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.

In this work we report a theoretical investigation on behavior of the elastic constant C-44 and the transverse optical phonon mode E(2)g of a-Hf under pressure within the density functional theory. In contrast to many other reported transition metals, the above two quantities do not show a synchronous relation as pressure increases. Below 13 GPa, an opposite shifting tendency has been observed. However, once the pressure is raised above 13 GPa, the trend is pulled back to be consistent. This anomalous behavior is figured out to be caused by the large lattice anisotropy of the c/a ratio along with the elastic anisotropy. The synchronous behavior is found to be in accordance with the behavior of c/a ratio with increased pressure. In our band-structure investigations the electronic topological transition has been discovered at 10 GPa, which relates to the change of c/a ratio suggested by recent literature. The presence of the Van Hove singularity shown in the densities of states has been identified and regarded as the origin of the variation of C-44 and E(2)g.

We have performed first-principles calculations based on density functional theory to investigate the doping characteristics of 31 different adatoms on stanene monolayer, which includes the elements of alkali metals (AM), alkaline earth metals (AEM), transition metals (TMs), and groups III-VII. The most stable configurations of all the dopants have been explored by calculating and comparing binding energies of all the possible binding sites. To comment on the uniform distribution of adatoms on stanene, the adsorption energies (E-ads) of adatoms have been compared with their experimental cohesive energies (E-c,) in the bulk phase.A further comparison reveals that the binding energies of most of the studied adatoms on stanene are much stronger than other group IV monolayers. Apart from structural and binding characteristics, bond lengths, adatom adatom distance, charge-transfer mechanism, electronic properties, and work function have also been explored in pristine and doped monolayers. The strong adsorption of adatoms on stanene, tunable electronic properties, and formation of dumbbell structures in the case of AEM and TM shows that doped stanene sheets are worth further exploration.

We present the formation possibility for Pd-hydrides and Pd-Rh hydrides system by density functional theory (DFT) in high pressure upto 50 GPa. Calculation confirmed that PdH2 in face-centered cubic (fcc) structure is not stable under compression that will decomposition to fcc-PdH and H-2. But it can be formed under high pressure while the palladium is involved in the reaction. We also indicate a probably reason why PdH2 can not be synthesised in experiment due to PdH is most favourite to be formed in Pd and H-2 environment from ambient to higher pressure. With Rh doped, the Pd-Rh dihydrides are stabilized in fcc structure for 25% and 75% doping and in tetragonal structure for 50% doping, and can be formed from Pd, Rh and H-2 at high pressure. The electronic structural study on fcc type PdxRh1-xH2 indicates the electronic and structural transition from metallic to semi-metallic as Pd increased from x = 0 to 1.

First-principles electronic structure calculations were carried out on hexagonal boron nitride (h-BN) sheets functionalized with small molecules, such as OLi, ONa, and Li2F, to study their hydrogen (H-2) storage properties. We found that OLi and ONa strongly adsorb on h-BN sheets with reasonably large inter-adsorbent separations, which is desirable for H-2 storage. Ab initio molecular dynamics (MD) simulations further confirmed the structural stability of OLi-BN and ONa-BN systems at 400K. On the other hand, Li2F molecules form clusters over the surface of h-BN at higher temperatures. We performed a Bader charge investigation to explore the nature of binding between the functionalized molecules and h-BN sheets. The density of states (DOS) revealed that functionalized h-BN sheets become metallic with two-sided coverage of each type of molecules. Hydrogenation of OLi-BN and ONa-BN revealed that the functionalized systems adsorb multiple H-2 molecules around the Li and Na atoms, with H-2 adsorption energies ranging from 0.20 to 0.28eV, which is desirable for an efficient H-2 storage material.

A great deal of interest has been paid to the application of carbon-based nano-and microstructured materials as electrodes due to their relatively low-cost production, abundance, large surface area, high chemical stability, wide operating temperature range, and ease of processing including many more excellent features. The nanostructured carbon materials usually offer various micro-textures due to their varying degrees of graphitisation, a rich variety in terms of dimensionality as well as morphologies, extremely large surface accessibility and high electrical conductivity, etc. The possibilities of activating them by chemical and physical methods allow these materials to be produced with further higher surface area and controlled distribution of pores from nanoscale upto macroscopic dimensions, which actually play the most crucial role towards construction of the efficient electrode/electrolyte interfaces for capacitive processes in energy storage applications. Development of new carbon materials with extremely high surface areas could exhibit significant potential in this context and motivated by this in present work, we report for the first time the utilization of ultralight and extremely porous nano-microtubular Aerographite tetrapodal network as a functional interface to probe the electrochemical properties for capacitive energy storage. A simple and robust electrode fabrication strategy based on surface functionalized Aerographite with optimum porosity leads to significantly high specific capacitance (640 F/g) with high energy (14.2 Wh/kg) and power densities (9.67x103 W/kg) which has been discussed in detail.