Organic–inorganic hybrid [NH₃(CH₂)₆NH₃]ZnBr₄ crystal: structural characterization, phase transitions, thermal properties, and structural dynamics

Abstract

Organic–inorganic hybrid [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were prepared by slow evaporation; the crystals had a monoclinic structure with space group P2₁/c and lattice constants a = 7.7833 Å, b = 14.5312 Å, c = 13.2396 Å, β = 90.8650°, and Z = 4. They underwent two phase transitions, at 370 K (T_C1) and 430 K (T_C2), as confirmed by powder X-ray diffraction patterns at various temperatures; the crystals were stable up to 600 K. The nuclear magnetic resonance spectra, obtained using the magic-angle spinning method, demonstrated changes in the ¹H and ¹³C chemical shifts were observed near T_C1, indicating changing structural environments around ¹H and ¹³C. The spin–lattice relaxation time, T_1ρ, increased rapidly near T_C1 suggesting very large energy transfer, as indicated by a large thermal displacement around the ¹³C atoms of the cation. However, the environments of ¹H, ¹⁴N, and C1 located close to NH₃ in the [NH₃(CH₂)₆NH₃] cation did not influence it significantly, indicating a minor change in the N–H⋯Br hydrogen bond with the coordination geometry of the ZnBr₄ anion. We believe that the information on the physiochemical properties and thermal stability of [NH₃(CH₂)₆NH₃]ZnBr₄, as discussed in this study, would be key to exploring its application in stable, environment friendly solar cells.

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1. Introduction

Solar cells based on organic–inorganic hybrid materials have been extensively studied. Typically, CH₃NH₃PbX₃ (X = Cl, Br, and I) thin-film photovoltaic devices have been used as solar cells;^1–6 however, these substances readily decompose in humid air and are toxic because of the presence of Pb. Therefore, it is necessary to develop eco-friendly hybrid perovskite solar cells. Recently, a new series of perovskite compounds [(CH₃)₂NH₂]Zn(HCOO)₃, consisting of an organic cation and a metal ion bridged by formate ions, has been reported.^7–13 They have the potential for application in novel memory storage and manipulation devices. In addition, research on type of organic–inorganic hybrid perovskites [NH₃(CH₂)_nNH₃]BX₄ (n = 2, 3, 4, …; B = Mn, Co, Cu, Zn, Cd; X = Cl, Br, I), with a focus on the optimization of the perovskite structure and dynamics investigation, is rapidly garnering attention. The physical properties of organic–inorganic hybrids depend on their organic cations, inorganic anion coordination geometry of the metal ions (BX₄²⁻ or BX₆²⁻), and halogen ions.^14–24 The organic cations of hybrid perovskites induce structural flexibility and nonlinear optical properties, whereas the inorganic anion is attributable for thermal and mechanical properties.^25,26 The crystal structures with B = Mn, Cu, and Cd consist of alternate octahedrons, (BX₆)²⁻, and organic chains, and is two-dimensional structure. The ammonium ion of the organic group is connected by a N–H⋯X hydrogen bond to the halide ion of the inorganic layer, rendering such structures good candidates for proton conduction.¹⁶ The isolated tetrahedral structures with B = Co and Zn are formed, where an inorganic layer of (BX₄)²⁻ is sandwiched between the organic cation, and zero-dimensional structure.^18,27–29 The crystal consists of unassociated tetrahedrally distorted (BX₄)²⁻ anions and cations linked by H bonds to X⁻ ions. Structural rearrangements due to conformational changes in the chains are important for long-chain alkylenediammonium materials [NH₃(CH₂)_nNH₃]BX₄, with n >> 4.³⁰ Among them, the organic–inorganic perovskite type [NH₃(CH₂)₆NH₃]ZnBr₄ (1, 6-hexanediammonium tetrabromozincate(II)), containing [NH₃(CH₂)₆NH₃] cations and layered ZnBr₄ anions (with Zn atoms surrounded by four Br atoms to form the ZnBr₄ tetrahedron), is an interesting hybrid material.

