XANES and wet-chemical analyses of the charge balance in (Hg,Pb)(Ba,Sr)₂Ca₂Cu₃O_8+z

Here we present the results revealed from iodometric, coulometric and cerimetric titrations and Cu L-edge and O K-edge X-ray absorption near-edge structure analyses of the concentration and distribution of holes in a series of polycrystalline (Hg,Pb)(Ba,Sr)₂Ca₂Cu₃O _8+z [(Hg,Pb)-1^(Ba,Sr)223] samples. By means of oxygen-content tuning only, the average valence of copper in the pristine Hg-1^(Ba)223 phase can be controlled in the range of 2.03-2.10. Substituting Hg partly with Pb increased considerably the amount of excess oxygen z in the (Hg,Pb)O_z charge-reservoir block. Lead was found to enter the phase with an oxidation state higher in average than III so as to increase the positive charge at the Hg site. However, the increase in the oxygen content counter-balanced the effect of higher-valent (Pb ^III/IV) for lower-valent (Hg^II) cation substitution such that the average CuO₂-plane hole concentration was found higher for the (Hg,Pb)-1^(Ba,Sr)223 sample with T_c = 115 K than for the optimally doped pristine Hg-1^(Ba)223 sample with T_c = 134.5 K.