Molecular motions and phase transitions in tris(methylammonium) nonabromodibismuthate (III), (CH 3 NH 3 ) 3 Bi 2 Br 9 , were investigated by measuring the temperature dependences of static and rotating frame 1 H spin-lattice relaxation times ( T 1 ans T 1ρ ) and the second moment ( M 2 ) of 1 H NMR absorptions. Near 80 K in the lowest-temperature phase IV, the orientation of the methylammonium cation is almost fixed but the cation itself is rapidly rotating about the C-N bond axis. On heating, gradual disordering of the cationic orientations takes place in phases IV and III for two thirds of crystallographically nonequivalent cations and in phase II for the remaining cations. Finally, all the cations become to be dynamically disordered and undergo rapid overall reorientations in the room-temperature phase I. The IV-III transition at 108 K may be connected to dynamics of the inorganic (Bi 2 Br 9 3- ) n subsystem rather than the ionic motions. An anomalous behavior of 1 H T 1ρ , attributable to cross relaxation between 1 H and Br nuclei, was observed.
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Kozioł et al. (1991) studied this question.