Abstract
The crystal symmetry of the reaction product of hydrogen with the intermetallic compound Mg2Ni has been determined to be tetragonal with unit cell dimensions: a=6.533 AA and c=7.499 AA, by in situ X-ray diffractometry at about 623K and under 4.6 MPa hydrogen pressure. The resultant hydride phase transforms at 503K to an orthorhombic structure with lattice dimensions: a=11.394 AA, b=11.196 AA and c=9.165 AA. This phase transition is of polymorphic type as is evident from the well defined exothermic (endothermic) peak in the pressure-differential scanning calorimetry curves obtained during cooling (heating) after the formation of the nearest stoichiometric hydride Mg2NiH4. The results are briefly compared with previously reported data.