Structural studies of ambient temperature plastic crystal ion conductors

A number of novel organic ionic compounds based on the pyrrolidinium cation are described which have been found to be ion conductors in their solid st ates around room temperature. The properties of the compounds are consisten t with their exhibiting plastic crystal phases. In order to understand some of the molecular origins of the plastic crystal behaviour and the ion cond uctivity that it promotes, a number of related compounds based on the imida zolium and ammonium cations are also described which have structural elemen ts in common with the pyrrolidinium cation, but which do not show the plast ic behaviour. It is found therefore that the nature of the cation is quite critical to the development of this behaviour. The alkyl methyl pyrrolidini um cation is found to produce plastic crystal phases when the alkyl chains are short, thereby preserving the ability of the cation to rotate with mini mal steric hindrance. The ammonium and imidazolium cations of comparable si ze and structure are less able to produce these plastic phases, in many cas es because the low temperature phase proceeds to melt rather than forming a stable rotator phase.