Phase Transitions in Dimethylammonium Tetrafluoroborate and Molecular Motions in Its Ionic Plastic Phase Studied by 1H and 19F NMR, Thermal Measurements, and X‐Ray Powder Diffraction Techniques

Three solid phases of (CH3)2NH2BF4 were found between 200 K and the melting point (375 K) by differential thermal analysis, differential scanning calorimetry, and 1H and 19FNMR measurements. The highest‐temperature solid phase obtainable above 283.5 K was shown to be an ionic plastic phase, where the cations as well as the anions perform rapid translational self‐diffusion and isotropic reorientation about their center of gravity. The crystal in this phase was revealed to form a CsCl‐type cubic structure with α = 5.35 Å at ca. 290 K by powder X‐ray diffraction. The two phase transitions, from the lowest‐temperature phase to the intermediate one and from the intermediate phase to the highest temperature phase, successively occur at nearly the same temperature of 283.5 ± 0.5 K. The intermediate phase could be clearly observed as an undercooled state.