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 dd 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.