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Hydrostatic pressure response of $\mathrm{LaAu}{\mathrm{Sb}}_{2}$ charge density wave (CDW) system is investigated using electrical transport, x-ray diffraction (XRD), and density-functional theory (DFT). Resistivity data at ambient pressure evidence a clear CDW transition at 83 K. X-ray diffraction at ambient pressure reveals that in plane and out of plane axes show opposite behavior with decreasing temperature, in particular, out of plane $c$ axis develops a distinct change at \ensuremath{\sim}250 K, much above the observed CDW transition at 83 K. The CDW transition shifts to low temperature with increasing pressure. Our resistivity data indicate a complete suppression of the CDW transition at \ensuremath{\sim}3.6 GPa. High-pressure XRD revealed a change from the linear trend for the out of plane ($c$) and the in plane ($a$) lattice parameters for pressure above 3.8 GPa. With compression, DFT indicated an anomaly in the c/a ratio around 8 GPa. The calculated electronic structure also indicated minor changes in the band structure in this pressure range. In addition, high-pressure DFT structural investigations reveal the $\mathrm{LaAu}{\mathrm{Sb}}_{2}$ system to be stable up to pressures as high as 150 GPa.

The power conversion of perovskite solar cells has increased steadily and reached 22.1%, recently. This has led the researchers to study lead-free halide perovskite materials like Cs2SnI6 due to the toxic nature of lead used in the record-breaking organic-inorganic perovskite structures. Since the presence of an intermediate band in semiconductors leads to enhancement of efficiency of the solar cells, we have also investigated the electronic structure of Cs2SnI6, to find whether the intermediate band is likely to exist in the middle of the band gap. The computational results for the electronic band structure and density of states indicate that Cs2SnI6 contains an intermediate band (IB). Additional absorption peaks that appear in the optical spectra of this material presented in this paper explicitly confirm this. All these studies and the presence of an IB that we identified in Cs2SnI6 suggests that Cs2SnI6 may have high potential used as intermediate band solar cell material.

In recent years, the scientific community has shown an increasing interest in regards to the investigation of novel materials for the intercalation of lithium atoms, suitable for application as cathodes in the new generations of Li-ion batteries. Within this framework, we have computed the relative structural stability, the electronic structure, the elastic and dynamic properties of Li2MSiO4 compounds (M = Mn, Co, Ni) by means of first-principles calculations based on density functional theory. The so-obtained structural parameters of the examined phases are in agreement with previous reports. The energy differences between different polymorphs are found to be small, and most of these structures are dynamically stable. The band structures and density of states are computed to analyse the electronic properties and characterise the chemical bonding. The single crystal elastic constants are calculated for all the examined modifications, proving their mechanical stability. These Li2MSiO4 materials are found to present a ductile behaviour upon deformation. The diffusion coefficients of Li ions, calculated at room temperature for all the examined modifications, reveal a poor conductivity for this class of materials.

The Ag-based delafossite transparent conducting oxides (TCO) are potential p-type materials for transparent electronics. However, they have attracted less attention compared with the Cu-based delafossite systems due to their difficult synthetic chemistry and relatively low conductivity. We present here a complete comparison of structural and electronic properties of these two families based on the results obtained from the periodic density functional calculation. The equilibrium structural parameters are obtained with the Perdew Burke Ernzerhof (PBE) exchange correlation functional in the calculation, while the electronic structure is investigated by using the screened hybrid functionals proposed by Heyd, Scuseria and Ernzerhof (HSE06). The structural stabilities of these two families of compounds are similar, being completely determined by the B-site metal ions. The density of states plots show that the valence band is relatively broader for the Ag-compounds. The Ag-4d orbitals are narrow and much lower in energy than the O p states. Therefore holes created at the oxygen site are highly localized and consequently have low mobility. The computed band gaps values are found to be in excellent agreement with the corresponding experimentally observed band gap values from optical measurements. The effective mass analysis suggests that for the Cu-compounds the conductivity follows the following trend: Sc > Ga ≈ Y > Al > In, in excellent agreement with the experimental observations. However, the calculated effective masses of the carriers suggest that the conductivity of the Ag-based compounds follows the following trend: Sc > Y > Ga > In > Al.

