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.
Abslracl. W e have measured the pressure effects on the superconducting T, and the current density/, of the YBaCuO (123) phase and the BiSrCaCuO (2223) phase up to 10 kbar. We have found that for Bi ceramics the increase in J , with pressure is related to the T, change, while for YBCO it is due to the Josephson medium weak-link eflects. This result provides the evidence that in Bi compounds strongcurrent Josephson contacts are possible. The influence of high magnetic fields on the Josephson critical current is briefly discussed.
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