Oxygen potential and defect structure of the solid solution, Mg-Gd-UO₂

For solid solutions Mg_yGd_zU_1-y-zO_2+x with y = 0.03, 0.06 and 0.10 and z = 0.142, oxygen potential, ΔḠ_O2, was measured as a function of O/M ratio (M = Mg+Gd+U) at temperatures between 1000°C and 1250°C. The O/M ratio which gives the steepest change of ΔḠ_O2 (referred to as GOM) was lowered with increasing magnesium concentration. The GOMs were 1.968, 1.940 and 1.914 for 3, 6 and 10 mol% Mg, respectively. There seemed to be no temperature dependence of GOM. As the oxygen partial pressure, p_O2, decreases, a part of the magnesium atoms rearrange from the substitutional 4a sites of Fm3̄m to the interstitial 4b sites as shown by density measurements, during which the magnesium solubility decreases from y = 1/3 to the minimum (<0.06) at around p_O2 = 10^-3 Pa. However, at p_O2 = 10^-8 Pa, the solubility again increases to >0.1. The ratio of interstitial magnesium to total magnesium was near 0.5 at 3-5 mol% Mg at GOM. The change rates of lattice parameter were found to be ∂a/∂x = -0.101 Å, ∂a/∂y = +0.290 Å for interstitial magnesium and ∂a/∂z = -0.187 Å.