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Study of phase transition mechanisms in [N(CH3)4]2ZnCl4 by static NMR and MAS NMR
Solid State Sciences 31 (2014) 70e74
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Study of phase transition mechanisms in [N(CH3)4]2ZnCl4 by static NMR and MAS NMR Ae Ran Lim a, *, Kye-Young Lim b a b
Department of Science Education, Jeonju University, Jeonju 560-759, Republic of Korea Department of Energy & Electrical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 4 March 2014 Accepted 5 March 2014 Available online 12 March 2014
The temperature dependences of chemical shifts, intensities, the spin-lattice relaxation time in laboratory frame T1, and in rotating frame T1r were measured for 1H and 13C in [N(CH3)4]2ZnCl4 by single-crystal NMR and MAS NMR. The unit cell in phase I contains two chemically inequivalent types of N(CH3)4 ions. However, the two chemically different ions N(CH3)4 cannot be practically identified from the 13C NMR spectrum. The structural changes for 1H and 13C were measured near Ti and TC4. The existence of chemically equivalent N(CH3)4 ions in phase I and the existence of the ferroelastic characteristic of the N(CH3)4 ions in phases IV and V are discussed. Ó 2014 Elsevier Masson SAS. All rights reserved.
Keywords: [N(CH3)4]2ZnCl4 Nuclear magnetic resonance CP/MAS NMR Phase transition Ferroelectrics Ferroelastics Crystal growth
1. Introduction Single crystals of tetramethylammonium tetrachlorozincate, [N(CH3)4]2ZnCl4, belong to the group of A2BX4-type crystals, many of which have an incommensurate (INC) phase in their phase transition sequence [1,2]. Crystals of this type have attracted considerable interest due to their multiple phase transitions. [N(CH3)4]2ZnCl4 undergoes five phase transitions, at 161 K (¼TC4), 181 K (¼TC3), 276.3 K (¼TC2), 279 K (¼TC1), and 296 K (¼Ti) [3e9]. These previous studies have concluded that there are six phases (I, II, III, IV, V, and VI in order of decreasing temperature) of [N(CH3)4]2ZnCl4 crystals. The crystal structure of phase I is orthorhombic with Z ¼ 4. The transition from the normal (I) to the incommensurate (II) phase occurs at Ti ¼ 296 K. In the following low-temperature phases, the cell parameter along the c-axis changes from the value c0 in the basic structure to 5c0, 3c0, c0, and 3c0 in phases III, IV, V, and VI, respectively. The corresponding symmetry changes are the following: the ferroelectric phase III is orthorhombic with space group P21cn and Z ¼ 20; the ferroelastic phase IV is monoclinic with space group P21/n and Z ¼ 12; the
* Corresponding author. Tel.: þ82 (0)63 220 2514; fax: þ82 (0)63 220 2053. E-mail addresses: [email protected], [email protected] (A.R. Lim). http://dx.doi.org/10.1016/j.solidstatesciences.2014.03.004 1293-2558/Ó 2014 Elsevier Masson SAS. All rights reserved.
ferroelastic phase V is monoclinic with space group P21/c and Z ¼ 4; and finally phase VI is orthorhombic with space group P212121 and Z ¼ 12 [7]. It should be noted that this material undergoes successive phase transitions that are similar to those of [N(CH3)4]2CoCl4 [10e12]. [N(CH3)4]2ZnCl4 has been studied using various experimental techniques [13e19]. According to the nuclear magnetic resonance (NMR) measurements previously reported, the 13C spin-lattice relaxation time by Blinc et al. [20,21] revealed reorientation motion of both the CH3 groups and the N(CH3)4 ions in [N(CH3)4]2ZnCl4. Also, Niemela and Heinila [22] reported on the 1H spin-lattice relaxation time in polycrystalline [N(CH3)4]2ZnCl4. They discussed the reorientation motion of CH3 groups about their C3 axes and the tumbling motion of the N(CH3)4 ions at low and high temperature, respectively. And, the internal motions and phase transition in [N(CH3)4]2ZnCl4 was revealed by 1H spin-lattice relaxation time [23]. In our previous studies, the 1H spin-lattice relaxation time in the laboratory frame, T1, was discussed near the phase transition temperatures in [N(CH3)4]2ZnCl4 crystals [24,25]; the T1 minimum in phase IV was attributed to molecular tumbling of all four CH3 groups in a given N(CH3)4 ion. To better elucidate the nature of the phase transitions in these crystals, and, in particular, to obtain a better understanding of the structural geometry, we measured the temperature dependences of
A.R. Lim, K.-Y. Lim / Solid State Sciences 31 (2014) 70e74
the cross-polarization/magic-angle spinning (CP/MAS) NMR spectrum and the nuclear spin-lattice relaxation times in the laboratory frame T1 and in the rotating frame T1r for 1H and 13C in [N(CH3)4]2ZnCl4. Based on these results, we will discuss the role of the N(CH3)4 ions near the phase transition temperature. 2. Crystal structure [N(CH3)4]2ZnCl4 in the highest temperature phase, phase I, has an orthorhombic structure with space group Pmcn. Its orthorhombic lattice constants are a ¼ 8.946 A, b ¼ 15.515 A, and c ¼ 12.268 A, which are slight different from those of the hexagonal form [8,26]. In this phase, a unit cell contains four formula units consisting of two inequivalent kinds of tetramethylammonium ions, abbreviated a-N(CH3)4 and b-N(CH3)4 hereafter, and one kind of ZnCl2 4 ion [8]. A projection of the structure in the normal phase I viewed from the c-direction is shown in Fig. 1. In phase I, the two stable configurations of ZnCl4 ion and the a-N(CH3)4 ion can be related to each other by a rotation about an axis passing through the center of mass, which lies almost parallel to the c-direction, while in b-N(CH3)4, the axis lies almost parallel to the b-direction. The ZnCl4 ion and the a-N(CH3)4 are positioned in a strongly correlated manner, while the b-N(CH3)4 is less correlated to the other kind of ions.
