Net Holes and Superconductivity in the Members with n>3 of Homologous Series, Tl- and Hg-m2(n-1)n

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All the Tl- and Hg-based superconducting cuprates can be classified as members of Tl/Hg-m2(n-l)n homologous series, with m being 1 or 2, and n increasing even up to 7. In the phases with n ≥ 3, holes may be inhomogeneously distributed between outer (Coordination Number = 5) and inner (CN = 4) CuO2 planes. In each series T c increases as n increases from 2 to 3, and then either saturates or slowly decreases with n > 3, proposing that the CuO2 planes predominantly controlling the value of T c are different in the n ≥ 3 phases from those in the structures with n = 1 (CN = 6) or 2 (CN = 5). Estimation of the distribution of holes between the inequivalent CuO2 planes utilizing existing crystallographic data and a bond-valence-sum calculation method, results in a somewhat surprising conclusion that the holes are confined in the innermost CuO2 planes. On the other hand, the result agrees with the empirical believe that the flatter the CuO2 planes the higher the T c..