Monoclinic-tetragonal phase transition in zirconia: mechanism, pretransformation and coexistence

The high temperature X-ray diffraction study of the monoclinic tetragonal phase transition in ZrO₂ showed that it is spread over a temperature range 930-1220°C. Anomalous intensity changes are observed in the pretransformation region 930-1100°C. Coexistence of phases through hybrid crystal formation in the region 1100-1220 °C and the mechanism of transition are discussed. The orientation relationship between the monoclinic (m) and the tetragonal (t) crystal structures consists in the parallelism of the (100)_m plane to (110)_t and of the b_m axis to the c_t axis. A drastic change in a small temperature range during the tetragonal-monoclinic transition is interpreted as a cooperative change in both short and long range interactions. The large thermal hysteresis is attributed to the difference in the mechanism of transition during heating and cooling.