Appl Magn Reson (2013) 44:1245–1252 DOI 10.1007/s00723-013-0478-2

Applied Magnetic Resonance

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Al Magic Angle Spinning Nuclear Magnetic Resonance Study of Al12xCrxK(SO4)212H2O (x 5 0, 0.07, and 0.2) Celesta L. Chang • Se-Young Jeong • Younkee Paik

Received: 13 April 2013 / Revised: 23 June 2013 / Published online: 17 July 2013 Ó Springer-Verlag Wien 2013

Abstract The physical properties of Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for 27Al. On the basis of the physical properties of pure AlK(SO4)212H2O, the effects of partially replacing Al3? with Cr3? ions were examined. Molecular motion changed with the concentration of Cr3? ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T1q changed with the concentration of paramagnetic ions.

1 Introduction The development of materials suitable for storing energy absorbed by solar collectors is attracting considerable interest. One area of research concerns storing energy chemically in reversible reactions or thermally by a phase change and/or a temperature increase in the storage material for domestic heating and hot water applications [1–4]. Some inorganic salt hydrates, known as the alums, with suitable melting temperatures and high enthalpies of fusion are among the most promising materials in this area. C. L. Chang Department of Physics, Yonsei University, Seoul 120-749, Korea e-mail: [email protected] S.-Y. Jeong School of Nanoscience and Technology, Pusan National University, Miryang 627-706, Korea Y. Paik (&) Daegu Center, Korea Basic Science Institute, Daegu 702-701, Korea e-mail: [email protected]

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The alums can be represented by the general formula M3?Me?(SO4)212H2O, where M is a trivalent cation such as Al, Fe, or Cr, and Me is a monovalent cation such as Na, K, Rb, Cs, or NH4 [5, 6]. It is well known that there are many M3?Me?(SO4)212H2O alums that exhibit ferroelectric activity. The ions M3? and Me? are each surrounded by six water molecules forming an octahedron. A complex network of H bonds is one of the main structural features of these alums. The crystallization of one such alum, AlK(SO4)212H2O, from aqueous solution has been widely studied for its thermodynamic properties and dehydration kinetics [4, 7–10]. AlK(SO4)212H2O crystals have a cubic structure at room temperature with ˚. space group Pa3 (point group m3) [10, 11], and lattice parameter a = 12.157 A 27 39 There are four formula units (Z = 4) in the unit cell. The Al and K ions in a AlK(SO4)212H2O crystal are each surrounded by six water molecules. The six molecules surrounding Al3? is known to form a nearly regular octahedron. Bloembergen [12] studied the spin–lattice relaxation time in the laboratory frame T1 for proton resonance in AlK(SO4)212H2O containing different amounts of Cr3? as a function of temperature. Furthermore, Ramesh et al. [13] presented the electronic spectrum of Cr3?-doped AlK(SO4)212H2O. Oldfield et al. [14] reported the complete resolution of both the (1/2, 3/2) and (3/2, 5/2) transitions of 27Al nuclei in a mixture of AlK(SO4)212H2O and AlNH4(SO4)212H2O. Furthermore, Lim et al. [15] described the relaxation mechanism of 27Al and 39K in pure AlK(SO4)212H2O crystals. Recently, the phase transition temperature and paramagnetic effects in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) crystals were studied by investigating the 27Al nuclear magnetic resonance (NMR) relaxation time in the laboratory frame [16]. However, the molecular motion and spin–lattice relaxation times in the rotating frame T1q for 27Al in AlK(SO4)212H2O containing Cr3? ions have not previously been reported. The spin–lattice relaxation time in the rotating frame T1q is similar to that in the laboratory frame T1. Measurements of T1q have the advantage that they probe the molecular motion in the kHz frequency range, whereas T1 reflects motion in the MHz range. To obtain information about molecular motion, phase transition temperature, and paramagnetic effects in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2), it is necessary to measure the 27Al magic angle spinning (MAS) NMR spectra and spin–lattice relaxation time in the rotating frame T1q. In this paper, the temperature dependence of the chemical shifts, intensities, and T1q values for the 27 Al nuclei in Al1-xCrxK(SO4)212H2O are investigated using pulse NMR spectrometry. From these results, we discuss the changes in the molecular motion, phase transition temperature, and T1q when paramagnetic Cr3? ions partly replace Al3? ions.

2 Experimental Method Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) single crystals were grown in the form of hexagons by slow evaporation from an aqueous solution. The colors of the Al1-xCrxK(SO4)212H2O crystals varied with the relative Cr3? ion composition. Pure AlK(SO4)212H2O crystals without paramagnetic impurities were colorless.

