A cooperative pseudo Jahn–Teller effect in three phases of CeAlO3

The sequence of phases of CeAlO3, which experimentally have been found by lowering the temperature: Pm3¯m→R3¯c→Imma→I4/mcm, can be characterized by cooperative pseudo Jahn–Teller effects due to interaction of the ground state (4f)1 with the excited state (5d)1 from the Ce3+-ion. The three phases, a tetragonal one, an orthorhombic one and a rhombohedral phase which are found from low temperature, successively to higher temperature, can be described by distortions from respectively a single T2u-component, a linear combination of two T2u-components and a linear combination of three T2u-components. Since in CeAlO3 the distortions are at the Ce3+-center, it is practical to name the distortion mode: T2u at the R-point (with respect to the center with a 12-environment of anions).The four short bonds around each Ce3+-ion in the tetragonal phase point to covalent bonding. This bonding is in the tetragonal structure stronger than in the rhombohedral phase. This can explain the observation that the rhombohedral phase, with three short bonds, occurs at higher temperature, and is stabilized by entropy.