Physicochemical properties and structural dynamics of organic–inorganic hybrid [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) crystals

Abstract

The physical properties of the organic–inorganic hybrid crystals having the formula [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl, Br) were investigated. The phase transition temperatures (T_C; 268K for Cl and 272K for Br) of the two crystals bearing different halogen atoms in their skeletons were determined through differential scanning calorimetry. The thermodynamic properties of the two crystals were investigated through thermogravimetric analysis. The structural dynamics, particularly the role of the [NH₃(CH₂)₃NH₃] cation, were probed through ¹H and ¹³C magic-angle spinning nuclear magnetic resonance spectroscopy as a function of temperature. The ¹H and ¹³C NMR chemical shifts did not show any changes near T_C. In addition, the ¹H spin–lattice relaxation time (T_1ρ) varied with temperature, whereas the ¹³C T_1ρ values remained nearly constant at different temperatures. The T_1ρ values of the atoms in [NH₃(CH₂)₃NH₃]ZnCl₄ were higher than those in [NH₃(CH₂)₃NH₃]ZnBr₄. The observed differences in the structural dynamics obtained from the chemical shifts and T_1ρ values of the two compounds can be attributed to the differences in the bond lengths and halogen atoms. These findings can provide important insights or potential applications of these crystals.

Introduction

Organic–inorganic compounds based on hybrid perovskites, particularly [(C_nH_2n+1NH₃)]₂BX₄ (n = 1, 2, 3,···; B = Mn, Co, Cu, Zn, Cd; X = Cl, Br) and [NH₃(CH₂)_nNH₃]BX₄ (n = 2, 3,…) have attracted considerable attention in recent years. Monoammonium series [(C_nH_2n+1NH₃)]₂BX₄^1,2,3,4,5 and diammonium series [NH₃(CH₂)_nNH₃]BX₄ have been extensively studied because of their relative stability and potential applications^{6,7,8,9,10,11}. The physical and chemical properties of the organic–inorganic hybrid perovskites depend on the characteristics of the organic cations, geometry of the inorganic anions (metal halide ions; (BX₆)²⁻ or (BX₄)²⁻), and reaction stoichiometry^{1,2,3,12,13,14,15,16,17,18}. For B = Mn, Cu, or Cd, the structure is consists of the corner shared octahedral (BX₆)²⁻ alternated with organic layers. While for B = Co or Zn, the structures are tetrahedral (BX₄)²⁻ sandwiched between layers of organic cations. These compounds have gained research attention because of the multiplicity of their crystal structures, which is correlated to the structural dynamics of the cations and anions. Organic–inorganic hybrid materials based on perovskite structures are of interest due to their potential applications^19,20,21. Recently, solid-state NMR on hybrid perovskite materials has also garnered considerable attention^22,23,24,25.

The [NH₃(CH₂)₃NH₃]ZnCl₄ (1,3-propanediammonium tetrachlorozincate (II)) complex (n = 3; B = Zn; X = Cl), a member of the diammonium [NH₃(CH₂)_nNH₃]BX₄ series, belongs to the monoclinic crystal system (space group: P2₁/n) at room temperature. Its lattice constants have been reported as a = 10.692 Å, b = 10.611 Å, c = 10.786 Å, β = 118.47°, and Z = 4²⁶. Figure 1 shows the structure of the [NH₃(CH₂)₃NH₃]ZnCl₄ crystal at 300 K (CCDC number : 1227730). The crystal structure of the [NH₃(CH₂)₃NH₃]ZnBr₄ (1,3-propanediammonium tetrabromozincate (II)) complex (n = 3, B = Zn; X = Br) is identical to that of the [NH₃(CH₂)₃NH₃]ZnCl₄ complex; both belong to the same space group. Its unit cell dimensions were determined as a = 11.084 Å, b = 10.968 Å, c = 11.185 Å, β = 117.07°, and Z = 4²⁷. The [ZnBr₄]²⁻ anion forms a tetrahedron interconnected via the N‒H···Br hydrogen bonds to the [NH₃(CH₂)₃NH₃] cation. There is no center of symmetry within the [NH₃(CH₂)₃NH₃] cation in [NH₃(CH₂)₃NH₃]ZnBr₄.

Figure 1

Crystal structure of [NH₃(CH₂)₃NH₃]ZnCl₄ at room temperature.

To date, the structures of [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) have been reported by Kallel et al.²² and Ishihara et al.²³, who studied the single crystals of the complexes through X-ray diffraction analysis. Although [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) has many applications, the physical properties and structural dynamics of its crystals have not been elucidated.

