Crystal growth, phase transition, and nuclear magnetic resonance of organic–inorganic hybrid perovskite NH₂(CH₃)₂CdCl₃

Abstract

Understanding the physicochemical properties of organic–inorganic hybrid materials is essential to promote their applications. In this study, a single crystal of NH₂(CH₃)₂CdCl₃ was grown, and it exhibited a monoclinic structure. Its phase transition temperatures were 460 and 470 K, and it showed sufficient thermal stability. The changes in the NMR chemical shifts of each atom in the crystal with increasing temperature were determined; the chemical shift of ¹H of NH₂ in the NH₂(CH₃)₂ cation changed with temperature, which was correlated to the changes in the chemical shift of ¹⁴N in NH₂. The change in ¹¹³Cd chemical shifts indicate the change of six Cl atoms around Cd in CdCl₆. Therefore, the change in the coordination geometry of CdCl₆ is attributed to the change in the N–H⋯Cl hydrogen bond between the NH₂(CH₃)₂ cation and CdCl₆ anion. In addition, the ¹³C activation energies E_a obtained from the spin-lattice relaxation time T_1ρ values are smaller than those of the ¹H E_a values, suggesting that is free compared to ¹H in the cation. We believe that this study furthers our fundamental understanding of organic–inorganic hybrid materials to promote their practical solar cell applications.

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1. Introduction

Organic–inorganic hybrid perovskite crystals based on metal halides have received increasing attention owing to their interesting optoelectronic, electrical, and magnetic properties.^1–5 The most common perovskite MAPbX₃ (MA = CH₃NH₃, X = I, Br, or Cl) based thin-film photovoltaic devices are used as solar cells.^6–14 However, MAPbX₃-based photovoltaics are highly unstable under ambient conditions, especially in the presence of moisture, and are highly toxic owing to Pb bioaccumulation. Recently, researchers have suggested substituting Pb with other low-toxicity or eco-friendly metals to develop Pb-free photovoltaic devices. Free-Pb [NH₃(CH₂)_nNH₃]BX₄ (n = 1, 2, 3…, B = Mn, Co, Cu, Zn, or Cd) and [C_nH_2n+1NH₃]₂BX₄ are organic–inorganic hybrids that have recently attracted considerable attention.^15–27 In addition, [NH₂(CH₃)₂]₂BX₄ and NH₂(CH₃)₂BX₃ are MA₂BX₄- and MABX₃-type complexes, where MA⁺ (= NH₂(CH₃)₂) is a univalent cation.^28–34 Here, the organic group of the organic–inorganic hybrid material determines the optical properties and structural flexibility of the material, whereas the inorganic group affects the mechanical and thermal properties. Hence, these organic–inorganic materials constitute an important area of research in the field of materials science.^35,36

X-ray studies on dimethylammonium trichlorocadmium crystals, NH₂(CH₃)₂CdCl₃, revealed their monoclinic structure with a P2₁/c space group and the lattice parameters a = 8.7377 Å, b = 13.2709 Å, c = 6.6910 Å, β = 98.371°, and Z = 4 at room temperature.³⁷ The structural unit is formed by the organic [NH₂(CH₃)₂]⁺ and the inorganic [CdCl₆]⁻ moieties. The infinite chains consist of face-sharing CdCl₆ octahedra, resulting in a 1D perovskite structure of the MABX₃ family, where MA = NH₂(CH₃)₂, B = Cd, and X = Cl. The six doubly bridging chloride ions link the adjacent Cd centers via Cd–Cl bonds.

