Optical and X‐Ray Single Crystal Studies of the Monoclinic ⇌ Tetragonal Transition in ZrO ₂

The monoclinic ⇌ tetragonal phase transition in ZrO 2 single crystals was studied at temperature by transmission optical microscopy and X‐ray diffraction techniques. A series of timelapse 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) m plane; simultaneous twinning also occurs behind the advancing interface. The tetragonal phase is usually twinned on the (1 2) bct or ( 12) 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) m plane when observations are made in the (100) m plane. Transformation of a twinned tetragonal crystal results in twins on the {110} m and {001} m planes. Orientation relations in the ZrO 2 transformation are: (100) m ‖(110) bct , [010] m ‖[001] bct , and by the virtue of twinning, (100) m ‖(110) bct , [001] m ‖[001] bct . During cooling the same topotaxial relations are maintained.