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set to 10 kHz and 7 kHz for 1H MAS and 13C CP/MAS, respectively, to minimize spinning sideband overlap. The p pulse times for 1H and 13 C were 5 ms and 20 ms, respectively, according to the spin-locking field strength of 50 kHz and 12.5 kHz. Here, the chemical shifts are obtained relative to the reference signal of tetramethylsilane (TMS). The heating rate for NMR is 1 C/min, and after a certain temperature was reached, the temperature was maintained constant about 5 min before measurement. T1r was measured by varying the duration of a 1H and 13C spin-locking pulse applied after a direct polarization of spins, i.e., p/2-spin lock-acquisition. The experimental temperatures were maintained at constant values within an accuracy of 0.5 K by controlling the nitrogen gas flow and heater current. The temperature-dependent NMR measurements were carried out in the temperature range from 140 to 450 K. 4. Experimental results and discussion Structural analysis of the protons in [N(CH3)4]2ZnCl4 was carried out by MAS NMR method at a frequency of 400.12 MHz. Fig. 2(a) shows the 1H MAS NMR spectrum of [N(CH3)4]2ZnCl4 at room temperature. The NMR spectrum consists of one peak, at a chemical shift of d ¼ 3.54 ppm. The spinning sidebands are marked with asterisks. The signal at a chemical shift of 3.54 ppm is assigned to the methyl proton. Also, as the temperature is raised, the relative intensity of the 1H signal decreases abruptly, as shown in Fig. 2(b).
3. Experimental procedure Single crystals of [N(CH3)4]2ZnCl4 were grown at room temperature by slow evaporation of aqueous solution containing ZnCl2 and N(CH3)4Cl in a molar ratio of 1 : 2. The prepared [N(CH3)4]2ZnCl4 crystals were transparent and colorless. The spin-lattice relaxation time in the laboratory frame, T1, for 1 H in [N(CH3)4]2ZnCl4 single crystal by static NMR was measured using Bruker 200 FT NMR spectrometer at Korea Basic Science Institute. The static magnetic field was 4.7 T, and the central radio frequency was set to u0/2p ¼ 200 MHz for 1H NMR. The 1H NMR experiments were performed using a ptp/2 pulse sequence. On the other hand, MAS NMR experiments were performed using a Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute. The magnetic field was 9.4 T, and 1H MAS NMR and 13C CP/ MAS NMR experiments were performed at the Larmor frequencies of 400.12 MHz and 100.61 MHz, respectively. The samples were placed in the 4-mm CP/MAS probe as powders. The MAS rate was
Fig. 1. Crystal structure of [N(CH3)4]2ZnCl4 in the paraelastic phase. Two inequivalent kinds of tetramethylammonium ions, a-N(CH3)4 and b-N(CH3)4, and one kind of ZnCl2 4 ion. The hydrogen atoms of the CH3 ion are not shown.
Fig. 2. (a) 1H MAS NMR spectrum for [N(CH3)4]2ZnCl4 at room temperature and (b) the relative intensity for 1H signal as a function of temperature.
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A.R. Lim, K.-Y. Lim / Solid State Sciences 31 (2014) 70e74
This is more so due to the isotropic reorientation of the N(CH3)4 molecules than the change in equilibrium between non-hydrogen bonded arrangements within the structure. The spin-lattice relaxation times in the rotating frame, T1r, were taken at several temperatures for the proton in [N(CH3)4]2ZnCl4 by MAS NMR. The nuclear magnetization recovery traces obtained for protons were described by the following single exponential function [27]:
MðtÞ ¼ M0 exp t T1r
(1)
where M(t) is the magnetization at time t, and M0 is the total nuclear magnetization of 1H at thermal equilibrium. The nuclear magnetization recovery traces for 1H showed a slightly nonexponential decay due to the correction in the relative positions of the three protons in each CH3 group, but the effects are too small for detection in our experiments, and we neglect them. The slopes of the recovery traces are different at each temperature. From these results, the temperature dependence of the 1H spin-lattice relaxation time in the rotating frame T1r is shown in Fig. 3. The proton T1r data shows no evidence of an anomalous change near the phase transition temperatures. On the other hand, the 1H NMR spectra of the [N(CH3)4]2ZnCl4 single crystals by static NMR consisted of a single resonance line at a frequency of 200 MHz. The inversion recovery traces of the magnetizations of these crystals were measured at several temperatures. The recovery traces for the 1H resonance lines can be expressed with the following equation [28]:
MðNÞ MðtÞ ¼ 2MðNÞexpðWtÞ;
(2)
where W is the transition probability for Dm ¼ 1. The relaxation time, T1 ¼ 1/W, can thus be determined directly from the slope of a plot of log([M(N)M(t)]/2M(N)) versus time (t). The inversion recovery traces could be expressed with a single exponential form at all temperatures investigated. The temperature dependences of the 1H spin-lattice relaxation time in the laboratory frame, T1, are shown in Fig. 3. Here, the magnetic field used in the NMR measurements was applied along the crystallographic c-axis. The discontinuity in the T1 curve near 161 K (¼TC4) corresponds to the phase transition [24,29]. Although a discontinuous change of T1 near Ti is not observed, the slopes near Ti are different. Also, the curve is more or less continuous at the other phase transition
Fig. 3. Temperature dependences of the 1H spin-lattice relaxation time in the laboratory frame, T1, and in the rotating frame, T1r, for [N(CH3)4]2ZnCl4.
