Pressure and temperature dependence of hydrogen-induced volume in fcc nickel hydride

In transition-metal hydrides, hydrogen occupies interstitial sites in the metal lattice, resulting in significant volume expansion. The difference in unit-cell volume between the metal and its hydride is denoted as hydrogen-induced volume (vH), and this value has been determined for various metal hydrides. However, systematic investigations of its effects on thermoelastic properties under pressure (P) and temperature (T) are scarce, and the P-T-dependence of vH is poorly constrained. Here, we report the P-V-T relations of nickel hydride (H/Ni ∼ 1) with a face-centered cubic structure, which is frequently observed in 3d transition metals and their hydrides. We collected x-ray diffraction data on nickel hydride at 7-16 GPa and 300-1000 K and determined its equations of state. We also conducted a melting experiment at 15 GPa and observed melting at 1450 K. The results show that nickel hydride is more compressible than pure Ni and exhibits a large thermal expansion, resulting in a negative dependence on pressure and a positive dependence on temperature of vH. This behavior originates from the reduction in bulk modulus and Debye temperature by hydrogenation, suggesting that interstitial H atoms weaken the Ni-Ni bonds through lattice expansion and possibly also through phonon softening, as evidenced by comparison at equivalent volumes.