Dislocations in perovskites BaTiO₃ and CaTiO₃

Abstract

Dislocations either grown-in or produced by plastic deformation have been studied in transmission electron microscopy (TEM) in two crystals with the perovskite structure, namely BaTiO₃ (P4mm at room temperature) and CaTiO₃ (Pcmn). Two distinct deformation processes have been used: (i) microindentation at room temperature and (ii) creep at high temperature for BaTiO₃. It is found that for both materials the easy glide systems at room temperature are {110}_PC 〈1$\bar 1$0〉_PC where the subscript PC refers to the pseudo-cubic lattices of these crystal structures. The corre-sponding Burgers vector 〈110〉_PC is not the shortest lattice translation but these glide systems are such that dislocation gliding does not involve bringing the various cations close to each other. In BaTiO₃, after creep at elevated temperature, the above glide systems still are observed but dislocations with Burgers vectors {100}_PC are also seen. Most often they are in climb configuration i.e. their line lies in a plane {100}_PC perpendicular to their Burgers vector. Furthermore they are dissociated by climb in the above plane following a reaction like [010]_PC→1/2[011]+1/2[01$\bar 1$] the two partial Burgers vectors being orthogonal. It is suggested that this climb dissociation is due to the precipitation of the supersaturated point defects when the samples are cooled down. The equilibrium concentration of these point defects as well as their mobility would be very large at high temperature thus allowing a high temperature deformation by dislocation climb rather than by dislocation glide as it is observed at room temperature.