Ishiha and Horiuchi³¹ measured the ⁸¹Br nuclear quadrupole resonance (NQR) frequencies of [NH₃(CH₂)₆NH₃]ZnBr₄ below room temperature. Four resonance signals were assigned to the four bromine atoms in the tetrahedron. The N–H⋯Br hydrogen bonds between the cations and anions were considered based on these temperature dependences. In addition, the phase transition temperature, measured by differential scanning calorimetry (DSC), was not observed below room temperature, but was observed at 380 K and 424 K above room temperature; the melting point was 475 K.³¹ The crystal structure and electronic properties of [NH₃(CH₂)₆NH₃]ZnCl₄ similar to the [NH₃(CH₆)NH₃]ZnBr₄ crystal were reported by Mostafa and El-khiyami.¹⁶ Although the NQR data and phase transition temperatures have been reported, to the best of our knowledge, investigation of crystal structure, thermodynamic stability and structural dynamics with respect to temperature change has not been widely conducted.

This study is the first to investigate the crystal structures, phase transition temperature (T_C), and thermodynamic properties of [NH₃(CH₂)₆NH₃]ZnBr₄ crystals with zero-dimensional. Secondly, nuclear magnetic resonance (NMR) chemical shifts and spin–lattice relaxation times (T_1ρ) for ¹H and ¹³C were also measured to understand the coordination geometry and molecular dynamics of the organic [NH₃(CH₂)₆NH₃] cation near T_C. In addition, the effect of temperature on the static ¹⁴N NMR spectra was investigated to elucidate the atomic configurations of the cation. The change in the coordination geometry in response to temperature change was explained by the changes of N–H⋯Br hydrogen bonds between the cations and tetrahedral ZnBr₄ anions. The investigation of the crystal structures and physicochemical properties of the phase transition mechanism conducted herein will expand the application scope of [NH₃(CH₂)₆NH₃]ZnBr₄ crystals in environmentally friendly solar cells.

2. Experimental

To obtain [NH₃(CH₂)₆NH₃]ZnBr₄ single crystals, an aqueous solution containing NH₂(CH₂)₆NH₂·2HBr (Aldrich, USA) and ZnBr₂ (Aldrich, 99.99%, USA) in 1 : 1 ratio was slowly evaporated in a thermostat at 300 K. Colorless single crystals (10 × 3 × 2 mm) were grown for approximately five weeks. The single crystal grown here was colorless and transparent, with a long rectangular shape.

Fourier transformation infrared (FT-IR) spectra in the 4000–500 cm⁻¹ range were measured using an FT-IR spectrometer (PerkinElmer, L1600300) with a compressed KBr pellet.

The lattice parameters at various temperatures were determined by single-crystal X-ray diffraction (XRD) at the Seoul Western Center of the Korea Basic Science Institute (KBSI). A colorless crystal block was picked up with paratone oil and mounted on a Bruker D8 Venture PHOTON III M14 diffractometer equipped with a graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation source and a nitrogen cold stream (−50 °C). Data was collected and integrated using SMART APEX3 (Bruker, 2016) and SAINT (Bruker, 2016). The absorption was corrected by a multi-scan method implemented in SADABS. The structure was solved using direct methods and refined by full-matrix least-squares on F² using SHELXTL.³² All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were added to their geometrically ideal positions. Additionally the powder XRD patterns of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were measured at several temperatures using an XRD system with a Mo-Kα radiation source.

The DSC experiments were performed using a DSC 25 apparatus (TA Instruments, USA) at heating and cooling rates of 10 K min⁻¹ from 200 to 570 K in a nitrogen gas atmosphere.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were performed on a thermogravimetric analyzer (TA Instruments, USA) at the heating rate identical to that utilized for DSC at 300–873 K under nitrogen gas.

The NMR chemical shifts of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were measured using a Bruker 400 MHz Avance II+ NMR spectrometer (Bruker, Germany) at KBSI. The Larmor frequency for ¹H magic-angle spinning (MAS) NMR was ω_o/2π = 400.13 MHz, and that for the ¹³C MAS NMR experiment was ω_o/2π = 100.61 MHz. To minimize spinning sidebands, the MAS speeds for ¹H and ¹³C were measured at 5 kHz. Tetramethylsilane (TMS) was used as a reference material for accurate NMR chemical shift measurements. The spin–lattice relaxation time, T_1ρ, values were obtained using a π/2–τ pulse, followed by a spin-lock pulse of duration τ, and the π/2 pulse widths for ¹H and ¹³C were measured by a previously published method.³³ Static ¹⁴N NMR resonance frequencies were recorded with a Larmor frequency of ω_o/2π = 28.90 MHz using the one-pulse method and NH₄NO₃ was used as the reference material. The temperature during the NMR experiment ranged from 180 to 430 K; owing to the limitations of the instrument, measurements at higher temperatures were not possible.

