Molecular motion in solid (CH3NH3)2TeX6 (X=Cl, Br, and I) was studied by measuring the spin-lattice relaxation time, T1, of 1H and 35Cl nuclei in several crystalline phases appearing below room temperature. A deep minimum of the proton T1 was observed in the low-temperature phase of all the compounds, indicating that the methylammonium ion performs a rapid reorientation as a whole about its three-fold axis. The activation energies, Ea, for the motion of cations are determined to be 7.0, 3.7, and 2.7 kJ/mol for X=Cl, Br, and I respectively. In (CH3NH3)2TeI6, which has the lowest Ea value, the relaxation process associated with the tunneling of the cation is apparent at low temperatures. For all the complexes studied, a shallow T1 minimum ascribable to the independent rotation of the CH3 and NH3 groups in the cation was observed on the high-temperature side of each deep T1 minimum. The 35Cl T1 of (CH3NH3)2TeCl6 exhibited a distinct minimum at the lower phase-transition point at 139 K and a discontinuity at the higher one near 230 K. The hindered rotation of the [TeCl6]2− octahedron occurs in the room-temperature phase, with an Ea value of 48 kJ/mol.
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Furukawa et al. (1981) studied this question.