Proton magnetic resonance study of the phase transitions of [N(CH3)4]2BCl4 (B=59Co, 63Cu, 67Zn, and 113Cd) single crystals

The proton spin-lattice relaxation rates in [N(CH3)4]2BCl4 (B=59Co, 63Cu, 67Zn, and 113Cd) single crystals grown using the slow evaporation method were investigated over the temperature range 120-400 K. It was found that the relaxation processes of 1H for all the [N(CH3)4]2BCl4 crystals can be described with single exponential functions. The changes in the 1H relaxation behavior in the neighborhood of the phase transition temperatures are used to detect changes in the state of internal motion. From the 1H spin-lattice relaxation rate measurements for [N(CH3)4]2BCl4 crystals, the activation energies were calculated for each phase. The large values of the activation energies indicate that the N(CH3)4 groups are significantly affected during the transitions. Although these [N(CH3)4]2BCl4 crystals all belong to the group of A2BX4-type crystals, their 1H spin-lattice relaxation rates have different temperature dependences and indicate the occurrence of different molecular motions within the crystals. We additionally show for the first time that the differences in 1H spin-lattice relaxation rates among the [N(CH3)4]2BCl4 (B=59Co, 63Cu, 67Zn, and 113Cd) single crystals arise from differences in the electron structures of the metal ions within the series.