Oxygen superstructures in semiconducting RBa 2Cu 3O 6+ x

Abstract

We study the ground state and weakly excited oxygen superstructures of RBA 2Cu 3O 6+ x , where R=Y or a rare earth, using a model lattice gas Hamiltonian introduced earlier by the authors. This model assumes that any two O ions of the CuO x planes repel each other with a screened Coulomb interaction, but the repulsion between second-neighbour O ions with a Cu ion in between is reduced by a factor f. The value of f was calculated using the extended Hubbard Hamiltonian in the strong-coupling limit. For large screening length λ, R= Y and x= 3 8 the predictions of the model are in excellent agreement with recent neutron measurements. Assuming that λ decreases strongly in the metallic phase, all previously observed diffraction patterns for R=Y can also be explained (x= 1 8 , 1 2 , 4 7 , 3 5 , 5 8 , 2 3 , 3 4 and 7 8 ) . We conclude that CuO chains can be observed almost exclusively in the metallic phase and predict superstructures that should be observed in semiconducting materials, in particular when x≥ 1 2 and the ionic radius of R 3+ is larger than that of Y 3+.