Stability of the hydrides REMgNi4H4 (RE = Y, Gd) from first principles

YMgNi4H4 and GdMgNi4H4 hydrides were studied from first principles. Starting from the experimental crystallographic values, the Equation Of State (EOS) of the intermetallics as well as the hydrides have been established. Considering the two possible structures reported for such pseudo Laves phase hydrides (i.e. cubic and orthorhombic), the relative stabilities of the hydrides have been compared. The orthorhombic configuration is confirmed for REMgNi4H4 (RE = Y, Gd) as seen experimentally. Nevertheless, in a larger cell (i.e. REMgNi4−xAlx solid solution), it has been shown experimentally that the hydride remains cubic, and the present calculations show that the hydrogen atoms have to be considered within the [RENi3] tetrahedrons.