The critical temperature dependence of the nuclear quadrupole-resonance 79 Br frequencies at the phase transition (71 = 163 K) was found in (CH3)4NCdBr3. It was concluded that bromine nuclei take part in the phase transition not directly but via the interaction with the crystal polarization. PACS numbers: 64.70.Kb Tetramethylammonium cadmium tribromide, (CH3)4NCdBr3 (TMCB), belongs to the compousss of the type (CH3)4NMX3 (M=Mn, Ni, Cd; X=Cl, Br, I), which also includes such widely investigated

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... rystals as (CH3)4NMnCl3 (TMMC) and (CH3)4NCdCl3 (TMCC). The peculiarity of their stucture is the existence of infinite chains (... M-X3-M-X3 ...) separated by tetramethylammonium ions. These ions are orientationally disordered at room temperature [1, 2] . At low temperatures these compousss are characterized by the structural phase transition of the order-disorder type, accompanied by ordering of the tetramethylammonium ions [3, 4] . At room temperature the TMCB crystal is isomorphous to the above-mentioned compounds and the first-order structural phase transition occurs at the temperature T = 163 K [5]. However, the NMR investigations did not reveal any anomalies in the temperature dependence of the proton spin-lattice relaxation time at 163 K [6]. This phenomenon may be explained by the fact that in this temperature range the relaxation mechanism is due to fast reorientation of the CH3-groups which masks slower motions connected with the change of stucture at the phase transition. (271)