W-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 an
d ReH0.4 with no change in the types of the host metal lattices was observe
d through the expansion of the host lattices. No structure changes were obs
erved 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 conducti
on electrons. The pressure dependence of the partial hydrogen volume in the
se hydrides supports the hypothesis of the apparent common-volume behaviour
of hydrogen in a metallic environment.