Theoretical Prediction of the Orthorhombic (II) Phase in the YBa2Cu3O7-δ System

Abstract
The existence of the orthorhombic(II) phase in the YBa2Cu3O7-δ system is derived by a model calculation of the oxygen order-disorder transformation on the Cu(1) plane. The repulsion interaction energy between the second-nearest-neighboring oxygen atoms contributes to the occurrence of the orthorhombic(I)-orthorhombic(II) transformation while the first-nearest interaction gives rise to the tetragonal-orthorhombic(I) transformation. The structure of the orthorhombic(II) phase is considered to be an alternately-ordered state of the oxygen atoms within the a-axis site.