3. Results and discussion

3.1. FT-IR spectra

The FT-IR spectrum within the 4000–500 cm⁻¹ range was recorded at room temperature. The result is shown in Fig. 1, and the peak near 3128 cm⁻¹ is assigned to the C–H mode. And, the peak at 3058 cm⁻¹ is due to the N–H⋯Br hydrogen bond, and those at 1577 and 1469 cm⁻¹ correspond to the asymmetric deformation of NH₃ and symmetric deformation of NH₃, respectively. The peaks near 1025 and 973 cm⁻¹ are defined to the C–N and C–C mode.

Fig. 1 FT-IR spectrum of [NH₃(CH₂)₆NH₃]ZnBr₄ at room temperature.

3.2. Crystal structure and phase transition

Single-crystal XRD patterns for [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were obtained at several temperatures. At 300 K, the hybrid was found to have crystallized as a monoclinic system with a P2₁/c space group and had lattice constants a = 7.7833 (2) Å, b = 14.5312 (4) Å, c = 13.2396 (3) Å, β = 90.8650 (1)°, and Z = 1. Table 1 lists single-crystal XRD and refinement data of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystal, and Fig. 2 shows its structure. The atomic numbering scheme and thermal ellipsoids for the H atoms are shown in Fig. 3, and their bond lengths and angles are summarized in Table 2. The Zn atom is coordinated by four Br atoms, forming a nearly regular tetrahedron of ZnBr₄. The hydrogen atoms of each formula unit are able to form hydrogen bonds N–H⋯Br. The changes in the lattice parameters at 230, 250, 270, 300, and 350 K are shown in Fig. 4, where a, b, and c have different thermal expansion upon with increasing temperature and β increases slightly with increasing temperature.

Table 1 Crystal data and structure refinement for [NH₃(CH₂)₆NH₃]ZnBr₄ at 300 K. The full data are available in the CIF files

Chemical formula	C6H18N2ZnBr4
Weight	503.23
Crystal system	Monoclinic
Space group	P21/c
T (K)	300
a (Å)	7.7833
b (Å)	14.5312
c (Å)	13.2396
β (°)	90.8650(10)
Z	4
V (Å^3)	1497.24
Radiation type	Mo-Kα
Wavelength (Å)	1.71073
Reflections collected	27 490
Independent reflections	3572 (Rint = 0.0452)
Goodness-of-fit on F^2	1.047
Final R indices [I > 2sigma(I)]	R1 = 0.0300, wR2 = 0.0633
R indices (all data)	R1 = 0.0421, wR2 = 0.0677

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Fig. 2 Crystal structure of [NH₃(CH₂)₆NH₃]ZnBr₄ at 300 K.

Fig. 3 Thermal ellipsoid plot (50% probability) for structure of [NH₃(CH₂)₆NH₃] ZnBr₄.

Table 2 Bond-lengths (Å) and bond-angles (°) at 300 K

Bond-length (Å) and bond-angle (°)
Br(1)–Zn(1)	2.4139 (5)	Br(3)–Zn(1)–Br(1)	110.07 (2)
Br(2)–Zn(1)	2.4333 (5)	Br(3)–Zn(1)–Br(4)	111.31 (2)
Br(3)–Zn(1)	2.3822 (6)	Br(1)–Zn(1)–Br(4)	108.08 (2)
Br(4)–Zn(1)	2.4264 (5)	Br(3)–Zn(1)–Br(2)	112.00 (2)
		Br(1)–Zn(1)–Br(2)	107.482 (19)
		Br(4)–Zn(1)–Br(2)	107.73 (2)
N(1)–C(1)	1.487 (5)
C(1)–C(2)	1.523 (5)
C(2)–C(3)	1.522 (5)
C(3)–C(3)	1.507 (7)

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Fig. 4 The lattice parameters a, b, c, and β of [NH₃(CH₂)₆NH₃]ZnBr₄ crystal according to the temperatures.

