Optical absorption spectroscopy of thorium dioxide. Analysis of oxidising and reducing anneals and gamma and UV radiation on both flux-grown and arc-fused single crystals

Abstract

The optical absorption spectra of crystals of ThO2 have been recorded from 40 000 to 5000 cm–1(5.0–0.6 eV). For the first time both flux-grown and arc-fused single crystals were compared under identical conditions of high temperature anneals, between 500 and 1000 °C, in air, oxygen and hydrogen. The effects of gamma and near UV radiation before and after various anneals were followed. Bands generated upon annealing were resolved into their Gaussian components. Flux-grown crystals, containing lead and bismuth impurities, shifted their absorption edge to lower energy with increasing anneal temperature and changed from yellow through red to black. The much purer arc-fused crystals only shifted their absorption edge slightly upon annealing, but bands on this edge arose and fell, and were subsequently resolved into their Gaussian components. Near UV radiation had no effect on the spectra of flux-grown crystals but did affect both oxidized and reduced arc-fused crystals. 60Co gamma radiation also affected flux-grown and arc-fused crystals, the effects decaying with time, and the rate increasing with near UV radiation. The effects were most noticeable on the bands produced by annealing the arc-fused crystals. The involvement of impurity ions in the colour changes in flux-grown crystals is discussed. For arc-fused crystals the difficulties in associating the shift of the fundamental absorption edge with temperature with Urbach's rule are given and suggestions made for the origin of the bands generated upon annealing. F and F+ centres are possibly formed by gamma radiation, but EPR studies did not support F+ formation. Previously unpublished data from Childs on pure arc-fused crystals are incorporated and discussed.