No abstract

A concise overview on the available theoretical and experimental data concerning the structure of oxygen vacancies in the oxide perovskites is given. In this context the statements made in the Comment are discussed.There is no doubt that neutral oxygen leaves an oxygen perovskite, OP, ͑ABO 3 ͒ crystal when annealed in a reducing atmosphere. For each oxygen atom lost two electrons remain in the crystal.What is the structure of the resulting defects? We start by characterizing the present knowledge concerning isolated oxygen vacancies as based on quantum chemical studies. Then we compare these different approaches and discuss the apparent discrepancies. In the light of these findings we assess the available relevant experimental data and point out directions for further research.Recently theoretical evidence cumulates (Refs. 1-4 and references therein) that the isolated oxygen vacancy ͑V O ͒ in the OPs has an electronic groundstate characterized by the full, unbroken, a 1g type, symmetry of the defect. This implies that there is equal electron density at both B-cation neighbors of V O , and finite density at this site is not forbidden. The results support earlier findings by Tsukada et al. 5 They are valid no matter whether V O contains one electron, as for BaTiO 3 1 and KNbO 3 , 2 or two electrons, investigated for KNbO 3 , 2 PbTiO 3 , 3 and SrTiO 3 . 4 These recent theoretical results, independently obtained by four groups employing rather flexible ab initio methods (for details see , are at variance with earlier ones using the Green function procedure (Refs. 6 and 7 and references therein). These predicted a groundstate symmetry having a nodal line along the axis of V O and xy-type symmetry at both B-neighbors, e.g., Ti 3+ ͑3d xy ͒. Based on phenomenological or semiempirical 7 arguments Prosandeyev et al. additionally claimed that the electron density is asymmetrically localized at only one of the two cation neighbors of V O . In a rather detailed way, Donnerberg et al. 1,8 modeled this situation, using their flexible method, and found that this corresponds to a higher energy than the a 1g state, ruling out the xy-type state as the theoretical groundstate of isolated V O . This argument was cited in the commented paper. 9,10 Not only the ground state symmetries but also the predicted level energies differ in both types of approaches: The a 1g states are deep, at least 0.4 eV below the conduction band edge, for the doubly filled V O in SrTiO 3 , 4 and they are found to be still lower for the other compounds and, of course, for the singly filled V O . 1,2 The xy-type states, on the other hand, are predicted to be shallow, about 0.3 eV below the conduction band edge, for V O containing one electron, and higher for the doubly filled one. 6,7 Electron paramagnetic resonance (EPR) can yield the most detailed structural information on defects. The oxygen vacancy in the OPs has been definitely identified in this way only when neighboring a paramagnetic transition metal ion, e.g., ͑Ni 3+ ͑3d 7 ͒ − V O ͒ 11 or other case...

No abstract

Application of uniaxial stress reveals the Jahn-Teller (JT) properties of the defect ions Ni + (3d 9 ), Rh 2+ (4d 7 ) and Pt 3+ (5d 7 ), all incorporated on Ti 4+ sites in BaTiO 3 . In all cases the vibronic ground states are stabilized by an E ⊗ e Jahn-Teller effect, here leading to tetragonally elongated defect-O 6 octahedra. Orbitals of (x 2 − y 2 ) type are thus lowest for the hole-like Ni + , and (3z 2 − r 2 ) for the electron-like strong-field cases Rh 2+ and Pt 3+ . The three systems are characterized by motionally averaged isotropic spectra at elevated temperatures. At low temperatures strong dependences on the distribution of internal strains are found, leading to a coexistence of isotropic averaged and quasi-static tetragonal situations for Ni + , becoming more static under uniaxial external stress. The latter behavior is also identified for Rh 2+ . For Rh 2+ and Pt 3+ the coupling of the JT systems to the external stress could be determined quantitatively, predicting rather low JT energies by a simple model calculation. The observation of tetragonal distortions of these defects thus points to the presence of regions with strong internal strains. The impurity ion Fe 5+ (3d 3 ), also replacing Ti 4+ , is found to go spontaneously off-center along [111] type directions. The defect axis orients parallel to the stress direction, indicating an axially compressed Fe 5+ -O 6 oxygen arrangement in the off-center situation.

An investigation of bound hole and free electron polarons in the oxide perovskites BaTiO 3 and KTaO 3 is presented. Their EPR study, also under application of uniaxial stress, allows to deconvolute the superposition of various polaron types, the local structure of the polarons and their JT-induced symmetries.