Molecular Motions and Phase Transitions in [(CH3)4N]2MBr6 (M: Pt, Te, Sn). A Proton Magnetic Relaxation, Differential Thermal Analysis, and Powder X‐Ray Diffraction Study

The measurements of the temperature dependences of the proton spin‐lattice relaxation time T1 and of the second moment of proton NMR absorptions as well as the experiments of DTA and X‐ray powder diffraction were carried out in tetramethylammonium(tetMA) hexabromo‐platinate(IV),‐tellurate(IV), and ‐stannate(IV). From the DTA experiments, various phases were found for (tetMA)2PtBr6 while three phases for both of the remaining complexes. At room temperature, the three complexes crystallize with the Fd3c cubic structure, while they form the Fm3m type cubic crystals in their highest‐temperature phase. The lattice constants of each complex in both of these phases were determined. The second moment study revealed that the reorientational motions of the cations in these complexes freeze for the most part at nearly the same temperature, but a small fraction of the motional freedom still remains even at 77 K. A broad and also deep T1 minimum accompanied by a shallow one on the low‐temperature side was obtained in the T1 measurements of all of these complexes. The broad T1 minimum can be attributed to both of the C3 reorientation of the four methyl groups and the isotropic reorientation of the cation, while the shallow one to a small‐angle reorientation of the methyl groups about their respective C3 axes through the all‐staggered conformation of the cation. The activation energies were evaluated for various motional modes detected.