temperatures, and hence these transitions could not be detected in the NMR results. As the temperature is increased, T1 decreases initially, reaches a minimum of 122 ms at 210 K, and then increases with further increase of the temperature. The T1 values can be related to the rotational correlation time, sC, which is the time that a molecule remains in a given state before it reorients. Thus sC is a direct measure of the time of motion. The experimental value of T1 can be expressed in terms of sC using the BloembergenePurcelle Pound (BPP) function [28]. According to the BPP theory, T1 for the spin-lattice interaction in the case of random motion is given by Ref. [30]
. i . . 2 h . sC 1 þ u20 s2C þ 4sC 1 þ 4u20 s2C T11 ¼ 9 10 g2 Z r 3 (3) where g is the gyromagnetic ratio for the 1H nuclei, r is the 1He1H distance, Z ¼ h/2p where h is Planck’s constant, sC is the correlation time of the random reorientation, and u0 is the resonance frequency of the proton spins. Our analyses of the data were carried out by assuming a minimum in T1 when u0sC ¼ 0.616, and that the BPP relation between T1 and the characteristic frequency of motion u0 can be applied. It was possible to determine the coefficient in the BPP formula. We were then able to calculate sC as a function of temperature. The temperature dependence of sC followed a simple Arrhenius expression:
sC ¼ s0 expðEa =RTÞ
(4)
Thus, the activation energy for the molecular motion, Ea, could be determined from the slope of the straight-line portion of a plot of log sC vs. 1000/T, as previously reported [24]. The activation energy was found to be 22.49 kJ/mol in phase IV, and the activation energy in phase I was 11.23 kJ/mol. The activation energy on the high temperature side of the T1 minimum is different from the activation energy on the low temperature side of the minimum, demonstrating the existence of several different motional processes with different correlation times and different activation energies. Structural analysis of the carbons in [N(CH3)4]2ZnCl4 was carried out using 13C NMR spectroscopy at a frequency of 100.61 MHz. Fig. 4 shows the 13C CP/MAS NMR spectra at 240 K and at 300 K. At room temperature, the 13C CP/MAS NMR spectrum for [N(CH3)4]2ZnCl4 shows one signal at a chemical shift of d ¼ 57.05 ppm, and this
Fig. 4.
13
C CP/MAS NMR spectrum for [N(CH3)4]2ZnCl4 at 240 K and 300 K.
A.R. Lim, K.-Y. Lim / Solid State Sciences 31 (2014) 70e74
signal represent the methyl carbons. As it can be seen from Fig. 4, only a single symmetric 13C line shifted by 57.05 ppm with respect to TMS, has been observed at room temperature. Also, Fig. 4 shows the 13C CP/MAS NMR spectrum at 240 K, and the 13C chemical shift can be seen to have split into two lines. The splitting of the 13C NMR line below TC2 was due to the ferroelastic twin property. The chemical shifts for 13C in [N(CH3)4]2ZnCl4 were measured for the temperature range of 170e380 K, as shown in Fig. 5(a). The 13 C CP/MAS NMR spectrum consists of one resonance line for one kind of N(CH3)4 in the temperature range 293e380 K. The chemical shifts of the CH3 in two inequivalent kinds of a-N(CH3)4 and bN(CH3)4 in [N(CH3)4]2ZnCl4 were not distinguishable in this measurement. At the transition point of 293 K, the 13C NMR chemical shift splits into two lines. This splitting indicates that at this temperature there is a phase transition to a new phase with monoclinic symmetry lower than orthorhombic symmetry. The III-IV transition results in an abrupt splitting of the 13C NMR line into two components, indicative of a ferroelastic property. The ferroelastic domain structure in phases IV and V of [N(CH3)4]2ZnCl4 were observed by employing an optical polarizing microscope previously reported [31]. Further, above 293 K, there were only continuous quantitative changes in the chemical shift. The 13C chemical shift slowly and monotonously increases with increasing temperature. This means that the structural geometry of 13C in N(CH3)4 continuously changed. On the other hand, the relative intensities of the 13C signals are shown as a function of temperature in Fig. 5(b). As the
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Fig. 6. Temperature dependences of the 13C spin-lattice relaxation time in the rotating frame, T1r, for [N(CH3)4]2ZnCl4.
