Within anharmonic phonon theory, however, the reverse tetragonal-to-cubic transition upon heating is not well described, and the tetragonal phase remains of lower free energy at all temperatures. Thus, it is expected that a further stabilisation mechanism is involved. From AIMD simulations, we show that spontaneous formation of Frenkel defect pairs occur in cubic zirconia at elevated temperatures. Thus, we propose that this defect formation is responsible for the stabilisation of cubic zirconia through a similar mechanism as that of YSZ. Interestingly, the transition is observed to occur at ⇠80% of the melting point temperature, which is similar to the Bredig transition temperature in other fluorite type structures, which is the temperature at which these structures become superionic conductors . Furthermore, a stabilisation mechanism involving the creation of defect pairs would explain the weak first-order transition behaviour observed experimentally as an enthalpy of transition , since defect creation requires energy. This suggests that the transition should not be considered a simple second-order group-subgroup transition purely driven by the soft phonon mode.