Structure and polytypes in thallium superconductors

Publisher Website
Google Scholar
Abstract
There have been a number of small contradictions in reports of the structure of the recently discovered thallium superconducting materials, in particular the phase Tl2Ba2Ca2Cu3Ox, hereafter abbreviated to 2223. The structure has been reported to be pseudo-tetragonal or 14/mmm with a c axis of 3.625, 3.61 or 3.59 nm, and an a axis of 0.54, 0.547 or 0.385 nm. The authors report results obtained by high-resolution electron microscopy and diffraction which show that these results are not inconsistent with each other. The a axis is around 0.385 nm, and the c/a axis ratio and the experimental images are genuinely variable, which they suspect is due to variations in the oxygen stoichiometry. The earlier reports of an a axis of 0.547 nm are due to loss of symmetry caused by intergrowths. They also report evidence for polytypism based around half unit cell blocks involving the local stoichiometries Tl2Ba2Ca1Cu2Ox (2212), Tl2Ba2Ca3Cu4Ox (2234), Tl2Ba2Ca4Cu5Ox (2245) and Tl2Ba2Ca5Cu6Ox (2256). In all these materials the a axis appears to remain the same but there is a small systematic contraction in the local Cu-Ca-Cu perovskite spacing along the c axis as the number of perovskite units increases, which they suggest could be linked to the superconducting transition temperature.