Introduction

Organic–inorganic hybrid compounds have been a field of great interest recently because of their potential applications. Their potential applications were mainly related to sensors, fuel cells, solar cells, light emitting transistor, and optical switches1,2,3,4,5. Recently, CH3NH3PbX3 (X = Cl, Br, I) has been used for solar cells6,7,8, but these materials were easily degraded in humid air and toxic due to the presence of Pb. Therefore, it is necessary to develop highly stable, eco-friendly hybrid perovskite solar cells. A new type of perovskite compound [(CH3)2NH2]Zn(HCOOO)3, consisting of a cation and a metal ion connected by formate ions, has been reported9,10,11,12. Especially, research on [NH3(CH2)nNH3]MX4 (n=2, 3, 4, …, M represents divalent metals; Mn, Co, Cu, Zn, Cd, Pb, and X halogens; Cl, Br, I), with a focus on the physicochemical properties of their perovskite structure and dynamics investigation, has been garnering much attention13,14,15,16. As a new alternative, compounds of [NH(CH3)3]2MX4 may be described as sequences of alternating organic–inorganic layers17,18,19,20,21,22,23. For these organic-inorganic materials, the ammonium ion of the organic group forms the N−H···X hydrogen bond with the halide ion of the inorganic layer. The individual MX4 tetrahedral anions in these materials were completely isolated and surrounded by organic [NH(CH3)3] cations. These substances were also expected to act as potential proton conductors owing to the existence of hydrogen bonds24,25.

Solid-state nuclear magnetic resonance (NMR) provides information for studying local structure and mobility. By considering the temperature dependence of spin-lattice relaxation time of each nucleus, information can be obtained on the dynamical motion occurring in different environments26,27,28,29,30,31,32,33. Solid-state NMR on hybrid perovskite materials has also garnered considerable attention.

According to the previously reported, the phase transition temperatures TC of [NH(CH3)3]2ZnCl4 crystal, trimethylammonium tetrachlorozincates, were reported by Kapustianik et al.34 at 276.5 K and 310 K. According to Sveleba et al.35, it was reported that the phase transition temperatures by dielectric properties occur at 269, 282, and 309 K. After that, the TC of [NH(CH3)3]2ZnCl4 defined by differential scanning calorimetry (DSC) has five phase transitions at 255 K, 282 K, 302 K, 320 K, and 346 K19. The [NH(CH3)3]2ZnCl4 crystal at 300 K has an orthorhombic structure with the space group Pnma, and its lattice constants are a=10.660 Å, b=9.629 Å, c=14.991 Å. The [NH(CH3)3]2ZnCl4 structure consists of trimethylammonium [NH(CH3)3] cation and [ZnCl4] tetrahedron. This crystal is characterized by hydrogen bonds N−H···Cl connecting the organic cation to the inorganic anion. And, the ferroelectric properties and characterization of phase transitions were discussed by Raman spectroscopy36.

In this study, the [NH(CH3)3]2ZnCl4 single crystals were grown using the aqueous solution method, and their structures, the phase transition temperatures, and thermal properties were investigated. Nuclear magnetic resonance (NMR) chemical shifts as a function of temperature were measured to investigate the coordination geometry for the 1H and 13C atoms in the cations of this crystal. From these results, the N‒H···Cl hydrogen bond according to the ligands between the cations and anions was considered. And, the spin-lattice relaxation times T1ρ representing the energy transfer around the 1H and 13C atoms were discussed as a function of temperature, and their activation energies Ea are determined. The results of the single-crystal structure, NMR chemical shifts, and T1ρ are predicted important information on the crystal configuration and the energy transfer mechanism for the potential applications.

Methods

Crystal growth

Single crystals of [NH(CH3)3]2ZnCl4 were prepared from [NH(CH3)3]Cl (Aldrich, 98%) and ZnCl2 (Aldrich, 98%) in a ratio of 2:1. The mixed compounds were heated to make a homogeneous solution. The mixed solution was filtered once through filter paper, and colorless single crystals grown by slow evaporation were obtained after few days in a thermostat of 300 K.

