High-Temperature Phase Transitions in ZrO2

An analysis of primary literature sources shows that there is only one reliable report on the observation of the cubic ZrO2 phase by the X-ray method under oxidizing conditions. This phase is observed at temperatures not lower than 2500 degrees C. All other reports on the observation of a phase transition around 2300 degrees 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 ZrO2 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.