Ordering of Tl and Bi in Bi-doped Tl-1223 high T-c superconductors

High T-c superconducting materials with nominal compositions of Tl1-xBix(Sr, Ba)(2)Ca(2)Cu3O(9) (0 less than or equal to x less than or equal to 0.5) have been investigated using x-ray powder diffractometry and electron microscopy. It was found that Tl and Bi cations were in an ordered arrangement in the (Tl, Bi)-O atomic layer. Although the proposed supercell is 4a X 4b X 4c derived from the basic unit cell of the undoped TlBa2Ca2Cu3O9 (Tl-1223), the basic two-dimensional superlattice in the (Tl, Bi)-O layer is 2a x 4b or 4a x 2b with Bi rations at the center and the corner of the rectangle, giving an ideal and preferred composition with the TI:Bi ratio of 3:1. Accordingly, the maximum Bi doping for Tl in the solid solution is 25%. The proposed model of the superstructure was supported by computer simulation of electron diffraction patterns. The mechanism of formation of the superstructure is discussed. (C) 1996 American Institute of Physics.