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

Diffusionless tetragonal ( t′ )  cubic ( c′ ) phase transformation in 65‐mol%‐CeO 2 ‐ZrO 2 was investigated around the c′‐t′ equilibrium temperature T 0 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′ ‐ZrO 2 broadened at 860° and 900°C. This indicated that the t′ ‐ZrO 2 transformed partially into c′ ‐ZrO 2 . The {400} peak profile of c′ ‐ZrO 2 also broadened at 860° and 900°C, which indicated that the c′ ‐ZrO 2 partially transformed to t′ ‐ZrO 2 . The finish point of t′→c′ transformation T f t′ → c′ was investigated by annealing the t′ ‐ZrO 2 containing 30–65 mol% CeO 2 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 T 0 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.