The trend is for higher symmetry at higher temperatures, as is usually the case.
A few percentage of the oxides of calcium or yttrium stabilize the cubic phase.
The very rare mineral tazheranite (Zr,Ti,Ca)O2 is cubic.
Unlike TiO2, which features six-coordinate Ti in all phases, monoclinic zirconia consists of seven-coordinate
zirconium centres.
This difference is attributed to the larger size of Zr atom relative to the Ti atom