PHASE-DIAGRAMS OF OXYGEN ORDERING IN HIGH-TEMPERATURE SUPERCONDUCTORS RBA2CU3O7-X

In the framework of the lattice-gas model the investigation of various possible forms of phase diagrams of RBa2Cu3O7-x (R = rare earth) superconductors is carried out. The calculations are performed by the modified cluster-variation method. The conditions are analyzed which lead to the occurrence of a two-phase region at low temperatures. The problems of RBa2Cu3O7-x system behavior are discussed when the values of the interaction parameters depend on the amount of oxygen. It is shown that at specific conditions imposed on the interaction parameters the region of tetragonal-phase existence can significantly increase at relatively low temperatures. The orthorhombic phase with the reduced wave vector (1/3,0,0) is found to occur when the interactions between the next-nearest O - Cu - O chains are considered.