Specific Heats of Delta-Phase Zr-H and Zr-D

The specific heats of fcc Zr${\mathrm{H}}_{1.58}$ and Zr${\mathrm{D}}_{1.58}$ have been measured in the temperature interval 30-500°C. The data are interpreted in terms of a harmonic oscillator model for the hydrogen and deuterium specific heat contributions. This model is in accord with recent inelastic neutron scattering studies and predicts an isotopic depression of the deuteride Einstein temperature by a factor of $\frac{1}{\sqrt{2}}$ relative to the hydride value. Over the interval 30-200°C, the data and model are quantitatively consistent. At higher temperatures, the deuteride specific heat is somewhat smaller than anticipated. Quantities of noncubic $\gamma$-phase material are known to be present in the samples used, particularly in the deuteride, and this is thought to be a likely cause for the deviations. The hydride Einstein temperature calculated from the difference between deuteride and hydride specific heats at 150°C is 1500±300°K, as compared with the inelastic neutron scattering value of 1500±60°K.