Twinning and Structure in YBCO Crystals

The twinning observed in superconducting YBaaCu3O7-x crystals and its correlation with crystal structure and preparative conditions are discussed. The existence of crystals showing predominant twinning domains together with the particular features of the twinning permitted to collect high-order X-ray diffraction data utilized for a redetermination of the crystal structure is discussed. The anisotropy of thermal parameters of the oxygens forming the linear chain of square planar coordinated Cu atoms, already observed in the first single crystal structure determination, is confirmed by high order diffraction. This, interpreted at a first time as a large librational movement of the chains, can be alternatively attributed to positional disorder of the oxygen atoms leading to incommensurate zig-zag chains along the b axis.