Characteristic thermodynamic properties of M_yU₁-_yO₂+_x (M = M⁴⁺, M³⁺ and M²⁺; x ≧< 0) solid solutions and the phase behavior of Mg solid solution in low oxygen pressures

After a short review on the existing regions and thermodynamic properties of essentially ionic ternary uranium dioxide solid solutions, M_yU_1-yO_2+x (x ≧< 0), classified by the valency of the M metals, a description is made for our study on the solubility and the crystal chemistry of Mg in the Mg solid solution in a range of low p_o2. The single phase region extended up to at least y = 0.1 at 1200°C in po = 10^-15 or ≤ 10^-19 atm. Density measurements revealed that the Mg atoms in the solid solution formed under the above conditions are about 60% in the interstitial sites. This experimental result was rationalized by the calculated ΔḠ_O2 change caused from the configurational partial molar entropy obtained through the lattice statistics of the occupation of Mg and oxygen atoms in the solid solution lattice sites.