Ab-initio study of the vacancy ordering in YBa2Cu3O7-x

With the aim of establishing a relationship between the electronic structur e and the site occupancy within the Cu-O chains, we perform total energy ca lculations for the sub-stoichiometric compound YBa2Cu3O6.5 for various patt erns of vacancy ordering. Supercells containing up to four formula units ar e considered. The effective interactions of O(1) atoms are extracted from t he total energies by means of the cluster expansion technique. The most sta ble configuration is found to be the experimentally observed ortho-ii struc ture. This finding is confirmed by Monte-Carlo simulations for a rigid latt ice using the pair interactions. Thermal disorder effects are studied. The influence of vacancy ordering on the total and partial densities of states is discussed.