Changes of structural properties in polycrystalline Y<sub>2</sub>O<sub>3</sub> under heating in air and vacuum

Polymorphic transformations of the first and second kind of ordered cubic structure of type C into a disordered type C1 were found during heating of samples of Y2O3 oxide grade (99,990 %) for phosphors in the range of 500‒1100 °C in air and vacuum. Phase transformations proceed without changing the chemical composition of oxygen in yttrium oxide. The mechanism of these phase transformations defined. The chemical composition of oxygen of the cubic structure of the C1 Y2O3 type is stable up to 1750 °C in air and up to 1500 °C in vacuum. The change in the chemical composition of Y2O3‒х with respect to the oxygen content in the C1 type cubic phase was studied. The cubic structure of the C1-type phase with a changed chemical composition in terms of the oxygen content in the lattice is stable up to 2200 °C in air and up to 1800 °C in vacuum and decomposes at these temperatures into an ordered type C and a monoclinic type B phase. For the first time it waхs theoretically substantiated. For the first time theoretically substantiated and experimentally confirmed, that when samples are heated in the range of 2100‒2200 °C in vacuum and deviate from stoichiometry to the composition YO1,37, the cubic phase of type Cх decomposes into 2 cubic phases: type F with the content of anionic vacancies and lattice parameter a = 0,5265 nm (space group Fm3m) and cubic phase of C11 type by the content of color centers and lattice parameter a = 1,0652 nm (space of group Ia3). The refractive indices of these phases were determined. The phases of type F and C11 are stable when heated in air to 1300 °C. In the range of 1300‒1400 °C in air, yttrium oxide samples are oxidized to the composition Y2O3. The energy of oxygen migration into the yttrium oxide lattice was defined and the rates diffusion of oxygen during oxidation was calculated and dependence from the temperature of oxidation was found. Ill. 7. Ref. 27.