Preparation and X-Ray Characterization of Highly Ordered Ortho-II Phase YBa₂Cu₃O_6.50 Single Crystals.
Ruixing Liang, D. A. Bonn, and Walter N. Hardy.
Published in Physica C 336, 57-62 (2000).

High purity, highly ordered and twin-free ortho-II phase YBa₂Cu₃O_6.5 crystals have been successfully prepared and their structural properties characterized by x-ray diffraction. Removal of the twin boundaries was found to be essential in order to obtain highly ordered ortho-II crystals. The ortho-II phase shows a sharp superconducting transition at 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. No doubling of the unit lattice along the c-direction was observed, indicating an attractive interaction between oxygen ions in adjacent basal planes.