ELSEVIER Physica B 230-232 (1997) 889-891 The electronic structure of Ba2Cu304C12 Helge Rosner*, RolandHayn MPG Research Group 'Electron Systems', University of Teehnolooy, D-01062 Dresden, German). Abstract Band structure calculations for Ba2Cu304C12 within the local density approximation (LDA) have been carried out and the total as well as partial densities of states have been calculated. The investigated compound is similar to the antiferromagnetic parent compounds of cuprate superconductors but contains additional Cu(B) atoms in the planes. Within the LDA metallic behaviour is found with two bands crossing the Fermi level. The result is in disagreement with the experimentally observed antiferromagnetic insulating state, showing the necessity to deal with the electron correlations in a more detailed way. To decide which orbitals should be correlated, a model with Cu-3d and O-2p orbitals was constructed. It is shown that the coupling between the Cu(A) and the additional Cu(B) subsystem is small. Keywords: Ba2Cu304C12; Band-structure calculation; Tight-binding model Layered copper oxides with CuO2-planes have been investigated extensively in recent years. In the undoped case, these materials are antiferromagnetic charge transfer insulators. Photoemission measure- ments of the hole dispersion in Sr2CuO2C12 [1] showed rough agreement with the dispersion expected from an extended t-J model. Recently, a similar dispersion was also measured in Ba2Cu304C12 [2] despite the existence of additional Cu(B) atoms in the plane. Moreover, this compound shows unusual mag- netic properties like two copper spin subsystems with Nrel temperatures different by one order of magni- tude [3]. Therefore, we will clarify here the influence of the additional Cu(B) atoms on the electronic struc- ture of Ba2Cu3OaC12 . One typical strategy to take into account the strong correlations in copper oxides is characterized as fol- lows: the relevant orbitals which determine the elec- tronic structure near the Fermi level are found from a band structure calculation. Using these orbitals a * Corresponding author. multi-band Hubbard model is constructed. In most cases it can be projected to an effective one-band t- J-like model. Here we want to present the first steps on this way: the band structure calculation, the deter- mination of the relevant orbitals and a first discussion how the known ideas for the CuO2-plane should be modified. We carried out a LDA band structure calcula- tion using the linear combination of atomic orbitals (LCAO). For the exchange and correlation poten- tial the parametrisation of von Barth and Hedin was chosen. We used a scalar relativistic scheme with a minimum basis. We have chosen the valence states Cu(4s,4p,3d), O(2s,2p), Ba(6s,6p,5d) and Cl(3s,3p), and the lower-lying states treated as core states. Ba2Cu3OaC12 occurs in bct-structure with a = 5.51 and c --- 13.822/~ [4]. It has a relatively open struc- ture. Therefore, we introduced four empty spheres. The main part of the potential, the Hartree contribu- tion, is constructed as a sum of overlapping spherical contributions. The exchange and correlation potential is calculated in atomic sphere approximation. 0921-4526/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0921-4526(96)00659-X