Moreover, Kalthoum et al.³⁷ investigated the application of NH₂(CH₃)₂CdCl₃, which has a large dielectric constant, in field-effect transistors. They also reported the optical properties of NH₂(CH₃)₂CdCl₃ for solar cell applications.³⁸ And, the effects for Zn²⁺ and Cu²⁺ doping of NH₂(CH₃)₂CdCl₃ for white-light emission and green solar cell applications were investigated by Jellai et al.³⁹

The minimum of nuclear magnetic resonance (NMR) spin-lattice relaxation time in the rotating frame (T_1ρ) depends on the frequency ω₁ rather than ω_o. Therefore, the minimum T_1ρ is observed at a lower temperature than the spin–lattice relaxation time in the laboratory frame (T₁). T_1ρ provides information on motion in the kHz region, and T₁ provides information on rapid motion in the order of tens or hundreds of MHz.⁴⁰

In this study, among the perovskite types applied to solar cell applications, single crystals of Pb-free and thermal stable materials are grown and their physicochemical properties are studied. Therefore, we grew a NH₂(CH₃)₂CdCl₃ single crystal via an aqueous solution method and analyzed its structure by single-crystal X-ray diffraction (SCXRD). The phase transition temperature (T_C) of this crystal was determined using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). Thermogravimetric analysis (TGA) was performed to determine the thermodynamic properties of the material. The structural environments of ¹H, ¹³C, ¹⁴N, and ¹¹³Cd in [NH₂(CH₃)₂] cation and CdCl₆ anion were analyzed based on the chemical shifts observed in magic angle spinning (MAS) NMR and static NMR spectra. Moreover, we determined the T_1ρ, which represents the energy transfer around the ¹H and ¹³C atoms of the cation, and activation energies (E_a). These physicochemical properties can provide important insights into the mechanisms of compound for potential applications in solar cell.

2. Experimental

2.1. Crystal growth

NH₂(CH₃)₂CdCl₃ single crystals were grown from a mixture of a 1 : 1 ratio of NH₂(CH₃)₂Cl (99%; Sigma-Aldrich) and CdCl₂ (99.99%; Sigma-Aldrich) in an aqueous solution. The mixture was heated and stirred to form a saturated solution. After filtering the prepared solution through a filter, the colorless and transparent single crystals were grown for a few weeks via slow evaporation at a constant temperature of 300 K.

2.2. Characterization

Lattice constants were obtained at 200 K and 300 K using the SCXRD instrument at the Seoul Western Center of the Korea Basic Science Institute (KBSI). A single crystal in Paratone oil was placed on a Bruker diffractometer (D8 Venture PHOTON III M14) equipped with a graphite-monochromated Mo-Kα radiation source (λ = 0.71073 Å) and a N₂ cold flow (−50 °C). The data were collected and integrated using SMART APEX3 (Bruker, 2016) and SAINT (Bruker, 2016). The absorption was corrected using the multiscan method implemented in SADABS. This structure was analyzed by full-matrix least-squares on F² using the SHELXTL software.⁴¹ All hydrogen atoms are represented by geometric positions. In addition, the PXRD patterns of the NH₂(CH₃)₂CdCl₃ were obtained at various temperatures with a Mo-Kα target.⁴²

DSC measurements (DSC 25, USA) were acquired in the range of 200–573 K at a heating rate of 5 °C min⁻¹ and 10 °C min⁻¹ under N₂ gas, respectively.

TGA (SDT 0650-0439, USA) and differential thermal analysis (DTA) were performed in the range of 300–873 K at a heating rate of 10 °C min⁻¹ under N₂ gas.