Li2FeSiO4 is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h/g). However, its development has faced significant challenges arising from structural complexity and poor ionic conductivity. In the present work, the relative stability, electronic structure, thermodynamics, and mechanical properties of potential cathode material Li2FeSiO4 and its polymorphs have been studied by state-of-the-art density-functional calculations. Among the 11 structural arrangements considered for the structural optimization calculations, the experimentally known monoclinic P21 modification is found to be the ground state structure. The application of pressure originates a sequence of phase transitions according to P21 - Pmn21 - I222, and the estimated values of the critical pressure are found to be 0.38 and 1.93 GPa. The electronic structures reveal that all the considered polymorphs have a non-metallic character, with band gap values varying between 3.0 and 3.2 eV. The energy differences between different polymorphs are small, and most of these structures are dynamically stable. On the other hand, the calculation of single crystal elastic constants reveals that only few Li2FeSiO4 polymorphs are mechanically stable. At room temperature, the diffusion coefficient calculations of Li2FeSiO4 in different polymorphs reveal that Li-ion conductivity of this material is destitute., Comment: 8 figures, 4 table

Modern density functional theory (DFT) provides an extremely valuable tool for predicting structures and energetics of new materials for both finite and periodic systems. In the present work the electronic structure, chemical bonding, and spectroscopic properties of AB2H2, and ABCH (A = Sr, Ba; B = Al, Ga; C = Si, Ge) phases have been studied by state-of-the-art density-functional calculations. In contrast with the previous theoretical studies, present electronic structures calculation using hybrid B3LYP reveal that AB2H2, phases are indirect bandgap semiconductors with estimated bandgap vary from 0.324 to 0.495 eV. In general, B3LYP gives a reasonable description of the electronic structure of semiconductors, semi-ionic oxides, sulfides, transition metal oxides and the ionic oxide. Similar to the AB2H2 phases ABCH phases are also indirect bandgap semiconductors and the magnitude is much higher than for the AB2H2 phases. We have also simulated the Raman and IR spectra, and calculated the NMR related parameters such as isotropic chemical shielding, quadrupolar coupling constant, and quadrupolar asymmetry for the AB2H2, and ABCH phases.

A systematic investigation of the crystal structure, chemical bonding, electronic structure, formation energy, and optical properties of Cd-MOF-5 via DFT calculations is presented. The calculated bulk modulus (13.4 GPa) indicates that Cd-MOF-5 is a soft material. The estimated band gap for Cd-MOF-5 is 3.6 eV, indicating semiconducting behavior. Moreover, large formation enthalpy (−41 kJ mol −1 ) indicates its high stability and could be synthesizable. The systematic investigation on the optical response properties of Cd-MOF-5 will trigger the experimental efforts in this direction. The detailed chemical bonding analysis reveals the nature of bonds, i.e., Cd–O having mainly ionic interaction whereas O–C, H–C and C–C exhibit mainly covalent interactions.

