The Crystal to Plastic Crystal Phase Transition of Tetraethylammonium Difluorophosphate and Tetrafluoroborate

Abstract

Ion configuration in the crystal structure of tetraethylammonium difluorophosphate ([TEA][PO₂F₂]) is highly related to that in tetraethylammonium tetrafluoroborate ([TEA][BF₄]). Both [TEA][PO₂F₂] and [TEA][BF₄] exhibit a crystal–plastic crystal phase transition above room temperature (323 and 340 K, respectively), and the plastic crystal phase has a NaCl-type structure. The TEA⁺ cation in both the salts adopts a trans–gauche conformer in the crystal structure, whereas the trans–trans conformer is dominant in the plastic crystal structure.