Characterization

The lattice parameters of [NH(CH3)3]2ZnCl4 crystals were determined by single-crystal X-ray diffraction (SCXRD) at the Seoul Western Center of the Korea Basic Science Institute (KBSI). The crystal block was mounted on diffractometer (Bruker D8 Venture PHOTON III M14) equipped with a graphite-monochromated Mo-Kα (λ=0.71073 Å) radiation source. Data using SMART APEX3 and SAINT was collected and integrated. The structure was solved using direct methods and refined by full matrix least-squares on F2 using SHELXTL37. Additionally the powder XRD (PXRD) patterns of the [NH(CH3)3]2ZnCl4 crystals were measured using an XRD system with the same Mo-Kα used in SCXRD.

DSC measurement was performed using a DSC instrument (TA Instruments, DSC 25) with a heating speed of 10 °C/min between the 200 and 573 K temperature range under a flow of nitrogen gas. The changes of the single crystal according to temperature change were measured using an optical polarizing microscope. A hot stage was used to change the temperature (Linkam THMS 600).

Thermogravimetric analysis (TGA) was also measured with a heating speed of 10 °C/min in the temperature range between 300 and 873 K under nitrogen gas.

The magic angle spinning (MAS) NMR chemical shifts and the spin-lattice relaxation time T1ρ of the [NH(CH3)3]2ZnCl4 crystals were measured using a solid-state NMR spectrometer (Bruker, AVANCE III+) at the same facility, Western Seoul Center of the KBSI. The Larmor frequency for 1H NMR experiment was 400.13 MHz, and that for the 13C NMR experiment was 100.61 MHz. The sample was placed in cylindrical zirconia rotors and measured with a spin speed of 10 kHz for the MAS NMR measurements, in order to the reduce the spinning sideband. Chemical shifts were referenced to adamantane and tetramethylsilane (TMS) for 1H and 13C, respectively, as standard materials in order to accurate the chemical shift measurements. 1D NMR spectrum for 1H and 13C was performed in the delay time of 2–20 s. The 1H T1ρ values were measured using π/2−τ spin-lock pulse for a duration of τ, and the π/2 pulse width was 3.65–4 μs. And, the 13C T1ρ values were obtained by varying the duration of a 13C spin-locking pulse after the CP preparation period. The 13C T1ρ values were obtained using CP-τ acquisition.

Experimental results

FT-IR spectra

The FT-IR spectrum at 300 K was recorded within the 4000–500 cm−1 range. The result is shown in Fig. 1, and the peaks near 813 and 978 cm−1 are assigned to the N–C mode. And, the peak at 1253 cm−1 is related to the deformation vibration N–C–H. The bands observed 1410 and 1468 cm−1 are assigned to the CH3 mode. The bands near 2769 and 3058 cm−1 are related to the stretching mode C–H, and the peak at 3493 cm–1 is the N–H stretching mode. This result agrees well the previously reported results of [NH(CH3)3]2CdCl423.

Figure 1
figure 1

FT-IR spectrum of [NH(CH3)3]2ZnCl4 at room temperature.

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Crystal structure

Single crystal XRD results for the [NH(CH3)3]2ZnCl4 crystal grown here were obtained at 300 K. The single crystal structure has an orthorhombic system with space group Pnma, lattice constants a = 10.6279(4) Å, b = 9.6297(4) Å, c = 14.9880(7) Å, and Z = 4, which are consistent with previously reported results19. Figure 2 shows the thermal ellipsoid and atomic number for each atom, and XRD data of [NH(CH3)3]2ZnCl4 crystals are shown in Table 1. The infinite chains consisted of face-shared ZnCl4 tetrahedra and four doubly bridging Cl− ions linked to adjacent Zn centers. As shown in Fig. 2, this compound is connected by three hydrogen bond N‒H···Cl between the [NH(CH3)3] cation and the [ZnCl4] anion. The bond-lengths for Zn‒Cl, N‒C, N‒H···Cl and bond-angles for Cl‒Zn‒Cl and N‒H···Cl are shown in Table 2. Here, the N‒H···Cl hydrogen bond consists of an angle greater than 120°.