The ¹H MAS and ¹³C cross-polarization (CP) MAS NMR spectra of NH₂(CH₃)₂CdCl₃ crystals were recorded at the Larmor frequencies of 400.13 and 100.61 MHz using a solid-state 400 MHz NMR spectrometer (AVANCE III+, Bruker, Germany) at the KBSI Seoul Western Center. The samples in the cylindrical zirconia rotors were spun at a rate of 10 kHz for the MAS NMR measurements to minimize the spinning sideband. Adamantane and tetramethylsilane (TMS) were selected as the internal standards for ¹H and ¹³C NMR, respectively. 1D NMR spectrum for ¹H and ¹³C was performed in the delay time of 1.5–15 s. The ¹H T_1ρ values were measured using π/2–τ spin-lock pulse for a duration of τ, and the π/2 pulse width was 4 μs. And, the ¹³C T_1ρ values were measured by varying the duration of a ¹³C spin-locking pulse applied after the CP preparation period. The ¹³C T_1ρ values were obtained using CP-τ acquisition. Static ¹⁴N NMR spectra of the NH₂(CH₃)₂CdCl₃ crystal were recorded using the one-pulse method with a Larmor frequency of 28.90 MHz, where NH₄NO₃ was the internal standard. Static ¹¹³Cd NMR spectra were recorded at a Larmor frequency of 88.75 MHz, where CdCl₂O₈·6H₂O was the internal standard. The NMR experiment above 430 K due to the limitations of the NMR instrument was not possible. The temperature was maintained nearly constant within the error range of ±0.5 °C, even when the rate of N₂ gas flow and the heater current were adjusted.

3. Results and discussion

3.1. Single crystal XRD

The SCXRD results at 200 K and 300 K showed that NH₂(CH₃)₂CdCl₃ crystallized into a monoclinic system with a P2₁/c space group. The cell constants were as follows: a = 8.6891 (13) Å, b = 13.212 (2) Å, c = 6.6851 (10) Å, β = 98.362° (2), and Z = 4. Fig. 1 shows the structure of the NH₂(CH₃)₂CdCl₃ crystal at 200 K, and the SCXRD data at 200 K and 300 K are presented in Table 1. The structure constitutes an organic [NH₂(CH₃)₂]⁺ cation and inorganic [CdCl₆]⁻ anion. The infinite chains consisted of face-shared CdCl₆ octahedra and six doubly bridging Cl⁻ ions linked to adjacent Cd centers. The bond lengths and angles are summarized in Table 2. Moreover, N–H⋯Cl hydrogen bonds connected the [NH₂(CH₃)₂]⁺ cations to the [CdCl₆]⁻ anions. Here, the N–H⋯Cl hydrogen bond consists of an angle greater than 120° as shown in Table 2.⁴³

Fig. 1 Crystal structure of NH₂(CH₃)₂CdCl₃ at 200 K (A.R. Lim CCDC 2258449).

Table 1 Crystal data and structure refinement for NH₂(CH₃)₂CdCl₃ at 200 K and 300 K. The full data available in the CIF files

Chemical formula	C2H8NCdCl3	C2H8NCdCl3
Weight	264.84	264.84
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	P21/c
T (K)	200	300
a (Å)	8.6891 (13)	8.7426 (5)
b (Å)	13.212 (2)	13.2834 (8)
c (Å)	6.6851 (10)	6.6950 (3)
β (°)	98.362 (2)	98.362 (2)
Z	4	4
V (Å^3)	759.0 (2)	769.23 (7)
Radiation type	Mo-Kα	Mo-Kα
Wavelength (Å)	0.71073	0.71073
Reflections collected	14 048	13 392
Independent reflections	1871 (Rint = 0.0582)	1932 (Rint = 0.0925)
Goodness-of-fit on F^2	1.078	1.079
Final R indices [I > 2sigma(I)]	wR1 = 0.0234, wR2 = 0.0615	wR1 = 0.0884, wR2 = 0.2946
R indices (all data)	wR1 = 0.0244, wR2 = 0.0621	wR1 = 0.0936, wR2 = 0.3043

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Table 2 Bond-lengths (Å) and bond-angles (°) for NH₂(CH₃)₂CdCl₃ at 200 K and 300 K

