Stability of monoclinic and tetragonal zirconia at low oxygen partial pressure

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