Optical and x-ray single crystal studies of the monoclinic in equilibrium tetragonal transition in ZrO2

The monoclinic reversible tetragonal phase transition in ZrO2 single crystals was studied at temperature by transmission optical microscopy and x-ray diffraction techniques. A series of time-lapse photographs illustrated the relations between the events that occur during the transition. The events themselves were recognized by direct observation using a high-temperature microscope stage and by scrutiny of several high-temperature Laue photographs. During heating the monoclinic phase transforms to the tetragonal by the motion of an interface parallel to the (100)/sub m/ plane; simultaneous twinning also occurs behind the advancing interface. The tetragonal phase is usually twinned on the (1 anti 1 2)/sub bct/ or (anti 1 1 2)/sub bct/ plane, and the extent of twinning is influenced by the heating rate. Cooling transforms the untwinned tetragonal form into a twinned monoclinic form with the orientation of the monoclinic twins parallel to the trace of the (001)/sub m/ plane when observations are made in the (100)/sub m/ plane. Transformation of a twinned tetragonal crystal results in twins on the (110)/sub m/ and (001)/sub m/ planes. Orientation relations in the ZrO2 transformation are: (100)/sub m/ parallel (110)/sub bct/, [010]/sub m/ parallel [001]/sub bct/, and by the virtue of twinning, (100)/sub m/ parallel (110)/sub bct/, [001]/sub m/ parallel [001]/sub bct/. During cooling the same topotaxial relations are maintained. 8 figures