The DSC analysis was performed on the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals at a heating rate of 10 K min⁻¹. Two strong endothermic peaks at 370 and 476 K and a weak peak at 430 K were observed (Fig. 5). A very small peak was also observed at 250 K. The phase transition temperatures were defined as T_C1 = 370 K and T_C2 = 430 K; 476 K was defined as the melting temperature, T_m. An analysis of the lattice parameters indicated that the peak at 250 K was independent of the phase transition temperature. According to the result previously reported by Ishiha and Horiuchi,³¹ [NH₃(CH₂)₆NH₃]ZnBr₄ undergoes structural phase transitions at 389 and 424 K, whereas no phase transition was observed below room temperature. The T_m was reported to be 475 K. The phase transition temperatures obtained in this study were 370 and 430 K, which were slightly different from the previously reported results,³¹ however, the T_m was remarkably similar. The slight differences in phase transition temperatures may vary depending on the heating rate in the DSC experiment and may also show some differences depending on the growth conditions of the crystal.

Fig. 5 DSC curve of [NH₃(CH₂)₆NH₃]ZnBr₄ crystal measured at heating rate of 10 K min⁻¹.

To further confirm the phase transitions, powder XRD patterns of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were obtained after heating; the results in the measuring range (2θ) of 5–50° are shown in Fig. 6.

Fig. 6 Powder XRD patterns of [NH₃(CH₂)₆NH₃]ZnBr₄ at 300, 380, 440, and 460 K.

The XRD pattern at 300 K (olive) was slightly different from that recorded at 380 K (red); this difference is related to T_C1. Further, the XRD pattern recorded at 380 K was different from those recorded at 440 K and 460 K exhibited a clear change in structure, which is attributed to T_C2. The phase transition temperatures shown in the XRD results are in reasonable agreement with the endothermic peaks in the DSC curves.

3.3. Thermal properties

The TGA and DTA results measured at a heating rate of 10 K min⁻¹ are shown in Fig. 7. As the temperature increased, the molecular weight of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals decreased. The molecular weight loss began at approximately 600 K, which was defined as the partial thermal decomposition temperature, T_d, where at 5% weight loss occurred. From the total molecular weight of 503.23 mg, the residual amounts after the decomposition of HBr and 2HBr were calculated using eqn (1) and (2), respectively:³³

{[NH₂(CH₂)₆NH₂·HBr]ZnBr₃ + HBr (g)}/[NH₃(CH₂)₆NH₃]ZnBr₄ = 92.75% (1)

{[NH₂(CH₂)₆NH₂]ZnBr₂ + 2HBr (g)}/[NH₃(CH₂)₆NH₃]ZnBr₄ = 85.51% (2)

Fig. 7 TGA and DTA curves of [NH₃(CH₂)₆NH₃]ZnBr₄.

Thus, molecular weight losses of 7% and 14% were due to the decomposition of HBr and 2HBr, respectively, and the decomposition temperatures of HBr and 2HBr obtained by TGA were 604 and 620 K, respectively. The 55% weight loss was mainly attributed to organic decomposition. The mass rapidly decreased in the 600–750 K range, with a corresponding mass loss of 95% near 750 K. Further, the endothermic peak at 476 K, obtained from DSC, was confirmed by a polarizing microscope, which showed that the single crystal had started to melt at 470 K; thus, the endothermic peak at 476 K was denoted as the melting temperature.

3.4. ¹H NMR chemical shifts and spin–lattice relaxation times

Initially, the ¹H NMR chemical shifts of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals with spinning speeds of 5 and 10 kHz at 300 K were measured (ESI 1†). However, the chemical shifts measured at the two different spinning rates were observed at the same position; thus, NMR chemical shifts with respect to temperature change were measured at 5 kHz only. The in situ ¹H chemical shifts according to temperature change are shown in Fig. 8(a), and the change in chemical shifts is shown in Fig. 8(b) for better understanding. Below 260 K, six resonance lines were observed instead of the two ¹H signals expected from NH₃ and CH₂. NH₃ denotes ¹H in NH₃, and the remaining four or five signals appeared on the right side represent ¹H in the six CH₂. At 300 K, the ¹H chemical shift for NH₃ was recorded at 7.27 ppm, and those for the CH₂ were obtained at 4.32, 3.42, 2.01, and 1.69 ppm. Two ¹H chemical shifts at approximately 4.5–5.5 ppm overlap into one signal at 260 K; in addition, two ¹H chemical shifts near 3.5 ppm overlap into one signal at 400 K. Below T_C1, one ¹H peak was observed for H around C3 at the center of cation as shown in the structure in Fig. 3, and four ¹H peaks were observed for H around C2 and C1 with different surrounding environments. However, above T_C1, the symmetry around ¹H improved such that one ¹H peak was observed for each of C1, C2, and C3. The ¹H chemical shifts for NH₃ are continuous regardless of T_C1, as expressed by the different colors in each phase (Fig. 8(a)), while those for CH₂ show a slight change near T_C1.

