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.

Density-functional-theory calculations within the generalized-gradient approximation are used to established the ground-state structure, optimized geometry, and electronic structure for Mg(AlH4)2 and Mg(BH 4) 2. Among 28 structural arrangements used as inputs for structural optimization calculations, the experimentally known framework is reproduced for Mg(AlH 4) 2 (space group P 3m1) with positional and unit-cell parameters in good agreement with the experimental findings. The crystal structure of Mg(BH4)2 is predicted, the ground-state framework being orthorhombic (space group P mc21; Pearson symbol oP 22) with a fascinating two-dimensional arrangement of Mg 2+ atoms and [BH 4 ] 2− tetrahedra. The formation energy for the predicted Mg(BH 4) 2 phase is investigated along different reaction pathways. The electronic structures reveal that Mg(AlH 4) 2 and Mg(BH 4) 2 are insulator with estimated band gap around 4.5 and 6.2 eV, respectively.

Using gradient-corrected, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides RTInH1.333 (R=La, Ce, Pr, or Nd; T= Ni, Pd, or Pt) possess unusually short H-H separations. The most extreme value (1.454 A) ever obtained for metal hydrides occurs for LaPtInH1.333. This finding violates the empirical rule for metal hydrides, which states that the minimum H-H separation is 2 A. The paired, localized, and bosonic nature of the electron distribution at the H site are polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. Also, R-R interactions contribute to shielding of the repulsive interactions between the H atoms.

The electronic structure of the perovskite La 1−x Sr x CoO 3 has been obtained as a function of Sr substitution and volume from a series of generalizedgradient-corrected, full-potential, spin-density-functional band structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy vs spin magnetic moment curve for LaCoO 3 the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment ∼1.2 µ B /f.u. From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO 3 originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO 3 is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding change of the lattice parameters we have observed the disappearence of the metamagnetic solution that is associated with the IS state. The FSM calculations on La 1−x Sr x CoO 3 suggest that the hole doping stabilizes the IS state and the calculated magnetic moments are in good agreement with the corresponding experimental values. Our calculations show that the HS state cannot be stabilized by temperature or hole doping since the HS state is significantly higher in energy than the LS or IS state. Hence the spin-state transition in LaCoO 3 by temperature/hole doping is from an LS to an IS spin state and the present work rules out the other possibilities reported in the literature. Typeset using REVT E X

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende-and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzitetype structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons at the conduction-band minimum and of the holes at the valence-band maximum have been calculated and show that the holes are much heavier than the conduction-band electrons in agreement with experimental findings. The calculations, moreover, indicate that the effective masses of the holes are much more anisotropic than the electrons. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The local-density-approximation with multiorbital mean-field Hubbard potential approach is found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement between the experimentally established location of the Zn-3d levels and that derived from pure LDA calculations.

Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende-and wurtzite-type structures were studied using the ab initio density functional method within the LDA, GGA, and LDA+U approaches. Calculations of the optical spectra have been performed for the energy range 0-20 eV, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

A detailed high-pressure study on LiAlH 4 has been carried out using the ab initio projected augmented plane-wave method. Application of pressure transforms ␣-to ␤-LiAlH 4 (␣-NaAlH 4 -type structure͒ at 2.6 GPa with a huge volume collapse of 17%. This abnormal behavior is associated with electronic transition from Al-s to -p states. At 33.8 GPa, a ␤ to ␥ transition is predicted from ␣-NaAlH 4 -type to KGaH 4 -type structure. Up to 40 GPa LiAlH 4 remains nonmetallic. The high weight percent of hydrogen, around 22% smaller equilibrium volume, and drastically different bonding behavior than ␣-phase indicate that ␤-LiAlH 4 is expected to be a potential hydrogen storage material.Metal hydrides which can accommodate more than 3 wt % hydrogen have been targeted in the Japanese WE-NET project MITI. 1 The parallel international cooperative project under IEA Task-12 is set up to develop storage materials which can store more than 5 wt % hydrogen. Several interstitial metal hydrides operate at around room temperature, but their reversible hydrogen storage capacity is limited to at most 2.5 wt %. 2 Recent interest is directed toward ternary aluminum hydrides as potential materials with enhanced storage capacity ͑e.g., LiAlH 4 and NaAlH 4 with 10.6 and 7.5 wt % theoretical hydrogen content, respectively͒ as solidstate sources for hydrogen cells ͑e.g., fuel reservoirs͒ etc. Hence, LiAlH 4 and NaAlH 4 could be viable candidates for practical usage as on-board hydrogen storage materials. However, a serious problem with these materials is poor kinetics and lacking reversibility with respect to hydrogen absorption/desorption. Improved understanding of the processes which occur in these hydrogen-containing materials during uptake and release of hydrogen are of considerable interest. Recent experimental evidences show that LiAlH 4 and NaAlH 4 after being subjected to mechano-chemical processing under ambient conditions in the presence of certain transition-metal catalysts 3-6 rapidly release 7.9 and 5.6 wt % of H, respectively. This represents nearly four to five times more stored hydrogen than LaNi 5 -based alloys which are presently used in nickel-based hydride batteries. The detailed crystal structure of LiAlH 4 is known, but a systematic highpressure study has not yet been reported. A theoretical investigation of LiAlH 4 assumes importance as high-pressure x-ray and neutron diffraction studies will experience difficulties in identifying more accurate positions for the hydrogen atoms. The present study concerns the phase stability and electronic structure of LiAlH 4 using first-principles ab initio calculations.LiAlH 4 crystallizes in the monoclinic ␣-LiAlH 4 -type structure with space group P2 1 /c and four formula units per unit cell. 7 Four hydrogen atoms are arranged around aluminum in an almost regular tetrahedral configuration. The structure consists of ͓AlH 4 ͔ Ϫ units well separated by Li ϩ ions. The Al-H distances vary between 1.59 and 1.64 Å, the Li-H separations between 1.83 and 1.97 Å, and the arrangement of the lithi...