Allotropic Transitions of Mg₂NiH₄

The crystallographic structure and thermal behavior of Mg₂NiH₄ have been studied in the temperature range 25-500 °C. Two allotropic forms of this hydride were identified. An orthorhombic structure with a = 11.36 Å, b = 11.16 Å, and c = 9.12 Å (P222₁) containing 16 formula units per cell is stable at ambient temperature. This structure transforms at 210-245 °C to a cubic pseudo-CaF₂-type structure with a = 6.490 Å and 4 formula units per cell. The allotropic transition is not accompanied by a change in hydrogen composition which remains the same for both structures (under about a 700-torr hydrogen atmosphere). The enthalpy change associated with this transition was estimated to be 0.80 ± 0.05 kcal/mol of H₂ (1.60 kcal/mol of Mg₂NiH₄).