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Al0.93Cr0.07K(SO4)212H2O appeared light purple. Al0.8Cr0.2K(SO4)212H2O appeared a darker purple. In order to obtain the chemical shifts and the spin–lattice relaxation time in the rotating frame, T1q, solid state NMR experiments were performed using a Bruker 400 MHz NMR spectrometer. 27Al MAS NMR experiments were performed at a Larmor frequency of 104.26 MHz. The samples were placed on the 4 mm CP/MAS probe in the form of powder. The MAS rate was set at 10 kHz for 27Al MAS to minimize spinning sideband overlap. T1q was measured by varying the duration of a 27 Al spin-locking pulse applied after a direct polarization, i.e. p/2-spinlock-acq. The field strength of the p/2 and spinlock pulses were the same, which was 89.3 kHz for 27 Al in AlK(SO4)212H2O and Al0.93Cr0.07K(SO4)212H2O and 78.1 kHz for 27Al in Al0.8Cr0.2K(SO4)212H2O. The experimental temperatures were maintained at constant values by controlling the nitrogen gas flow and heater current within an accuracy of ±0.5 K. 27Al NMR spectra were referenced to 1.0 M AlCl3 (aq) at 0 ppm.

3 Experimental Results Structural analysis of 27Al in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) was carried out by a solid state NMR method. The 27Al MAS NMR spectrum of pure AlK(SO4)212H2O at room temperature is shown in Fig. 1. The 27Al NMR spectrum consists of a single peak at a chemical shift of d = -0.21 ppm. The spinning sidebands are marked with asterisks. The chemical shifts for 27Al MAS NMR spectrum of Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) are shown in Fig. 2 as a function of temperature. Here, the chemical shifts for 27Al change abruptly near TC. The chemical shifts for pure AlK(SO4)212H2O and Al0.93Cr0.07K(SO4)212H2O change near 345 K, whereas that of Al0.8Cr0.2K(SO4)212H2O changes near 355 K.

Fig. 1

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Al MAS NMR spectrum of AlK(SO4)212H2O at room temperature

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Fig. 2 Chemical shifts of the 27Al MAS NMR spectrum in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) as a function of temperature

In addition, the changes in chemical shifts for the three materials are very similar. These changes are consistent with previously reported phase transition temperatures [16]. The intensities of the 27Al NMR signal in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) are shown in Fig. 3. Although the intensity for the three materials shows very similar patterns, the intensity near TC changes abruptly. The large changes in the intensity for 27Al NMR line are associated with structural changes of the loss of H2O. The interactions determining the intensity for 27Al are coupled with water molecules surrounding Al3?. This result would explain the loss of H2O, which disrupts the water molecules surrounding Al3?. And three materials show different TC values as the chemical shifts of Fig. 2. Here, the intensities for 27Al decreased with an increasing proportion of Cr3? ions, consistent with the relative decrease of Al3? ions. The 27Al spin–lattice relaxation times in the rotating frame, T1q, were taken at several temperatures in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2). The nuclear magnetization recovery traces obtained for 27Al were described by the following single exponential function, S(t) = S(0)exp(-t/T1q) [17]. The recovery traces showed a single exponential decay at all temperatures, and the slopes of the recovery traces are different at each temperature. From these results, the temperature dependence of the 27Al spin–lattice relaxation time in the rotating frame, T1q, is shown in Fig. 4. The value for T1q of 27Al had a similar trend, and that of 27Al in Al0.8Cr0.2K(SO4)212H2O with 0.2 mol % of Cr3? ions was shorter than that of 27Al in pure AlK(SO4)212H2O. T1q for 27Al in the three materials strongly depended on the temperature. The T1q values take on maxima near 260 K and a minimum at 320 K. The three materials showed different depths for the minimum point; T1q values for the relaxation time of 27Al are 11.74, 6.12, and 4.12 ms for x = 0, 0.07, and 0.2 mol % at 320 K, respectively. The form of this 27Al T1q versus

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Fig. 3 Intensities of the 27Al MAS NMR spectrum in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) as a function of temperature

Fig. 4 Temperature dependences of the 27Al spin–lattice relaxation time in the rotating frame T1q in Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2)

temperature curve suggests that the relaxation process is affected by molecular motion. 27Al T1q for Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) has a welldefined minimum, as described by the Bloembergen–Purcell–Pound (BPP) theory [18]. According to the BPP theory, the T1q value for a spin–lattice interaction in the case of random motion is given by

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2 1 3 nT1q h=rAlH ½4A þ B þ 3C þ 6D þ 6E; ¼ 0:05 cAl cH 

