Collective States in Single and Mixed Valence Metal Chain Compounds

Abstract

The purpose of the present chapter is to survey the structures of inorganic compounds containing chains of interacting metal atoms, to try to see what the molecular requirements are for building up crystals containing such chains, and then to seek correlations between the structures of the chains and the resulting physical properties of the solid. Only a very small fraction of substances containing metal atom chains could in any sense be called one-dimensional metals, and one of the things we wish to do is to see if it is possible to find rules which will help us in looking for potentially metallic systems. Metallic conducting behaviour, as we shall see, is only the limiting case in a sequence of continuously increasing strengths of interaction between adjacent metal ions and evidence for metal-metal interaction stopping short of direct electron exchange can be found in the physical properties of many metal chain compounds. Indeed, in a trivial sence, the electronic states of any assembly of atoms whose positions are governed by translational symmetry have to be called ‘collective’, because the individual constituent atoms are in principle indistinguishable. However, collective states are not by any means necessarily ones in which electron transfer is freely permitted, and what we shall call ‘collective effects’ for our present purpose are simply any noticeable departures in the physical properties of the aggregate from what might have been anticipated simply by summing the contributions of the component atoms or ions.