Studies of the anisotropic self-diffusion and reorientation of butylammonium cations in the rotator phase of butylammonium chloride using 1H magnetic resonance, electrical conductivity and thermal measurements

Abstract

Butylammonium chloride crystals annealed by slowly scanning through the phase transitions many times have been shown to exhibit a simple thermogram, having a single phase transition at 241 K (T_tr) on heating from ca. 100 K up to the melting temperature T_m(487 K). Another additional thermal treatment was made on the annealed crystals, then measurements of ¹H NMR spin–lattice relaxation times T₁, T_1ρ and the second moment M₂ were measured over the same temperature range. These results showed that the cationic axial reorientation mainly contributes to ¹H T₁ around room temperature, while the cationic self-diffusion plays an important role in the high-temperature range near T_m. AC electrical conductivity measurements on single crystals revealed that ionic conduction takes place in the 2D layers of the room-temperature phase (rotator phase) which have a lamellar-type double-layer structure. The observed T₁ and T_1ρ data were explained well by applying the theory of 2D diffusion by MacGillivray and Sholl in the low vacancy concentration limit. The average jump times for the cationic self-diffusion were derived from T₁ and T_1ρ data to be 5 × 10^–3 s at room temperature and 3 × 10^–7 s at T_m. The transition entropies at T_tr and T_m determined by differential scanning calorimetry were 26 and 19 J K^–1 mol^–1, respectively. These results imply that the rotator phase is a kind of plastic crystal.