The equilibrium solid-state structure, electronic structure, formation enthalpy, chemical bonding, and optical properties of IRMOF-10 and its alkaline earth metal analogues M-IRMOF-10 (M = Cd, Be, Mg, Ca, Sr, Ba) have been investigated with density functional calculations. The unit cell volume and atomic positions were fully optimized with the GGA functional. This supplements the incomplete experimental structural parameters available for Zn-IRMOF-10. The calculated bulk moduli decrease monotonically from Zn to Cd, and from Be to Ba, and indicate that Zn-IRMOF-10 and its analogues are relatively soft materials. The estimated bandgap values are in the range 2.9 to 3.0 eV, indicating nonmetallic character. Importantly, the bandgaps within the M-IRMOF-10 series (containing a rather long 4,4′-biphenyldicarboxylate linker) are smaller than those within the M-IRMOF-1 series (shorter benzene dicarboxylate linker). The optical properties (dielectric function e(ω), refractive index n(ω), absorption coefficient α(ω), optical conductivity σ(ω), reflectivity R(ω), and electron energy-loss spectrum L(ω)) of the M-IRMOF-10 series were computed. The observation of very small reflectivities over a wide energy range suggests possible uses in hybrid solar cell applications. The main characteristics of the optical properties are similar for the whole series although differences are seen in the details. An analysis of chemical bonding in the M-IRMOF-10 series reveals as might be anticipated that M–O bonds are largely ionic whereas C–O, C–H and C–C exhibit mainly covalent interactions. The BOP values of M–O decrease through the series when going from Zn to Cd, and from Be to Ba, i.e. the ionicity increases and the covalency decreases for the M–O bonds.

Using total energy calculations within the generalized-gradient approximation the high pressure phases, optimized geometry, and electronic structure of AlH 3 are established. Among the 58 structural arrangements considered for the structural optimization calculations, the experimentally known β modification becomes the ground-state structure. Application of pressure makes the sequence of phase transitions from β → α' → α → hpl (P63/m) → hp2 (Pm3n) modification and the estimated transition pressures are 2.4, 4.3, 64, and 104 GPa, respectively. The coordination of Al has been changed from six to nine in the newly identified high pressure hpl and to twelve in hp2 polymorph. The electronic structures reveal that a, a', β, and y polymorphs are nonmetals with calculated band gap varying between 1.99 and 3.65 eV, whereas the hpl and hp2 phases possess semiconducting and metallic behavior, respectively.

The structural stability of the MMgH3 (M = Li, Na, K, Rb, Cs) series has been investigated using the density-functional projector-augmentedwave method within the generalized-gradient approximation. Among the 24 structural arrangements used as inputs for the structural optimization calculations, the experimentally known frameworks are successfully reproduced, and positional and unit-cell parameters are found to be in good agreement with the experimental findings. The crystal structure of LiMgH3 has been predicted, the most stable arrangement being the trigonal LiTaO3-type (R3c) structure, which contains highly distorted octahedra. The formation energy for all members of the MMgH3 series is investigated along different reaction pathways. The electronic density of states reveals that the MMgH3 compounds are wide-band-gap insulators. Analyses of chemical bonding in terms of charge density, charge transfer, electron-localization function, Born effective charge, and Mulliken population show that these hydrides are basically saline hydrides similar to the parent alkali-/alkaline-earth mono-/di-hydrides.

The crystal and electronic structures of the entire series of alkali aluminum and alkali gallium tetrahydrides (ABH4 ;A ) Li, Na, K, Rb, or Cs; B ) Al or Ga) are systematically investigated using an ab initio projected augmented plane-wave method. For structural stability studies, we have considered several possible structural modifications, and reproduced successfully the equilibrium structures for the known phases LiAlH4, NaAlH4, KAlH4, NaGaH4, and KGaH4. Moreover, we predict the equilibrium structures of the other unknown members of this series. RbAlH4, CsAlH4, RbGaH4, and CsGaH4 should crystallize with the KGaH4-type structure, and LiGaH4 should crystallize with the NaGaH4-type structure. According to the density of states, all these compounds have nonmetallic character with a finite band gap of around 5 eV. Charge-density plot and electron localization function analyses show that the (BH4) subunits almost look like a separate molecular species spread over the A matrix. An ionic type of interaction is present between the A and the (BH4) units. Crystal orbital Hamilton population analyses reveal that the interaction between the B and H atoms is stronger than the other interactions present in these compounds.