Temperature	200 K	300 K
Cd–Cl(1)	2.6062 (6)	2.5927 (18)
Cd–Cl(2)	2.5872 (6)	2.6076 (19)
Cd–Cl(3)	2.6613 (6)	2.659 (2)
Cd–Cl(1)#2	2.6788 (6)	2.6803 (18)
Cd–Cl(2)#1	2.6223 (6)	2.6226 (19)
Cd–Cl(3)#1	2.6860 (6)	2.6933 (19)
N(1)–C(1)	1.472 (4)	1.456 (12)
N(1)–C(2)	1.479 (3)	1.463 (13)
N(1)–H(1AN)	0.9100	0.8900
N(1)–H(1BN)	0.9100	0.8900
C(1)–H(1)	0.9800	0.9600
C(2)–H(2)	0.9800	0.9600
Cl(2)–Cd–Cl(1)	99.17 (2)	98.83 (6)
Cl(2)–Cd–Cl(3)	84.59 (2)	84.67 (6)
Cl(1)–Cd–Cl(3)	176.126 (18)	176.38 (6)
H(1AN)–Cl(2)	2.792	2.697
H(1BN)–Cl(3)	2.551	2.590
N(1)–H(1AN)–Cl(2)	122.09	141.02
N(1)–H(1BN)–Cl(3)	148.93	150.24

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3.2. Phase transition temperatures and thermal properties

DSC results were obtained in the temperature range of 200–573 K at a heating rate of 5 and 10 °C min⁻¹ by putting powder sample of 15.2 mg and 5.7 mg in a capsule, respectively. Two weak endothermic peaks at 461 and 470 K and one strong endothermic peak at 549 K were shown in Fig. 2. The enthalpies for the three observed peaks in the heating rate of 10 °C min⁻¹ were 20.44, 73.16, and 690.68 kJ mol⁻¹.

Fig. 2 Differential scanning calorimetry curves of NH₂(CH₃)₂CdCl₃ measured with the heating rates of 5 and 10 °C min⁻¹ (inset: thermogravimetry and differential thermal analysis curves of NH₂(CH₃)₂CdCl₃).

In addition, TGA experiment with the amount of 8.35 mg was conducted at a heating rate of 10 °C min⁻¹ the same as the DSC experiment, and the results are shown in Fig. 2 in detail. The crystal was thermally stable up to 524 K with a weight loss of ∼2%. The endothermic peak at ∼547 K confirms the peak observed in the DSC curve. Moreover, the fact that the crystal became opaque at ∼550 K suggests that ∼550 K is the decomposition temperature (T_d) of the crystal. From the total molecular weight of 264.86 mg, the amounts of residual produced by the decomposition of HCl and [NH₂(CH₃)₂Cl] were considered. The molecular weight losses of 14 and 31% were due to the decomposition of HCl and [NH₂(CH₃)₂Cl], respectively. As the two step decomposition processes, the first undergo a weight loss of 14% near 573 K, and the second undergo a weight loss of 31% near 784 K.

In order to accurately confirm the three peaks shown in the DSC results of Fig. 2, we observed the changes in the single crystal with temperature using an optical polarization microscope. No significant changes were observed when the temperature was increased from 300 to 500 K, following which the crystal gradually became opaque at higher temperatures. The single crystal became completely opaque at ∼550 K and melted at 600 K.

PXRD patterns were acquired at different temperatures in the 2θ range of 8–60° as shown in Fig. 3. The PXRD patterns obtained at 300, 350, and 400 K (blue color) of phase III differ slightly from that recorded at 465 K (red color) of phase II; this difference is associated with T_C2 (= 461 K). Furthermore, the PXRD pattern recorded at 465 K of phase II differs from those recorded above 480 K (olive color) of phase I, indicating a clear change in T_C1 (= 470 K). These results are consistent with those of the DSC experiments. Additional, the theoretical XRD pattern based on the cif file at 300 K is shown in Fig. 3, which agrees well with the experimental pattern. The peaks observed in this diffractogram are indexed with Mercury program.

Fig. 3 Powder X-ray diffraction patterns of NH₂(CH₃)₂CdCl₃ at phases III, II, and I. 300 K (cal.) is the theoretical XRD pattern at 300 K.

Therefore, the DSC, PXRD, and polarizing microscopy experiments suggest that the phase transition temperatures were T_C2 = 461 K and T_C1 = 470 K, whereas the decomposition temperature was T_d = 549 K.