temperature is increased, the intensity of the signal increases and changes near Ti and TC3, which indicates that N(CH3)4 plays an important role in this phase transition. The HartmaneHahn match is sensitive to power levels when the dipolar coupling between the 1 H and 13C is comparable to the MAS spinning speed. The dipolar coupling between the 13C and 1H can be dependant on the chemical environment of the 13C nucleus. The 13C spectrum was dependant on the quality of the HartmaneHahn match. Therefore, the proper HartmaneHahn match in 13C CP/MAS NMR is important. The spin-lattice relaxation times in the rotating frame, T1r, by MAS NMR were taken for each carbon in the [N(CH3)4]2ZnCl4 at several temperatures, with variable spin locks on the carbon channel following cross-polarization. The 13C magnetization was generated by cross-polarization, after spin locking of the protons. The proton field was then turned off for a variable time t, while the 13 C rf field remained on. Finally, the 13C free induction decay was observed under high-power proton decoupling, and was subsequently Fourier transformed. Values of T1r could be selectively obtained by the Fourier transformation of the free-induction decay (FID), after the spin locking and repetition of the experiment with variations in the time t. All of the traces obtained for the carbons were described by a single exponential function of the form of Eq. (1). The T1r values for 13C are shown in Fig. 6. The T1r values are more or less continuous in the phase transition temperatures. The T1r values for two 13C signals in the ferroelastic phase are nearly the same within the experimental error range. 5. Conclusion
Fig. 5. (a) Chemical shifts of the 13C CP/MAS NMR spectrum as a function of temperature and (b) the relative intensities for 13C signals as a function of temperature.
The structural geometry near the phase transition temperature was studied by observing the chemical shifts, the relative intensities, T1, and T1r for 1H and 13C in [N(CH3)4]2ZnCl4 as a function of temperature. In the high-temperature phase I, the unit cell of [N(CH3)4]2ZnCl4 contains two nonequivalent types of N(CH3)4 ions, a-N(CH3)4 and b-N(CH3)4. The chemical shift for 13C signal obtained in [N(CH3)4]2ZnCl4 shows one signal at d ¼ 57.05 ppm. Therefore, the two crystallographically different ions a-N(CH3)4 and bN(CH3)4 cannot be separately identified using 13C CP/MAS NMR spectroscopy. This observation is in agreement with the Raman measurements previously reported [32], which showed the absence of a doublet structure of Raman lines in the internal N(CH3)4 modes in spite of their crystallographic inequivalence. From these results, the environments of the a-N(CH3)4 and b-
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N(CH3)4 groups may be chemically equivalent. The structural changes to 1H and 13C were obtained near Ti and TC3. These results are compatible with the existence of chemically equivalent N(CH3)4 ions in phase I and the existence of ferroelastic properties of N(CH3)4 ions in phases IV and V. On the other hand, the temperature dependences of the T1 and T1r for 1H and 13C reflect the modulation of the inter-CH3 dipolar interactions due to N(CH3)4 motion. It is apparent that T1 and T1r for 1 H are not governed by the same mechanism. T1r is affected by slower molecular motions compared with T1, and so the T1r measurements provide additional information that can be used for a more reliable check on various models of motion. Acknowledgment This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2012001763). References [1] J. Berger, J.P. Benoit, C.W. Garland, P.W. Wallace, J. Phys. 47 (1986) 483. [2] A.P. Levanyuk, in: R. Blinc, A.P. Levanyuk (Eds.), Incommensurate Phases in Dielectrics, Part I, Foundamentals, Noth-Holland, Amsterdam, 1986. [3] I.R. Larrea, A. Lopez Echarri, M.J. Tello, J. Phys. C. Solid State Phys. 14 (1981) 3171. [4] M. Hohioka, J. Phys. Soc. Jpn. 52 (1983) 4056.
