Stability of monoclinic and tetragonal zirconia at low oxygen partial pressure

We have found that both tetragonal and monoclinic zirconia annealed at temp eratures in the range of 1100 to 1300 degrees C in atmospheres of low oxyge n partial pressures (down to 10(-26) Pa) transform slowly to an apparently cubic phase. The transformation can be reversed by increasing the oxygen pa rtial pressure sufficiently, i.e., exposing the sample to air again. These observations were made by In-111/Cd perturbed angular correlation (PAC) mea surements of undoped zirconia samples. Upon annealing under various reducin g atmospheres PAC spectra show a steadily increasing fraction of Cd probe a toms in a locally cubic environment with the fraction of probe atoms in tet ragonal or monoclinic sites decreasing accordingly.