3.3. ¹H and ¹³C MAS NMR chemical shifts

Two ¹H NMR spectra by NH₂ and CH₃ in NH₂(CH₃)₂CdCl₃ should be obtained, but only one signal was obtained, and these results are shown in Fig. 4(a). At low temperatures, the two ¹H NMR signals completely overlapped. The sideband of the ¹H NMR spectrum is marked by *. As the temperature increased, they separated slightly.

Fig. 4 (a) ¹H MAS NMR chemical shifts for NH₂(CH₃)₂CdCl₃ as a function of temperatures. Asterisks are sidebands for ¹H NMR signal (inset: halfwidths A and B of full width at half maximum at 180, 300, and 400 K), and (b) ¹H MAS NMR chemical shift for NH₂ and CH₃ at 430 K.

One sideband appears on the right side and two sidebands appear on the left side. The appearance of two sidebands (oval on the left side) indicated that the two ¹H signals in NH₂ and CH₃ did not completely overlap. That is, at 430 K, the ¹H chemical shift in CH₃ was 3.24 ppm and the ¹H chemical shift in NH₂ was ∼5 ppm as shown in Fig. 4(b). Here, the sidebands for NH₂ and CH₃ are marked by open circles and crosses, respectively; moreover, the half widths (A and B) of full width at half maximum (FWHM) at 180, 300, and 400 K were not symmetrical (inset of Fig. 4(b)). The ¹H peak of CH₃ hardly changed with temperature, suggesting that the structural environment around ¹H in CH₃ does not change with temperature. However, the fact that the sideband on the left side appears as one at low temperatures and two at high temperatures indicates that the position of the ¹H signal in NH₂ changes slightly with temperature. This result indicates that the environments around ¹H in NH₂ vary slightly with the temperature.

The ¹³C MAS NMR chemical shifts of NH₂(CH₃)₂CdCl₃ were measured at increasing temperatures, as shown in Fig. 5. Two signals for ¹³C were recorded at 220 K, indicating that the two ¹³C atoms of the cation had different environments. At 260 K, only one signal is observed, indicating that the environment surrounding ¹³C remains the same. However, at 300 K and 340 K, two separate ¹³C spectra were observed, indicating that the surrounding environments of ¹³C in the cation were different from each other. In other words, the observation of the two ¹³C NMR signals means that the surrounding environments of the two CH₃ located on both sides of N in the NH₂(CH₃)₂ cation are different each other. And, the observation of one ¹³C NMR signal means that the surrounding environments of the two CH₃ located on both sides of N are the same each other. Finally, at temperatures above 380 K, a single spectrum was observed, and the line width became very narrow. Hence, the environments around ¹³C in CH₃ vary considerably depending on the temperature. ¹³C NMR chemical shifts were observed at 39.84 and 37.97 ppm at 300 K, and the line width was narrower (1.54 ppm) than that (32 ppm) in the ¹H NMR of NH₂ and CH₃, indicating that the molecular motion of ¹³C located at the end of the [NH₂(CH₃)₂] cation is very free.

Fig. 5 ¹³C MAS NMR chemical shifts for NH₂(CH₃)₂CdCl₃ as a function of temperature (inset: ¹³C MAS NMR chemical shifts for NH₂(CH₃)₂CdCl₃ at 220, 260, 300, 340, 380, and 430 K).

