Oxygen Ordering in YBa2Cu3O6+δ: A Phase Diagram Calculation
The phase diagram of the ordered phases in YBa2Cu3O6+δ as a function of oxygen concentration δ is calculated. The model takes as units the octahedra clusters formed by the O(1), O(4) and O(5) sites coordinated to the Cu(1) sites. A ground state analysis is carried out for δ in the range 0 to 1 assuming that the only structural units present are octahedra occupied by 0, 1 and 2 oxygen atoms in the basal plane, in addition to a repulsive Coulomb interaction between O atoms not mediated by Cu(1) atoms. The model accounts for several of the experimentally observed ordered structures. At finite temperatures, the model Hamiltonian is solved within the octahedron approximation of the cluster variation method. The effect of the O(4) vacancies on the equilibrium phase diagram is analysed.
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