A MODEL FOR THE LOCAL-STRUCTURE OF TWIN DOMAINS AND DOMAIN BOUNDARIES IN DOPED YBA2(CU1-XMX)3OY SUPERCONDUCTORS

The structural transformations of the orthorhombic YBa2Cu3O7 phase under influence of impurity M atoms are considered. The local structure of twin domains and domain boundaries is analyzed for the case when M is substituted for Cu in Cul sites and the impurity induced "tetragonal" structure appears. In the structural transformations the important role of the additional oxygen captured by the impurity atoms is verified. A model of the new orthorhombic phase for the impurity containing YBa2(CuM)3O7 superconductors is constructed. In this phase half of the Cu(1) sites has five-fold oxygen coordinations and can be occupied by the impurity atoms. The conditions for coexistence of the new phase with the known YBa2Cu3O7, YBa2Cu3O6.5 and YBa2Cu3O6 phases are considered in both domains and domain boundaries. A structural model of the YBa2(CuM)3Oy crystal explaining many experimental data is suggested.