The domain structure in tetragonal zirconia (t-ZrO_2) formed by the diffusionless cubic-to-tetragonal transformation have been examined in ZrO_2-3 mol% Y_2O_3-12 mol% TiO_2, which was a wide single t-ZrO_2 field. During annealing in the single-phase field, the domain boundaries with antiphase-boundary-like contrast tend to be facetted with a habit plane close to one of the {111} planes. Transmission electron microscopy-energy-dispersive spectroscopy analysis shows that yttrium ions segregate to the domain boundaries. Since the domain boundaries have a cubic structure, they must be stabilized by the segregation of yttrium ions, which are known to stabilize cubic zirconia.
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