Fig. 8 (a) In situ ¹H MAS NMR spectra of NH₃ and CH₂ in [NH₃(CH₂)₆NH₃]ZnBr₄ according to the temperature change. (b) ¹H MAS NMR chemical shifts of NH₃ and CH₂ in [NH₃(CH₂)₆NH₃]ZnBr₄ according to the temperature change.

The ¹H MAS NMR spectra were measured by changing the delay time at each temperature, and the plots of spectral intensity as a function of delay times were expressed as mono-exponential curves.

The recovery traces of the magnetization were characterized by the spin–lattice relaxation time, T_1ρ, according to eqn (3):^29,34–36

I_H(τ) = I_H(0)exp(−τ/T_1ρ) (3)

where I_H(τ) and I_H(0) are the signal intensities for the protons at time τ and τ = 0, respectively. The ¹H T_1ρ values were determined for NH₃ and CH₂ from eqn (3); the ¹H T_1ρ results as a function of inverse temperature are shown in Fig. 9. The ¹H T_1ρ values were strongly dependent on temperature change, in the order of 40–450 ms. As the temperature increased, the ¹H T_1ρ values of NH₃ rapidly increased from 35 ms at 180 K to 254 ms at 320 K. Near T_C1, T_1ρ shows a continuous change, similar to the change in the ¹H chemical shift near T_C1. Moreover, T_1ρ follows the trend of the Bloembergen–Purcell–Pound (BPP) phenomenon, indicating molecular motion above 320 K. The ¹H T_1ρ values for CH₂ showed a similar trend to that of the ¹H values for NH₃.

Fig. 9 ¹H NMR spin–lattice relaxation times of NH₃ and CH₂ in [NH₃(CH₂)₆NH₃]ZnBr₄ as a function of inverse temperature.

3.5. ¹³C NMR chemical shifts and spin–lattice relaxation times

The ¹³C chemical shifts in the MAS NMR spectra with respect to increasing temperature are shown in Fig. 10. The ¹³C chemical shift for TMS at 300 K was recorded at 38.3 ppm, and this value was set as the standard reference. In the [NH₃(CH₂)₆NH₃] cation structure (inset of Fig. 10), CH₂ close to NH₃ at both ends of the cation was labeled C1, CH₂ at the center of the six CH₂ chains was labeled C3, and CH₂ between C3 and C1 was labeled C2. From 180 to 350 K, the chemical shifts showed a slight change with temperature, while those near T_C1 abruptly changed. At 300 K, the ¹³C chemical shifts were, 41.69 and 39.86, 29.55 and 27.56, and 24.94 and 20.50 ppm for C1, C2, and C3, respectively. The ¹³C chemical shifts at 360 K were recorded at 42.48 and 41.36 ppm for C1, 27.93 and 27.00 ppm for C2, and 26.01 ppm for C3. All ¹³C chemical shifts for C1, C2, and C3 in the cation rapidly changed near T_C1, indicating that the structural environment around ¹³C changed near T_C1. The ¹³C peaks corresponding to the chemical shifts for C1, C2, and C3 (Fig. 10) below T_C1, including those on the opposite side of C3 at the center of cation, were observed far away from each other. In contrast, the ¹³C peaks for C1, C2, and C3 above T_C1 were found to be close to each other. This result implies that the symmetry of the environment around C1, C2, and C3 above T_C1 is improved as indicated by the ¹H chemical shifts.

Fig. 10 In situ ¹³C MAS NMR spectra of [NH₃(CH₂)₆NH₃]ZnBr₄ at several temperatures (inset: structure of cation).