[5] S. Sawada, I. Yamaguchi, H. Suzuki, F. Shimizu, J. Phys. Soc. Jpn. 54 (1985) 3129. [6] K. Parlinski, F. Denoyer, J. Phys. C. Solid State Phys. 18 (1985) 293. [7] G. Madariaga, F.J. Zuniga, J.M. Perez-Mato, M.J. Tello, Acta Cryst. B 43 (1987) 356. [8] K. Hasebe, H. Mashiyama, N. Koshiji, S. Tanisaki, J. Phys. Soc. Jpn. 56 (1987) 3543. [9] P.S. Gregoire, K. Tchakpele, T. Assih, Ferroelectrics 126 (1992) 319. [10] A.G. Cuevas, A.L. Echarrie, M.J. Tello, J.R. Olalla, Ferroelectrics 36 (1981) 384. [11] H. Zangar, J.L. Miane, A. Daoud, J. Phys. C. Solid State Phys. 16 (1983) 6875. [12] C.L. Folcia, J.M. Perez-Mato, Phys. Rev. B 42 (1990) 8499. [13] S.M. Silence, K.A. Nelson, J. Berger, Phys. Rev. B 46 (1992) 2714. [14] E. Dijkstra, A. Janner, H. Meekes, J. Phys. Condens. Matter 4 (1992) 693. [15] E. Dijkstra, M. Kremers, H. Meekes, J. Phys. Condens. Matter 4 (1992) 715. [16] R.P. Rathore, T. Chakraborty, A.L. Verma, Phase Trans. 42 (1993) 155. [17] J. Hencoque, A. Hedoux, J.L. Sauvajol, Ferroelectrics 145 (1993) 205. [18] A. El-Korashy, J. Mat. Sci. Lett. 19 (2000) 29. [19] A. Abu El-Fadl, A. El-Korashy, H. El-Zahed, J. Phys. Chem. Solids 63 (2002) 375. [20] R. Blinc, M. Burgar, J. Slak, V. Rutar, F. Milia, Phys. Stat. Solidi A 56 (1979) k65. [21] J. Dolinsek, D. Hanzel, R. Blinc, M. Koren, Phys. Stat. Solidi A 95 (1986) k53. [22] L.K.E. Niemela, J.E. Heinila, Chem. Phys. Lett. 15 (1981) 182. [23] C.S. Sundaram, J. Ramakrishna, K. Chandrasekhar, V.S.S. Sastry, Ferroelectrics 69 (1986) 299. [24] A.R. Lim, K.W. Hyung, K.S. Hong, S.Y. Jeong, Phys. Stat. Solidi B 219 (2000) 389. [25] A.R. Lim, K.-Y. Lim, Solid State Commun. 147 (2008) 11. [26] F.J. Zuniga, G. Madariaga, J.M. Perez-Mato, Acta Cryst. B 45 (1989) 462. [27] J.L. Koenig, Spectroscopy of Polymers, Elsevier, New York, 1999. [28] B. Cowan, Nuclear Magnetic Resonance and Relaxation, Cambridge Univ., UK, 1997. [29] A.R. Lim, W.K. Jung, J. Phys. Chem. Solids 66 (2005) 1795. [30] N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679. [31] A.R. Lim, D.K. Park, J.-H. Chang, S.-Y. Jeong, J. Phys. Soc. Jpn. 70 (2001) 1937. [32] V.I. Torgashev, Yu I. Yuzyuk, L.M. Rabkin, Yu I. Durnev, L.T. Latush, Phys. Stat. Solidi B 165 (1991) 305.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Study of phase transition mechanisms in [N(CH3)4]2ZnCl4 by static NMR and MAS NMR Ae Ran Lim a, *, Kye-Young Lim b a b
Department of Science Education, Jeonju University, Jeonju 560-759, Republic of Korea Department of Energy & Electrical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 November 2013 Received in revised form 4 March 2014 Accepted 5 March 2014 Available online 12 March 2014
The temperature dependences of chemical shifts, intensities, the spin-lattice relaxation time in laboratory frame T1, and in rotating frame T1r were measured for 1H and 13C in [N(CH3)4]2ZnCl4 by single-crystal NMR and MAS NMR. The unit cell in phase I contains two chemically inequivalent types of N(CH3)4 ions. However, the two chemically different ions N(CH3)4 cannot be practically identified from the 13C NMR spectrum. The structural changes for 1H and 13C were measured near Ti and TC4. The existence of chemically equivalent N(CH3)4 ions in phase I and the existence of the ferroelastic characteristic of the N(CH3)4 ions in phases IV and V are discussed. Ó 2014 Elsevier Masson SAS. All rights reserved.
Keywords: [N(CH3)4]2ZnCl4 Nuclear magnetic resonance CP/MAS NMR Phase transition Ferroelectrics Ferroelastics Crystal growth
1. Introduction Single crystals of tetramethylammonium tetrachlorozincate, [N(CH3)4]2ZnCl4, belong to the group of A2BX4-type crystals, many of which have an incommensurate (INC) phase in their phase transition sequence [1,2]. Crystals of this type have attracted considerable interest due to their multiple phase transitions. [N(CH3)4]2ZnCl4 undergoes five phase transitions, at 161 K (¼TC4), 181 K (¼TC3), 276.3 K (¼TC2), 279 K (¼TC1), and 296 K (¼Ti) [3e9]. These previous studies have concluded that there are six phases (I, II, III, IV, V, and VI in order of decreasing temperature) of [N(CH3)4]2ZnCl4 crystals. The crystal structure of phase I is orthorhombic with Z ¼ 4. The transition from the normal (I) to the incommensurate (II) phase occurs at Ti ¼ 296 K. In the following low-temperature phases, the cell parameter along the c-axis changes from the value c0 in the basic structure to 5c0, 3c0, c0, and 3c0 in phases III, IV, V, and VI, respectively. The corresponding symmetry changes are the following: the ferroelectric phase III is orthorhombic with space group P21cn and Z ¼ 20; the ferroelastic phase IV is monoclinic with space group P21/n and Z ¼ 12; the
* Corresponding author. Tel.: þ82 (0)63 220 2514; fax: þ82 (0)63 220 2053. E-mail addresses: [email protected], [email protected] (A.R. Lim). http://dx.doi.org/10.1016/j.solidstatesciences.2014.03.004 1293-2558/Ó 2014 Elsevier Masson SAS. All rights reserved.
