THE 2-ROOT-2-A(0)-X-ROOT-2-B(0)-X-C(0) HERRINGBONE PHASE OF YBA2CU3O6- NO OXYGEN ORDERING BUT AN ALIEN OXIDE, BACU3O4(X )

A synchrotron-radiation study has been carried out on a crystal of YBa 2Cu3O6+x with a low oxygen content (x = 0.15-0.20) to investigate the so-called 2 root 2 a(0) x root 2b(0) x c(0) phase, attributed up to no w to oxygen ordering in the YBCO CuOx layers. This crystal was grown f rom a flux and quenched from 900 degrees C. It is shown that neither t he ordering of Cu-O-Cu monomers in the herringbone-like scheme nor any other kind of oxygen ordering in the YBCO lattice can explain the obs erved reflections that actually correspond to the unit cell 2 root a(0 ) x root 2b(0) x c approximate to c(0)/3. A structural model is propos ed that gives a very good description of the diffraction pattern; it c orresponds to a parasitic phase of formula BaCu3O4 that is characteriz ed by a stacking of Cu3O4 layers connected by barium atoms and that pr obably grows by epitaxy upon the CuO2 planes of YBCO.