The Role of Bulk Stiffening in Reducing the Critical Temperature of the Metal-to-Hydride Phase Transition and the Hydride Stability: The Case of Zr(Mo_xFe_1−x)₂-H₂

This study aims to shed light on the unusual trend in the stabilities of Zr(Mo_xFe_1−x)₂, 0 ≤ x ≤ 1, hydrides. Both the rule of reversed stability and the crystal volume criterion correlate with the increased hydride stabilities from x = 0 to x = 0.5, but are in contrast with the destabilization of the end member ZrMo₂ hydride. The pressure-composition isotherms of ZrMo₂-H₂ exhibit very wide solid solubility regions, which may be associated with diminished H–H elastic interaction, u_elas. In order to discern this possibility, we measured the elastic moduli of Zr(Mo_xFe_1−x)₂, x = 0, 0.5, 1. The shear modulus, G, shows a moderate variation in this composition range, while the bulk modulus, B, increases significantly and monotonically from 148.2 GPa in ZrFe₂ to 200.4 GPa in ZrMo₂. The H–H elastic interaction is proportional to B and therefore its increase cannot directly account for a decrease in u_elas. Therefore, we turn our attention to the volume of the hydrogen atom, (Formula presented.), which usually varies in a limited range. Two coexisting phases, a Laves cubic (a = 7.826 Å) and a tetragonal (a = 5.603 Å, c = 8.081 Å) hydride phase are identified in ZrMo₂H_3.5, obtained by cooling to liquid nitrogen temperature at about 50 atm. The volume of the hydrogen atom in these two hydrides is estimated to be 2.2 ų/(H atom). Some very low (Formula presented.) values, have been reported by other investigators. The low (Formula presented.) values, as well as the one derived in this work, significantly reduce u_elas for ZrMo₂-H₂, and thus reduce the corresponding critical temperature for the metal-to-hydride phase transition, and the heat of hydride formation. We suggest that the bulk stiffening in ZrMo₂ confines the corresponding hydride expansion and thus reduces the H-H elastic interaction.