The ferroelectric phase transition in tridymite type BaAl2O4 studied by electron microscopy

The paraelectric ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl2O4 was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accom panied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400-670 K) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6(3)22, a approximate to 5.22 Angstrom, c approximate to 8.8 Angstrom) and of the FE phase (space group P6(3), a = 10.4469(1)Angstrom, c = 8.7927(1)Angstrom) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.