A considerable Tl deficiency was detected in this superconducting phase, which formed in thin films and polycrystals. Transmission electron microscopy was used to characterize the structure of this Tl-deficient phase. It was found that the space group changed to I4mm; which appeared to be different to the I4/mmm which was usually reported for the material. The change in the space group was attributed to a lack of (001) mirrors, in the defective structure, which could be directly imaged using high-resolution electron microscopy. Image simulation confirmed a proposed structural model in which inhomogeneously distributed Tl vacancies and Tl disorder in the Tl-O layers were considered to be responsible for the symmetry change.

A Defective Structure for Tl-Deficient Tl2-xBa2CaCu2Oy Superconductors. X.F.Zhang: Philosophical Magazine A, 1998, 78[6], 1339-51