Lattice constants ofβ−PdHxandβ−

Lattice constants of the $\ensuremath{\beta}$ phase of $\mathrm{Pd}{\mathrm{H}}_{x}$ and $\mathrm{Pd}{\mathrm{D}}_{x}$ for the concentration range $0.8<x<0.98$ have been measured at 77 K using a powder x-ray diffraction technique. The lattice constant ${a}_{0}$ of $\mathrm{Pd}{\mathrm{H}}_{x}$ is slightly larger (\ensuremath{\sim} 0.1%) than that of $\mathrm{Pd}{\mathrm{D}}_{x}$ and $\frac{d\mathrm{ln}{a}_{0}}{\mathrm{dx}}=0.044$ for both $\mathrm{Pd}{\mathrm{H}}_{x}$ and $\mathrm{Pd}{\mathrm{D}}_{x}$; extrapolated ${a}_{0}$ values for $x=1.0$ are 4.090 and 4.084 A for PdH and PdD, respectively. This study shows that the inverse isotope effect in the superconducting transition temperature is not simply a result of the relative volume of $\mathrm{Pd}{\mathrm{H}}_{x}$ and $\mathrm{Pd}{\mathrm{D}}_{x}$.