If the Y2O3 content is more than 8 mol %, the yttria stablized zirconia turns to cubic.

refer to the phase diagram in http://elib.dlr.de/56086/1/Poster.pdf

Pure ZrO2 undergoes a phase transformations from monoclinic to tetragonal and then to cubic as:

monoclinic (1173 C) - tetragonal (2370 C) - cubic (2690 C) - melt

It is not possible to obtain stable sintered ZrO2 because of the large volume change accompanying the transition from tetragonal to monoclinic (about 9%). The ionic radius of Zr(IV)[ 0.82A] being too small to bring about stabilization to the cubic polymorph of ZrO2 and thereby form an ideal FLUORITE LATTICE for the tetragonal ZrO2. But if lightly larger ions like Y(III)[).96 A] are doped to the extent of of appxo.8% , then i ZrO2 becomes all the more stable.

ZrO2 is one of the most important technical ceramic materials; its excellent mechanical properties ensure broad range of applications. It exists in three crystallinephases. At a temperature below 1127 ºC, the monoclinic phase (М) is thermodynamically stable; from 1127 to 2297 ºC, zirconia exists in the tetragonal phase (Т); and above 2227 ºC up to the melting point, it has a cubic structure (C). Pure zirconia is almost not used due to the destructive phase transition monoclinic to tetragonal. The effect of this transformational transition can be eliminated by adding the corresponding amount of MgO, CaO, Y2O3, or oxides of other rare earth metals to zirconia to obtain the solid solutions at room temperature. Their phase composition and structure depend on both the concentration and type of the stabilizing admixture and the means of their production.