Using generalized-gradient-corrected full-potential density-functional calculations, we have studied the electronic structure and magnetic properties of YBaMn2O5 ,S r 4Fe4O11, and Ca3Co2O6. In these phases, the 3d transition metal ions have dual valence. We have studied the electronic structure using site-, angular momentum-, and orbital-projected density of states. The charge and orbital ordering are analyzed in terms of the calculated electron-density distribution, charge density, and electron localization function. The oxygen vacancy, cation radii, and crystal-field effects are found to play an important role for the various ordering phenomena in these compounds. © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

The self-consistent tight-binding linear muffin-tin orbital method was employed to calculate the electronic structure and the total energy of ${\mathrm{Ti}}_{2}X\mathrm{Al}$ $(X=\mathrm{N}\mathrm{b},\mathrm{}\mathrm{V},\mathrm{}\mathrm{Zr})$ in $B2,$ ${D0}_{19},$ and O (orthorhombic) phases and the results were used to study the phase stability and cohesive properties of these intermetallic compounds. Our theoretical calculation shows that the $B2$ phase is the most stable phase of ${\mathrm{Ti}}_{2}\mathrm{NbAl}$ as observed by experimentalists. However, the three phases are close in energy indicating the possibility of the presence of all these phases in equilibrium over a range of temperatures, which is in accordance with experimental observations. Our calculations predict that ${\mathrm{Ti}}_{2}\mathrm{VAl}$ is more stable in the $B2$ phase whereas ${\mathrm{Ti}}_{2}\mathrm{ZrAl}$ is more stable in the ${D0}_{19}$ phase. We also report the calculated equilibrium lattice parameters, cohesive energies, heats of formation, and bulk modulii of these systems, and a possible comparison of the calculated quantities with the available experimental data is made. From our studies we are made to conclude that the concept of pseudogaps which has been very much emphasized for binary intermetallics does not carry so much significance with respect to ternary systems.

The electronic structure and structural stability of BeH2 are studied using first-principles density-functional calculation. The calculated structural parameters for α-BeH2 at the equilibrium volume are in very good agreement with experiments. At higher pressures α-BeH2 successively undergoes four structural transitions: (i) α- to β-BeH2 at 7.07 GPa; (ii) β- to γ-BeH2 at 51.41 GPa; (iii) γ- to δ-BeH2 at 86.56 GPa; and (iv) δ- to e-BeH2 at 97.55 GPa [an effective two-phase (γ and δ) region is found at 73.71–86.56 GPa]. Density of states studies reveal that BeH2 remains insulating up to 100 GPa whereupon anomalous changes are seen in the band-gap region with increasing pressure.

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.

The electronic structure, lattice dynamics, and mechanical properties of AlH(3) phases have been studied by density functional calculations. The chemical bonding in different polymorphs of AlH(3) are evaluated on the basis of electronic structures, charge density analysis, and atomic charges, as well as bond overlap population analysis and the Born effective charges. The phonon dispersion relations and phonon density of states of all the polymorphs of AlH(3) are calculated by direct force-constant method. Application of pressure induces seqauence of phase transitions in β-AlH(3) which are understood from the phonon dispersive curves of the involved phases. The previously predicted phases (Chem. Mater. 2008, 20, 5997) are found to be dynamically stable. The calculated single crystal elastic constants reveal that all the studied AlH(3) polymorphs are easily compressible. The chemical bonding of these polymorphs have noticeable covalent character (except the hp2 phase) according to the present chemical bonding analyses. For all these polymorphs, the NMR-related parameters, such as isotropic chemical shielding, quadrupolar coupling constant, and quadrupolar asymmetry, are also calculated. All IR- and Raman-active phonon frequencies, as well as the corresponding intensities, are calculated for all the AlH(3) polymorphs and are compared with available experimental results.

