographically approved
7. Water adsorption on ZnO(10(1)over-bar0): The role of intrinsic defects
Open this publication in new window or tab >>Water adsorption on ZnO(10(1)over-bar0): The role of intrinsic defects
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2012 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 97, no 1, 17014- p.Article in journal (Refereed) Published
Abstract [en]

Density functional theory (DFT) calculations have been performed to investigate the interaction of water molecules with bare and defective ZnO(10 (1) over bar0) surfaces. We show that at high coverages water molecules avoid adsorption close to defect sites, whereas at low coverages adsorption on defective surfaces show a similar adsorption pattern to those adsorbed on the defect-free surface, adsorbing in a molecular fashion. Finally we show that the electronic structure of the defective non-polar surface is not much affected by the adsorption of water, with exception of the O-defect surfaces.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-170802 (URN)10.1209/0295-5075/97/17014 (DOI)000300250800045 ()
Available from: 2012-03-13 Created: 2012-03-13 Last updated: 2017-12-07Bibliographically approved
8. Functionalization of hydrogenated silicene with alkali and alkaline earth metals for efficient hydrogen storage
Open this publication in new window or tab >>Functionalization of hydrogenated silicene with alkali and alkaline earth metals for efficient hydrogen storage
2013 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 43, 18900-18905 p.Article in journal (Refereed) Published
Abstract [en]

First principles density functional theory has been employed to investigate the electronic structure along with the stability, bonding mechanism, band gap and charge transfer of metal functionalized hydrogenated silicene (SiH), or silicane, in order to envisage the hydrogen storage capacity. Various metal adatoms including Li, Na, K, Be, Mg and Ca have been doped on the most stable chair like configuration of silicane. The corresponding binding energies and charge transfer mechanism have been discussed from the perspective of H-2 storage ability. The Li and Na metal adatoms have been found to be ideally suitable not only for their strong metal to substrate binding and uniform distribution over the substrate but also for their high capacity for storage of hydrogen. The stability of both Li and Na functionalized SiH has also been confirmed by MD simulations. It was found that both Li+ and Na+ adsorbed four H-2 molecules attaining reasonably high storage capacities of 6.30 wt% and 5.40 wt% respectively with average adsorption energies lying within the range suitable for practical H-2 storage applications, in contrast with alkaline earth metals.

Keyword
Silicane, Hydrogen Storage, Adsorption Energy
National Category
Natural Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-206631 (URN)10.1039/c3cp52830h (DOI)000325943200017 ()
Available from: 2013-09-02 Created: 2013-09-02 Last updated: 2017-12-06Bibliographically approved
9. Electronic structure and ionic diffusion of green battery cathode material: Mg2Mo6S8
Open this publication in new window or tab >>Electronic structure and ionic diffusion of green battery cathode material: Mg2Mo6S8
2014 (English)In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 261, 17-20 p.Article in journal (Refereed) Published
Abstract [en]

We report ab-initio density functional theory calculations of crystal and electronic structure of Mg2Mo6S8, a candidate material to be used in rechargeable magnesium batteries, by employing hybrid exchange-correlation functionals. We find that Mg2Mo6S8 crystalizes in a triclinic geometry and it is a semiconductor with an indirect band gap. Ab-initio molecular dynamics shows that Mg ions present progressive diffusion starting at 200 K with a preferable path through the channel between Mo6S8 blocks along the [010] direction. The intercalation voltage of the system is also determined and the results show that the voltage evaluated by PBE and hybrid functionals likely implies the lower and the upper limit of the experimental value. Lastly, we confirm the dynamical stability of the crystal structure by the calculated phonon dispersion relation. 

Keyword
Mg-ion batteries, First-principle calculations, Hybrid functionals, Molecular dynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-229710 (URN)10.1016/j.ssi.2014.03.023 (DOI)000338819500004 ()
Available from: 2014-08-18 Created: 2014-08-12 Last updated: 2017-12-05Bibliographically approved

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