High‐Temperature X‐ray Study of the Cubic–Tetragonal Diffusionless Phase Transition in the ZrO₂─ErO_1.5System: II, Temperature Dependences of Oxygen Ion Displacement and Lattice Parameter of Compositionally Homogeneous 12 mol% ErO_1.5─ZrO₂

The tetragonal‐to‐cubic phase transition of rapidly quenched 12 mol% ErO_1.5─ZrO₂ was investigated in situ using high‐temperature X‐ray diffraction. Rapid quenching of the melt yielded a small oxygen ion displacement and a small tetragonality, axial ratio c/a. Although the tetragonality of the as‐quenched specimen increased with temperature up to about 1000°C, it became independent of temperature on cooling to room temperature from about 1000°C and reheating up to 1000°C. The tetragonality decreased continuously with temperature above 1000°C and became unity at about 1400°C. The atomic coordination z for the oxygen ion, which expresses the displacement from its ideal site in the fluorite‐type structure, increased with temperature and became 1/4 at about 1400°C. The tetragonality increased with annealing time through a thermal activation, which could explain the compositional dependence of the tetragonality in rapidly quenched ZrO₂─RO_1.5 samples (R = rare earths).