The results of investigations of te defects induced by light illumination in single crystals of PbTiO_3 and KtaO_3 are reported. Measurements were performed by ESR in temperature interval (4,2-77) K. In both the ABO_3 perovskites the centres with photoelectrons trapped on B cations were revealed nearby some lattice defects as oxygen vacancies V_O, OH~- groups and unavoidable impurities. Analysis of temperature dependence of the intensities of of ESR spectra of these centres had shown that the aforementioned centres ae shallow donors. For example, the energy levels of Ta~4~+-V_O and Ta~4+-V_O-Me~4+ (Me - Ti, Sn) centres are situated at 26 and 8 meV below the bottom of the conduction band, respectively. Comparative analysis of hte results obtained in the considered perovskites is performed. In particular we observed O~- photochromic centres in KTaO_3 crystal, whereas they were absent in PbTiO_3 because of peculiarities of the valence zone structure in lead-cotnaining perovskites. On the other hand, contrary to ferroelectric PbTiO_3 in incipient ferroelectric KTaO_3 centres like Ta~4+ -V_O are symmetry-breaking and so they can be the sources of polarization. Theoretical calculations had shown that similar photoinduced centres have to appear in many other perovskites at low temperature. The proposed physical mechanism that underlie the symmetry break is analogous to pseudo-Jan-Teller effect. The calculations of the criteria of appearance of this effect are performed. The role of the revealed defects in photoinduced phenomena such as photocurrent, photoluminescence etc. is discussed.
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