Nanovoids and double volumetric phase transitions in a bulky ionic liquid: Trihexyl(tetradecyl)phosphonium tetrafluoroborate

The phase behaviors of a bulky ionic liquid (IL), trihexyl(tetradecyl)phosphonium tetrafluoroborate ([P666,14][BF 4 ]), were investigated using synchrotron small- and wide-angle X-ray scattering at low temperature (LT) and high pressure (HP). A reversible phase transition was observed at LT. The nanovoid space in the LT phase was visualized in the unit cell. Under HP, the IL underwent a large volume contraction via two-step nanovoid contraction. The flexible crystal (HP-β phase) was transformed to an ionic liquid crystal at 5.3 GPa ( P ILC ). Above P ILC , the lattice constant b changed slightly. Upon compression, a flexible–rigid phase transition of the bulky IL was induced at P ILC . Upon decompression, HP-β phase was recovered despite the maximum pressure of 7.3 GPa.