Introduction

The hybrid organic–inorganic compounds, [(CH2)n(NH3)2]MX4 (M = Mn, Fe, Co, Cu, and Cd, X = Cl, Br, n = 2, 3 …), are well-known, and have been studied extensively for groups of these crystals. Most of these structures exhibit ferroelastic or ferroelectric phase transitions. The physical properties and phase transitions are related to their structure and the interaction between cationic and anionic sublattices. An interesting family of hybrid compounds is perovskite-type crystals with (CH2)n(NH3)2 and MX4-layered metal-halogen anionic sublattice1,2,3,4,5,6,7,8. In [(CH2)n(NH3)2]MX4, the hydrogen bonds form between the NH3 groups at both ends of the aliphatic chains and X-atoms of the perovskite-type layer. Hybrid organic–inorganic materials based on the perovskite structures are interesting owing to their potential applications9,10,11,12,13,14,15. On the one hand, the ferroelastic orientation state in a material is determined by its spontaneous strain tensor, similar to how spontaneous polarization leads to ferroelctricity16. Moreover, ferroelasticity is commonly observed in materials with a perovskite crystal structure. Recently, the ferroelastic twin domain observed in hybrid organic–inorganic perovskite has also garnered much attention17,18,19.

Among these materials, [(CH2)3(NH3)2]CuCl4 [bis (propylene-1, 3-diammonium) tetrachlorocuprate] with n = 3 and M = Cu undergoes two phase transitions, at temperatures of 333 K (= TC2) and 434 K (= TC1)20.

The crystal at room temperature has an orthorhombic structure with a space group Pnma. The unit cell dimensions are a = 7.202 Å, b = 18.260 Å, c = 7.515 Å, and Z = 421. The crystal structure consists of chloro-bridged deformed tetragonal (CuCl4)2- to form two-dimensional layers. The chlorocuprate sheet is hydrogen bonded to [(CH2)3(NH3)2] in alternating layers. The structural geometry of the [(CH2)3(NH3)2]CuCl4 is shown in Fig. 121. Extensive hydrogen bonding of the N–H···Cl occurs between the Cu–Cl layer and the alkylammonium chain. The organic chains are extended along the a direction. The organic chains NH3–CH2–CH2–CH2–NH3 are almost identical, and the skeleton N–C–C–C–N is planar. Above 434 K, the symmetry is monoclinic with space group B2/m and lattice constants a = 7.309 Å, b = 8.866 Å, c = 7.614 Å, α = 95.365°, and Z = 222. The lattice constants a and c in the monoclinic structure are comparable with those in the room temperature structure, whereas the b parameter in the monoclinic structure is half of that in the room temperature structure.

Figure 1
figure 1

Structural geometry of [(CH2)3(NH3)2]CuCl4 at room temperature. Here, CH2 between CH2 and CH2 is named CH2-1, and CH2 close to NH3 is named CH2-2.

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According to the previously reported, the Phelps et al.21 and Czupinski et al.23 determined the structural phase transition for (CH2)3(NH3)2CuCl4. And, the structural, dielectric, and conductive properties were discussed by Mostafa et al.20. In addition, the structural phase transition was analysed by x-ray and optical studies22, where ferroelastic multidomain walls were observed in the orthorhombic phase. Iqbal et al.1 reported Raman scattering results at various temperatures above and below the respective magnetic ordering temperature (149 K) and in a magnetic field up to 10 kg. The crystal structure, magnetic and optical properties have been studied by only a few researchers. In addition, the thermal properties, the structural and molecular dynamics of the [(CH2)3(NH3)2]CuCl4 crystal have not been studied in detail.

Here, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) experiments were performed to provide a better understanding of the phase transition temperatures and thermal properties of [(CH2)3(NH3)2]CuCl4. In addition, the structural characterizations of the [(CH2)3(NH3)2] cation were studied in detail by magic angle spinning (MAS) nuclear magnetic resonance (NMR) and static NMR methods. The temperature dependences of the chemical shifts and spin–lattice relaxation times T1ρ were measured by 1H MAS NMR and 13C cross-polarization (CP)/MAS NMR to highlight the role of the cation in [(CH2)3(NH3)2]CuCl4. In addition, 14N static NMR spectra of [(CH2)3(NH3)2]CuCl4 single crystals were acquired. Based on these results, the structural characterizations for NH3–CH2–CH2–CH2–NH3 are discussed as a function of temperature. In particular, the hydrogen bonding of the N–H···Cl between the Cu–Cl layer and the alkylammonium chain within the [(CH2)3(NH3)2]CuCl4 is expected to give important information regarding the fundamental mechanisms that enable various potential applications.

