Introduction

Some inorganic salt hydrates have optimal melting temperatures and high enthalpies of fusion and are thus suitable for storing the energy absorbed by solar collectors [1, 2]. For domestic heating and hot-water supplies, this energy might be stored chemically in the form of reversible reactions, thermally in the form of phase changes, or as temperature increases in the storage material. Therefore, further studies of the physical properties of salt hydrates are needed, in particular, to obtain precise structural information for metal–water bonded systems [2]. Tutton salts are an isomorphous series of monoclinic crystals with the general formula M I2 MII(SO4)2·6H2O. They contain two octahedral hexahydrate complexes [MII(H2O)6]2+ in the crystal unit cell, where MII is a divalent cation (Co, Zn, Fe, or an ion of the 3d group), and MI is a monovalent cation (Na, K, Rb, or Cs) [3–11]. The unit cell dimensions and molecular structures of the crystals of this family are very similar. Montgomery and Lingafelter [12] described the structural characteristics of the crystals in this series, including the details of their hydrogen bond networks. One M2Zn(SO4)2·6H2O compound, K2Zn(SO4)2·6H2O, has a monoclinic structure with space group P21/a. The unit cell contains two Zn2+ ions, each surrounded by six water molecules forming an octahedron, as shown in Fig. 1. K2Zn(SO4)2·6H2O is composed of Zn(H2O)6 octahedra and SO4 square planar forms. The Zn(H2O)6 octahedral form is highly distorted, as indicated by the Zn–O bond. Each [Zn(H2O)6]2+ complex cation is surrounded by four sulfate anion acceptor groups and four K cation donor groups.

Fig. 1
figure 1

Crystal structure of K2Zn(SO4)2·6H2O projected onto the ab plane

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The purpose of this study was to investigate the structural and thermodynamic properties of K2Zn(SO4)2·6H2O single crystals using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). In addition, the temperature dependence of the spin–lattice relaxation time in the laboratory frame, T 1, and the spin–lattice relaxation time in the rotating frame, T 1ρ , for the 1H and 39K nuclei in K2Zn(SO4)2·6H2O was investigated using a pulse nuclear magnetic resonance (NMR) spectrometer to obtain detailed information about the physical properties. This was the first investigation of the structural changes in K2Zn(SO4)2·6H2O crystals. We used the results to analyze the environments of their 1H and 39K nuclei. We also compared our results to those obtained with Na2Zn(SO4)2·6H2O, Rb2Zn(SO4)2·6H2O, and Cs2Zn(SO4)2·6H2O, which have been reported previously. Further studies of the physical properties of salt hydrates are needed, in particular, to obtain precise structural information for metal–water bonded systems.

Experimental method

Single crystals of K2Zn(SO4)2·6H2O were grown by slow evaporation from an aqueous solution at 293 K. The resulting single crystals were colorless and transparent with dimensions 4 × 6 × 3 mm.

The NMR spectrum of the 1H and 39K nuclei for the K2Zn(SO4)2·6H2O single crystals in the laboratory frame was measured by using the Varian 200 FT NMR and Bruker 400 FT NMR spectrometers at the Korea Basic Science Institute Seoul Western Center. The central radio frequency was set at ω 0/2π = 200 MHz for the 1H nucleus and ω 0/2π = 18.67 MHz for the 39K nucleus. A probehead with a solenoid coil was used. The spin–lattice relaxation time in the laboratory frame T 1 was measured by applying the pulse sequences π–t-π/2-acquisition for 1H and π/2–t–π/2-acquisition for 39K. The nuclear magnetizations S(t) of the 1H nucleus at time t after the π pulse were determined from the saturation recovery sequence following the pulse, whereas those of the 39K nucleus at time t after the π/2 pulse were determined from the inversion recovery sequence following the pulse. The width of the π/2 pulse was 3.1 μs for 1H and 33.3 μs for 39K.

The 1H magic angle spinning (MAS) NMR experiments in the rotating frame were also performed using the Varian 200 FT NMR and Bruker 400 FT NMR spectrometers at the Korea Basic Science Institute. The magnetic fields were 4.7 T and 9.4 T, and the 1H MAS NMR experiments were performed at the Larmor frequencies of ω 0/2π = 200 MHz and ω 0/2π = 400 MHz, respectively. A MAS probehead with a 4-mm zirconia rotor was used. The MAS rate was set to 10 kHz to minimize spinning sideband overlap. The spin–lattice relaxation times in the rotating frame T 1ρ were measured by varying the duration of a spin-locking pulse applied after a direct polarization of spins, i.e., π/2—spin lock-acquisition. The width of the π/2 pulse was 3 μs for 1H at ω 0/2π = 200 MHz and 5 μs for 1H at ω 0/2π = 400 MHz. The temperature-dependent NMR measurements were obtained over the temperature range of 180–420 K. The samples were maintained at constant temperatures by controlling the flow of nitrogen gas and the heater current.

