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

Choline dihydrogen phosphate ([N1.1.1.2OH]DHP) and 1-butyl-3-methylimidazolium dihydrogen phosphate ([C4mim]DHP) were synthesized as a new class of proton-conducting ionic plastic crystals. Both [N1.1.1.2OH]DHP and [C4mim]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 [N1.1.1.2OH]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 [C4mim]DHP showed about 10−5 S cm−1 in the plastic crystalline phase. [N1.1.1.2OH]DHP showed one order of magnitude higher ionic conductivity than [C4mim]DHP in the temperature range where the plastic phase is stable.