However, although the understanding for these basic facts exists for a relatively long time, there is no single coherent theory which would be applicable irrespectively of crystalline symmetry, degree of diffraction-line broadening, and so on. BaTiO 3 polycrystals are studied increasingly because the particle size, which has important influence on structure and physical properties, is controlled easily. The existence of a distinct severely textured and strained surface layer was detected. Most likely, this layer has the same tetragonal structure as the main fraction. The inhomogeneous strain upon poling is about the same order of magnitude as the strain caused by electrostriction during poling and indicates a substantial increase of the dislocation density.
Dislocations play an important role in bulk microstructural changes that occur upon poling.
This implies that the application of an external poling field generates defects in the structure and increases the internal stress. The increase of internal stress influences the ferroelectric and phase-transition temperatures. Moreover, the increase of both internal stress and defect concentration may have adverse consequences on both polycrystalline and epitaxial thin films through the accelerated degradation of dielectric properties. Because some dislocation reactions lead to microcracking, this may even result in the mechanical failure. Tc superconductors contain numerous crystalline defects, such as substitutions, vacancies, twin and stacking faults, and dislocations simultaneously.