Temperature-triggered reversible ferroelastic phase transition in an 1:1 inclusion complex of 18-crown[6] with 4-ethylanilinium tetrafluoroborate

A new 1:1 inclusion complex, 4-ethylanilinium tetrafluoroborate–1,4,7,10,13,16–hexaoxacyclooctadecane(1/1), formed between 18-crown[6] and 4-ethylanilinium tetrafluoroborate undergoes a reversible ferroelastic phase transition at ca. 180 K, as evidenced by DSC measurement with a hysteresis of 1.3 K and the variable-temperature crystal structure determinations. The single crystal X-ray diffraction data obtained at 113 and 298 K suggests that the phase transition undergoes from a room temperature paraelectric phase with a space group of Pnma to a low temperature paraelectric one with a space group of P21/c, the symmetry breaking occurs with an Aizu notation of mmmF2/m. The phase transition seems to be associated with slight swing of the ethyl group of the cation brought about by reduced strength of the hydrogen bonds at elevated temperature.