3.4. ¹⁴N and ¹¹³Cd static NMR chemical shift

The ¹⁴N static NMR spectrum of NH₂ in the NH₂(CH₃)₂CdCl₃ single crystal was recorded in the temperature range of 180–420 K as shown in Fig. 6. The ¹⁴N NMR spectrum was less sensitive than ¹H, but has a larger chemical shift range. The magnetic field direction relative to the single-crystal direction was measured in an arbitrary direction. The NMR spectrum due to the spin number of ¹⁴N (I = 1)⁴⁴ was predicted by the two resonance lines owing to the quadrupole interaction. The Larmor frequency for ¹⁴N NMR spectrum was extremely low as 28.90 MHz, which made it difficult to obtain a signal. Moreover, distinguishing the signals was difficult owing to the low peak intensities and wide line widths. Four ¹⁴N NMR signals were observed, which were divided into two sets: N(1) and N(2). The chemical shift of N(1) slightly increased with increasing temperature, whereas that of N(2) decreased and then slightly increased. At 300 K, the line width was extremely broad (∼64 ppm) and indicated the presence of two sets of N sites. The continuous changes in the N(1) and N(2) chemical shifts with increasing temperature indicated a change in the coordination geometry of the local environment of ¹⁴N atom. SCXRD results did not indicate different N sites; thus, these two sets of ¹⁴N NMR signals suggest a twin structure due to the property of ferroelastic materials.

Fig. 6 The static ¹⁴N NMR chemical shifts of NH₂(CH₃)₂CdCl₃ single crystal as a function of temperature (inset: ¹⁴N NMR spectrum at 300, 360, and 420 K).

The ¹¹³Cd signal was distributed over a wide chemical shift range and exhibited good intensity owing to its relatively high natural abundance; therefore, it was not difficult to record. From the chemical shifts of static ¹¹³Cd NMR, the structural environment around ¹¹³Cd in CdCl₆ was determined based on the temperature change. As the number of spins in ¹¹³Cd (I = ½),⁴⁴ only one resonance signal was expected. At 300 K, the ¹¹³Cd chemical shift was 215.26 ppm as shown in Fig. 7, and the ¹¹³Cd chemical shifts changed with temperature, suggesting that the ¹¹³Cd environment in CdCl₆ changed with temperature. The line widths of the ¹¹³Cd NMR spectra are shown in detail in Fig. 7. At 300 K, the line width was ∼39 ppm, which is wider than ¹H line widths, and hardly changed within the error range depending on the temperature.

Fig. 7 Static ¹¹³Cd NMR chemical shifts for NH₂(CH₃)₂CdCl₃ as a function of temperature (inset: ¹¹³Cd NMR chemical shifts and line widths for NH₂(CH₃)₂CdCl₃ as a function of temperature).

3.5. ¹H and ¹³C NMR spin-lattice relaxation times

The ¹H and ¹³C T_1ρ values of NH₂(CH₃)₂CdCl₃ crystals were collected using a spin-lock pulse and subsequent FID. The intensity changes in the measured magnetization are calculated using the following equation:^45,46

S(t)/S(0) = exp(-t/T_1ρ) (1)

where S(t) is the intensity of the resonance line at delay time t and S(0) is the intensity of the resonance line at delay time t = 0. The experiment was repeated several times with different t values, and the T_1ρ values were obtained from the slopes of the resulting intensities vs. the t values in eqn (1). The T_1ρ values for ¹H and ¹³C NMR of NH₂(CH₃)₂CdCl₃ as a function of 1000/temperature are shown in Fig. 8 and 9, respectively. The ¹H NMR T_1ρ values increased slightly with increasing temperature but decreased rapidly at temperatures above 220 K with increasing temperature and subsequently increased rapidly. The minimum ¹H T_1ρ value of 0.9 ms was exhibited at 300 K, and the pattern of ¹H T_1ρ values shown in Fig. 8 suggests that molecular motion fits the Bloembergen–Purcell–Pound (BPP) theory. The experimental T_1ρ value is related to the correlation time, τ_C, according to BPP theory, and the T_1ρ value for molecular motion is expressed by eqn (2) as shown below:⁴⁰where R (= μ₀ħγ_Hγ_C/πr³)² is a constant; ω₁ is the spin-lock field; and ω_H and ω_C are the Larmor frequencies for proton and carbon, respectively. The minimum T_1ρ value is satisfied when ω₁τ_C = 1, and the constant R value (= 350 × 10⁵ for ¹H; 133 × 10⁵ and 48 × 10⁵ for ¹³C) can be calculated by using the τ_C obtained from ω₁τ_C = 1. Using R and ω₁ values as well as the ω_H, ω_C, and T_1ρ values obtained from the experiment, we calculated τ_C values of molecular motion as a function of temperature. The local field fluctuation is described by the thermal motion of the protons, which is caused by thermal energy. The correlation time τ_C is represented by the Arrhenius dependence of the activation energy as shown below:⁴⁵

