Low‐Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System

Abstract

Low‐temperature phase equilibria ranging from 1000° to 1200°C in the ZrO2–CeO2 system were investigated by annealing compositionally homogeneous ZrO2–CeO2 solid solutions in a Na2B2O7.1 NaF flux. The 5 mol% CeO2 samples decomposed into monoclinic (m) and tetragonal (t) phases during annealing at 1100°2 and 1120°C, and the t‐phase transformed diffusionlessly into monoclinic (m′) symmetry during quenching. A eutectoid reaction, t→ (m + c), was confirmed to occur at 1055°± 10°C, where the equilibrium compositions of the t‐, m‐, and c‐phases were 11.2 ± 2.8, 0.9 ± 0.9, and 84 ± 1 mol% CeO2, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate withouts affecting the equilibration. The previous data are discussed using metastable–stable phase diagrams. The discrepancies of the low‐temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggish kinetics.