Electronic Structure of Intrinsic Point Defects in Perovskite‐Type Crystals

Abstract

Using a tight-binding model for the transition-metal perovskite-type oxides ABO, a method of electronic Green function calculation is developed. In the framework of this technique the analytical equations for the local levels of atomic vacancies are obtained. The vacancy potential is calculated in the dielectric approximation including the ion displacements on the long ranges from the vacancy site and the electronic screening in the vicinity of the defect. It is shown that the appearance of the local levels in the forbidden gap is caused by the perturbation potential on the first neighbours of the vacancies. The energies of electronic excitations from the local levels are evaluated using the generalized transition state scheme. Donor levels close to the conduction band for the oxygen vacancy and acceptor levels for the A and B ion vacancy are obtained. The correlation between the results obtained and the physical properties of the perovskite oxides is discussed.Mit dem ,,tight-binding"-Mode11 fur die tfbergangsmetalloxide ABO, vom Perovskit-Typ wird eine Methode der Berechnung der elektronischen Greenschen Funktion entwickelt. Im Rahmen dieser Technik werden die analytischen Gleichungen fur die lokalen Niveaus der atomaren Leerstellen erhalten. Das Leerstellcnpotential wird in dielektrischer Naherung unter EinschluD der Ionenverlagerungen vom Leerstellenplatz auf den langen Ketten und EinschluB der elektronischen Abschirmung in der Nahe des Defekts berechnet. Es wird gezeigt, da13 das Auftreten von lokalen Niveaus in der Energieldcke durch das Storungspotential an den ersten Nachbarn der Leerstellen verursacht wird. Die Energien der elektronischen Anregungen der lokalen Niveaus werden mit dem verallgemeinerten Ubergangszustandsschema berechnet. Donatorniveaus in der Nahe des Leitungsbandes fur die Sauerstoffleerstelle und Akzeptorniveaus fur die Aund B-Ionenleerstelle werden erhalten. Die Korrelationen zwischen den erhaltenen Ergebnissen und physikalischen Eigenschaften von Perovskit-Oxiden werden diskutiert. l) Engelsstr. 105, 344006 Rostov-on-Don, USSR.

High Energy Efficiency and Thermal Stability of BaTiO3–BiScO3 Thin Films Based on Defects Engineering

“…This explains the critical role of Sc Ti ′ in predominantly determining the overall film conductivity. Such behavior can be understood by the role of lower valent acceptor centers, such as Sc Ti × , which form more robust hole trapping centers as compared to A-site vacancies. − Moreover, a significant improvement in insulating properties with even a modest increase in Sc Ti ′ concentration is also consistent with earlier observations that even small concentrations of lower valent acceptor centers dominate defect chemistry in BaTiO 3 . − Figure a–c schematically illustrates the role of Sc Ti × in enhancing the high-temperature stability of the BSBT films.…”

“…Such broadening of the P – E hysteresis loop at high temperatures can be likely attributed to the destabilization of the hole trapping centers. It was shown in past studies that A-site vacancies in perovskite ferroelectrics have shallow acceptor energy levels in perovskite oxides, whereas transition-metal substitutions at the B-site, such as Sc, generate deep levels inside the band gap. − This indicates that increased conductivity at high temperatures in BSBT thin films is likely due to the destabilization of trap centers associated with A-site vacancies. In other words, although the introduction of A-site vacancies can be beneficial in terms of inducing relaxor behavior, their presence can also be a limiting factor for high-temperature (>170 °C) operations.…”

Study of intrinsic point defects in oxides of the perovskite family: I. Theory

“…It was shown in [5][6][7][8][9][10][11] that it is convenient to apply the Green function method to the calculation of the electronic structure of point defects in OPFs. In this approach, the complex problem of solving the Schrödinger equation for the whole crystal containing a point defect is reduced to the comparatively simple problem of describing the scattering of the Bloch waves by the defect.…”

“…Owing to this simplification, the electron energy as well as the density of the electron states can be described by analytical expressions (the Wolfram-Ellialtioglu [17] model). This circumstance helps us to construct convenient analytical formulae for calculating the ideal crystal Green functions [5][6][7][8][9][10][11]. In the present work, we shall employ just this approach.…”

“…The electronic structure of V O was studied within different cluster approaches [1][2][3][4] as well as in the framework of the Green function method [5][6][7][8][9][10][11]. All these studies were performed on the assumption that the symmetry of V O coincides with the point symmetry of the oxygen site in the ideal crystal.…”