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Abstract
The chemical bond in ferroelectric ABO3 perovskites has been considered as a mixed ionic-covalent one. The effect of covalency on the structure has been analyzed using valence bond theory. A particular attention has been paid to the π bond formation and resulting distortion of the lattice. A role of two ferroelectricity-driving parameters, ionic effective charges and oxygen octahedron volume, has been underlined in the light of overlapping of the orbitals forming (d←p)π bond. The phase transitions, which may occur in perovskite crystals, have been discussed from the point of view of electron charge localization.