Experimental

Crystals of [(CH2)3(NH3)2]CuCl4 were prepared by mixing equimolar amounts of NH2(CH2)3NH2·2HCl and CuCl2 (1:1 ratio) in aqueous solution. Then, the resulting mixture was allowed to slowly evaporate at 300 K. The crystals grew as rectangular parallelepipeds, with dimensions of 7 × 7 × 2 mm3.

The structure of the [(CH2)3(NH3)2]CuCl4 crystal at 300 K was analysed using an X-ray diffraction system equipped with a Cu-Kα radiation source at the KBSI, Seoul Western Center. DSC (TA Instruments, DSC 25) was conducted at a heating rate of 10 °C/min from 190 to 600 K under nitrogen gas. TGA was performed using a thermogravimetric analyser (TA Instruments) from 300 to 680 K at the same heating rate. The sample weights used for DSC and TGA experiments were 6.23 and 7.53 mg, respectively. Optical observations were performed using an optical polarized microscope in the temperature range of 300–600 K, where the as-grown crystals were placed on a Linkam THM-600 heating stage.

NMR spectra of [(CH2)3(NH3)2]CuCl4 crystals were obtained using a 400 MHz Avance II + Bruker solid-state NMR spectrometer, equipped with 4 mm CP/MAS probes (at the KBSI, Seoul Western Center). The Larmor frequencies to 1H MAS NMR and 13C CP/MAS NMR experiments were at ω0/2π = 400.13 and 100.61 MHz, respectively. A MAS rate of 10 kHz was used to minimize the spinning sideband. The NMR chemical shifts were recorded using tetramethylsilane (TMS) as the standard. The T1ρ values were measured using a π/2 − t sequence by changing the spin-locking pulses, and the width of the π/2 pulse was 3.3 μs. The spin-lock power on the 1H and 13C channel was 75.76 kHz. The 13C T1ρ values were obtained by changing the duration of the 13C spin-locking pulse applied after the CP preparation period. In addition, 14N NMR spectra of a [(CH2)3(NH3)2]CuCl4 single crystal were measured with a Larmor frequency of 28.90 MHz. The resonance frequency was referenced with respect to NH3NO3 as a standard sample. The 14N NMR experiments were performed using a solid-state echo sequence: 8 μs—tau (16 μs) – 8 μs—tau (16 μs). NMR data could not be obtained because the NMR spectrometer could not operate at temperatures above 430 K. The true temperature at spinning condition of 10 kHz was adjusted based on the sample temperature, suggested by Guan and Stark24. The temperature change was maintained within the error range of ± 0.5 K while adjusting nitrogen gas flow and heater current.

Experimental results

The X-ray powder diffraction pattern of the [(CH2)3(NH3)2]CuCl4 crystal at room temperature is displayed in Fig. 2, and this result was consistent with that reported by Czapla et al.23 The results of the DSC analysis of [(CH2)3(NH3)2]CuCl4 under a nitrogen atmosphere are shown in Fig. 3. An endothermic peak at 434 K and a exothermic peak at 539 K where observed. However, the peak around 334 K reported previously20 was not observed. To confirm that the DSC peaks at 434 K and 539 K were consistent with the structural phase transition, TGA was performed. The measured TGA curves are also shown in Fig. 3. Good thermal stability was observed up to around 480 K; above this temperature, the first signs of weight loss were observed, indicating the onset of partial thermal decomposition. The crystalline structure of the compound [(CH2)3(NH3)2]CuCl4 (M = 280.49 mg) breaks down at high temperatures. Considering the TGA results and possible chemical reactions, the solid residue amounts were calculated. The weight loss of 13% at around 539 K (see Fig. 3) was likely due to the decomposition of the HCl moieties, which is consistent with the exothermic peak at the same temperature in the DSC curve. The weight sharply decreased between 500 and 600 K, with a corresponding weight loss of 65% around 650 K. This result is consistent with previous TGA data23. Further, optical polarizing microscopy was used to understand the crystal’s phase transition, thermal decomposition, and melting mechanism. The color of the crystal was dark brown at room temperature, as illustrated in the inset of Fig. 3. While there were no changes observed from room temperature to 523 K, it began to melt slightly at approximately 539 K. Above 600 K, the crystal emitted an odour, and its surface and edges melted considerably (see Suppplementary Information 1).

