Diffuse reflectance spectroscopy of tetravalent neptunium and plutonium ions in ThO2

Near infrared (4000-12 000 cm−1) absorption spectra of Pu4+ and Np4+ dilutely incorporated in air-fired polycrystalline ThO2 were collected at ambient temperature and showed several well-defined absorption bands having widths of a few hundred cm−1. These bands are ascribed to f-f single-ion electronic transitions and increase in intensity with Np and Pu concentration up to 0.03 formula units (f.u.). Reheating samples in H2/N2 atmosphere at 1400 °C did not produce obvious spectral change for Np-doped ThO2 and therefore no evidence of the formation of Np3+. However, the spectral lines attributed to Pu4+ decreased by approximately a factor of six in intensity, which implies reduction of Pu4+ to Pu3+.