Formation energies, chemical bonding, electronic structure, and optical properties of metal-organic frameworks of alkaline earth metals, A-IRMOF-1 (where A = Be, Mg, Ca, Sr, or Ba), have been systemically investigated with DFT methods. The unit cell volumes and atomic positions were fully optimized with the Perdew-Burke-Ernzerhof functional. By fitting the E-V data into the Murnaghan, Birch and Universal equation of states (UEOS), the bulk modulus and its pressure derivative were estimated and provided almost identical results. The data indicate that the A-IRMOF-1 series are soft materials. The estimated bandgap values are all ca. 3.5 eV, indicating a nonmetallic behavior which is essentially metal independent within this A-IRMOF-1 series. The calculated formation energies for the A-IRMOF-1 series are -61.69 (Be), -62.53 (Mg), -66.56 (Ca), -65.34 (Sr), and -64.12 (Ba) kJ mol(-1) and are substantially more negative than that of Zn-based IRMOF-1 (MOF-5) at -46.02 kJ mol(-1). From the thermodynamic point of view, the A-IRMOF-1 compounds are therefore even more stable than the well-known MOF-5. The linear optical properties of the A-IRMOF-1 series were systematically investigated. The detailed analysis of chemical bonding in the A-IRMOF-1 series reveals the nature of the A-O, O-C, H-C, and C-C bonds, i.e., A-O is a mainly ionic interaction with a metal dependent degree of covalency. The O-C, H-C, and C-C bonding interactions are as anticipated mainly covalent in character. Furthermore it is found that the geometry and electronic structures of the presently considered MOFs are not very sensitive to the k-point mesh involved in the calculations. Importantly, this suggests that sampling with Γ-point only will give reliable structural properties for MOFs. Thus, computational simulations should be readily extended to even more complicated MOF systems.

The ground-state structure, equilibrium structural parameters, electronic structure, and thermodynamical properties of MCaH(3) (M=Li, Na, K, Rb, and Cs) phases have been investigated. From the 104 structural models used as inputs for structural optimization calculations, the ground-state crystal structures of MCaH(3) phases have been predicted. At ambient condition, LiCaH(3), NaCaH(3), and KCaH(3) crystallize in hexagonal, monoclinic, and orthorhombic structures, respectively. The remaining phases RbCaH(3) and CsCaH(3) crystallize in a cubic structure. The calculated phonon spectra indicate that all the predicted phases are dynamically stable. The formation energy for the MCaH(3) phases have been calculated along different reaction pathways. The electronic structures reveal that all these phases are insulators with an estimated band gap varying between 2.5 and 3.3 eV.

Density functional theory calculations within the generalized-gradient approximation are used to establish the ground-state structure, equilibrium structural parameters, and electronic structure for MBeH(3) phases. From the 24 structural arrangements used as inputs for structural optimization calculations, the ground-state crystal structures of MBeH(3) phases have been predicted. At ambient conditions, LiBeH(3) and NaBeH(3) crystallize with perovskite-related orthorhombic and cubic structures, respectively. The remaining phases KBeH(3), RbBeH(3), and CsBeH(3) crystalize in a monoclinic structure. In the predicted phases one can store up to 15.93 wt % of hydrogen. The formation energy for the MBeH(3) phases have been investigated along different reaction pathways. The electronic structures reveal that all these phases are insulators with estimated band gaps varying between 1.79 and 3.44 eV.

¯ are indirect band gap semiconductors, while the other phase of space group Ia3 is having direct band gap. The calculated carrier effective masses for all these three phases are compared with available experimental and theoretical values. From charge-density and electron localization function analysis, it is found that these phases have dominant ionic bonding with noticeable covalent interaction between indium and oxygen. The magnitudes of the absorption and reflection coefficients for In2O3 with space groups Ia3 and R3 are small in the energy range 0 – 5 eV, indicating that these phases can be regarded and classified as transparent.

The nature of the bonding in ${\mathrm{Li}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ has been re-examined with additional tools integrated in density-functional programs. From partial density of states, charge density distribution, charge transfer, electron localization function, crystal orbital Hamilton population, Mulliken population, and Born effective charge analyses, it is concluded that the interaction between Li and $\mathrm{Al}{\mathrm{H}}_{6}$ in ${\mathrm{Li}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ is ionic as earlier advocated [Phys. Rev. B 69, 020104(R) (2004)]. Based on charge density distribution, electron localization function, and density-of-states analyses we earlier suggested [Phys. Rev. B 69, 020104(R) (2004)] that the interaction between Al and H is largely of the covalent type. However, additional analyses indicate that the interaction between Al and H in the $\mathrm{Al}{\mathrm{H}}_{6}$ structural subunit is of a more mixed covalent-ionic (iono-covalent) character.

