Phase relation and defect structures of nonstoichiometric U4O9±y and UO2+x at high temperatures

Phase relations in the composition range from $\text{UO}{}_{\mathrm{2+x}}$ to $\text{U}{}_3\text{O}{}_{\mathrm{8?z}}$ were studied by electrical-conductivity measurements and X-ray diffraction in the ranges $\text{1025°C ? T ? 1140°C}$ and . The plot of log σ versus log showed straight lines with distinct slopes, which corresponded to four regions ($\text{UO}{}_{\mathrm{2+x}}$, $\text{U}{}_4\text{O}{}_{\mathrm{9?y}}$, $\text{U}{}_4\text{O}{}_{\mathrm{9+y}}$ and $\text{U}{}_3\text{O}{}_{\mathrm{8?z}}$). The existence of the hyperstoichiometric $\text{U}{}_4\text{O}{}_{\mathrm{9+y}}$ phase was suggested in the temperature range from 1025 to 1126°C. The peritectoid temperature ($\text{U}{}_4\text{O}{}_{\mathrm{9\pm y}}\text{= UO}{}_{\mathrm{2+x}}\text{+ U}{}_3\text{O}{}_{\mathrm{8?z}}$) was estimated to be present between 1126 and 1131°C. The partial free enthalpies and entropies for the two-phase equilibrium ($\text{U}{}_4\text{O}{}_{\mathrm{9+y}}\text{+ U}{}_3\text{O}{}_{\mathrm{8?z}}$, and $\text{U}{}_4\text{O}{}_{\mathrm{9?y}}\text{+ UO}{}_{\mathrm{2+x}}$) were calculated and compared with previous results. From the dependence of the electrical conductivity on the oxygen partial pressure the nonstoichiometric defect structures of $\text{UO}{}_{\mathrm{2+x}}$ and $\text{U}{}_4\text{O}{}_{\mathrm{9\pm y}}$ were interpreted as consisting of doubly charged oxygen interstitials (O_i'') and doubly charged oxygen vacancies (V_O''). At room temperature, the homogeneity range of the U₄O₉ phase was investigated with a Debye-Scherrer camera.