ELECTRONIC STRUCTURE OF HIGH-TEMPERATURE SUPERCONDUCTORS II. La-Ba-Cu-O SYSTEMS MARTIN BREZA Department os Chemical Physics, Slovak Technical University, SK-812 37 Bratislava, Slovakia Received 9 May 1995 Electronic structure of La2_xBa~CuO4 superconductors has been studied. Model sys- tems Lal0CuO~-~~, Laa0cuO+~ and LagBaCuOl+43 with various Ba positions have been investigated using the quasirelativistic INDO molecular orbital method. The oxidation state of copper atoms corresponds to Cu(II). The influence of Ba/La substitution on copper atoms is much lower than on their oxygen ligands sphere. Introduction Quantum-chemical model studies may bring useful results in the research of mi- croscopic properties connected with local defects in materials. In the first part of this series [1] the electronic structure of Y-Ba-Cu-O superconductors has been in- vestigated. High-temperature superconductivity (transition temperature Tc above 30 K) was initially observed [2] in a multiphase La-Ba-Cu-O system. The supercon- ducting compound was identified [3] as La2-xBaxCuO4 (x = 0.15 shows maximum To) with the distorted K2NiF4 structure, The experiments showed that Ba 2+ doping (substitution of La 3+) is essential for the occurrence of superconductivity. The crystal structure of La2CuO4 may be described by Abma (DI~) space group [4]. The formal oxidation state of copper atoms corresponds to Cu(II). Cu atoms are coordinated by four equatorial O(e) and two axial O(a) atoms (dcu-O(~) = 1.907 • 10- l~ dcu-O(~) = 2.459 x 10- l~ These tilted elongated CuO6 octahedra with common O(e) atoms form an infinite two-dimensional structure (curly plane). La atoms are coordinated by six oxygens into a trigonal prism and three additional neighbouring O atoms increase the coordination number of La to nine. The quantum-chemical study on La-Ba-Cu-O superconductors meets many dif- ficulties connected with the method of calculation (relativistic effects for heavy atoms) as well as the size and the charge of the model cluster used (the inconsis- tency of the electroneutrality condition with the unit-cell symmetry conservation). The reasonable compromises are necessary (quasirelativistic semiempirical method of calculation, minimization of the model cluster charge). The aim of this study is to investigate the influence of the Ba2+/La 3+ substitu- tion on the electronic structure of copper-oxide polyhedra relevant for the high-Tr superconductivity. Czechoslovak Journal of Physics, Vol. 46 (1996), No. 5 503