The crystal structure of K2Zn(SO4)2·6H2O was determined using an X-ray diffractometer system with Bruker AXS GMBH at the Korea Basic Science Institute Western Seoul Center. The crystal was mounted on a Bruker SMART CCD diffractometer equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). Data collection and integration were performed with SMART (Bruker 2000) and SAINT-Plus (Bruker 2001) [13]. The structure of the K2Zn(SO4)2·6H2O crystals at room temperature exhibits monoclinic symmetry with cell parameters a = 9.041 Å, b = 12.310 Å, c = 6.182 Å, α = γ = 90°, and β = 104.777°. These results are consistent with the data of Montgomery and Lingafelter [12]. In addition, the phase transition temperatures of the crystals were determined by DSC measurements with a DuPont 2010 DSC instrument at a heating rate of 10 °C min−1. Two endothermic peaks were observed at 386 and 410 K, as shown in Fig. 2. TG was then used to determine whether these high-temperature transformations are structural phase transitions or chemical reactions. The curve of K2Zn(SO4)2·6H2O is shown in Fig. 3. The first mass loss begins around 353 K and reaches 4 and 16 % for K2Zn(SO4)2·5H2O and K2Zn(SO4)2·2H2O, respectively. Near 438 K, the thermal decomposition enters a new stage, and the residue of the final products reaches a value of 75.64 %, along with the escape of H2O. The bulk mass of K2Zn(SO4)2·6H2O decreases at 353 K (T d) and reaches complete thermal decomposition into K2Zn(SO4)2 around 438 K. In TG of Fig. 3, the mass loss continues from T d to 440 K at a heating rate of 10 °C min−1. The DSC, TG, and optical polarizing microscopy results for K2Zn(SO4)2·6H2O crystals show that the mass loss around 353 K (=T d) is due to the onset of partial thermal decomposition. The two endothermic peaks at 386 and 410 K in DSC are not polymorphic phase transitions but dehydration accompanied by loss of H2O.

Fig. 2
figure 2

DSC curve of K2Zn(SO4)2·6H2O

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Fig. 3
figure 3

Thermogravimetric analysis (TG) of K2Zn(SO4)2·6H2O

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Experimental results and discussion

Spin–lattice relaxation time for 1H of K2Zn(SO4)2·6H2O in the laboratory and rotating frames

The values for 1H spin–lattice relaxation time in the laboratory frame T 1 were obtained by a static NMR method at a frequency of 200 MHz. The saturation recovery traces of the magnetizations for 1H in K2Zn(SO4)2·6H2O crystals were determined at several different temperatures. The relaxation time was then determined directly from the slope of each log [S(∞) − S(t)]/S(∞) versus t plot [14]. The temperature dependence of T 1 for 1H is shown in Fig. 4; T 1 decreases with increasing temperature. The T 1 values for 1H are more or less continuous near T d.

Fig. 4
figure 4

1H T 1 and T 1ρ in K2Zn(SO4)2·6H2O by static NMR and MAS NMR as a function of temperature

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A structural analysis of 1H in K2Zn(SO4)2·6H2O was conducted using MAS NMR experiments at two different Larmor frequencies, 200 MHz and 400 MHz. The 1H MAS NMR spectrum of a K2Zn(SO4)2·6H2O crystal at 300 K is shown in Fig. 5. The spectrum consists of one peak at a chemical shift of δ = 6.49 ppm; this signal is attributed to water protons. The spinning sidebands are marked with asterisks. The values of the spin–lattice relaxation times in the rotating frame T 1ρ were obtained for the protons in K2Zn(SO4)2·6H2O by MAS NMR experiments at several temperatures. The nuclear magnetization recovery traces obtained for the protons have been described by the following single exponential function,

Fig. 5
figure 5

Chemical shifts of 1H MAS NMR in K2Zn(SO4)2·6H2O at 300 K (ω 0/2π = 400 MHz)

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$$ S\left( t \right) \, = \, S\left( 0 \right)\exp \left( {{-}t/T_{1\rho } } \right), $$
(1)

where S(t) is the magnetization at delay time t, and S(0) is the total nuclear magnetization of 1H at thermal equilibrium [15]. The recovery traces showed single exponential decay at all temperatures. The temperature dependence of T 1ρ for 1H is shown in Fig. 4. The slope of the proton T 1ρ data changes abruptly near T d. As mentioned above, this temperature is associated with the onset of partial thermal decomposition. The variation of T 1ρ with temperature exhibits a minimum: 7.15 ms at 230 K for 200 MHz and 9.64 ms at 220 K for 400 MHz. This behavior of T 1ρ indicates that a distinct molecular motion is present. The minimum temperature is different in each case owing to the Larmor frequency; T 1ρ depends on the frequency.

