Structural stability of alkali boron tetrahydrides ABH4 (A = Li, Na, K, Rb, Cs) from first principle calculation

The ground-state crystal structures of the boron hydride series ABH4 (A = Li, Na, K, Rb, Cs) have been established from first principle projected-augmented plane-wave calculations by considering different possible structural arrangements and subjecting the test structures to structural relaxations. LiBH4 crystallizes with orthorhombic symmetry (KGaH4-type; Pnma), NaBH4, KBH4, and RbBH4 with tetragonal symmetry (γ-NaBH4-type; P42/nmc), and CsBH4 in another tetragonal space group (β-NaBH4-type; P4¯21c). A possible reason for the transitions from lower- to higher-symmetric structures along the series has been advanced. The theoretically derived positional parameters for LiBH4 gives an almost ideal tetrahedral configuration for the [BH4]− complex whereas the experimental results show that these units are strongly distorted. The theoretically derived structures for KBH4, RbBH4, and CsBH4 differ appreciably from the experimentally established low-temperature structures. From total-energy calculations it seems that syntheses under appropriate pressure and temperature conditions may be able to generate a number of metastable phases of the ABH4 series. The density-of-states characteristics reveal that all these ABH4 compounds exhibit insulating behavior with band gaps varying between 5.5 and 7.0 eV.