Structure and bulk modulus of Ln-doped UO2 (Ln = La, Nd) at high pressure

The structure of lanthanide-doped uranium dioxide, Ln(x)U(1-x)O(2-0.5x+y) (Ln = La, Nd), was investigated at pressures up to similar to 50-55 GPa. Samples were synthesized with different lanthanides at different concentrations (x similar to 0.2 and 0.5), and all were slightly hyperstoichiometric (y similar to 0.25-0.4). In situ high-pressure synchrotron X-ray diffraction was used to investigate their high-pressure phase behavior and determine their bulk moduli. All samples underwent a fluorite-to-cotunnite phase transformation with increasing pressure. The pressure of the phase transformation increased with increasing hyper-stoichiometry, which is consistent with results from previous computational simulations. Bulk moduli are inversely proportional to both the ionic radius of the lanthanide and its concentration, as quantified using a weighted cationic radius ratio. This trend was found to be consistent with the behavior of other elastic properties measured for Ln-doped UO2, such as Young's modulus. (C) 2017 Elsevier B.V. All rights reserved.