The defects of structural and phase transformations in polycrystalline cerium dioxide under heating in vacuum and in air

Abstract

Structural changes in cerium dioxide, when heated in vacuum in the range of 25‒1600 °C, in air in the range of 25‒1500 °C, and during successive annealing in the range of 1600‒2100 °C in air, followed by quenching in water, were studied. In the crystal lattice of CeO2‒х, the F → F1 phase transformation in vacuum proceeds in the range of 1100‒1600 °C; additionally, at 1200 °C, X-ray lines of the C-type Ce2O3 phase appear. The thermal expansion coefficient of phases of the fluorite type F and F1 in the range of 25‒1500 °C in air, as well as phases of the fluorite type F, F1 and type C Ce2O3 in the range of 25‒1600 °C are determined in vacuum and their dependence on the change in the oxygen content in the CeO2‒х crystal lattice was found. The kinetic conditions for reduction of cerium dioxide in vacuum and oxidation in air are different. The cubic structure of the fluorite type F CeO2‒х, when the samples are heated in air, is preserved up to 1800 °C with the content of anionic vacancies, at 1900 °C the transformation F → F1 occurs. The formation of loops, edge and screw dislocations in the structure of cerium dioxide grains after annealing of samples in the range of 1900‒2100 °C in air were discovered for the first time. The decomposition of the structure F1 into cerium oxide phases of types F and C proceeds at 2100 °C along the height and boundaries of screw dislocations. It was found that fragments of the C-type phase of cerium oxide are located on loops along the height of screw dislocations, which indicates the movement and evaporation of these fragments. When the samples are oxidized at 1600 °C in air, the black-colored C Ce2O3‒х phase in a gradient of different concentrations moves along certain trajectories to opposite ones. grain boundaries, abuts against dislocation loops, bends them, and oxidizes to phases F1 and F. In the structure of polycrystalline cerium dioxide, when heated in vacuum and in air, certain concentrations of defects control phase transformations.