CRYSTALLOGEOMETRY RELATIONS AND LATENT PHASES IN THE Y-BA-CU-O COMPOUNDS

This paper deals with a new description of the structure of the high-i r, superconductors. In particular, we show that the structure of the Y -Ba-Cu-O compounds can be described as the result of an order-disorder phase transition from a high-symmetry latent phase Pm (3) over bar m. The crystallogeometry relations between the high-symmetry phase Pm (3 ) over bar m and the low-symmetry phases P4/mmm and Pmmm give, in part icular, the changing of the symmetry of translations and the equivalen t positions of the different atoms. This point of view allows us to de scribe the modification of the structure, associated with changing che mical composition with the critical formula YBa2Cu3O6.44, in agreement with the experimental phase diagram.