Thermodynamic stability of metastable tetragonal t′-Ce0.5Zr0.5O2 phase in the CeO2-ZrO2 system

According to the XRD analysis and emf measurements employing a solid electrochemical cell, we have compared the thermodynamic stabilities of metastable t'-Ce0.5Zr0.5O2 phase, ZrO2-based monoclinic and CeO2-based cubic phases (m+c) mixture, and ZrO2-based tetragonal and CeO2-based cubic phases (t+c) mixture. The present experiments have confirmed that,the t' phase is metastable at higher temperatures than 1373 K, and the stable state is (t+c) mixture. The t' phase is metastable at lower temperatures than 1173 K, and the stable state is (m+c) mixture. These results are consistent with the equilibrium phase diagram of CeO2-ZrO2 system. According to the emf measurements, it was found that the thermodynamic stability of the t' phase lies between those of (t+c) and (m+c) at lower temperatures than 1173 K. It was concluded that the t' phase is metastable, but its thermodynamic stability is close to those of (t+c) and (m+c).