THE EFFECT OF THALLIUM AND OXYGEN STOICHIOMETRY ON STRUCTURE AND TC IN TL-2201 AND TL-2212

By a novel method we have prepared the high-T(c) superconductors Tl2-x BaCuO6-d Tl-2201) and Tl2-xBa2CaCu2O8-d (Tl-2212) in a reproducible wa y with respect to purity, thallium and oxygen content, crystal structu re, and superconducting properties. The changes in structure induced b y changes in thallium or oxygen content were studied by X-ray and neut ron powder diffraction techniques. Tl-2201 crystallizes in either the tetragonal or orthorhombic system, and both forms can be made supercon ducting. The detailed atomic order in the Tl2O4 layers seems to be det ermined by the thallium as well as by the oxygen content. A high thall ium and oxygen content stabilizes the orthorhombic form. Tl-2212 has t etragonal symmetry irrespective of thallium or oxygen content. In samp les low in thallium content no single layer thallium compounds are for med, the thallium deficiency being accommodated as statistically distr ibuted vacancies in the Tl2O4 layers. The oxygen content is the main f actor in determining T(c) in Tl-2201 and Tl-2212. Low thallium content promotes higher T(c)'s, whereas addition of oxygen to the as prepared samples decreases T(c). The c-axis length of Tl-2201 decreases by abo ut 0.06 angstrom when annealed in oxygen at 300-degrees-C. In Tl-2212 the shortening of the c-axis length is about 0.08 angstrom for the sam e annealing treatment. The change in c-axis length and the atom displa cements, in particular in the Tl-2212 system, can be explained in term s of a charge redistribution as for the Y-123 system. An increase in o xygen content leads to a shift of Ba2+ toward the Tl2O4 layers, and to an increase in the CuO2 plane puckering as expected from an increase in hole concentration in the CuO2 planes. (C) 1994 Academic Press, Inc .