Figure 2
figure 2

X-ray diffraction pattern of the [(CH2)3(NH3)2]CuCl4 crystal at 300 K.

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Figure 3
figure 3

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) thermogram of [(CH2)3(NH3)2]CuCl4 (Inset: photograph of the crystal at 300 K).

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The chemical shifts of the 1H NMR spectrum of [(CH2)3(NH3)2]CuCl4 crystals were obtained with increasing temperature, as shown in Fig. 4. Two peaks in the NMR spectra are indicated in the figure; the spinning sidebands for CH2 are represented with crosses, and those for NH3 are marked with open circles. At 300 K, the 1H NMR chemical shift for CH2 was observed at δ = 2.76 ppm, whereas that for NH3 was at δ = 11.48 ppm. Below 300 K, the signal for 1H of CH2 had very low intensity and could not be easily identified. The 1H peak for CH2 did not significantly change with increasing temperature, while for NH3, the change in the chemical shift was dependent on temperature (see the Supplementary Information 2).

Figure 4
figure 4

1H MAS NMR spectra for CH2 and NH3 of [(CH2)3(NH3)2]CuCl4 at various temperatures (spinning sidebands are indicated by cross and open circles).

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The 1H NMR spectra were also measured with several delay times, and the intensity of NMR spectra as a function of delay time followed a single exponential function. The rate of decay of the spin-locked proton magnetization is characterized by T1ρ25,26,27:

$${\text{I}}\left( t \right) \, = {\text{ I}}(0){\text{exp}}( - t/{\text{T}}_{{1\rho}} ),$$
(1)

where I(t) and I(0) are the signal intensity at time t and t = 0, respectively. The 1H NMR signals of CH2 and NH3 measured at 300 K were plotted as a function of delay time over the range of 0.2–80 ms, as shown in the inset of Fig. 5. It can be seen that the 1H NMR signal intensities varied with the delay time. From the slope of the intensity vs. delay time curve, 1H T1ρ values for [(CH2)3(NH3)2]CuCl4 were obtained from the CH2 and NH3 peaks as a function of inverse temperature. Changes in T1ρ values above TC1 were not observed outside this temperature because of the limitation of the NMR spectrometer. The 1H T1ρ values for CH2 and NH3 were of the order of 10 ms, and their values were almost independent of temperature (see Fig. 5).

Figure 5
figure 5

1H NMR spin–lattice relaxation times T1ρ for CH2 and NH3 ions of [(CH2)3(NH3)2]CuCl4 as a function of inverse temperature (inset: 1H NMR spectrum at several delay times at 300 K).

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The 13C NMR chemical shifts for CH2 in [(CH2)3(NH3)2]CuCl4 were measured as a function of temperature, as shown in Fig. 6. At all temperatures, the 13C MAS NMR spectra showed two resonance signals. The 13C MAS NMR spectrum for TMS was observed at 38.3 ppm at 300 K, which was used to calibrate the device to 0 ppm for determining the chemical shift in 13C28. Here, CH2 between CH2 and CH2 is named CH2-1, and CH2 close to NH3 is named CH2-2. At 300 K, the two resonance signals were recorded at chemical shifts of δ = 28.78 and δ = 124.97 ppm for CH2-1 and CH2-2, respectively. The 13C chemical shifts for CH2 were different CH2-1 far away from NH3 and CH2-2 close to NH3. The small 13C resonance peaks indicated by arrow at 420 K and 430 K were attributed to a splitting of the CH2-2. The 13C chemical shift for CH2-1 increased slowly and monotonously without an anomalous change with increasing temperature, whereas those for CH2-2 moved abruptly to the lower side with increasing temperature compared to CH2-1, as shown in the inset in Fig. 6.

Figure 6
figure 6

13C chemical shifts for CH2-1 and CH2-2 of (CH2)3(NH3)2CuCl4 as a function of temperature.

