Temperature dependence of the PdK-edge extended x-ray-absorption fine structure ofPdCx(x∼0.13)

Pd K-edge extended x-ray-absorption fine-structure (EXAFS) and x-ray-absorption near-edge-structure (XANES) measurements were performed on a Pd carbide phase, ${\mathrm{PdC}}_{\mathit{x}}$ (with x∼0.13), and metallic Pd powder at temperatures between 40 and 423 K. The data were analyzed by nonlinear least-squares fitting using an ab initio theoretical standard. The average lattice expansion of the carbide phase, 2.7±0.1% relative to Pd metal, agrees well with previous x-ray- and neutron-diffraction results. The temperature dependence of the Pd-Pd nearest-neighbor mean-squared relative displacement (${\mathrm{\sigma }}^2$) yields a Debye temperature, ${\mathrm{\Theta }}_{\mathit{D}}$=295±10 K for Pd powder. There is an additional static-disorder contribution in ${\mathrm{\sigma }}^2$ of 0.0009±0.0002 A${\mathrm{̊}}^2$, relative to bulk Pd metal. The Debye temperature of ${\mathrm{PdC}}_{0.13}$ powder is also 295±10 K. There is, however, an additional static-disorder contribution to ${\mathrm{\sigma }}^2$ of 0.0012 A${\mathrm{̊}}^2$, relative to Pd powder, probably the result of local distortion of the Pd lattice by interstitial C. The Pd-C coordination is not directly observed in a Fourier transform of the EXAFS. The location of C in octahedral interstices is evident by its effect, through multiple scattering, on the magnitude of the second Pd-Pd peak in the Fourier transform magnitude of the EXAFS data, which is greatly reduced. The Pd K-edge XANES spectrum of ${\mathrm{PdC}}_{0.13}$ is nearly identical to that of Pd metal. There is no significant chemical shift (≤0.5 eV) of the Pd K edge of ${\mathrm{PdC}}_{\mathrm{\sim }0.13}$.