Effects of High Pressure on the Structure of VH0.5 and NbH0.75

The effect of pressure on the crystal structure and lattice parameter of VH 0.5 , and NbH 0.75 has been investigated at room temperature by using a diamond-anvil cell up to the pressure of 60 GPa. It was found that the volume increase due to hydrogen, Δ v H , is practically incompressible, and that NbH 0.75 transforms at about 20 GPa into a bct structure isomorphous with β-VH 0.5 . The invariance of Δ v H under pressure is examined in the more general context of the apparent invariance of Δ v H in any host metals, and a tentative explanation for it in terms of local electronic states is proposed.