Electronic structure and optical properties of complex hydrides LiBH4 and NaAlH4 compounds

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The properties of the complex hydrides LiBH4 and NaAlH4 such as structural, electronic, and optical properties were calculated using the augmented plane wave plus local orbitals (APW + lo) method. The three phases α, β, and γ of LiBH4 and NaAlH4 hydrides are studied, the β-phase is transformed at the high-pressure to γ phase for the compound LiBH4 with the generalized gradient approximation (GGA) with a 16% volume decrease. However, NaAlH4 compound did not show any phase transition. The interaction between the boron (aluminum) atoms and the hydrogen atoms in the [AlH4]− ([BH4]−) complexes is strongly covalent, and between the complexes [AlH4]− and the lithium Li+ cations for the LiBH4 compound and between the complexes [AlH4]− and the sodium Na+ cations for the NaAlH4 compound, the bond is ionic. For the most stable phases of the two complex hydrides, different optical parameters have been calculated as a function of the photon energy. The plasma frequency ωp is calculated from the mean peak of the energy loss function. Therefore, by way of example, the plasma energy ħωp of its peak position is 13.23 (12.93) eV for α-NaAlH4 (α-LiBH4).