Ti-alloys have many qualities making them ideal for use in aerospace applications, medical implants and chemical industries such as high strength to weight ratio, good high temperature strength and chemical stability. One downside to Ti-alloys is, however, that they are considered difficult to machine. Several investigations have been made in order to understand the wear mechanisms present in machining of Ti-alloys and the most common understanding is a combination of attrition and dissolution-diffusion. Observations by Odelros et al. [1] have shown that there exists a small layer of pure bcc W on top of the outermost WC grains after turning of Ti-6Al-4V. In order for such a layer to form C has to diffuse away from the WC leaving behind only W. In this work Density Functional Theory (DFT) is used together with Harmonic Transition State Theory (HTST) to investigate the prefactors and barriers for C diffusion into and within two different WC/W interfaces, [0001]/[111] and [10 (1) over bar0]/[100]. The diffusion into the interfaces show that the barrier for the [0001]/[111] interface is more than twice as high as the barrier for the [10 (1) over bar0]/[100] interface. Diffusion within the interfaces show, on average, slightly higher barriers for the [0001]/[111] interface.

The 'missing Xe paradox' is one of the phenomena at the Earth's atmosphere. Studying the 'missing Xe paradox' will provide insights into a chemical reaction of Xe with C. We search the ground-state structure candidates of xenon carbides using the Universal Structure Predictor: Evolutionary Xtallography (USPEX) code, which has been successfully applied to a variety of systems. We predict that XeC2 is the most stable among the convex hull. We find that the I((4) over bar)2m structure of XeC2 is the semiconducting phase. Accurate electronic structures of tetragonal XeC2 have been calculated using a hybrid density functionals HSE06, which gives larger more accurate band gap than a GGA-PBE exchange-correlation functional. Specifically, we find that the I((4) over bar)2m structure of XeC2 is a dynamically stable structure at high pressure. We also predict that the P6/mmm structure of XeC2 is the superconducting phase with a critical temperature of 38 K at 200 GPa. The ground-state structure of xenon carbides is of critical importance for understanding in the missing Xe. We discuss the inference of the stable structures of XeC2. The accumulation of electrons between Xe and C led to the stability by investigating electron localization function (ELF).

Recent upsurge in the two-dimensional (2D) materials have established their larger role on energy storage applications. To this end, Mxene represent a new paradigm extending beyond the realm of oft-explored elemental 2D materials beginning with graphene. Here in, we employed first principles modelling based on density functional theory to investigate the role of S-functionalized Nitride Mxenes as anodes for Li/Na ion batteries. To be specific, V2NS2 and Ti2NS2 have been explored with a focus on computing meaningful descriptors to quantify these 2D materials to be optimally performing electrodes. The Li/Na ion adsorption energies are found to be high (> -2 eV) on both the surfaces and associated with significant charge transfer. Interestingly, this ion intercalation can reach up to multilayers which essentially affords higher specific capacity for the substrate. Particularly, these two 2D materials (V2NS2 and Ti2NS2) have been found to be more suitable for Li-ion batteries with estimated theoretical capacities of 299.52 mAh g(-1) and 308.28 mAh g(-1) respectively. We have also probed the diffusion barriers of ion migration on these two surfaces and these are found to be ultrafast in nature. All these unique features qualify these Mxenes to be potential anode materials for rechargeable batteries and likely to draw imminent attention.

We report the evolution of charge density wave states under pressure for two NbS3 phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above similar to 7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the similar to 6-8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.

We calculate the formation enthalpies of PdHx (x = 0-3) by cluster expansion (CE) and calculations based on density functional theory. CE predicts the stable palladium hydride structures PdH, PdH2.62, and PdH2.75. The band structures and density of states indicate that the amount of hydrogen in the palladium lattice does not alter the metallic character of the palladium significantly. However, all PdH X structures with x > 1 have greater formation enthalpies than that of the given reaction path 4PdH(2) = 2PdH + 2Pd + 3H(2) and thus they are thermodynamically unstable. The shorter bond length of Pd-H and the smaller bond angle of Pd-H-Pd imply a higher cohesive energy in zincblende (ZB) PdH than that in rocksalt (RS) PdH. Bader charge analysis shows a stronger electronegativity of H atoms in ZB-PdH than that in RS-PdH. This results in a stronger Pd-H bond in ZB-PdH than that in RS-PdH. Thus ZB-PdH has lower formation enthalpy than that of RS-PdH. However, regarding the dynamic stability, we conclude that hydrogen atoms prefer to occupy the octahedral sites of the palladium lattice because of the lower zero-point energy and vibration free energy than that of occupying the tetrahedral sites. 

