Structure and bonding in BAlH5 (B=BeB=Be, Ca, Sr) from first-principle calculations

Prediction of structures for the hitherto hypothetical compounds BeAlH5, CaAlH5, and SrAlH5 have been attained through minimization of the total energy for 50 different guess structures. In addition to the ground-state phases (here designated α modifications), BeAlH5 and CaAlH5 are also predicted to form high-pressure modifications (termed β modifications). Bonding analysis shows that Ca and Sr can, to a reasonable approximation, be regarded as divalent ions, while Be forms iono-covalent (sp3sp3-hybridized) bonds with H in tetrahedral configuration. The β-BeAlH5, α-CaAlH5, β-CaAlH5, and SrAlH5 structures exhibit chains of corner-sharing AlH6 octahedra, while α-BeAlH5 forms layers of alternating sheets of corner-sharing AlH6 octahedra and twin chains of corner-sharing BeH4 tetrahedra. At ambient conditions CaAlH5 and Ca(AlH4)2 are predicted to be thermodynamically metastable, while SrAlH5 is anticipated to be stable. However, indications suggest that CaAlH5 can be stabilized under pressure.