Defects in Perovskites Induced by Illumination

Abstract

The results of investigations of the defects induced by light illumination in single crystals of PbTiO₃ and KTaO₃ are reported. Measurements were performed by ESR in temperature interval (4,2-77) K. In both the ABO₃ perovskites the centres with photoelectrons trapped on B cations were revealed nearby some lattice defects as oxygen vacancies Vo, 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 are shallow donors. For example, the energy levels of Ta⁴⁺-V_o and Ta⁴⁺-Vo-Me⁴⁺ (Me — Ti, Sn) centres are situated at 26 and 8 meV below the bottom of the conduction band, respectively. Comparative analysis of the results obtained in the considered perovskites is performed. In particular we observed O^— photochromic centres in KTaO₃ crystal, whereas they were absent in PbTiO₃ because of peculiarities of the valence zone structure in lead-containing perovskites. On the other hand, contrary to ferroelectric PbTiO₃ in incipient ferroelectric KTaO₃ centres like Ta⁴⁺-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.