τ_C = τ_C(0)exp(-E_a/k_BT) (3)

where τ_C(0), E_a, k_B, and T are the pre-correlation time, activation energy, Boltzmann constant, and temperature, respectively. The magnitude of E_a depends on molecular dynamics. The plot of τ_C on a logarithmic scale vs. 1000/T for ¹H is shown in Fig. 8. τ_C was determined to be 44.84 ± 2.26 kJ mol⁻¹ from the slope of the blue dotted line.

Fig. 8 ¹H T_1ρ values and correlation times of NH₂(CH₃)₂CdCl₃ as a function of inverse temperature. Blue dot line is represented the activation energy E_a.

Fig. 9 ¹³C T_1ρ values and correlation times of NH₂(CH₃)₂CdCl₃ as a function of inverse temperature. Two solid lines are represented the activation energies E_a.

On the other hand, the two ¹³C T_1ρ values shown in Fig. 9 showed similar trends at all temperatures, and the minimum T_1ρ value of CH₃ was observed at 280 K. The minimum ¹³C T_1ρ values of 6.54 and 2.32 ms were observed at 280 K. The ¹³C T_1ρ values varied considerably with temperature. These values first gradually increased with increasing temperature up to 220 K, above which they decreased rapidly to a minimum and finally rapidly increased. Similar to the ¹H T_1ρ values, the ¹³C T_1ρ values exhibit molecular motion according to the BPP theory. The ¹³C τ_C values were calculated using the same method as that used for the ¹H τ_C value based on the temperature change, as shown in Fig. 9. The ¹³C E_a values obtained from the plots of τ_C vs. 1000/T were 40.77 ± 2.82 and 37.19 ± 1.91 kJ mol⁻¹; these two values are similar within the error range.

4. Conclusions

In this study, an organic–inorganic hybrid NH₂(CH₃)₂CdCl₃ crystal was grown and its crystal structure, phase transition temperature, thermal behavior, and structural dynamics were investigated. The structure of this crystal was monoclinic, as evidenced by SCXRD at 200 K and 300 K, and the phase transition temperatures T_C of 460 and 470 K were determined using DSC and PXRD. The thermal properties determined by TGA showed that the crystal began decomposing at 540 K and was thermally stable. Although the surrounding environments by the ¹H, ¹³C, ¹⁴N, and ¹¹³Cd NMR chemical shifts varied with increasing temperature, this is independent of the phase transition. It is noteworthy that the ¹H and ¹⁴N NMR chemical shifts bonded to NH₂ in the NH₂(CH₃)₂ cation were changed more as the temperature changes, and the change in ¹¹³Cd NMR chemical shift indicate changes in the surrounding environment of the six Cl atoms around the Cd in CdCl₆. These experimental results suggest that the N–H⋯Cl hydrogen bonding between the NH₂(CH₃)₂ cation and CdCl₆ anion changed with temperature. In addition, T_1ρ values which represent the energy transfer around the ¹H and ¹³C atoms of the cation, varied significantly with temperature. ¹H and ¹³C T_1ρ values were comparable in the measured temperature range because ¹H and ¹³C connect the same bond in the cation. Moreover, ¹³C E_a values were smaller than those of ¹H, suggesting that ¹³C at the end of the cation is free compared to ¹H in the cation. The physicochemical properties will help a fundamental understanding of Pb-free organic–inorganic perovskite type solar cell.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the national Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2023R1A2C2006333). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (2016R1A6A1A03012069).

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Footnote

† Electronic supplementary information (ESI) available. CCDC The datasets generated and/or analysed during the current study are available in the CCDC 2258449. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3ra04381a

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