Thermodynamic behavior of various phases appearing in the CeZrO₄-CeZrO_3.5 system and the formation of metastable solid solutions

In order to investigate the thermodynamic behavior of t́, t*, and κ phases with the composition, CeZrO₄, the equilibrium oxygen partial pressure, p_O2, over their mixtures with pyrochlore (Ce₂Zr₂O_7+x) was measured utilizing an electrochemical cell: Pt, {CeZrO₄(t′, t*, or κ phase) + Ce₂Zr₂O_7+x}/ZrO₂(+Y ₂O₃)/air, Pt. The conclusions described below were derived: the thermodynamic stability of the κ phase is the lowest in the CeO₂-ZrO₂ system. The κ phase forms metastable solid solutions with the pyrochlore phase; it was virtually stable around 1123 K. Two kinds of tetragonal phases exist, tet. (high temp.) and tet. (low temp.), which may correspond to t′ and t*, respectively. A change in p_O2 corresponding to the phase transitions: κ → t′ and t′ ⇄ t* was observed. The standard Gibbs energies of formation, ΔG°, of κ(CeZrO₄) and t*(CeZrO₄) for the reaction: 1/2Ce₂Zr₂O₇ + 1/4 O₂ → CeZrO₄ were evaluated from the emf data.