Pressure-induced frustration-frustration process in 1-butyl-3-methylimidazolium hexafluorophosphate, a room-temperature ionic liquid.

We have found that the room-temperature ionic liquid (RTIL) reveals outstanding pressure-induced phase changes from a liquid state to a crystal polymorph and finally to a glass form upon compression by up to 8 GPa. The RTIL is 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], which offers the opportunity to investigate a variety of fluctuations in one system and can be completely recovered without dissociation or polymerization, even after decompression. Similar to charge frustration, spin ice-like frustration, and geometric frustration in high potential spintronics/multiferroic materials, the RTIL frustrations are classified into charge (scalar), orientation (vector), and coordination number (topology). Degrees of freedom at each state of [C4mim][PF6] are described by charge balancing, molecular orientational order/disorder, molecular conformations of the C4mim(+) cation, and the coordination number. Here, we show a novel "conformation glass" induced by high pressure.