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The 13C full-width at half-maximum (FWHM) values of the NMR peaks for CH2-1 and CH2-2 decreased with increasing temperature. Broader line widths are observed for more rigid lattices, where motional narrowing is quenched, as shown by the increase in line widths at lower temperatures. The line widths of 13C for CH2-1 and CH2-2 were the same within experimental uncertainty, where the line width narrowed from 30 to 10 ppm with increasing temperature from 180 to 430 K, respectively (see the Supplementary Information 3).

The integration change of the 13C NMR spectra obtained by increasing the delay time was measured. All decay curves for CH2-1 and CH2-2 were described by a single exponential function, as shown by Eq. (1). 13C T1ρ values were measured by the spin-locking pulse sequence with a locking pulse of 75.76 kHz. From the slope of their recovery traces, the 13C T1ρ values were obtained for the CH2-1 and CH2-2 as a function of 1000/temperature, as shown in Fig. 7. Although no change in T1ρ values was observed near TC2, T1ρ values measured for 180–430 K indicated a much slower dynamics of carbon motion. The T1ρ vs. temperature curve showed minima of 16.32 and 18.87 ms for CH2-1 and CH2-2 at 200 K, respectively. This trend indicates that distinct molecular motions exist, where the minimum T1ρ was attributed to the uniaxial rotation of CH2 ions. The T1ρ values were described by the correlation time τC for molecular motion, based on the theory of Bloembergen–Purcell–Pound (BPP). The T1ρ value for the molecular motion is given by27,29:

$${\text{T}}_{{1\rho}}^{{ - {1}}} = {\text{ C }}(\gamma_{{\text{I}}}^{{2}} \gamma_{{\text{e}}}^{{2}} \mu_{{\text{B}}}^{{2}} {\text{S}}\left( {{\text{S}} + {1}} \right)/r^{{6}} )\left[ {{4}f_{{\text{a}}} + f_{{\text{b}}} + { 3}f_{{\text{c}}} + { 6}f_{{\text{d}}} + { 6}f_{{\text{e}}} } \right]$$
(2)

where fa = τC /[1 + ω12τC2], fb = τC /[1 + (ωC ‒ ωe)2τC2], fc = τC /[1 + ωC2τC2], fd = τC /[1 + (ωC + ωe)2τC2], and fe = τC /[1 + ωe2τC2]. Here, C is a coefficient, γe is the gyromagnetic ratio of the electron, S is the spin number of the paramagnetic ion, r is the distance between the paramagnetic ion and the carbon, ωe is the Larmor frequency of electron, and ω1 is the spin-lock field. When ωCτC = 1, T1ρ is at its minimum, so a relationship between T1ρ and ω1 was applied to obtain the coefficient in Eq. (2). Using this coefficient, we calculated τC as a function of temperature. According to BPP theory, the local field fluctuation is governed by the thermal motion of CH2-1 and CH2-2, which is activated by thermal energy. In this case, τC is described by Arrhenius behaviour: τC = τoexp(‒Ea/kBT), where τo, Ea, and kB are the pre-correlation time, activation energy of the motions, and Boltzmann constant, respectively27. As the magnitude of Ea depends on the molecular dynamics, we plotted τC vs. 1000/T on a logarithmic scale (inset of Fig. 7), which gave Ea values for CH2-1 and CH2-2 of 8.93 ± 0.54 and 6.85 ± 0.48 kJ/mol, respectively.

Figure 7
figure 7

13C NMR spin–lattice relaxation times T1ρ for CH2-1 and CH2-2 of [(CH2)3(NH3)2]CuCl4 as a function of inverse temperature (Inset: Correlation times for CH2-1 and CH2-2 in [(CH2)3(NH3)2]CuCl4 as a function of inverse temperature. Solid lines represent the activation energies).

