The high-temperature polymorphism of ZrO$_{2}$

An analysis of primary literature sources shows that there is only one reliable report on the observation of the cubic ZrO$_{2}$ phase by the X-ray method under oxidizing conditions. This phase is observed at temperatures not lower than 2500$^\circ$C. All other reports on the observation of a phase transition around 2300$^\circ$C are reliable only in relation to the existence of a transition at this temperature, but this transition is to the $t$' tetragonal phase rather than to the cubic one. The conclusion is that the following four polymorphic modifications are present in ZrO$_{2}$ under the standard pressure: monoclinic $(m)$, two tetragonal ($t$ and $t$'), and cubic $(c)$ phases that are converted into each other with an increase in the temperature.