Figure 2
figure 2

Crystal structure of [NH(CH3)3]2ZnCl4 at 300 K.

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Table 1 Crystal data and structure refinement for [NH(CH3)3]2ZnCl4 at 300 K.
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Table 2 Bond-lengths (Å) and bond-angles (°) for [NH(CH3)3]2ZnCl4 at 300 K.
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Phase transition temperatures

The DSC thermogram for powder [NH(CH3)3]2ZnCl4 was measured in the temperature range from 200 to 573 K with the heating rate of 10 ℃/min. Figure 3 shows the four endothermic peaks at 257 K, 286 K, 326 K, and 348 K. And, the one strong endothermic peak at 553 K was obtained. The enthalpies for the five peaks were 2.54, 7.99, 7.53, 1.86, and 17.58 kJ/mol, respectively. Starting from 200 K, these five phases were denoted as phase V below 257 K, phase IV between 257 and 286 K, phase III between 286 and 326 K, phase II between 326 and 348 K, and phase I above 348 K, shown in Fig. 3.

Figure 3
figure 3

Differential scanning calorimetry curve of [NH(CH3)3]2ZnCl4 measured at a heating rate of 10 °C/min.

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In order to know whether the five endothermic peaks shown in the DSC results shown in Fig. 3 are the phase transition temperatures or melting temperature, the changes of a single crystal were observed using an optical polarizing microscope with increasing temperature. Until the temperature rises to 530 K, the single crystal is almost unchanged, but the single crystal began to melt above 550 K.

In addition, a powder XRD experiment was measured according to the temperature change. The PXRD patterns in the range of 8°–50° (2θ) are shown at various temperature in Fig. 4. The PXRD patterns below 330 K (black) differ from that recorded above 330 K (red); this difference is related to the structural phase transition in TC2 (= 326 K). Furthermore, the XRD patterns recorded above 330 K differed from those recorded at 350 K (olive), and this difference is related to the phase transition in TC1 (= 348 K). These results are consistent with those of the DSC result. And, the theoretical XRD pattern at 300 K, which agrees well with the experimental pattern, is shown in Fig. 4.

Figure 4
figure 4

Powder X-ray diffraction patterns of [NH(CH3)3]2ZnCl4 at phases III, II, and I. The blue colour is the theoretical powder pattern at 300 K.

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The phase transition temperatures and melting temperature shown in the PXRD and polarizing microscope results are agree well with the endothermic peaks obtained in the DSC curve. From the DSC, PXRD, and optical polarizing microscopy results, the phase transition temperatures were determined as TC4 = 257 K, TC3 = 286 K, TC2 = 326 K, TC1 = 348 K, and the melting temperature was Tm = 553 K.

Thermal property

TGA curves shown in Fig. 5 were obtained with the increasing temperature. In the TGA curve, the partial decomposition temperature Td representing a weight loss of 2 % was 495 K, and this material was thermal stable up to 495 K. The molecular weight of the [NH(CH3)3]2ZnCl4 crystal as the temperature increased was abruptly decreased by the partial decomposition. The molecular weight losses of 11 % and 22 % calculated from the total molecular weight were by the partial decomposition of HCl and 2HCl, respectively. The initial weight loss (45 %) was occurred in the range of 500–630 K. On the other hand, an endothermic peak at 342 K appeared in the differential thermal analysis (DTA) curve, which is shown as a differential form of TGA, was in good agreement with the phase transition temperature TC1 shown in the DSC result. In addition, it was found that total weight loss occurred at temperatures near 800 K.

