Motion of methylammonium ions and the mechanism of phase transitions in solid methylammonium bromide as studied by differential thermal analysis, infrared, and proton magnetic resonance experiments

The measurement of DTA and the temperature variation study of IR revealed that the reported stable phases α, α′, and β of methylammonium bromide (CH₃NH₃Br) exist above 387 K, between 387 and 281 K, and below 281 K, respectively. The metastable δ phase prepared by rapidly cooling the sample to 77 K seems to be transformed into α′ at 196 K, and, immediately above this temperature, it is transformed again to β. The temperature dependences of the ¹H spin-lattice relaxation time T₁ at 20 MHz and of the second moment of ¹H NMR absorptions were determined for the bromide and also its partially deuterated analogues CD₃NH₃Br and CH₃ND₃Br in their respective phases. An analysis of the CH₃ and NH₃ rotational reorientations about the C-N axis occurring in the β phase was accomplished by using the T₁ data of the partially deuterated salts. The onset of both reorientations in the δ phase occurs at approximately the same temperature which is lower than those found in the β phase. The motional parameters of the cation in the β and δ phases could be derived from the present results of ¹H NMR.