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The phase transitions in TiO2–Sc2O3 (40–50 mol % Sc2O3) samples prepared from coprecipitated and mechanically activated oxide mixtures are studied. It is shown that heat treatment below 1000°C leads to the formation of nonstoichiometric, metastable cubic phases with a distorted fluorite structure. With increasing temperature, this structure transforms into a stable fluorite-like structure through (low-symmetry) orthorhombic and hexagonal phases, which is associated with the incorporation of OH groups. Characteristically, the phases studied contain nanoscale regions differing in the degree of ordering, which result from internal stress. The internal stress in mechanically activated phases is shown to be fully relieved by about 1300°C.