It was recalled that the existence of twin domains had been confirmed by many experiments. However, the details of the twin domains had not been previously reported. After studying the O content, Cu and Co valences, and Co coordination in the samples, a domain model was proposed which consisted of 2 pairs of parallel zig-zag Co-O chains which surrounded orthorhombic cells. One was oriented along the <110> direction, and the other along the <1¯10> direction. The zig-zag Co-O chains were the twin walls; with neighbouring walls perpendicular and opposite ones parallel. The distance between the parallel walls (zig-zag Co-O chains) depended upon the Co concentration of the sample. The twin-domain model predicted the geometry, size and the absence of systematic symmetry of the domains.

A Model of Twin Domains in YBa2(Cu,Co)3Oy. W.M.Chen, X.X.Yao, Y.C.Guo, H.K.Liu, S.X.Dou: Journal of Superconductivity, 2000, 13[1], 129-36