The electronic structure and ground state properties of AlB 2 type transition metal diborides TMB 2 ͑TMϭSc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta͒ have been calculated using the self consistent tight-binding linear muffin-tin orbital method. The equilibrium volume, bulk moduli (B 0), pressure derivative of bulk moduli (B 0 Ј), cohesive energy (E coh), heat of formation (⌬H), and electronic specific heat coefficient (␥) are calculated for these systems and compared with the available experimental and other theoretical results. The bonding nature of these diborides is analyzed via the density of states ͑DOS͒ histogram as well as the charge density plots, and the chemical stability is analyzed using the band filling principle. The variation in the calculated cohesive properties of these materials is correlated with the band filling effect. The existence of a pseudogap in the total density of states is found to be a common feature for all these compounds. The reason for the creation of the pseudogap is found to be due to the strong covalent interaction between boron p states. We have made spin polarized calculations for CrB 2 , MnB 2 , and FeB 2 and found that finite magnetic moments exist for MnB 2 and CrB 2 whereas FeB 2 is nonmagnetic.

Structural phase stability, electronic structure, optical properties, and high-pressure behavior of polytypes of In 2 O 3 in three space group symmetry I2 1 3, Ia3 and R3 are studied by first-principles density functional calculations. From structural optimization studies lattice and positional parameters have been calculated, which are found to be in good agreement with the corresponding experimental data. In 2 O 3 of space group symmetry I2 1 3 and Ia3 are shown to undergo a pressureinduced phase transition to IO3 at pressures around 3.83 GPa. From analysis of band structure it is found that In 2 O 3 of space group symmetry I2 1 3 is indirect band gap semiconductors, while the other phase of space group Ia3 is direct band gap. The calculated carrier effective masses for all these three phases are compared with available experimental and theoretical values. From chargedensity and electron localization function analysis it is found that these phases have dominant ionic bonding. The magnitude of the absorption and reflection coefficients of In 2 O 3 with space group Ia3 and R3 are small in the energy range 0-5 eV, so that these materials can re regarded and classified as transparent.

The chemical bonding, electronic structure, and optical properties of metal-organic framework-5 (MOF-5) were systematically investigated using ab initio density functional calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional leading to a good agreement between the experimental and the theoretical equilibrium structural parameters. The calculated bulk modulus indicates that MOF-5 is a soft material. The estimated band gap from a density of state (DOS) calculation for MOF-5 is about 3.4 eV, indicating a nonmetallic character. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, and electroluminescence cells, the optical properties of this material were investigated. The detailed analysis of chemical bonding in MOF-5 reveals the nature of the Zn-O, O-C, H-C, and C-C bonds, that is, Zn-O having mainly ionic interaction whereas O-C, H-C, and C-C exhibit mainly covalent interactions. The findings in this paper may contribute to a comprehensive understanding about this kind of material and shed insight into the synthesis and application of novel and stable MOFs.

Crystallite size effects can influence the performance of battery materials by making the structural chemistry deviate from what is predicted by the equilibrium phase diagram. The implications of this are profound: the properties of many battery materials should be reassessed. Sodium ion battery anodes made from nanocrystalline bismuth form different phases during electrochemical cycling compared to anodes with larger crystallites. This is due to the formation of a metastable cubic polymorph of Na 3 Bi on the crystallite surfaces. The structural differences (weaker Na−Bi bonds, different coordination of Na to Bi) between the metastable cubic Na 3 Bi phase found in the nanocrystals and the hexagonal equilibrium polymorph which dominates the larger crystallites offer an explanation for the improvements in cycling behavior observed for the nanostructured anode.

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Our recent electronic structure studies on series of transition metal diborides indicated that the electron phonon coupling constant is much smaller in these materials than in superconducting intermetallics. However experimental studies recently show an exceptionally large superconducting transition temperature of 40 K in MgB 2 . In order to understand the unexpected superconducting behavior of this compound we have made electronic structure calculations for MgB 2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB 2 compared with other superconducting intermetallics and the exceptionally high T c in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly-degenerate quasi-two dimensional key-energy band in the vicinity of E F along Γ-A direction of BZ (having equal amount of B p x and p y character) which play an important role in deciding the superconducting behavior of this material. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB 2 , CaB 2 , SrB 2 , LiBC and MgB 2 C 2 and found that MgB 2 C 2 is one potential material from the superconductivity point of view. We have also investigated closely related compound MgB 4 and found that its E F is lying in a pseudogap with a negligibly small density of states at E F which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB 2 ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB 2 by the accurate full-potential method and derived the directional dependent linear compressibility, Young's modulus, shear modulus and relevant elastic properties from these results. We have observed large anisotropy in the elastic properties consistent with recent high-pressure measurements. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB 2 possesses a mixed bonding character and this has been verified from density of states, charge density and

The electronic structure and structural stability of the technologically interesting material NaAlH4 are studied using an ab initio projected augmented plane-wave method for different possible structure modifications. We predict that α-NaAlH4 converts to β-NaAlH4 at 6.43 GPa with a 4 % volume contraction. Both modifications have nonmetallic character with finite energy gaps, the calculated band gap for β-NaAlH4 being almost half of that for the α phase. β-NaAlH4 stores hydrogen more volume efficient than the α phase and would if stabilized at ambient conditions be an interesting candidate for further studies with regard to hydrogen absorption/desorption efficiency.

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