Structural and Phase Transformation Defects Within Polycrystalline Cerium Dioxide on Heating in Vacuum and in Air

Abstract

Structural changes in cerium dioxide on heating in a vacuum in the range 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 water quenching, are studied. In the CeO_2–x crystal lattice the F ⟶ F¹ phase transformation in a vacuum proceeds in the range of 1100 – 1600°C; in addition, 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 a vacuum and their dependence on the change in oxygen content in the CeO_2–x crystal lattice is found. Kinetic conditions for reduction of cerium dioxide in a vacuum and oxidation in air are different. The cubic structure of the fluorite type F CeO_2–x, when specimens are heated in air, is preserved up to 1800°C with the content of anionic vacancies, and at 1900°C the transformation F ⟶ F¹ occurs. Formation of loops, edge and screw dislocations within the structure of cerium dioxide grains after specimen annealing in the range of 1900 – 2100°C in air are discovered for the first time. 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 is found that fragments of the C-type phase of cerium oxide are located in loops over the height of screw dislocations, which indicates movement and evaporation of these fragments. During specimen oxidation at 1600°C in air, a black-colored type C Ce₂O_3–x phase in a gradient of different concentrations moves along certain trajectories to opposite grain boundaries, abuts against dislocation loops, bends them, and is oxidized to phases F¹ and F. Within the structure of polycrystalline cerium dioxide, during heating in a vacuum and in air, certain concentrations of defects control phase transformations.