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14N NMR investigations were performed using a [(CH2)3(NH3)2]CuCl4 single crystal over the temperature range of 180–430 K. The 14N spectra were obtained using the solid-state echo method by static NMR at a Larmor frequency of 28.90 MHz. Two 14N NMR signals were derived from the quadrupole interactions due to the spin number I = 1. Near 333 K (= TC2), the number of resonance lines and resonance frequency of the NMR spectrum showed abruptly changes, as shown in Fig. 8. Above TC2, the spectrum showed one pair of lines, whereas below TC2 it showed two pairs. The lines with the same colour below TC2 indicate the same pairs for 14N. The changes in the 14N resonance frequency as a function of temperature were attributed to variations in the structural geometry, corresponding to changes in the quadrupole coupling constant30,31. The resonance frequency of the 14N signals below TC2 changed almost continuously, and those of the 14N signal above this temperature also varied abruptly. Near TC2, the electric field gradient tensors at N sites varied, reflecting changes in the atomic configuration around the nitrogen atom. Although the phase transition temperature at TC2 reported previously20 was not observed in our DSC experimental results, the 14N NMR spectrum showed changes near TC2. The phase transition at TC2 exists, and 14N in the NH3 groups plays an import role in this phase transition. In contrast, the two different 14N spectra below TC2 are thought to have two inequivalent N sites or be due to twin domains. However, according to the previously reported X-ray results21, there have been no reports of two different N sites, and twin domains have been reported22. Czapla et al.22 suggested that the ferroelastic domains observed in the orthorhombic phase could be connected to a prototype tetragonal phase. Here, the [(CH2)3(NH3)2]CuCl4 crystal existed in three crystallographic phases: monoclinic (2/m) above 434 K, tetragonal (4/mmm) between 334 and 434 K, and orthorhombic (mmm) below 334 K. For the transition from the 4/mmm of the tetragonal phase to the mmm of the orthorhombic phase, the domain wall orientations were expressed as x = 0 and y = 0. According to Aizu32 and Sapriel33, the equations of the twin domain walls reflected the ferroelasticity of the 4/mmmFmmm. Hence, our results are thought to support the mechanism of ferroelastic twin domains. As a result, the separation of two 14N NMR lines into four 14N NMR lines under TC2 was due to the ferroelastic twin domain structure.

Figure 8
figure 8

Temperature dependences on 14N resonance frequency of [(CH2)3(NH3)2] CuCl4 single crystal (Inset: static 14N NMR spectrum at 300 K).

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Conclusion

To investigate the physical properties of [(CH2)3(NH3)2]CuCl4 perovskite crystals, we performed DSC, TGA, optical polarizing microscopy, and NMR spectroscopy. The structural roles of the [(CH2)3(NH3)2]+ cation in [(CH2)3(NH3)2]CuCl4 crystals were investigated by 1H MAS NMR, 13C CP/MAS NMR, and 14N static NMR as a function of temperature. The changes in chemical shifts in the 1H and 13C NMR spectra indicated changes in crystallographic symmetry. The NMR chemical shifts were related to the local field at the location of the resonating nucleus in the crystals. The 1H NMR chemical shift for NH3 changed more significantly with temperature than that of CH2 because being H-bonded, the 1H NMR chemical shift of the NH3 moiety is much more sensitive to temperature fluctuations, and varies significantly due to the variation in H-bond length with temperature. The 13C NMR chemical shift for CH2-1 increased slowly with increasing temperature, without any anomalous change. However, the shift for CH2-2, moved significantly to lower values with increasing temperature compared to CH2-1. The 13C NMR chemical shifts of CH2-2 closer to the N–H···Cl bonds were higher those of CH2-1. In addition, the abrupt change in the resonance frequency of the 14N nuclei observed near TC2 was attributed to a ferroelastic phase transition. The previously reported phase transition at TC220 was not observed in DSC, but the 14N NMR data clearly indicated changes in atomic configuration at this temperature. The NH3 groups are coordinated by N–H···Cl bonds; thus, atomic displacements with temperature in the environment of the 14N nuclei are correlated with CuCl4.

1H and 13C T1ρ have lower values in the order of milliseconds because T1ρ is inversely proportional to the square of the magnetic moment of paramagnetic ions. The T1ρ values for CH2 and NH3 protons were almost independent of temperature, but the CH2 moiety located in the middle of the N–C–C–C–N bond undergoes tumbling motion according to the BPP theory. The increase in 13C T1ρ at high temperatures may be simply due to the change in distance rather than the change in correlation time. More importantly, the total correlation time τC is dominated by the electric relaxation correlation time, rather than the rotational correlation time of the paramagnetic.