Raman spectroscopic studies of defect structures and phase transition in hyper-stoichiometric UO2+x

Abstract

A method to determine the defect structures in hyper-stoichiometric UO_2+x using a combination of XRD and Raman spectroscopy has been developed. A sequence of phase transitions, from cubic to tetragonal symmetry, occurs with increasing degree of non-stoichiometry. This sequence proceeds from a cubic phase through an intermediate t″-type tetragonal (axial ratio c/a = 1) phase to a final t-type tetragonal (c/a ≠ 1) phase. Four distinct structural defect regions can be identified in the stoichiometry range, UO₂ to U₃O₇: (i) a random point defect structure (x (in UO_2+x) ≤ 0.05); (ii) a non-stoichiometry region (0.05 ≤ x ≤ 0.15) over which point defects are gradually eliminated and replaced by the Willis 2 : 2 : 2 cluster; (iii) a mixture of Willis and cuboctahedral clusters (0.15 ≤ x ≤ 0.23); (iv) the cuboctahedral cluster (x ≥ 0.23). The geometry and steric arrangement of these defects is primarily determined by the concentration of the excess-oxygen interstitials.