Local structure and vibrational properties ofα′−Pumartensite in Ga-stabilized δ-Pu

Abstract

Extended x-ray absorption fine structure spectroscopy (EXAFS) is used to investigate the local atomic environment and vibrational properties of plutonium and gallium atoms in the ${\ensuremath{\alpha}}^{\ensuremath{'}}$ and \ensuremath{\delta} phases of a mixed phase Pu-Ga alloy. EXAFS results measured at low temperature compare the structure of the mixed phase sample with a single-phase \ensuremath{\delta}-Pu sample. EXAFS spectral components attributed to both ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Pu}$ and \ensuremath{\delta}-Pu were observed in the mixed phase sample. Ga K-edge EXAFS spectra indicate local atomic environments similar to the Pu ${L}_{\mathrm{III}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ EXAFS results, which suggests that Ga is substitutional for Pu atoms in both the monoclinic ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Pu}$ and the fcc \ensuremath{\delta}-Pu structures. In \ensuremath{\delta}-Pu, we measure a Ga-Pu bond length contraction of 0.11 \AA{} with respect to the Pu-Pu bond length. The corresponding bond-length contraction around Ga in ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Pu}$ is only 0.03 \AA{}. Results from temperature-dependent Pu ${L}_{\mathrm{III}}\ensuremath{-}\mathrm{e}\mathrm{d}\mathrm{g}\mathrm{e}$ EXAFS measurements are fit to a correlated Debye model, and a large difference in the Pu-Pu bond Debye temperature is observed for the ${\ensuremath{\alpha}}^{\ensuremath{'}}$ and \ensuremath{\delta} phases: ${\ensuremath{\theta}}_{\mathrm{cD}}({\ensuremath{\alpha}}^{\ensuremath{'}})=159\ifmmode\pm\else\textpm\fi{}13\mathrm{K}$ versus ${\ensuremath{\theta}}_{\mathrm{cD}}(\ensuremath{\delta})=120\ifmmode\pm\else\textpm\fi{}3\mathrm{K}.$ The corresponding analysis for the Ga K EXAFS determines a Ga-Pu bond Debye temperature of ${\ensuremath{\theta}}_{\mathrm{cD}}(\ensuremath{\delta})=188\ifmmode\pm\else\textpm\fi{}12\mathrm{K}$ in the \ensuremath{\delta}-Pu phase. These results are related to the observed solubility of Ga in \ensuremath{\delta}-Pu, the ``stabilization'' of \ensuremath{\delta}-Pu by Ga at room temperature, and the insolubility of Ga in ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{Pu}.$