Using full-potential density-functional calculations we show that oxygen stoichiometry plays an important role on spin, charge, and orbital ordering in manganites. The electronic structure and magnetic properties of LaBaMn2O51d have been studied for d50, 0.5, and 1; for d50 and 0.5 the system exhibits charge, orbital, and antiferromagnetic spin ordering, whereas at d51 the charge and orbital orderings disappear but the spin ordering remains. We also bring out an insulator-to-metal transition upon going from d50 to 1. The study suggests that one can manipulate the charge and orbital orderings in certain perovskite-like oxides by merely varying the oxygen stoichiometry and hence design oxides with desired electrical and magnetic properties.

The crystal structure of KAlH 4 is determined by the first principle projected augmented plane wave method taking different possible structural arrangements into consideration. From the total energy it is concluded that KAlH 4 crystallizes in the orthorhombic KGaH 4 -type structure (space group Pnma ) with unit-cell dimensions a =9.009, b =5.767, c =7.399 A. A metastable modification with α-NaAlH 4 -type structure is predicted in energetic close proximity to the stable phase. The electronic structure shows that KAlH 4 has a non-metallic character with a band gap of approximately 5.5 eV.

The structural stability and electronic structure of the potential hydrogen-storage material ${\mathrm{Li}}_{3}{\mathrm{AlH}}_{6}$ have been investigated up to 90 GPa using density-functional total-energy calculations. At ambient conditions ${\mathrm{Li}}_{3}{\mathrm{AlH}}_{6}$ stabilizes in space group $R3\ifmmode\bar\else\textasciimacron\fi{}.$ The structure consists of isolated, close-to-regular $[{\mathrm{AlH}}_{6}{]}^{3\ensuremath{-}}$ octahedra, which are connected via six-coordinated Li. We predict that this $\ensuremath{\alpha}$ phase of ${\mathrm{Li}}_{3}{\mathrm{AlH}}_{6}$ undergoes three successive structural phase transitions on application of pressure: $\ensuremath{\alpha}$ to $\ensuremath{\beta}$ at 18.64 GPa, $\ensuremath{\beta}$ to $\ensuremath{\gamma}$ at 28.85 GPa, and $\ensuremath{\gamma}$ to $\ensuremath{\delta}$ at 68.79 GPa. All modifications of ${\mathrm{Li}}_{3}{\mathrm{AlH}}_{6}$ should have nonmetallic character with estimated bandgaps varying between 2.72 and 4.12 eV.

~Received 2 July 2002; revised manuscript received 19 September 2002; published 15 January 2003! First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (R5La, Ce, and Nd!, and their saturated hydrides ( R3Ni3In3H45RNiInH1.333) are performed using a full-potential linear muffin-tin orbital approach. This series of phases crystallizes in a ZrNiAl-type structural frame-work. When hydrogen is introduced in the RNiIn matrix, anisotropic lattice expansion is observed along @001# and lattice contraction along @100#. In order to establish the equilibrium structural parameters for these compounds we have performed force minimization as well as volume and c/a optimization. The optimized atomic positions, cell volume, and c/a ratio are in very good agreement with recent experimental findings. From the electronic structure and charge density, charge difference, and electron localization function analyses the microscopic origin of the anisotropic change in lattice parameters on hydrogenation of RNiIn has been identified. The hydrides concerned, with their theoretically calculated interatomic H-H distances of ;1.57 A, violate the ‘‘2-A rule’’ for H-H separation in metal hydrides. The shortest internuclear Ni-H separation is almost equal to the sum of the covalent radii. H is bonded to Ni in an H-Ni-H dumbbell-shaped linear array, with a character of NiH2 subunits. Density of states, valence charge density, charge transfer plot, and electron localization function analyses clearly indicate significant ionic bonding between Ni and H and weak metallic bonding between H-H. The paired and localized electron distribution at the H site is polarized toward La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. The calculations show that all these materials have a metallic character.

