Compressibility of hydrogen in Ni and Re hosts at pressures to 123 GPa

Ni-H and Re-H binary systems have been studied at room temperature by X-ray diffraction in a diamond-anvil cell. The formation of the hydrides NiH and ReH_0.4 with no change in the types of the host metal lattices was observed through the expansion of the host lattices. No structure changes were observed in the metal sub-lattice of either hydride under increasing pressure and the equations of state have been obtained to 123 GPa. The difference in the partial hydrogen volume as a function of pressure between the Ni-H and Re-H systems can be understood with reference to the behaviour of conduction electrons. The pressure dependence of the partial hydrogen volume in these hydrides supports the hypothesis of the apparent common-volume behaviour of hydrogen in a metallic environment.