ELSEVIER Physica B 210 (1995) 121-130 Origin of the shallow oxygen levels in perovskite structures M.D. Glinchuk, R.O. Kuzian, A.A. Karmazin Institute for Material Sciences, Ukrainian Academy of Sciences, Krjijanovskogo Street 3, 252180 Kiev, Ukraine Received 5 August 1994 Abstract A mechanism of shallow oxygen level formation in band gap of perovskites is proposed. It is shown that local lattice distortions lead to such levels. Three types of neutral impurities as well as BO 6 cluster ionization were considered as the reason of oxygen ion displacements. The electronic structure calculation of distorted crystal was carried out analytically in the framework of LCAO model. It was seen that energy level values depend on geometry of distortions. Applicability of the obtained result to the explanation of shallow O- levels observed in BaTiO3 recently is discussed. 1. Introduction In the last years several experimental evidences about existence of shallow impurities levels in the band gap of the dielectric crystals with perovskite structure appeared. Nowadays these shallow levels are known to play an important role in photocon- ductivity, photoluminescence and photorefractive effects. E.g. transformation of blue photolumines- cence line into yellow one was observed in undoped KTaO3 at T I> 50 K so that there was only yellow line at T/> 80 K [1]. The authors explained these results by existence of unknown nature shallow (~< 80 K) levels in KTaO3 band gap. Under the light beam these levels trap holes from valence band at low temperature and lose them at T higher than 80 K. The explanation of observed depend- ence of photoconductivity on light intensity in BaTiO3 also requires that one of the centers is connected with shallow levels [2]. On the other hand ESR signals of O- paramagnetic centers were identified recently in BaTiO3 and other single crys- tals (see Ref. [3] and references therein) after low temperature illumination. The existence of three types of O- centers with energies about 25, 40 and 65 K, respectively, was shown in nominally undoped BaTiO3. The distribution of internal strains was supposed to be the reason for O- centers variety. In another nominally undoped BaTiO3 it was iden- tified that holes created by low temperature illu- mination were captured at 0 2- ions next to A13 ÷ substituted for Ti 4 +. Two shallow O- trapped hole centers in BaTiO3 were identified [4] to arise from association with Na ÷ and Pt x ÷. After ),-irradiation O - paramagnetic centers were observed in congru- ent LiNbO3 [5]. Recently observation of photo- conductivity in undoped Kl-xLixTaO3 at low temperature region (T < 80 K) was explained with the help of oxygen shallow levels originating from the shifted 0 2 - ions next to off-center Li + ions [6]. In this work we have shown that in the common case shallow oxygen levels appear in the band gap of the crystals of a perovskite structure with defects due to oxygen ions displacements. Three types of neutral impurities as well as BO 6 cluster ionization were considered as the reason for oxygen ions dis- placements. The calculation of electronic structure was carried out analytically in the framework of LCAO model allowing for interactions between nearest and next-nearest neighbors. It was shown 0921-4526/95/$09.50 © 1995ElsevierScienceB.V. All rights reserved SSDI 0921-4526(94)00938-4