ferroelastic phase V is monoclinic with space group P21/c and Z ¼ 4; and finally phase VI is orthorhombic with space group P212121 and Z ¼ 12 [7]. It should be noted that this material undergoes successive phase transitions that are similar to those of [N(CH3)4]2CoCl4 [10e12]. [N(CH3)4]2ZnCl4 has been studied using various experimental techniques [13e19]. According to the nuclear magnetic resonance (NMR) measurements previously reported, the 13C spin-lattice relaxation time by Blinc et al. [20,21] revealed reorientation motion of both the CH3 groups and the N(CH3)4 ions in [N(CH3)4]2ZnCl4. Also, Niemela and Heinila [22] reported on the 1H spin-lattice relaxation time in polycrystalline [N(CH3)4]2ZnCl4. They discussed the reorientation motion of CH3 groups about their C3 axes and the tumbling motion of the N(CH3)4 ions at low and high temperature, respectively. And, the internal motions and phase transition in [N(CH3)4]2ZnCl4 was revealed by 1H spin-lattice relaxation time [23]. In our previous studies, the 1H spin-lattice relaxation time in the laboratory frame, T1, was discussed near the phase transition temperatures in [N(CH3)4]2ZnCl4 crystals [24,25]; the T1 minimum in phase IV was attributed to molecular tumbling of all four CH3 groups in a given N(CH3)4 ion. To better elucidate the nature of the phase transitions in these crystals, and, in particular, to obtain a better understanding of the structural geometry, we measured the temperature dependences of
A.R. Lim, K.-Y. Lim / Solid State Sciences 31 (2014) 70e74
the cross-polarization/magic-angle spinning (CP/MAS) NMR spectrum and the nuclear spin-lattice relaxation times in the laboratory frame T1 and in the rotating frame T1r for 1H and 13C in [N(CH3)4]2ZnCl4. Based on these results, we will discuss the role of the N(CH3)4 ions near the phase transition temperature. 2. Crystal structure [N(CH3)4]2ZnCl4 in the highest temperature phase, phase I, has an orthorhombic structure with space group Pmcn. Its orthorhombic lattice constants are a ¼ 8.946 A, b ¼ 15.515 A, and c ¼ 12.268 A, which are slight different from those of the hexagonal form [8,26]. In this phase, a unit cell contains four formula units consisting of two inequivalent kinds of tetramethylammonium ions, abbreviated a-N(CH3)4 and b-N(CH3)4 hereafter, and one kind of ZnCl2 4 ion [8]. A projection of the structure in the normal phase I viewed from the c-direction is shown in Fig. 1. In phase I, the two stable configurations of ZnCl4 ion and the a-N(CH3)4 ion can be related to each other by a rotation about an axis passing through the center of mass, which lies almost parallel to the c-direction, while in b-N(CH3)4, the axis lies almost parallel to the b-direction. The ZnCl4 ion and the a-N(CH3)4 are positioned in a strongly correlated manner, while the b-N(CH3)4 is less correlated to the other kind of ions.
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set to 10 kHz and 7 kHz for 1H MAS and 13C CP/MAS, respectively, to minimize spinning sideband overlap. The p pulse times for 1H and 13 C were 5 ms and 20 ms, respectively, according to the spin-locking field strength of 50 kHz and 12.5 kHz. Here, the chemical shifts are obtained relative to the reference signal of tetramethylsilane (TMS). The heating rate for NMR is 1 C/min, and after a certain temperature was reached, the temperature was maintained constant about 5 min before measurement. T1r was measured by varying the duration of a 1H and 13C spin-locking pulse applied after a direct polarization of spins, i.e., p/2-spin lock-acquisition. The experimental temperatures were maintained at constant values within an accuracy of 0.5 K by controlling the nitrogen gas flow and heater current. The temperature-dependent NMR measurements were carried out in the temperature range from 140 to 450 K. 4. Experimental results and discussion Structural analysis of the protons in [N(CH3)4]2ZnCl4 was carried out by MAS NMR method at a frequency of 400.12 MHz. Fig. 2(a) shows the 1H MAS NMR spectrum of [N(CH3)4]2ZnCl4 at room temperature. The NMR spectrum consists of one peak, at a chemical shift of d ¼ 3.54 ppm. The spinning sidebands are marked with asterisks. The signal at a chemical shift of 3.54 ppm is assigned to the methyl proton. Also, as the temperature is raised, the relative intensity of the 1H signal decreases abruptly, as shown in Fig. 2(b).
3. Experimental procedure Single crystals of [N(CH3)4]2ZnCl4 were grown at room temperature by slow evaporation of aqueous solution containing ZnCl2 and N(CH3)4Cl in a molar ratio of 1 : 2. The prepared [N(CH3)4]2ZnCl4 crystals were transparent and colorless. The spin-lattice relaxation time in the laboratory frame, T1, for 1 H in [N(CH3)4]2ZnCl4 single crystal by static NMR was measured using Bruker 200 FT NMR spectrometer at Korea Basic Science Institute. The static magnetic field was 4.7 T, and the central radio frequency was set to u0/2p ¼ 200 MHz for 1H NMR. The 1H NMR experiments were performed using a ptp/2 pulse sequence. On the other hand, MAS NMR experiments were performed using a Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute. The magnetic field was 9.4 T, and 1H MAS NMR and 13C CP/ MAS NMR experiments were performed at the Larmor frequencies of 400.12 MHz and 100.61 MHz, respectively. The samples were placed in the 4-mm CP/MAS probe as powders. The MAS rate was
Fig. 1. Crystal structure of [N(CH3)4]2ZnCl4 in the paraelastic phase. Two inequivalent kinds of tetramethylammonium ions, a-N(CH3)4 and b-N(CH3)4, and one kind of ZnCl2 4 ion. The hydrogen atoms of the CH3 ion are not shown.