Herein, the phase transition temperatures and thermodynamic properties of the crystals of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ complexes are determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structural dynamics of the crystals of [NH₃(CH₂)₃NH₃]ZnX₄ (particularly the role of the [NH₃(CH₂)₃NH₃] cation) were probed by ¹H magic-angle spinning nuclear magnetic resonance (MAS NMR) and ¹³C MAS NMR spectroscopy as a function of temperature. The ¹H and ¹³C NMR spectral profiles were recorded to determine the changes in the chemical shifts. A change in the chemical shift values reflects a change in the structural environment. In addition, the spin–lattice relaxation times (T_1ρ) in the rotating frame were discussed according to the change of temperature. Based on the MAS NMR results, the effects of different halogen atoms on the hydrogen bond and carbon atoms in the [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ crystals were investgated. Moreover, comparison of the physical properties of the two crystals revealed important information regarding the basic mechanisms.

Experimental

An aqueous solution containing NH₂(CH₂)₃NH₂·2HCl and ZnCl₂ was slowly evaporated at 300 K to produce the single crystals of [NH₃(CH₂)₃NH₃]ZnCl₄. Similarly, the slow evaporation at 300 K of an aqueous solution containing NH₂(CH₂)₃NH₂·2HBr and ZnBr₂, produced the single crystals of [NH₃(CH₂)₃NH₃]ZnBr₄. Transparent hexagonal crystals of [NH₃(CH₂)₃NH₃]ZnCl₄ and thin-plate-shaped crystals of [NH₃(CH₂)₃NH₃]ZnBr₄ were produced.

The structures of the [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) crystals at 298 K were analysed using an X-ray diffraction system equipped with a Cu-Kα radiation source. And, the lattice parameters were determined by single-crystal X-ray diffraction methods at the Western Seoul Center of Korea Basic Science Institute. The crystals were mounted on a Bruker D8 Venture equipped with IμS micro-focus sealed tube Mo-Kα and a PHOTON III M14 detector.

DSC (TA, DSC 25) experiments were carried out at a scanning speed of 10 K/min in the temperature range of 190–620 K under an atmosphere of nitrogen. TGA experiments were performed on a thermogravimetric analyzer (TA Instrument) in the temperature range of 300–870 K. The same heating rate was maintained. The amounts of [NH₃(CH₂)₃NH₃]ZnCl₄ used for the DSC and TGA experiments were 6.04 and 6.72 mg, respectively and the amounts of [NH₃(CH₂)₃NH₃]ZnBr₄ used were 6.40 and 8.96 mg, respectively.

The NMR spectra of the crystals of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ were recorded on a 400 MHz Avance II + Bruker solid-state NMR spectrometer equipped with 4 mm MAS probes (at KBSI, Western Seoul Center). The MAS ¹H and ¹³C NMR experiments were conducted at the Larmor frequency of 400.13 and 100.61 MHz, respectively. It was observed that an MAS rate of 10 kHz can minimize the spinning sidebands. Tetramethylsilane (TMS) was used as the standard to record the NMR spectra. As preparation for the MAS NMR experiments, the single crystal was ground into powder. The T_1ρ values were obtained using a variable length spin lock pulse. For the two compounds, the width of the π/2 pulse for ¹H was 3.5 μs and the width of the π/2 pulse for ¹³C was 3.96–4.3 μs. The radiofrequency power of the spin-lock pulses was 71.42 kHz for ¹H and 62.50 kHz for ¹³C. An almost constant temperature (error ± 0.5 K) was maintained even when the rate of flow of nitrogen gas and the heater current were adjusted.

Experimental results

X-ray results

The X-ray powder diffraction patterns of the [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) crystals at 298 K are displayed in Fig. 2. And, the lattice constants for [NH₃(CH₂)₃NH₃]ZnCl₄ were determined to be a = 10.670 Å, b = 10.576 Å, c = 10.755 Å, and β = 118.477°, and those for [NH₃(CH₂)₃NH₃]ZnBr₄ were determined to be a = 11.146 Å, b = 10.995 Å, c = 11.188 Å, and β = 117.215°. These results are consistent with those reported previously^26,27.

Figure 2

X-ray diffraction patterns of the [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ at 300 K.

Thermal properties

Two endothermic peaks at 268 and 597 K were observed in the DSC curves of [NH₃(CH₂)₃NH₃]ZnCl₄, as shown in Fig. 3. The DSC curves of [NH₃(CH₂)₃NH₃]ZnBr₄ revealed the presence of two endothermic peaks at 272 and 589 K, as shown in Fig. 3. The results obtained from the DSC experiments revealed that the peaks that appeared at ~ 590 K were significantly larger than the other peaks. An additional TGA and differential thermal analysis (DTA) experiments were performed to determine whether these endothermic peaks are related to the structural phase transitions or melting. The results of the TGA and DTA experiments conducted with the two crystals are presented in Fig. 4a,b. The onset of thermal decomposition temperature (= T_d) was observed at temperatures > 550 K. This was characterized by a loss in the weight of the compounds. It was observed that [NH₃(CH₂)₃NH₃]ZnCl₄ (Mw = 283.34 mg) and [NH₃(CH₂)₃NH₃]ZnBr₄ (Mw = 461.15 mg) started losing weight at higher temperatures. The amounts of solid residues obtained were calculated from the molecular weights of the compounds and balanced chemical reactions. The amounts were estimated from Eqs. (1), (2), (3), and (4)²⁸.

