Chemical structure and superconductivity

Consideration of the chemical bonding topology of the conducting skeletons in the Chevrel phases MMo6S8 and the lanthanide rhodium borides LnRh(4)B(4) indicates that the conducting skeletons consist of networks of edge-locali zed bonded atoms. These observations suggest that superconductivity is favo red when electron mobility is restricted to fewer than three dimensions. Th us, the chemical bonding topologies of the highest T-c, superconductors con tain one of the following features: (1) edge-localized M-M bonding in the c onducting skeletons of ternary and quaternary solid-state superconductors o r related, partially localized bonding in superconducting transition metals and their alloys; (2) localized rather than fully delocalized bonding in c opper oxide superconductors containing Cu-O-Cu linkages with appreciable Cu Cu antiferromagnetic interactions; or (3) surface-localized bonding in ful lerene superconductors containing direct C ... C bonds such as K3C60 In add ition, heavy fermion superconductors with considerably lower T(c)s values a re known in which the conducting skeleton consists of variable oxidation st ate lanthanide or actinide cations diluted by an anionic network of other m etal atoms so that the lanthanide or actinide atoms are not within banding distance of each other.