Crystal structure and phase transition in tert-butylammonium tetrafluoroborate studied by single crystal X-ray diffraction

Crystal structures of (CH₃)₃CNH₃BF₄ were analyzed at 296, 270, 240, 180, and 40 K by single crystal X-ray diffraction in order to investigate the mechanism of the phase transition occurring at 219 K. All the structures analyzed are monoclinic, belonging to the same space group of P2₁/c (#14) with Z = 8. The asymmetric unit of the crystals is composed of two (CH₃)₃CNH₃⁺ ions and two BF₄^- ions. One of the two anions in the asymmetric unit is disordered in four orientations in the temperatures of 180-296 K, and in two orientations at 40 K. For the other anion and one of the two cations, disordering in two orientations was also detected at 40 K. A continuous change in the orientations of the two anions was observed from room temperature to 40 K. The phase transition is mainly concerned with the change in the number of disordered orientations of one of the two anions, as well as the change in the orientations of the two anions.