Phase-transition mechanisms of [N(C2H5)4]2BCl4 and [N(CH3)4]2BCl4 (B= 63Cu and 67Zn) single crystals studied by proton NMR

The proton line widths and spin-lattice relaxation rates of [N(C2H5)4]2BCl4 (hereafter denoted [TEA]2BCl4, where B=63Cu or 67Zn) and [N(CH3)4]2BCl4 (hereafter denoted [TMA]2BCl4) single crystals were investigated in the temperature range 150–430 K. Our experimental results show that the relaxation rates of [TEA]2CuCl4 and [TMA]2CuCl4 differ from those of [TEA]2ZnCl4 and [TMA]2ZnCl4. In the case of [TEA]2ZnCl4 and [TMA]2ZnCl4, the maxima in the spin-lattice relaxation rates are attributed to the molecular motions of the ethyl and methyl groups in the [TEA]+ and [TMA]+ ions, respectively. The NMR line widths indicate that the ethyl groups in the [TEA]2BCl4 compounds have one more degree of freedom than the methyl groups in the [TMA]2BCl4 compounds. The differences between the 1H spin-lattice relaxation rates of these compounds are probably due to differences between the electronic structures of the metal ions, in particular between the structures of the d electrons, which screen the nuclear charge from the motion of the outer electrons. Further, the 1H spin-lattice relaxation behaviors of the Cu-based compounds are driven by fluctuations of the dipolar magnetic field of the Cu2+ ion, and are dominated by Cu spin fluctuations.