A new class of proton-conducting ionic plastic crystals based on organic cations and dihydrogen phosphate

Abstract

Choline dihydrogen phosphate ([N 1.1.1.2OH]DHP) and 1-butyl-3-methylimidazolium dihydrogen phosphate ([C 4mim]DHP) were synthesized as a new class of proton-conducting ionic plastic crystals. Both [N 1.1.1.2OH]DHP and [C 4mim]DHP showed solid–solid phase transition(s) and showed a final entropy of fusion lower than 20 J K −1 mol −1 which is consistent with Timmerman’s criterion for molecular plastic crystals. The ionic conductivity of [N 1.1.1.2OH]DHP was in the range of 10 −6 S cm −1–10 −3 S cm −1 in the plastic crystalline phase. On the other hand, the ionic conductivity of [C 4mim]DHP showed about 10 −5 S cm −1 in the plastic crystalline phase. [N 1.1.1.2OH]DHP showed one order of magnitude higher ionic conductivity than [C 4mim]DHP in the temperature range where the plastic phase is stable.