A defective structure for Tl-deficient Tl_2-xBa₂CaCu₂O_y superconductors

Abstract

Considerable Tl deficiency has been detected in the Tl₂Ba₂CaCu₂O_y (Tl-2212) superconducting phase formed in thin films and polycrystals. Transmission electron microscopy was used to characterize the structure of this Tl-deficient phase. It has been found that the space group changes to be I4mm which is apparently different from I4/mmm as usually reported for the Tl-2212 phase. The change in the space group is due to a lack of the (001) mirrors in the defective Tl-2212 structure which can be directly imaged by high-resolution electron microscopy. Image simulations confirmed a proposed structure model in which inhomogeneously distributed Tl vacancies and Tl disorder in the Tl-O layers are considered to be responsible for the symmetry change. The stability of such a defective Tl-2212 phase will be discussed.