Ab-initio Study of the Vacancy Ordering in Yba₂Cu₃O_7−x

Abstract

With the aim of establishing a relationship between the electronic structure and the site occupancy within the Cu-O chains, we perform total energy calculations for the sub-stoichiometric compound Yba ₂ Cu ₃ O _6.5 for various patterns of vacancy ordering. Supercells containing up to four formula units are considered. The effective interactions of O(1) atoms are extracted from the total energies by means of the cluster expansion technique. The most stable configuration is found to be the experimentally observed ortho-II structure. This finding is confirmed by Monte-Carlo simulations for a rigid lattice using the pair interactions. Thermal disorder effects are studied. The influence of vacancy ordering on the total and partial densities of states is discussed.