Lattice polarization in oxides of the perovskite family was shown to give rise to symmetry-breaking of 1-electron orbitals at F centers. This provided an explanation for previous experimental data on this para-electric material in which it had been demonstrated, by using the second-harmonic generation method, that some 1018/cm3 of dipole centers were present. Their concentration depended upon reduction and oxidation processes.

S.A.Prosandeyev, I.A.Osipenko: Physica Status Solidi B, 1995, 192[1], 37-43