ON THE QUESTION OF OVERLAP BETWEEN TL 6S AND CU 3D BANDS IN THE TL1-YLABA1-ZSRZCUO5-X SYSTEM

Wet-chemical analysis was used to charcterize the thallium and oxygen contents y and x of the system Tl1-yLaBa1-zSrzCuO5-x before and after annealing in an O2 atmosphere at 500-degrees-C. The values of y and x were found to vary sensitively with the conditions of the initial synt hesis, and the oxygen content increased after the 500-degrees-C anneal in O2. Single-phase samples were obtained for y less-than-or-equal-to 0.36; the CuO2 sheets were never reduced below (CuO2)2-, and the abil ity to reduce to this state decreased with increasing y. Bulk supercon ductivity may be obtained for all values of z in the range 0 less-than -or-equal-to z less-than-or-equal-to 1, and the change in Meissner fra ction and super-conductive onset temperature T(on) varies with the oxi dation state of the CuO2 sheets in the same manner as the system La2-x SrxCuO4 in the range 0 less-than-or-equal-to x less-than-or-equal-to 0 .16, provided the oxidation state of the CuO2 sheets is calculated on the basis that all the thallium are present as Tl3+. There is no evide nce of an electron transfer from the CuO2 sheets to Tl-6s states at lo w temperatures in this Tl monolayer system, which distinguishes it fro m the Tl bilayer systems. These observations are at variance with spec ulation in the literature; they also demonstrate how experimental obse rvations in the thallium cuprates may be misleading if they are not ac companied by chemical analysis. (C) 1994 Academic Press, Inc.