Fig. 2. (a) 1H MAS NMR spectrum for [N(CH3)4]2ZnCl4 at room temperature and (b) the relative intensity for 1H signal as a function of temperature.
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This is more so due to the isotropic reorientation of the N(CH3)4 molecules than the change in equilibrium between non-hydrogen bonded arrangements within the structure. The spin-lattice relaxation times in the rotating frame, T1r, were taken at several temperatures for the proton in [N(CH3)4]2ZnCl4 by MAS NMR. The nuclear magnetization recovery traces obtained for protons were described by the following single exponential function [27]:
MðtÞ ¼ M0 exp t T1r
(1)
where M(t) is the magnetization at time t, and M0 is the total nuclear magnetization of 1H at thermal equilibrium. The nuclear magnetization recovery traces for 1H showed a slightly nonexponential decay due to the correction in the relative positions of the three protons in each CH3 group, but the effects are too small for detection in our experiments, and we neglect them. The slopes of the recovery traces are different at each temperature. From these results, the temperature dependence of the 1H spin-lattice relaxation time in the rotating frame T1r is shown in Fig. 3. The proton T1r data shows no evidence of an anomalous change near the phase transition temperatures. On the other hand, the 1H NMR spectra of the [N(CH3)4]2ZnCl4 single crystals by static NMR consisted of a single resonance line at a frequency of 200 MHz. The inversion recovery traces of the magnetizations of these crystals were measured at several temperatures. The recovery traces for the 1H resonance lines can be expressed with the following equation [28]:
MðNÞ MðtÞ ¼ 2MðNÞexpðWtÞ;
(2)
where W is the transition probability for Dm ¼ 1. The relaxation time, T1 ¼ 1/W, can thus be determined directly from the slope of a plot of log([M(N)M(t)]/2M(N)) versus time (t). The inversion recovery traces could be expressed with a single exponential form at all temperatures investigated. The temperature dependences of the 1H spin-lattice relaxation time in the laboratory frame, T1, are shown in Fig. 3. Here, the magnetic field used in the NMR measurements was applied along the crystallographic c-axis. The discontinuity in the T1 curve near 161 K (¼TC4) corresponds to the phase transition [24,29]. Although a discontinuous change of T1 near Ti is not observed, the slopes near Ti are different. Also, the curve is more or less continuous at the other phase transition
Fig. 3. Temperature dependences of the 1H spin-lattice relaxation time in the laboratory frame, T1, and in the rotating frame, T1r, for [N(CH3)4]2ZnCl4.
temperatures, and hence these transitions could not be detected in the NMR results. As the temperature is increased, T1 decreases initially, reaches a minimum of 122 ms at 210 K, and then increases with further increase of the temperature. The T1 values can be related to the rotational correlation time, sC, which is the time that a molecule remains in a given state before it reorients. Thus sC is a direct measure of the time of motion. The experimental value of T1 can be expressed in terms of sC using the BloembergenePurcelle Pound (BPP) function [28]. According to the BPP theory, T1 for the spin-lattice interaction in the case of random motion is given by Ref. [30]
. i . . 2 h . sC 1 þ u20 s2C þ 4sC 1 þ 4u20 s2C T11 ¼ 9 10 g2 Z r 3 (3) where g is the gyromagnetic ratio for the 1H nuclei, r is the 1He1H distance, Z ¼ h/2p where h is Planck’s constant, sC is the correlation time of the random reorientation, and u0 is the resonance frequency of the proton spins. Our analyses of the data were carried out by assuming a minimum in T1 when u0sC ¼ 0.616, and that the BPP relation between T1 and the characteristic frequency of motion u0 can be applied. It was possible to determine the coefficient in the BPP formula. We were then able to calculate sC as a function of temperature. The temperature dependence of sC followed a simple Arrhenius expression:
sC ¼ s0 expðEa =RTÞ
(4)
Thus, the activation energy for the molecular motion, Ea, could be determined from the slope of the straight-line portion of a plot of log sC vs. 1000/T, as previously reported [24]. The activation energy was found to be 22.49 kJ/mol in phase IV, and the activation energy in phase I was 11.23 kJ/mol. The activation energy on the high temperature side of the T1 minimum is different from the activation energy on the low temperature side of the minimum, demonstrating the existence of several different motional processes with different correlation times and different activation energies. Structural analysis of the carbons in [N(CH3)4]2ZnCl4 was carried out using 13C NMR spectroscopy at a frequency of 100.61 MHz. Fig. 4 shows the 13C CP/MAS NMR spectra at 240 K and at 300 K. At room temperature, the 13C CP/MAS NMR spectrum for [N(CH3)4]2ZnCl4 shows one signal at a chemical shift of d ¼ 57.05 ppm, and this
Fig. 4.
13
C CP/MAS NMR spectrum for [N(CH3)4]2ZnCl4 at 240 K and 300 K.
