Changes in the structural properties of polycrystalline oxide La₂O₃ at heating it in air and in vacuum

Abstract

Changes in the phase’s structure of polycrystalline La2O3 upon heating in air and vacuum have been established. For the first time, on La2O3 samples after annealing in the range 700‒1900 оC in air, the microstructure of a monoclinic phase of type B and a hexagonal phase of type A with the presence of texture was obtained. For the first time, after annealing La2O3 samples at 1900 °C in air, the formation of loops and screw dislocations on La2O3‒х grains was discovered, through which the evaporation of crystals of different colors and the decomposition of hexagonal phase grains into cubic phase crystals of type F with an fcc lattice and type C with a bcc lattice occurred. For the first time, it has been theoretically substantiated and experimentally confirmed that heating lanthanum oxide samples at 1750 and 1900 °C leads to the decomposition of the hexagonal phase of type A with the presence of texture into two cubic phases: type F with the content of anionic vacancies and a crystal lattice parameter a = 0.5677 nm (Fm3m) and type C containing color centers and a = 1.14838 nm (Ia3). The refractive indices of these phases were determined. The energy of formation of anion vacancies in the lattice of the hexagonal phase of type A is calculated and the critical concentration of anion vacancies leading to the decomposition of this phase is determined.