NMR study of molecular dynamics and phase transitions in dimethyl ammonium hexabromo selenate [NH2(CH3)2]2SeBr6 Original Research Article

1H spin lattice relaxation study in the temperature range 439–77 K has been carried out in dimethyl ammonium (DMA) hexabromo selenate. A high temperature phase transition is observed as a dip in T1 around 360 K, followed by a slope change at 222 K, and a discontinuous jump in T1 at 154 K, which are also attributed to phase transitions. DMA diad axis motion and spin–rotation interaction are found to be the dominant mechanisms responsible for the observed T1 behaviour above 222 K, while between 222 and 154 K, DMA torsional oscillations contribute significantly. Below 154 K the relaxation is governed by the methyl group dynamics. The motional effects are also reflected in the second moment variation with temperature.