The α → β phase transition in quartz and AlPO4 as studied by electron microscopy and diffraction

The α → β phase transition in quartz and in the isostructural AlPO4 has been studied in situ in an electron microscope. A very high density of defects was observed close to the transition temperature. A diffraction contrast analysis allowed us to identify these defects as dauphiné twins. In the neighbourhood of the transition the twins form columnar domains parallel with the c-axis; these triangular prisms are arranged following regular (hexagonal) networks, their mesh width becomes smaller close to the transition temperature. Particular “defects” in the networks are analysed. The domain walls (dauphiné twin boundaries) are constantly vibrating, thus transforming continuously the α1 orientation into α2. These observations are suggestive for the interpretation of the β phase as being a time average of α1 and α2 orientations. The particular geometry of the diffuse intensity observed in the electron diffraction patterns is related to the phonon mode that drives the transition from the α to the β phase. In AlPO4 also antiphase boundaries were observed as expected from the structure model.