Spin–lattice relaxation time for 39K of K2Zn(SO4)2·6H2O in the laboratory frame

The NMR spectrum of 39K (I = 3/2) in K2Zn(SO4)2·6H2O single crystals was obtained at a frequency 18.67 MHz. When such crystals are rotated about the crystallographic axis, crystallographically equivalent nuclei may give rise to three lines in the spectrum: one central line and two satellite lines. The NMR spectrum of 39K consists of two central lines at room temperature, as shown in Fig. 6. The magnitudes of the quadrupole parameters of 39K nuclei are on the order of megahertz; hence, only central lines are usually obtained. The satellite resonance lines for 39K nuclei corresponding to transitions between the levels (+3/2 ↔ 1/2) and (–1/2 ↔ –3/2) are out of the frequency range of the NMR probe. These two K signals are the 39K central NMR lines due to the two inequivalent 39K nuclei, K(1) and K(2). Montgomery and Lingafeller [12] have previously reported that the K nuclei in the crystal structure are crystallographically equivalent. From these results, we are led to believe that two types of magnetically inequivalent K nuclei exist in the unit cell. As was previously reported, the resonance lines of several groups for M nuclei in M2Zn(SO4)2·6H2O (M = Na, K, Rb, and Cs) were caused by magnetically inequivalent but chemically equivalent sites: Na(1) and Na(2) for 23Na nuclei [16], K(1) and K(2) for 39K nuclei, Rb(1) and Rb(2) for 87Rb nuclei [16], and Cs(1) and Cs(2) for 133Cs nuclei [17]. The 39K spectrum consists of two lines displaced to lower frequencies relative to the reference signal, which is the 39K line obtained from an aqueous solution of KBr. The resonance frequencies of the K(1) and K(2) signals are shown as a function of temperature in the inset in Fig. 7. The frequencies of the K(1) and K(2) signals increase slowly with increasing temperature. Above 370 K, the K resonance lines suddenly disappear due to line broadening.

Fig. 6
figure 6

NMR spectrum of K(1) and K(2) in K2Zn(SO4)2·6H2O single crystals at 300 K

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Fig. 7
figure 7

Spin–lattice relaxation times in the laboratory frame for K(1) and K(2) in K2Zn(SO4)2·6H2O by static NMR as a function of temperature (inset resonance frequency of K(1) and K(2) NMR in K2Zn(SO4)2·6H2O as a function of temperature)

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The nuclear magnetization recovery curves of the 39K nuclei were obtained by measuring the nuclear magnetization at several temperatures. The inversion recovery trace for the central resonance line of 39K, with dominant quadrupole relaxation, can be represented by the combination of two exponential functions [14]:

$$ \left[ {S\left( \infty \right) - S\left( t \right)} \right]/2S\left( \infty \right) = 0.5\left[ {{ \exp }\left( { - 2W_{1} t} \right) + \exp \left( { - 2W_{2} t} \right)} \right] $$
(2)

The temperature dependence of the 39K spin–lattice transition rates W 1 and W 2 was obtained, and W 1 is nearly equal to W 2 over the whole temperature range. If W 1 and W 2 have the same values in the recovery traces of 39K, we can define a relaxation time T 1 as T 1 = 1/2W 1. When W 1 is not equal W 2, the constant 5/[2(W 1 + W 2)] is introduced instead of T 1; this constant is equal to T 1 when W 1 = W 2. We measured the relaxation times of the central lines for K(1) and K(2) with increasing temperature. The trends in the relaxation times for K(1) and K(2) are very similar; their values are consistent within the error range. The temperature dependence of T 1 for 39K nuclei is shown in Fig. 7.

Conclusions

The thermodynamic properties of K2Zn(SO4)2·6H2O were investigated. The first mass loss occurs near 353 K (T d), which is interpreted as the onset of partial thermal decomposition. The mass loss is continuous changed from T d to 440 K; the H2O content in the crystal decreases with increasing temperature. The phase transitions are restricted to changes in structure only, without any changes in composition [18]. As shown in Fig. 3, the mass loss continues, and it means that thermal decomposition of K2Zn(SO4)2·6H2O crystals takes place continuously via dehydrations. The transformation anomalies at 383 and 410 K are decomposition stage in two consecutive steps. Therefore, the transformation anomalies at 383 and 410 K are not related to structural phase transitions. Near T d, the relaxation time for the 1H nuclei slowly decreases. This is related to the beginning of the loss of H2O as observed in the TG results and indicates that the forms of the octahedra of water molecules surrounding Zn2+ ions might be disrupted. The disruption of the octahedral is due to proton hopping and breaking of hydrogen bonds.

The experimental results for K2Zn(SO4)2·6H2O were compared with those for Na2Zn(SO4)2·6H2O, Rb2Zn(SO4)2·6H2O, and Cs2Zn(SO4)2·6H2O, as shown in Fig. 8 [16, 17]. The T 1 values for the M nuclei are different owing to differences in the local environments of these ions. The difference between the spin–lattice relaxation times in the laboratory frame of these materials can be attributed to the different electric quadrupole moments (Table 1). This suggests that the differences in the chemical properties of M (=Na, K, Rb, and Cs) are responsible for the variations in the nature of the dehydration in these materials.

Fig. 8
figure 8

Spin–lattice relaxation times in the laboratory frame T 1 for M in M2Zn(SO4)2·6H2O (M = Na, K, Rb, and Cs) as a function of temperature

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Table 1 Spin–lattice relaxation times/T 1 and electric quadrupole moments/Q in M2Zn(SO4)2·6H2O (M = Na, K, Rb, and Cs)
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