Figure 5
figure 5

Thermogravimetric analysis and differential thermal analysis curves of [NH(CH3)3]2ZnCl4.

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1H and 13C MAS NMR chemical shifts

The NMR chemical shifts for 1H in the [NH(CH3)3]2ZnCl4 crystal were recorded at phases V, IV, III, II, and I, as shown in Fig. 6. The 1H NMR spectra for NH and CH3 were obtained, and their sidebands for 1H spectrum were represented as the open circles and asterisks, respectively. At 300 K, the 1H chemical shift for NH was recorded about 7.78 ppm and the 1H chemical shift for CH3 was obtained about 3.21 ppm. Depending on the temperature change, there is no change near TC4, TC2, and TC1, but the 1H chemical shift for NH near TC3 shows a change. As shown inside of Fig. 6, the 1H chemical shifts for CH3 were hardly change as the temperature increased, whereas the 1H chemical shifts for NH were changed, and this result means that the coordination geometry around 1H for CH3 does not changes according to the temperature change, whereas the coordination geometry around 1H for NH was changes.

Figure 6
figure 6

1H MAS NMR chemical shifts of NH and CH3 in [NH(CH3)3]2ZnCl4 at phases V, IV, III, II, and I. The open circles are sideband for NH, and the asterisks are is sideband for CH3 (inset: the 1H chemical shifts for NH and CH3 near phase transition temperatures).

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The 13C NMR chemical shifts of [NH(CH3)3]2ZnCl4 were measured in phases V, IV, III, II, and I with increasing temperature as shown in Fig. 7. In the structure of the crystal shown in Fig. 2, three 13C atoms are bonded with 1H and 14N. The 13C chemical shifts for CH3 at 220 K of phase V showed three signals (46.98, 46.24, and 45.87 ppm), it reduced to two signals at 300 K of phase III (48.05 and 47.44 ppm), and it also reduced to one signal at 420 K of phase I (47.89 ppm). Although 14N and 1H are bonded around 13C in the cation, the environments around 13C can be different depending on the nearby Cl−. That is, the change of 13C chemical shifts was not seen near TC4, whereas the change of chemical shift was large in TC3. In addition, two signals were obtained between TC3 and TC2, and at the temperature above TC1, only one signal was obtained. The change in the number of 13C chemical shifts can be explained as follows. That is, in phases V and IV, there are three CH3 with different environments, and in phases III and II, there are two CH3 with different environments. In particular, in phase I, it was found that all the environments around 13C in CH3 were the same. The number of peaks in the 13C NMR spectra decreases with increasing temperature, indicating an increase in coordination symmetry around the [NH(CH3)3] cations.

Figure 7
figure 7

13C MAS NMR chemical shifts in [NH(CH3)3]2ZnCl4 at phases V, IV, III, II, and I (inset: the 13C chemical shifts for CH3 as a function of temperature).

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1H and 13C NMR spin-lattice relaxation times

To understand the spin-lattice relaxation time T1ρ, the signal intensities of 1H and 13C NMR spectra were measured according to the change of the delay times. The decay curves by the change in the intensities and delay times are represented as following equation38,39,40:

