Solubility of magnesium in uranium dioxide

The solubility of magnesium in uranium dioxide under low oxygen pressures was studied at 1200°C. Magnesium was found to dissolve up to y > 0.1 (and below y = 0.15) of the apparent formula, Mg_yU_{1 - y}O_{2 + cursive chi} (cursive chi ^≤_≥ 0) on heating at pP_O2 = 10^-15 and ≤ 10^-19 atm. The formed solid solution in such a low p_O2 region was of the type (Mg_aU_{1 - a}){Mg_b}O_{2 + c}, in which the magnesium atoms partly occupy the interstitial sites together with the substitutional sites for uranium atoms. The ratio of interstitial atoms to the total magnesium atoms increased from 0.23 (y = 0.05) or 0.39 (y = 0.1) at p_O2 = 10^-15 atm with decreasing oxygen partial pressure to 0.62-0.63 (y = 0.05 and 0.1) at p_O2 ≤ 10^-19 atm. The lattice parameter of the (Mg_aU_{1 - a}){Mg_b}O_{2 + c}. solid solutions was represented as a linear equation of a, b and c. The interstitial magnesium caused an increase in the lattice parameter, in contrast to the substitutional magnesium which largely decreases the lattice parameter. It is possible that the uranium atoms in the solid solutions prepared at low oxygen partial pressures (≤ 10^-19 atm) were reduced to slightly less than the tetravalent state.