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

Abstract

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..