Diffusionless Tetragonal–Cubic Transformation Temperature in Zirconia–Ceria Solid Solutions

Diffusionless tetragonal (t′)  cubic (c′) phase transformation in 65‐mol%‐CeO₂‐ZrO₂ was investigated around the c′‐t′ equilibrium temperature T₀^c′‐t′ using powder X‐ray diffraction, where c′ was defined as a tetragonal or cubic phase with an axial ratio of unity. The {400} peak profile of t′‐ZrO₂ broadened at 860° and 900°C. This indicated that the t′‐ZrO₂ transformed partially into c′‐ZrO₂. The {400} peak profile of c′‐ZrO₂ also broadened at 860° and 900°C, which indicated that the c′‐ZrO₂ partially transformed to t′‐ZrO₂. The finish point of t′→c′ transformation T_f^t′→^c′ was investigated by annealing the t′‐ZrO₂ containing 30–65 mol% CeO₂ at various temperatures. The T′_f^t′→c′ line existed in the vicinity of the cubic solubility limit within the t + c two‐phase region. This T_f^t′→c′ location, which should have been located in the vicinity of T₀^c′‐t′ could not be explained by a simple regular solution model. However, it was described successfully by a thermodynamic model based on Landau's phenomenologic theory.