Special features of atomic vacancies in oxides of the perovskite family have been studied within the framework of the Green function method. It has been shown that, in accordance with experiments, the oxygen vacancy has both shallow and comparatively deep levels corresponding to different charge states. The single-charged oxygen vacancy has a deep level and, hence, it could be a good trap for electrons. We shall show that, owing to interaction with the lattice polarization, an electron is captured by one of the neighbours to the vacancy ions. As a result the single-charged oxygen vacancy proves to be a dipole centre. Peculiarities in the optical absorption spectra connected with the single-charged oxygen vacancy are discussed.

The electrical conductivity of and perovskite-type oxides converted by vacuum annealing into states with weak and strong compensation between donors and acceptors has been measured. It was shown that the reduced samples had both shallow and comparatively deep levels which were attributed to the neutral and singly-charged oxygen vacancy, respectively. The experimentally determined activation energies are consistent with the results of the theoretical computation reported in Part I.

The lattice polarization in oxides of the perovskite family is demonstrated to give rise to symmetry breaking of one-electron orbitals at F centers. This can provide an explanation of recent experiments on paraelectric KTaO, where, by the second harmonic generation method, approximately lo'* cm-3 dipole centers were found, the concentration of which depends on reduction and oxidation processes. I) Sorge str. 5, 344 104 Rostov-on-Don, Russia.

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