The ¹³C MAS NMR spectra measured the change in the intensities with increasing delay time at a spinning rate of 5 kHz at each temperature. The recovery traces of the ¹³C nuclei for the delay times ranging from 0.1 to 70 ms at 300 K are represented in the inset of Fig. 11. All decay curves for C1, C2, and C3 were described by a mono-exponential function, and the ¹³C T_1ρ values from the slopes of their recovery traces were obtained as a function of 1000/T (Fig. 11). The ¹³C T_1ρ values rapidly decreased with increasing temperature, while the values near T_C1 increased 10 times in 5 ms, similar to the rapid change in the ¹³C chemical shifts. Here, the T_1ρ values for C1, C2, and C3 were similar within the error range.

Fig. 11 ¹³C NMR spin–lattice relaxation times of C1, C2, and C3 in [NH₃(CH₂)₆NH₃]ZnBr₄ as a function of inverse temperature (inset: the ¹³C recovery traces according to the delay times between 0.1 and 70 ms at 300 K).

3.6. Static ¹⁴N resonance frequency

The static NMR spectra for ¹⁴N at both ends of the cation in the [NH₃(CH₂)₆NH₃]ZnBr₄ single crystals are shown inset of Fig. 12 (circled in red). The spectra in the temperature range of 180–400 K were obtained, and the direction of the applied magnetic field was measured with respect to the arbitrary direction of the single crystal. ¹⁴N has a spin number of 1, and two resonance signals are expected by the quadrupole interaction.³³ The ¹⁴N resonance frequency was very low at 28.90 MHz, and it was not easy to obtain a resonance signal by wiggling of the base line. However, because the intensity was small and the line width was relatively broad, it was difficult to distinguish the signals (inset of Fig. 12). The resonance frequencies for the ¹⁴N NMR spectra are shown in Fig. 12 at several temperatures. It can be seen that two resonance lines slightly increased with increasing temperature. The same pairs for ¹⁴N are indicated by symbols of the same color. However, the ¹⁴N signals were difficult to observe at temperatures above 350 K. In the vicinity of T_C1, the line width of the ¹⁴N signal rapidly widened, making it difficult to detect. The continuous change in the ¹⁴N resonance frequency with temperature change indicated a change in the coordination geometry of the environment around N, implying a change in the quadrupole coupling constant, e²qQ/h.

Fig. 12 ¹⁴N resonance frequency of [NH₃(CH₂)₆NH₃]ZnBr₄ as a function of temperature (inset: structure of cation, the ¹⁴N NMR spectrum at 330 K).

4. Conclusions

The crystal structures, phase transitions, structural geometries, thermal stabilities, and molecular dynamics of the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals were investigated through XRD, DSC, TGA, and NMR experiments. It was discovered that the crystals belong to a monoclinic system with a P2₁/c space group at 300 K, and the lattice constants are a = 7.7833 Å, b = 14.5312 Å, c = 13.2396 Å, β = 90.8650°, and Z = 4. The crystals undergo two phase transitions, at 370 K (T_C1) and 430 K (T_C2), as determined by their powder XRD patterns. Our results showed that the thermal property is stable, with a thermal decomposition temperature of approximately 600 K. In the NMR spectra, the changes in the ¹H and ¹³C chemical shifts were observed near T_C1, indicating that the structural environment around ¹H and ¹³C changed. This further suggests that the energy transfer above T_C1 is very large, indicated by the large thermal displacement around the ¹³C atoms. The influence of ¹H, ¹⁴N, and C1 located close to NH₃ in the [NH₃(CH₂)₆NH₃]ZnBr₄ crystals was not significant, indicating a minor change in the N–H⋯Br hydrogen bond related to the coordination geometry of the ZnBr₄ anion.

This material is lead-free, environmentally friendly, and stable at relatively high temperature; therefore, it is potentially applicable in solar cells.

Author contributions

A. R. Lim performed NMR experiments and wrote the manuscript. H. Ju performed XRD experiment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

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 (2018R1D1A1B07041593 and 2016R1A6A1A03012069).

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Footnote

† Electronic supplementary information (ESI) available: XRD data, bond distances and angles, and hydrogen-bond geometrics of the crystal structure. CCDC 2169730. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ra04834e

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