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(MoxFe1−x)2-H2

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This study aims to shed light on the unusual trend in the stabilities of Zr(MoxFe1−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, uelas. In order to discern this possibility, we measured the elastic moduli of Zr(MoxFe1−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 uelas. Therefore, we turn our attention to the volume of the hydrogen atom, vH, 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 ZrMo2H3.5, 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 Å3/(H atom). Some very low vH values, have been reported by other investigators. The low vH values, as well as the one derived in this work, significantly reduce uelas 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.