Thermal phase behavior and ion hopping in a 1,2,4-triazolium perfluorobutanesulfonate protic organic ionic plastic crystal

Abstract

Critical aspects of thermal behavior and the electrolytic properties of solid-state Protic Organic Ionic Plastic Crystals (POIPCs) are unknown. We present molecular dynamics (MD) simulations on a perfect crystal and a vacancy model to probe such physical phenomena in POIPCs using 1,2,4-triazolium perfluorobutanesulfonate ([TAZ][pfBu]) as an example. The results show the existence of a rotator phase wherein the cations, although translationally ordered are disordered rotationally and exhibit a tumbling motion which significantly affects hydrogen bond lifetimes. van Hove correlation functions characterize the concerted hopping of ions (cation or anion) at 500 K. These results are substantiated by calculated free energy barriers (cation = 2.5 kcal mol⁻¹ and anion = 6 kcal mol⁻¹) and suggest that proton and ion transport influenced by facile hydrogen bond dynamics in the rotator phase contribute to the solid-state conductivity of POIPCs.