Thermodynamics of (Mg,Ce,U)O_2+x(x</>0) solid solutions

Quaternary solid solutions, Mg_yCe_zU_1-y-zO_2+x(x</>0), were prepared and equilibrated under various oxygen partial pressures. The solubility of magnesium in Ce_0.1U_0.9O_2+x was about y = 0.05, and above this concentration MgO precipitated, forming a two-phase mixture. In Mg_yCe_zU_1-y-zO_2+x with y ≤ 0.05, a part of the dissolved magnesium atoms occupy the interstitial 4b position of Fm3̄m. The ratio of the interstitial magnesium to the total magnesium increases from about 0.3 to 0.6 as the oxygen partial pressure decreases to 10^-19-10^-21atm. The rates of change of the lattice parameter, ∂a/∂y, ∂a/∂z and ∂a/∂x, for x greater than 0 were somewhat smaller in absolute values than literature values. For x < 0, the rates ∂a/∂z and ∂a/∂x were in good agreement with the literature values. However, ∂a/∂y was much smaller as absolute value due to interstitial magnesium. A plot of oxygen potential vs O/M ratio at 1000°C showed a marked shift to lower x for the Mg_yCe_zU_1-y-zO_2+x solid solution. The shift rate was -0.01 per 1 mol% magnesium.