Diffuse reflectance spectra of U ions in ThO2

Diffuse reflectance spectra obtained at ambient temperature for polycrystalline UxTh(1−x)O2+y ceramics fired at 1500 °C in 3.5% H2/N2 show several well-defined absorption bands of a 500 cm−1 width in the range of 4000–120 00 cm−1, extending the range of previous measurements. These bands are ascribed to interconfigurational d–d single-ion transitions of U4+and increase in intensity with U concentration for x ⩽ 0.1. For higher x values, the intensities of the U4+ bands peak at x ∼ 0.2 and then decrease with increasing x to small values only at x = 1. Firing in argon at 1500 °C produced enhanced absorption in the 6000–7000 cm−1 range, indicative of U5+ formation. Further oxidation to U5+ was produced by firing in air. The detailed shape of the U5+ peaks depended upon the degree of oxidation and the U/Th ratio.