We present the results of a theoretical investigation on the electronic structure, bonding nature, and ground-state properties of Th 2 Al and Th 2 AlH 4 using generalized-gradient-corrected first-principles full-potential density-functional calculations. Th 2 AlH 4 has been reported to violate the "2-A rule" for H-H separation in hydrides. From our total-energy as well as force-minimization calculations, we found a shortest H-H separation of 1.95 A in accordance with recent high-resolution powder neutron-diffraction experiments. When the Th 2 Al matrix is hydrogenated, the volume expansion is highly anisotropic, which is quite opposite to other hydrides having the same crystal structure. The bonding nature of these materials is analyzed in terms of density of states, crystal-orbital Hamiltonian population, and valence-charge-density analyses. Our calculation predicts a different nature of the bonding between the H atoms along a and c. The strongest bonding in Th 2 AlH 4 is between Th and H along c which form dumbbell-shaped H-Th-H subunits. Due to this strong covalent interaction there is a very small amount of electrons present between the H atoms along c. This reduces the repulsive interaction between the H atoms along c and explains why Th 2 AlH 4 has a shorter H-H separation than most other metal hydrides. The large difference in the interatomic distances between the interstitial regions where one can accommodate H in the ac and ab planes along with the strong covalent interaction between Th and H are the main reasons for highly anisotropic volume expansion on hydrogenation of Th 2 Al.

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 MgB2. In order to understand the unexpected superconducting behavior of this compound we have made electronic structure calculations for MgB2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB2 compared with other superconducting intermetallics and the exceptionally high Tc in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identied a doubly-degenerate quasi-two dimensional key-energy band in the vicinity ofEF along -A direction of BZ (having equal amount of B px and py 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 BeB2, CaB2 ,S rB 2 ,L iBC and MgB 2C2 and found that MgB2C2 is one potential material from the superconductivity point of view. We have also investigated closely related compound MgB4 and found that its EF is lying in a pseudogap with a negligibly small density of states at EF which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB2 ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB2 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. MgB2 possesses a mixed bonding character and this has been veried from density of states, charge density and

The electronic structure and ground state properties of ${\mathrm{AlB}}_{2}$ type transition metal diborides ${\mathrm{TMB}}_{2}$ (TM=Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta) 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 ${(B}_{0}^{\ensuremath{'}}),$ cohesive energy ${(E}_{\mathrm{coh}}),$ heat of formation $(\ensuremath{\Delta}H),$ and electronic specific heat coefficient $(\ensuremath{\gamma})$ 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 (DOS) 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 ${\mathrm{CrB}}_{2},$ ${\mathrm{MnB}}_{2},$ and ${\mathrm{FeB}}_{2}$ and found that finite magnetic moments exist for ${\mathrm{MnB}}_{2}$ and ${\mathrm{CrB}}_{2}$ whereas ${\mathrm{FeB}}_{2}$ is nonmagnetic.

The chemical bonding, electronic structure, and optical properties of the experimentally available metal-organic framework IRMOF-14 and its metal-substituted analogues M-IRMOF-14 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba), which contain a pyrene-2,7-dicarboxylate linker group, have been systematically investigated using DFT calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional and showed good agreement between experimental and theoretical equilibrium structural parameters for Zn-IRMOF-14. The calculated bulk moduli indicate that the whole M-IRMOF-14 series are soft materials. The estimated band gap from DOS calculations for the M-IRMOF-14 series is ca. 2.5 eV, essentially independent of the metal ion and indicative of nonmetallic character. The band gap value is distinctly different from those calculated previously for the M-IRMOF-1 (benzene-1,4-dicarboxylate linker; ca. 3.5 eV) and M-IRMOF-10 (biphenyl-4,4'-dicarboxylate linker; ca. 3.0 eV) series and this confirms that the identity of the linker is a key parameter to control band gaps in an isoreticular series of main-group MOFs. In view of potential uses of MOFs in organic semiconducting devices such as field-effect transistors, solar cells, and organic light-emitting devices, the linear optical properties of these materials were also investigated. Comparisons are made with the M-IRMOF-1 and M-IRMOF-10 series.

