Lattice constants of $\beta -\mathrm{P}\mathrm{d}{\mathrm{H}}_x$ and $\beta -\mathrm{P}\mathrm{d}{\mathrm{D}}_x$ with $x$ near 1.0

Lattice constants of the $\beta$ phase of $\mathrm{Pd}{\mathrm{H}}_x$ and $\mathrm{Pd}{\mathrm{D}}_x$ for the concentration range $0.8<x<\mathrm{}0.98\mathrm{}$ 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 (∼ 0.1%) than that of $\mathrm{Pd}{\mathrm{D}}_x$ and $\frac{d\ln a_0}{\mathrm{dx}}=0.044\mathrm{}$ for both $\mathrm{Pd}{\mathrm{H}}_x$ and $\mathrm{Pd}{\mathrm{D}}_x$; extrapolated $a_0$ values for $x=1.0\mathrm{}$ 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 $\mathrm{Pd}{\mathrm{D}}_x$.