Crystallographic Phase Transitions in Laser Irradiated Cerium Dioxide

Abstract

Two polymorphic Ce₂O₃ compounds are reported: A-Ce₂O₃ (hexagonal, space group P 3ml) and C-Ce₂O₃ (cubic, space group T ⁷_h or Ia3) [1]. If A-Ce₂O₃ was obtained by difficult carbothermic reduction reactions, the C-Ce₂O₃ occurrence is yet controversial [2]. Structure and sintaxy of R₂O₃ (R = rare earth element) oxides and their relationship to the fluorite structure of RO₂ have been investigated in great detail. The intermediate oxides associated with various rare-earth elements (including cerium) can he described by the equivalent basic formulae: RO_x (1.5 ≤ x ≤ 2) and R_nO_2n−2 (4 ≤ n ≤∞) where n=4 leads to R₂O₃ and n=∞ to RO₂. The homologous series R_nO_2n−2 contains a great number of structures (especially hexagonal, monoclinic and triclinic) built on the fluorite-like lattice of RO₂ compound, which are due to various oxygen-vacancy rearrangement mechanisms. Some papers were devoted to the preparation of intermediate oxides [3] and the direct reduction of higher oxides during heating by means of an electron beam [4]. EM experiments demonstrated that electron beam irradiation induced unexpected phase transitions in.thin films of rare earth oxides [5].