A method to obtain the equivalent Poisson's ratio in chemical bonds as classical beams with finite element method was proposed from experimental data. The UFF (Universal Force Field) method was employed to calculate the elastic force constants of Zr-O bonds. By applying the equivalent Poisson's ratio, the mechanical properties of single-wall ZrNTs (ZrO2 nanotubes) were investigated by finite element analysis. The nanotubes' Young's modulus (Y), Poisson's ratio (nu) of ZrNTs as function of diameters, length and chirality have been discussed, respectively. We found that the Young's modulus of single-wall ZrNTs is calculated to be between 350 and 420 GPa.

A planar honeycomb monolayer of siligraphene (SiC7) could be a prospective medium for clean energy storage due to its light weight, and its remarkable mechanical and unique electronic properties. By employing van der Waals-induced first principles calculations based on density functional theory (DFT), we have explored the structural, electronic, and hydrogen (H-2) storage characteristics of SiC7 sheets decorated with various light metals. The binding energies of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca),scandium (Sc), and titanium (Ti) dopants on a SiC7 monolayer were studied at various doping concentrations, and found to be strong enough to counteract the metal clustering effect. We further verified the stabilities of the metallized SiC7 sheets at room temperature using ab initio molecular dynamics (MD) simulations. Bader charge analysis revealed that upon adsorption, due to the difference in electronegativity, all the metal adatoms donated a fraction of their electronic charges to the SiC7 sheet. Each partially charged metal center on the SiC(7)sheets could bind a maximum of 4 to 5 H-2 molecules. A high H-2 gravimetric density was achieved for several dopants at a doping concentration of 12.50%. The H-2 binding energies were found to fall within the ideal range of 0.2-0.6 eV. Based on these findings, we propose that metal-doped SiC7 sheets can operate as efficient H-2 storage media under ambient conditions.

Ab initio random structure searching (AIRSS) technique is used to identify the high-pressure phases of lithium (Li). We proposed the transition mechanism from the fcc to host-guest (HG) structures at finite temperature and high pressure. This complex structural phase transformation has been calculated using ab initio lattice dynamics with finite displacement method which confirms the dynamical harmonic stabilization of the HG structure. The electron distribution between the host-host atoms has also been investigated by electron localization function (ELF). The strongly localized electron of p bond has led to the stability of the HG structure. This remarkable result put the HG structure to be a common high-pressure structure among alkali metals.

Ab initio random structure searching (AIRSS) technique is predicted a stable structure of arsenic (As). We find that the body-centered tetragonal (bct) structure with spacegroup I4(1)/acd to be the stable structure at high pressure. Our calculation suggests transition sequence from the simple cubic (sc) structure transforms into the host-guest (HG) structure at 41 GPa and then into the bct structure at 81 GPa. The bct structure has been calculated using ab initio lattice dynamics with finite displacement method confirm the stability at high pressure. The spectral function alpha F-2 of the bct structure is higher than those of the body-centered cubic (bcc) structure. It is worth noting that both bct and bcc structures share the remarkable similarity of structural and property. Here we have reported the prediction of temperature superconductivity of the bct structure, with a T-c of 4.2 K at 150 GPa.

The host-guest structure of scandium is described as being built of two penetrating substructures with the incommensurate periods of the channels along the c axis. We present the ideal commensurate value of 4/3 in Sc-II using ab initio calculations. We reveal that the 3c(H) and 4c(G) structures do interpenetrate and combine to the commensurate value of 4/3 of Sc-II at a pressure of 70 GPa. Ab initio molecular dynamics confirms the stability of the commensurate value 4/3 of the host-guest structure at 300 K and 72 GPa. The pressure-induced structural phase transitions in scandium under high pressure up to 200 GPa are investigated. We use ab initio random structure searching to predict the crystal structure of Sc-III: it is the tetragonal structure with space group P4(1)2(1)2. Our calculations show that superconductivity arises in the P4(1)2(1)2 structure. This high pressure structure is not only a superconducting phase but also has been reported for the first time in this group of elements.