The structure of PSZ crystals have been studied in relation to the content of the stabilizing impurity (Y2O3) by X-ray diffraction and transmission electron microscopy. The measurement of both hardness and fracture toughness by microindentation have been carried out. Studies have shown that crystals of PSZ obtained by directional solidification of the melt consist of two tetragonal phases (t and t’), with varying degrees of tetragonality. The yttrium-enriched phase t’ is “untransformable” in contrast to the t phase, with a lower content of yttrium, which, under the influence of mechanical stress, undergoes a martensitic transition to the monoclinic form. Increasing the stabilizing impurity concentration leads to an increase in the volume fraction of the “untransformable” phase. Increasing the concentration of Y2O3 also affects the form and dispersion of the twin domains. The character of the twinned structure changes depending on the concentration of the stabilizing impurity and the hierarchy of the twinning disappears above 3mol.% Y2O3. In this work it is shown that the quantity of hardening (fracture toughness) is proportional to the content of the transformable t phase.