ð1Þ

where A = sC/[1 ? x21s2C], B = sC/[1 ? (xH - xAl)2 s2C], C = sC/[1 ? x2Als2C), D = sC/[1 ? (xH ? xAl)2 s2C], E = sC/[1 ? x2Hs2C]. Here, cAl and cH are the gyromagnetic ratio for the 27Al and 1H nuclei, respectively, n is the number of directly bound protons, r is the Al–H internuclear distance, h = h/2p (where h is Planck’s constant), xAl and xH are the Larmor frequencies of 27Al and 1H, respectively, and x1 is the spin-lock field. The analysis of our data was carried out assuming that T1q takes on a minimum when x1sC = 1. The BPP relation between T1q and the characteristic frequency of motion x1 was also assumed to be applied. Since the T1q curves were found to exhibit minima, it was possible to determine the coefficient in the BPP formula. Having determined this coefficient, we were then able to calculate the parameter sC as a function of temperature [19]. The temperature dependence of sC followed a simple Arrhenius expression, sC = soexp(-Ea/RT), where T is the temperature, R is the gas constant, and Ea is the activation energy. Thus, the slope of the straight line portion of a semilog plot could be used to determine the activation energy, Ea. Figure 5 shows the activation energy for the molecular motion of rotating obtained from the log sC versus 1,000/T curve. The activation energies for 27Al in pure AlK(SO4)212H2O and Al0.93Cr0.07K(SO4)212H2O were found to be 38.37 ± 6.73 kJ/mol and 31.91 ± 1.83 kJ/mol, respectively. The activation energy for 27Al in Al0.8Cr0.2K (SO4)212H2O was 27.37 ± 2.08 kJ/mol. The results showed that the activation energy for the pure sample was larger than that for Al0.93Cr0.07K(SO4)212H2O and Al0.8Cr0.2K(SO4)212H2O. In addition, large drop in the T1q values near TC is shown in Fig. 4, and this discontinuity in the T1q curve near TC corresponds to a first-order phase transition. TC was 345 K for pure AlK(SO4)212H2O and Al0.93Cr0.07K (SO4)212H2O, whereas the value for Al0.8Cr0.2K(SO4)212H2O was 355 K. These results are consistent with

Fig. 5 Arrhenius plot of the natural logarithm of the correlation times for 12H2O (x = 0, 0.07, and 0.2) as a function of inverse temperature

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Al in Al1-xCrxK(SO4)2

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the previously reported spin–lattice relaxation time in the laboratory frame T1 [16]. Figure 4 reveals that these paramagnetic effects on the phase transition temperature and T1q are strongest for 27Al in Al0.8Cr0.2K(SO4)212H2O with 0.2 mol % of Cr3? ions. The significant drop in T1q above TC, on the other hand, reflects transformations of the environments around Al3?.

4 Discussion The physical dynamics of Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) can be explained by the replacement of Al3? with Cr3? ions. We considered the influence of the Cr3? ions by estimating the relative contribution of the Al relaxation time in Al1-xCrxK(SO4)212H2O. Although the ionic radius of the Al3? ion is quite similar to that of the Cr3? ion, the presence of Cr3? ions in pure AlK(SO4)212H2O can change the molecular motion, phase transition temperature, and spin–lattice relaxation time T1q. At the phase transition temperature, the change of intensity is due to the internal molecular motion. We propose that the change of the intensity of the signal from 27Al with temperature is related to the loss of H2O. Also, the abrupt changes in the chemical shifts, the intensities, and T1q near the phase transition temperature can be explained by a structural phase transition, meaning that the structural geometry depends strongly on the concentration of Cr3? ions. An increase in temperature causes the loss of H2O, which disrupts the water molecules surrounding Al3?, thus altering their original octahedral formation. Because T1q should be inversely proportional to the concentration and to the square of the magnetic moment of the paramagnetic ions, the T1q values of samples containing paramagnetic ions are generally shorter than those of samples without paramagnetic ions. Therefore, 27Al T1q is driven in these systems by the fluctuations of the magnetic dipole of the Cr3? paramagnetic ions. A short T1q indicates rapid energy transfer from the nuclear spin system to the surrounding environment. The T1q values decrease when paramagnetic Cr3? ions replace diamagnetic Al3?.

5 Conclusions The molecular motion, phase transition temperature, and paramagnetic effects of Al1-xCrxK(SO4)212H2O (x = 0, 0.07, and 0.2) were studied by observing the chemical shifts and the intensities of 27Al MAS NMR, and the spin–lattice relaxation time in the rotating frame as a function of temperature. On the basis of the physical properties of pure AlK(SO4)212H2O, the variation in T1q is larger than that in T1 due to the effects of the partial replacement of Al3? ions with Cr3? ions. The relaxation processes in the rotating frame near 320 K are affected by molecular motion described by the BPP theory. Here, the molecular motion changes with the concentration of Cr3? ions. Also, the activation energies for 27Al decreased according to the amount of Cr3? ions. Additionally, the phase transition temperature TC changed with the concentration of paramagnetic ions. The T1q values of samples containing Cr3? ions were shorter than that of samples without paramagnetic ions.

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Acknowledgments This work was supported by an NRF grant funded by the MEST (No. 2011-0012663). Celesta L. Chang thanks the KBSI for an X-Science College-Student-Internship program.

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