Structural studies of ambient temperature plastic crystal ionconductors

Abstract

A number of novel organic ionic compounds based on the pyrrolidinium cationare described which have been found to be ion conductors in their solidstates around room temperature. The properties of the compounds areconsistent with their exhibiting plastic crystal phases. In order tounderstand some of the molecular origins of the plastic crystal behaviourand the ion conductivity that it promotes, a number of related compoundsbased on the imidazolium and ammonium cations are also described which havestructural elements in common with the pyrrolidinium cation, but which donot show the plastic behaviour. It is found therefore that the nature ofthe cation is quite critical to the development of this behaviour. Thealkyl methyl pyrrolidinium cation is found to produce plastic crystalphases when the alkyl chains are short, thereby preserving the ability ofthe cation to rotate with minimal steric hindrance. The ammonium andimidazolium cations of comparable size and structure are less able toproduce these plastic phases, in many cases because the low temperaturephase proceeds to melt rather than forming a stable rotator phase.