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

where P(t) is the intensity of the spectrum at time t and P(0) is the intensity of the spectrum at time t = 0. The T1ρ values for 1H and 13C in [NH(CH3)3]2ZnCl4 are obtained using Eq. (1), and their results are represented in Fig. 8 at phases V, IV, III, II, and I. 1H and 13C T1ρ at phases V and IV increases as the temperature increases and show a maximum value near 300 K. The T1ρ values at phase I show a tendency to rapidly decrease again. The similar tendency of the two 1H and 13C T1ρ values means that 1H and 14N around 13C are bonded, and 13C and 14N around 1H are bonded, so it is thought that they do the same motions. And, the fact that the 13C T1ρ value is longer than the 1H T1ρ value means that the energy transfer of 1H bonded to the end of 13C is easy. Near TC1, TC2, and TC4, the T1ρ values for 1H and 13C are more or less continuous, but T1ρ values near TC3 are change discontinuous. To determine the magnitude of Ea depending the molecular dynamics, the logarithmic scale of T1ρ values vs. 1000/T are shown as the solid lines in Fig. 8; Ea for 1H and was found to be 10.16 ± 0.88 kJ/mol at phases V and IV. And, the Ea for 1H was 37.08 ± 2.21 kJ/mol at phase I. On the other hand, the Ea for 13C obtained from the slope of T1ρ as the function of inverse temperature at phases V and IV was 9.80 ± 0.45 and 9.83 ± 0.42 kJ/mol, whereas it for 13C was 45.04 ± 1.99 kJ/mol at phase I. Although the 13C chemical shifts are slightly different, Ea at phases V and IV is almost the same within the error range. It is noteworthy that the differences in Ea at phases V, IV, and I are very large.

Figure 8
figure 8

1H and 13C NMR spin–lattice relaxation times T1ρ in [NH(CH3)3]2ZnCl4 at phases V, IV, III, II, and I. The slopes of lines are represented the activation energies by the T1ρ as a function of inverse temperature.

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The 1H and 13C T1ρ values shows a tendency to increase rapidly at phases V and IV, whereas those shows decrease abruptly at phase I. The behaviours of the T1ρ for Arrhenius-type molecular motions are separate into fast- and slow-motion parts. Fast motion is expressed as ω1τC << 1, T1ρ–1~ exp(Ea/kBT), and the slow motion as ω1τC >> 1, T1ρ~ω1–2 exp(–Ea/kBT)38. At the boundary of 300 K, it is divided into fast and slow motion. The 1H and 13C T1ρ values at phases V and IV were in the fast-motion regime, whereas the 1H and 13C T1ρ values at phase I were attributed to the slow-motion regime.

Conclusion

Investigating of the growth, phase transition temperatures, and thermodynamics of the organic–inorganic hybrid [NH(CH3)3]2ZnCl4 crystals were considered. The orthorhombic structure of this crystal was determined by SCXRD, and the four phase transition temperatures of 257 K (= TC4), 286 K (= TC3), 326 K (= TC2), and 348 K (= TC1) were defined using DSC and PXRD results. The previously reported19,34,35 phase transition temperatures may slightly differ depending on the crystal growth conditions. This crystal had the thermal stability of about 495 K, and weight loss resulting in the loss of the HCl and 2HCl moieties was observed with increasing temperature owing to thermal decomposition. From the chemical shifts caused by the local field around 1H, the environments around 1H for CH3 does not changes according to the temperature change, whereas the environments around 1H for NH was changes. And, the coordination geometry of 13C becomes high symmetry as the temperature rises. As a result, the change in 1H chemical shifts around NH in the cation is suggest be due to the change in the hydrogen bond N‒H···Cl, which is related to the change in Cl around Zn in the anion. Finally, 1H T1ρ and 13C T1ρ values, which represent the energy transfer for the 1H and 13C atoms of the cation are changed significantly with temperature. The activation energies considered from the NMR T1ρ values for molecular motion were very high at high temperature of phase I than at phase V and IV. Additionally, based on the relaxation time T1ρ measurements in the rotational system for 1H and 13C at different temperatures, activation barriers for the molecular reorientation of the cation in phases I, IV, and V were determined. The activation energy barriers in phases IV and V are approximately four times lower than that in phase I. The relaxations show that in phases IV and V the energy barrier was related to the reorientation of methyl groups around the triple symmetry axis, and in phase I the reorientation of methyl groups of the cation was related to the entire rotation. This work provides an understanding of the fundamental properties for applications in organic–inorganic hybrid materials.