Intermediate band (IB) materials are attractive for multiple photon harvesting in solar cells thus increasing their efficiency beyond the Shockley–Quassier limit. However, it has so far been demonstrated that this can only be achieved in a few inorganic solids by appropriate doping. Here we demonstrate that it may be possible to achieve intermediate band materials with the isoreticular metal–organic framework IRMOF-993 and metal-substituted analogues. The equilibrium crystal structures, electronic structures, formation enthalpies, chemical bonding, and optical properties of M-IRMOF-993 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba) were systematically investigated using density functional theory methods. The unit cell volume and atomic positions were optimized with the Perdew–Burke–Ernzerhof (PBE) functional; there was good agreement between the current theoretical equilibrium structural parameters and previously reported structural data for Zn-IRMOF-993. The calculated bulk moduli indicate that Zn-IRMOF-993 and its analogues are soft materials. The estimated fundamental bandgap values from the electronic structure studies for the whole series are ca. 3.5–3.6 eV, indicating a semiconducting character. The bandgap values estimated from the bottom of the IB to the top of VB are ca. 1.5–1.6 eV, and those from the top of IB to the bottom of CB are ca. 2.0 eV, suggesting that these materials may be suitable for enhancing the efficiency of solar cells. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, electroluminescence cells, organic semiconducting devices such as field-effect transistors, and organic light-emitting devices, the optical properties and chemical bonding of M-IRMOF-993 were also systematically investigated.

The phase stability and electronic structure of alpha- LiBH(4)-derived nanostructures and possible low energy surfaces of thin films have been investigated using the ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that, for the alpha- LiBH(4) phase, the (010) surface is the most stable of the possible low-energy surfaces. The predicted critical sizes of the nano-cluster and nano-whisker for alpha- LiBH(4) are 1.75 and 1.5 nm, respectively. Similarly, the bond distances in the surfaces of a nano-whisker are found to be higher than that in the bulk material. The calculated hydrogen site energies suggest that it is relatively easier to remove hydrogen from the surface of the clusters and nano-whiskers than from bulk crystals.

The phase stability, chemical bonding, and electronic structure of MgH(2) nanowires and possible low energy surfaces of α-MgH(2) thin films have been investigated using the ab initio projected augmented plane-wave method. Structural optimizations based on total energy calculations predicted that, for the α-MgH(2) phase, the (101) surface is more stable among the possible low energy surfaces. The electronic structure study reveals that the nanowires also have nonmetallic character similar to that of the bulk and thin film phases. Bonding analysis shows that the character of chemical bonding in nanowires has been considerably changed compared with that in bulk phases. Similarly, the bond distances in the surfaces of nanowires are found to be higher than in the bulk material, suggesting that it is possible to remove hydrogen from the nanowires considerably more easily than from bulk crystals.

Density-functional-theory calculations within the generalized-gradient approximation are used to establish the ground-state structure, optimized geometry, and electronic structure for Mg(AlH4)2 and Mg(BH4)2. Among 28 structural arrangements used as inputs for structural optimization calculations, the experimentally known framework is reproduced for Mg(AlH4)2 (space group P3¯m1) 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 Pmc21; Pearson symbol oP22 with a fascinating two-dimensional arrangement of Mg2+ ions and [BH4]2− tetrahedra. The formation energy for the predicted Mg(BH4)2 phase is investigated along different reaction pathways. The electronic structures reveal that Mg(AlH4)2 and Mg(AlH4)2 are insulators with calculated band gaps of around 4.5 and 6.2eV, respectively.