JOURNAL OF SOLID STATE CHEMISTRY 96, 464-467 (1992) On the Determination of Hole Concentration in Thallium Cuprate Superconductors A. MANTHIRAM, M. PARANTHAMAN, AND J. B. GOODENOUGH Center for Materials Science and Engineering, ETC 5.160, The University of Texas at Austin, Austin, Texas 78712-1084 Communicated by J. M. Honig, November 18, 1991 We comment on a recent report of a wet-chemical procedure for determining the hole concentration in thallium cuprate superconductors. That report failed to address (a) the oxidation of reduced thallium by the bromine liberated by the oxidized copper and (b) the variation with T1 concentration of the overlap of the Fermi energy by the T1 6s band in the double-Tl-layer compounds 9 1992 Academic Press, Inc. The known copper-oxide superconduc- tors all have intergrowth structures con- sisting of CuO2 sheets alternating with other layers along the c-axis. Oxidation/reduction of the CuO~ sheets above/below the formal Cu valence 2 + is one of the necessary con- ditions to suppress the long-range antiferro- magnetic order and induce superconductiv- ity. Muon spin relaxation experiments have shown (1) that Tc varies in p-type supercon- ductors with the mobile charge-carrier den- sity p and its effective mass m* as Tc ~ p/m* (1) until it saturates at a higher p/m* value. Although the Tc vs p relationship has been established for simple systems like Laz_y SryCuO 4 (2), such a correlation was hin- dered for thallium cuprate superconduc- tors--TlzBazCa,_ iCu~O2~+4 and TIBa 2 Ca,,_lCunO2~+3--mainly because of the in- ability to know the thallium and oxygen con- 0022-4596/92 $3.00 Copyright 9 1992 by Academic Press, Inc. All rights of reproduction in any form reserved, tents in the material. Recently we showed (3, 4) that the thallium and oxygen contents can be determined without any ambiguity by simple wet-chemical procedures. The thallium content is determined by a selective precipitation of T1 as a perchlo- rate-thiourea complex followed by filtering and dissolving the precipitate in HC1, oxi- dizing all TI* to T13+ with bromine, treating with excess KI, and titrating the liberated iodine with thiosulfate solution. During this iodometric titration all T13+ is quantitatively reduced to T1 + according to the reaction T13 + + 2I --~ T1 + + 12 (2) and from the titer value the total thallium content can be determined. The validity of this procedure was confirmed by carrying out the experiment under identical condi- tions with known quantities of either TI20 3 or a mixture ofT1203 and YBa2Cu306.94. The 464