H-1, (19)E and N-14 NMR, powder X-ray diffraction, and differential scanning calorimetry (DSC) were studied for trimethylethylammonium tetrafluoroborate, (CH3)(3)NC2H5BF4. Five solid phases, named I, II, III, IV, and V in the order of decreasing temperature, were obtained in the temperature range 77 - 630 K. The transition temperatures and the corresponding enthalpy changes determined by DSC were 222 K (0.16 kJ mol(-1)), 239 K (1.8 kJ mol(-1)), 293 K (2.1 kJ mol(-1)), and 362 K (4.5 kJ mol(-1)). X-ray powder patterns showed that Phase I and II form an NaCl-type cubic lattice (n = 10.14 Angstrom, Z = 4) and a tetragonal lattice (a = 8.59, c = 6.24 Angstrom, Z = 2), respectively. The NMR study revealed that the cationic orientation in Phase I and II is dynamically disordered. Moreover, since the self-diffusion of anions and the isotropic rotation of cations were observed in Phase I, this phase can be considered as a mesophase very close to the plastic phase. In Phases II, III, IV, and V we have detected the isotropic reorientation of anions, the anisotropic tumbling of cations, the C-3' reorientation of the whole (CH3)(3)N group about C-N bond axis, and the C-3 reorientation of the CH3 groups. The motional parameters were evaluated for these cationic and anionic motions.
