High hydrogen content complex hydrides: A density-functional study

Abstract

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.