Topological Aspects of Chemical Bonding in Superconductors

Abstract

The conducting skeletons of the ternary molybdenum chalcogenide (Chevrel phase) MnMo6S8 and the ternary lanthanide rhodium boride LnRh4B4 superconductors consist of infinite three-dimensional networks of metal polyhedra (Mo6 octahedra or Rh4 tetrahedra) with the intrapolyhedral metal-metal bonding confined to the polyhedral edges, the interpolyhedral distances short enough for some chemical bonding, and oxidation of the closed shell electron configuration to create the partially filled valence band required for p-type conductivity. The Cu-O conducting planes of the high critical temperature copper oxide superconductors exhibit closely related chemical bonding topologies with metal-oxygen-metal bonds and antiferromagnetic metal-metal interactions rather than direct metal-metal bonds. The much higher critical temperatures of the copper oxide superconductors relative to superconductors based on metal cluster structures can then be related to the much higher ionic character and thus much lower polarizability of metal-oxygen bonds relative to metal-metal bonds. Assuming that Tc scales linearly with the hole concentration leads to a simple formula for estimating Tc's of members of homologous series such as MIII2MII2Can-1CunO5+2n-1+x (MIII = Bi, MII = Sr or MIII = Tl, MII = Ba). This formula suggests that the maximum Tc's obtainable from copper oxide superconductors will be far below room temperature and probably no more than 180 K.