Mixed Conductors: Synthesis, Properties, Applications

Abstract

Only in the last two decades has the full realization been made that many materials can incorporate atoms or ions into their structures around room temperature. This incorporation frequently occurs with minimal structural changes so that the reaction can be reversed by appropriate chemical or electrical means. These materials include metals, inorganics, and organics. The driving force for reaction is a gain in free energy and is frequently associated with a transfer of electron density between the guest and host species. Thus by definition the host material must contain an electronic structure that can be readily oxidized or reduced, and hence for inorganic materials normally contains transition metals with their variable valences, or for organics a delocalized electron system.The systems described here frequently exhibit both electronic and ionic conductivity, i.e., they are mixed conductors over at least part of their composition range. They tend to have variable composition; this contrasts with solid electrolytes such as β-alumina which may also be nonstoichiometric but are of fixed composition at normal temperatures. Materials in this last category include the β-aluminas and aluminosilicates such as vermiculite and montmorillonite, both of which can be used as electrolytes due to their high ionic conductivity and low electronic conductivity.