Halogen-Bridged Metal Complexes: Model Compounds for High-TcSuperconductors?

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Abstract
A simple model for describing the family of quasi-one-dimensional halogen-bridged metal complexes is presented which takes both electron-phonon and electron-electron interactions into account. A competition between chemical bonding and charge disproportionation occurs in these MX chains. The charge-density wave ground state appears to be the rule, but minor modifications, e.g., replacing chlorine by iodine, can strongly change its character from a large amplitude mixed valence limit to that of a small amplitude Peierls distortion. Correspondingly, the polaronic defect states which are very localized in one regime become quite extended in the other. Experimental data for two specific materials are analysed in terms of the two limiting cases. Finally the recently discovered quasi-two-dimensional high-Tc superconductors and the related three-dimensional material BaPb1-xBixO3 are considered as interpenetrating MX chains. It is proposed that in these compounds the oxygen plays a similar role as the halogen in the MX chain in delocalizing the electronic states at the Fermi energy.