Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178Â K

We have investigated the molecular motions of TRIS+ ([(CH2OH)3CNH3]+) and SiF62− ions in the [(CH2OH)3CNH3]2SiF6 crystal below room temperature from the measurements of the spin-lattice relaxation time T1 and the NMR absorption line of 1H and 19F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of Tc=178 K. The narrowing of the 19F-NMR line was observed around Tc=178 K and the reorientation of the SiF62− anion appears above Tc. Moreover, from the analysis of the temperature dependence of T1, we have observed that the activation energy of the reorientational motion of SiF62− ions changes from 0.168 eV (T>Tc) to 0.185 eV (T