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signal represent the methyl carbons. As it can be seen from Fig. 4, only a single symmetric 13C line shifted by 57.05 ppm with respect to TMS, has been observed at room temperature. Also, Fig. 4 shows the 13C CP/MAS NMR spectrum at 240 K, and the 13C chemical shift can be seen to have split into two lines. The splitting of the 13C NMR line below TC2 was due to the ferroelastic twin property. The chemical shifts for 13C in [N(CH3)4]2ZnCl4 were measured for the temperature range of 170e380 K, as shown in Fig. 5(a). The 13 C CP/MAS NMR spectrum consists of one resonance line for one kind of N(CH3)4 in the temperature range 293e380 K. The chemical shifts of the CH3 in two inequivalent kinds of a-N(CH3)4 and bN(CH3)4 in [N(CH3)4]2ZnCl4 were not distinguishable in this measurement. At the transition point of 293 K, the 13C NMR chemical shift splits into two lines. This splitting indicates that at this temperature there is a phase transition to a new phase with monoclinic symmetry lower than orthorhombic symmetry. The III-IV transition results in an abrupt splitting of the 13C NMR line into two components, indicative of a ferroelastic property. The ferroelastic domain structure in phases IV and V of [N(CH3)4]2ZnCl4 were observed by employing an optical polarizing microscope previously reported [31]. Further, above 293 K, there were only continuous quantitative changes in the chemical shift. The 13C chemical shift slowly and monotonously increases with increasing temperature. This means that the structural geometry of 13C in N(CH3)4 continuously changed. On the other hand, the relative intensities of the 13C signals are shown as a function of temperature in Fig. 5(b). As the
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Fig. 6. Temperature dependences of the 13C spin-lattice relaxation time in the rotating frame, T1r, for [N(CH3)4]2ZnCl4.
temperature is increased, the intensity of the signal increases and changes near Ti and TC3, which indicates that N(CH3)4 plays an important role in this phase transition. The HartmaneHahn match is sensitive to power levels when the dipolar coupling between the 1 H and 13C is comparable to the MAS spinning speed. The dipolar coupling between the 13C and 1H can be dependant on the chemical environment of the 13C nucleus. The 13C spectrum was dependant on the quality of the HartmaneHahn match. Therefore, the proper HartmaneHahn match in 13C CP/MAS NMR is important. The spin-lattice relaxation times in the rotating frame, T1r, by MAS NMR were taken for each carbon in the [N(CH3)4]2ZnCl4 at several temperatures, with variable spin locks on the carbon channel following cross-polarization. The 13C magnetization was generated by cross-polarization, after spin locking of the protons. The proton field was then turned off for a variable time t, while the 13 C rf field remained on. Finally, the 13C free induction decay was observed under high-power proton decoupling, and was subsequently Fourier transformed. Values of T1r could be selectively obtained by the Fourier transformation of the free-induction decay (FID), after the spin locking and repetition of the experiment with variations in the time t. All of the traces obtained for the carbons were described by a single exponential function of the form of Eq. (1). The T1r values for 13C are shown in Fig. 6. The T1r values are more or less continuous in the phase transition temperatures. The T1r values for two 13C signals in the ferroelastic phase are nearly the same within the experimental error range. 5. Conclusion
Fig. 5. (a) Chemical shifts of the 13C CP/MAS NMR spectrum as a function of temperature and (b) the relative intensities for 13C signals as a function of temperature.
The structural geometry near the phase transition temperature was studied by observing the chemical shifts, the relative intensities, T1, and T1r for 1H and 13C in [N(CH3)4]2ZnCl4 as a function of temperature. In the high-temperature phase I, the unit cell of [N(CH3)4]2ZnCl4 contains two nonequivalent types of N(CH3)4 ions, a-N(CH3)4 and b-N(CH3)4. The chemical shift for 13C signal obtained in [N(CH3)4]2ZnCl4 shows one signal at d ¼ 57.05 ppm. Therefore, the two crystallographically different ions a-N(CH3)4 and bN(CH3)4 cannot be separately identified using 13C CP/MAS NMR spectroscopy. This observation is in agreement with the Raman measurements previously reported [32], which showed the absence of a doublet structure of Raman lines in the internal N(CH3)4 modes in spite of their crystallographic inequivalence. From these results, the environments of the a-N(CH3)4 and b-
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N(CH3)4 groups may be chemically equivalent. The structural changes to 1H and 13C were obtained near Ti and TC3. These results are compatible with the existence of chemically equivalent N(CH3)4 ions in phase I and the existence of ferroelastic properties of N(CH3)4 ions in phases IV and V. On the other hand, the temperature dependences of the T1 and T1r for 1H and 13C reflect the modulation of the inter-CH3 dipolar interactions due to N(CH3)4 motion. It is apparent that T1 and T1r for 1 H are not governed by the same mechanism. T1r is affected by slower molecular motions compared with T1, and so the T1r measurements provide additional information that can be used for a more reliable check on various models of motion. Acknowledgment This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2012001763). References [1] J. Berger, J.P. Benoit, C.W. Garland, P.W. Wallace, J. Phys. 47 (1986) 483. [2] A.P. Levanyuk, in: R. Blinc, A.P. Levanyuk (Eds.), Incommensurate Phases in Dielectrics, Part I, Foundamentals, Noth-Holland, Amsterdam, 1986. [3] I.R. Larrea, A. Lopez Echarri, M.J. Tello, J. Phys. C. Solid State Phys. 14 (1981) 3171. [4] M. Hohioka, J. Phys. Soc. Jpn. 52 (1983) 4056.
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