Oxygen ordering in YBa2Cu3O7-y in terms of the Landau theory

Oxygen ordering in nonstoichiometric compositions of YBa2Cu3O7-y is conside red in terms of the Landau theory of second-order phase transitions. It is shown that there can only be ten types of homogeneous long-range order, of which two types correspond to the OI and OII phases. The Landau theory pred icts that the filling of vacant positions 2(f)D-4h(1) by oxygen must follow a scenario that is far from forming chains but compatible with known facts . Such a scenario of filling vacancies with oxygen was not considered earli er. It is demonstrated that the predicted structures can be experimentally identified from the spectra of copper ions. In the course of identification of diffraction patterns, the symmetry-allowed displacement of copper ions from centrosymmetric positions in a Cu(1)(O(x)square(1- x))(2) layer should be taken into account. (C) 2000 MAIK "Nauka/ Interperiodica".