Defect structure and the phase diagram of uranium dioxide

Positions of the phase boundaries of pure uranium dioxide $\mathrm{UO}_{2-x}$ are calculated under the assumption that damage (melting) of the crystal lattice is caused by filling it with defects. The conditions of complete filling of the crystal lattice with individual defects are analyzed. The equilibrium reaction rate constants are determined for the high-temperature forms of interaction of the crystal with oxygen of ambient gaseous medium. The position of the monotectic point is found for $x>0$. The melting point parameters are determined for stoichiometric uranium dioxide and uranium dioxide with the maximum dense packing of the defects. A retrograde behavior of the solidus curve in the region $x>0$ and anisotropy of properties near the point of congruent melting are predicted. The complex character of the behavior of the boundaries of $\mathrm{U}_4\mathrm{O}_9$ and $\mathrm{U}_3\mathrm{O}_8$ phases is explained. The reason of discrepancy between the results of two different measurements of solidus temperature in the region $x<0$ is explained qualitatively, and the possibility of the existence of solid uranium dioxide up to $3400$ K under certain conditions is predicted.