Neutron powder diffraction study of the crystal structure of the oxycarbonate phase YBa2Cu2.85(CO3)(0.15)O-6.73

The oxycarbonate phase YBa2Cu2.85(CO3)(0.15)O-6.73 has been analyzed by neu tron powder diffraction. The phase crystallizes with the symmetry of space group P4/mmm and with unit cell parameters a = 3.8717(3), c = 11.60(1) Angs trom. Rietveld refinements show that the basic structure of the oxycarbonat e is the same as that of the 193 superconductor YBa2Cu3O6+w. The CO32- ions are located on the basal plane of the unit cell, with the carbon atoms rep lacing an equal number of "chain" copper atoms. The presence of the CO32- d efects explains why the oxygen stoichiometry of the oxycarbonate can he lar ger than seven atoms per formula unit. (C) 2000 Elsevier Science B.V. All r ights reserved.