Preparation and X-ray characterization of highly ordered ortho-II phase YBa2Cu3O6.50 single crystals

High purity, highly ordered and twin free ortho-II phase YBa2Cu3O6.50 cryst als have been successfully prepared and their structural properties charact erized by X-ray diffraction. Removal of twin boundaries was found to be ess ential in order to obtain highly ordered ortho-II crystals. The ortho-II ph ase shows a sharp superconducting transition at T-c = 62 K, 5 K higher than the ortho-I phase of the same oxygen content. The ordering was found to be three-dimensional with long correlation lengths xi(a) = 148 Angstrom, xi(b ) = 430 Angstrom and xi(c) = 58 Angstrom. No doubling of the unit lattice a long the c-direction was observed, indicating an attractive interaction bet ween oxygen ions in adjacent basal planes. (C) 2000 Elsevier Science B.V. A ll rights reserved.