Zirconia–Ceria Solid Solution Synthesis and the Temperature–Time–Transformation Diagram for the 1:1 Composition

Zirconia–ceria solid solutions of tetragonal symmetry ( t 'ZrO₂) containing 30 to 65 mol% ceria were prepared by annealing c '-ZrO₂ at 627°C samples sintered at 1660–1760°C, where c '-ZrO₂ is defined as a cubic or tetragonal phase whose axial ratio c/a (tetragonality) is equal to 1. The lattice parameters and the cube root of the unit cell volumes of the annealed samples increased linearly with the ceria content but, on the other hand, the axial ratio c/a (tetragonality) decreased to 1.000 at 70 mol%. During the annealing of each sample, the cell volume decreased because the residual Ce³⁺ ions were oxidized to the smaller Ce⁴⁺. The temperature–time–transformation (TTT) diagram of the 1:1 solid solution was investigated for the c'-t' diffusionless phase transition and the c'–(t + c) diffusional reaction. The c'–t' transformation was found to behave as a thermally activated process with an activation energy estimated to be 113 kJ/mol. This value may indicate that c'–t' transformation is controlled by oxygen diffusion accompanied by dimensional changes of the cerium ions.