Proton NMR study of the effect of paramagnetic impurities in the mixed crystals [N(CH3)4]2Zn1−xCoxCl4 (x=0,0.1,and 1) and [N(CH3)4]2Zn1−xCuxCl4 (x=0,0.1,and 1)
Introduction
[N(CH3)4]2BCl4 (B=59Co, 63Cu, and 67Zn) single crystals belong to the group of -type crystals [1], [2]. Crystals of this type have attracted considerable interest due to their multiple phase transitions. For example, as summarized in Table 1, [N(CH3)4]2CoCl4 crystals undergo six phase transitions, at 122, 192, 276, 277.6, 280.1, and 293 K [3], [4], [5]; [N(CH3)4]2CuCl4 crystals undergo four phase transitions, at 200, 263, 291, and 301 K [6], [7], [8], [9], [10], [11], [12]; and the phase behavior of [N(CH3)4]2ZnCl4 is quite similar to that of [N(CH3)4]2CoCl4 single crystals, with successive phase transitions at 161, 181, 276, 279, and 293 K [13], [14], [15], [16], [17], [18], [19]. At room temperature, all of the [N(CH3)4]2BCl4 single crystals are taken on an orthorhombic structure. Although [N(CH3)4]2BCl4 crystals have been examined in many studies, no previous study has used nuclear magnetic resonance (NMR) to investigate the effects of adding a paramagnetic impurity to these crystals.
The magnetic resonance relaxation times of nuclear spin systems have been widely used to elucidate various types of atomic motion in crystals, because it is thought that these relaxation times are related to the structures and internal motions of solids. A prominent feature of the solids studied in this manner is that their protons act as resonant nuclei and that the relaxation times of these nuclei reflect the ionic motions in the solid. Previous studies have established that near the phase transition temperatures of [N(CH3)4]2BCl4 crystals, the values of the magnetic resonance relaxation times and their slopes when plotted as a function of temperature undergo abrupt changes [20]; based on this information, it has been concluded that these phase changes are always accompanied by changes in molecular motion.
In the present study, we examined the effect of substituting Zn ions in [N(CH3)4]2ZnCl4 with Co or Cu ion impurities, with a focus on the effects of these impurities on the molecular motion and activation energy. Specifically, we investigated the isomorphic mixed systems [N(CH3)4]2Zn1−xCoxCl4 (, 0.1, and 1) and [N(CH3)4]2Zn1−xCuxCl4 (, 0.1, and 1). The effects of replacing Zn with Co or Cu in these crystals are expected to stem mainly from differences in the ionic size and electron structures of Zn2+, Co2+, and Cu2+, since all three ions are divalent. We used 1H NMR to measure the temperature dependences of the spin–lattice relaxation time, , and the spin–spin relaxation time, , in the mixed crystals. The variation of the activation energy as a function of impurity concentration in the mixed system is interpreted in terms of the differences in size and electron structure between the host and impurity ions. We strove in particular to explain the role of CH3 in the mechanism of molecular motion in the systems containing paramagnetic Co or Cu impurities, based on the 1H NMR data. The results of this study should be applicable to other isomorphic mixed systems.
Section snippets
Experimental procedure
Crystals of [N(CH3)4]2Zn1−xCoxCl4 (, 0.1, and 1) and [N(CH3)4]2Zn1−xCuxCl4 (, 0.1, and 1) were grown by slow evaporation at room temperature of an aqueous solution containing the appropriate stoichiometric proportions. The prepared crystals were transparent, and varied in color according to the impurity level: [N(CH3)4]2ZnCl4 was colorless, [N(CH3)4]2Zn0.9Co0.1Cl4 was light blue, [N(CH3)4]2CoCl4 was dark blue, [N(CH3)4]2Zn0.9Cu0.1Cl4 was yellow, and [N(CH3)4]2CuCl4 was bright orange.
Experimental results and analysis
The temperature dependences of the 1H spin–lattice relaxation time for the mixed crystals [N(CH3)4]2Zn1−xCoxCl4 (, 0.1, and 1) are displayed in Fig. 1. In this figure, the arrows indicate the phase transition temperatures for [N(CH3)4]2ZnCl4 and [N(CH3)4]2CoCl4, respectively. In the case of pure [N(CH3)4]2ZnCl4, the discontinuities in the curve near 161 and 181 K correspond to phase transitions [22]; however, the remaining three phase transitions could not be detected. The variation of
Conclusions
In this study we systematically investigated the temperature dependences of the 1H relaxation times in the mixed crystals [N(CH3)4]2Zn1−xCoxCl4 (, 0.1, and 1) and [N(CH3)4]2Zn1−xCuxCl4 (, 0.1, and 1) in order to elucidate the effects of adding paramagnetic Co or Cu impurities into a [N(CH3)4]2ZnCl4 single crystal. We found that the addition of these impurities did not affect the phase transition temperatures and the lattice constants of the mixed crystals, but did cause a change in the
Acknowledgment
This work was supported by a Jeonju University Faculty Research Grant.
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