Examination of U valence states in the brannerite structure by near-infrared diffuse reflectance and X-ray photoelectron spectroscopies

The valence state of uranium doped into a f0 thorium analog of brannerite (i.e., thorutite) has been examined using near-infrared (NIR) diffuse reflectance (DRS) and X-ray photoelectron (XPS) spectroscopies. NIR transitions of U4+, which are not observed in spectra of brannerite, have been detected in the samples of UxTh1−xTi2O6, and we propose that strong specular reflectance is responsible for the lack of U4+ features in UTi2O6. Characteristic U5+ bands have been identified in samples in which sufficient Ca2+ has been added to nominally effect complete oxidation to U5+. XPS results support the assignments of U4+ and U5+ by DRS. The presence of residual U4+ bands in the spectra of the Ca-doped samples is consistent with segregation of Ca2+ to the grain boundaries during high temperature sintering.