Extended defects in hexagonal BaTiO₃

Abstract

At 1460°C in air, BaTiO₃ shows a phase transition from the cubic to the hexagonal phase. Under reducing conditions, however, the phase transition temperature can be lowered to 1330°C. After cooling, the hexagonal phase is preserved at room temperature. Hexagonal BaTiO₃ ceramics prepared in a reducing atmosphere were analysed by transmission electron microscopy. Stacking faults in the basal plane extending over several hundred micrometres were found with a density of 10⁶ m⁻¹. The density of dislocations was approximately 10⁸ cm⁻² and was much higher than in cubic BaTiO₃ ceramics processed by the same heat treatment. Perfect and partial dislocations lying in the basal plane were analysed by their diffraction contrast. Perfect dislocations have Burgers vectors b of〈100〉 which is the shortest lattice vector of this structure.

Perfect dislocations dissociate into Shockley partial dislocations. Diffraction contrast analysis revealed that stacking faults have shear vectors R of ⅓〈111〉. Such defects are structural defects only and cannot accommodate non-stoichiometry. The diffraction contrast images of perfect and partial dislocations were simulated under two-beam conditions using with good results the elastic constants of the cubic phase for hexagonal BaTiO₃.