Thermodynamics of the UO2 solid solution with magnesium and europium oxides

Magnesium–europium solid solutions of uranium dioxide, MgyEuzU1−y−zO2+x, with y=0.05, 0.1 and z=0.05, 0.1 were prepared, and their oxygen potential, ΔḠO2, was measured as a function of O/M ratio (M=Mg + Eu + U) at temperatures of 1000°C, 1100°C and 1200°C. The O/M ratio which gave the steepest change of ΔḠO2 (referred to as GOM) was 1.955 for y=0.05, z=0.1 (and y=0.05, z=0.05), but it decreased to 1.908 at higher Mg2+ concentrations of y=0.1, z=0.05. The GOM did not change with temperature in the range 1000–1200°C. The solid solutions showed distinct ΔS̄O2 peaks of near 0 J mol−1 K−1 at GOM, which was the same for ΔH̄O2 though the peaks were much smaller. The rates ∂a/∂y and ∂a/∂z for the lattice parameter change were −0.162 and −0.154 Å, respectively. At GOM, the Mg2+ ions of a fraction of 0.473 of total Mg2+ ions occupy the interstitial position of the fluorite lattice. The averaged numbers of U5+ ions bonded to one Mg2+ ion, α, were very small, i.e. 0.1–0.4. The large leftward shift in the curves of ΔḠO2 vs. O/M ratio for this quarternary solid solution was considered to be caused by these small α values.