Thermodynamic properties of Mg_2.0Ni hydrides

Thermodynamic properties of hydrides of Mg_2.0Ni alloy produced by bulk mechanical alloying were determined from the pressure-composition (PC) isotherms over the temperatures from 623 to 423 K. In-situ X-ray diffraction studies indicated the hydride formation is accompanied with structural change of Mg_2.0Ni host lattice. The van't Hoff plots for the plateau pressures on isotherms clearly indicated the existence of the structural transformation of the hydride at around 525 K. It is believed that the transformation corresponds to the order/disorder transformation in the hydrogen sublattice. In this paper the difference in the thermodynamic properties between the high and low temperature hydrides will be discussed in relation to the hydrogen order/disorder phenomena presumed by studies on structure of Mg_2.0Ni hydrides.