Lattice constants of β-PdHx and β-PdDx with x near 1.0

Lattice constants of the β phase of PdH_x and PdD_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 a₀ of PdH_x is slightly larger (~ 0.1%) than that of PdD_x and dlna₀dx=0.044 for both PdH_x and PdD_x; extrapolated a₀ 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 PdH_x and PdD_x.