First-principles investigations of the MMgH3 (, Na, K, Rb, Cs) series

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.