The structural classification of the highly disordered crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6] salts (Nn+ = tetraalkylammonium and Pn+ = tetraalkylphosphonium)

Abstract

The structures of 16 symmetric tetraalkylammonium (N_n⁺) and tetraalkylphosphonium (P_n⁺) salts ([N_n][BF₄], [N_n][PF₆], [P_n][BF₄], and [P_n][PF₆], where n = 1 to 4, and denotes the number of carbon atoms in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size on the disordered structure of organic salts. All the salts exhibit one or more solid–solid phase transitions in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The structures are classified into 5 different types: CsCl′, NaCl, NaCl′, inverse NiAs, and TBPPF₆. The CsCl′-type whose octant corresponds to the original CsCl unit cell is observed for [N₁][PF₆] owing to the orientational difference for the cation or the anion. The NaCl-type structure is observed for the N₂⁺ and P₂⁺ salts while the NaCl′-type structure is observed for [N₃][PF₆], where the configuration of ions is based on the NaCl-type but the four equivalent positions in the original NaCl lattice split into two sets of equivalent positions (three and one). The inverse NiAs structure is observed for [P₃][PF₆]. Single-crystal X-ray diffraction reveals that the disordering of ions in [P₄][PF₆] becomes more significant with increasing temperature. The new structure of a cubic phase, the TBPPF₆-type structure, is found for the salts with long alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.