STRUCTURE AND SUPERCONDUCTIVITY IN TL2BA2CACU2O8

The structure of superconducting Tl2Ba2CaCu2O8 (T(c) = 110 K) was refined from single-crystal X-ray diffraction data measured at temperatures 296, 160, 130, 90 and 60 K. The refinements indicate a positional disorder of the thallium and oxygen atoms in the Tl-O layers, resulting in locally orthorhombic symmetry. The macroscopic symmetry of the specimen remains tetragonal through the transition to the superconducting state. Some of the structure parameters show anomalous behaviour in the vicinity of T(c). A possible mechanism of the transition to the superconducting state and its relation to the observed structural changes are discussed.