Figure 3

Differential scanning calorimetry (DSC) thermogram of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃] ZnBr₄.

Figure 4

(a) Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of [NH₃(CH₂)₃NH₃]ZnCl₄. (b). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of [NH₃(CH₂)₃NH₃]ZnBr₄.

[NH ₃ (CH ₂ ) ₃ NH ₃ ]ZnCl ₄

$$\left[ {{\text{NH}}_{3} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{3} } \right]{\text{ZnCl}}_{4} \to \left[ {{\text{NH}}_{2} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{2} \cdot2{\text{HCl}}} \right]{\text{ZnCl}}_{2} \to \left[ {{\text{NH}}_{2} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{2} \cdot{\text{HCl}}} \right]{\text{ZnCl}}_{2} \left( {\text{s}} \right) + {\text{HCl}}\left( {\text{g}} \right).$$

The amount of residue obtained is determined as follows:

$$\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} \cdot{\text{HCl}}} \right]{\text{ZnCl}}_{{2}} \left( {\text{s}} \right){/}\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnCl}}_{{4}} = 87.13\% ,$$

(1)

$$\left[ {{\text{NH}}_{{3}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{3}} } \right]{\text{ZnCl}}_{{4}} \to \left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} \cdot{\text{2HCl}}} \right]{\text{ZnCl}}_{{2}} \to \left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnCl}}_{{2}} \left( {\text{s}} \right) + 2{\text{HCl }}\left( {\text{g}} \right){.}$$

The amount of residue obtained is determined as follows:

$$\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnCl}}_{{2}} \left( {\text{s}} \right)/\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnCl}}_{4} = 74.26\% .$$

(2)

[NH ₃ (CH ₂ ) ₃ NH ₃ ]ZnBr ₄

$$\left[ {{\text{NH}}_{3} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{3} } \right]{\text{ZnBr}}_{4} \to \left[ {{\text{NH}}_{2} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{2} \cdot2{\text{HBr}}} \right]{\text{ZnBr}}_{2} \to \left[ {{\text{NH}}_{{\text{2}}} \left( {{\text{CH}}_{{\text{2}}} } \right)_{3} {\text{NH}}_{2} \cdot{\text{HBr}}} \right]{\text{ZnBr}}_{2} \left( {\text{s}} \right) + {\text{HBr}}\left( {\text{g}} \right).$$

The amount of residue obtained is determined as follows:

$$\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} \cdot{\text{HBr}}} \right]{\text{ZnBr}}_{2} \left( {\text{s}} \right)/\left[ {{\text{NH}}_{2} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{2} } \right]{\text{ZnBr}}_{4} = 82.45\% ,$$

(3)

$$\left[ {{\text{NH}}_{{\text{3}}} \left( {{\text{CH}}_{{\text{2}}} } \right)_{{\text{3}}} {\text{NH}}_{3} } \right]{\text{ZnBr}}_{4} \to \left[ {{\text{NH}}_{2} \left( {{\text{CH}}_{2} } \right)_{3} {\text{NH}}_{2} \cdot2{\text{HBr}}} \right]{\text{ZnBr}}_{2} \to \left[ {{\text{NH}}_{{\text{2}}} \left( {{\text{CH}}_{{\text{2}}} } \right)_{{\text{3}}} {\text{NH}}_{{\text{2}}} } \right]{\text{ZnBr}}_{{\text{2}}} \left( {\text{s}} \right) + 2{\text{HBr}}\;\left( {\text{g}} \right).$$

The amount of residue obtained is determined as follows:

$$\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnBr}}_{{2}} \left( {\text{s}} \right){/}\left[ {{\text{NH}}_{{2}} \left( {{\text{CH}}_{{2}} } \right)_{{3}} {\text{NH}}_{{2}} } \right]{\text{ZnBr}}_{{4}} = {64}{\text{.91}}\% {.}$$

(4)

[NH₃(CH₂)₃NH₃]ZnCl₄ was found to lose approximately 13% and 26% of its weight when the temperature was approximately 606 and 621 K, respectively. [NH₃(CH₂)₃NH₃]ZnBr₄ was found to lose approximately 17% and 35% of its weight when the temperature was approximately 622 and 649 K, respectively. The weight loss can be potentially attributed to the decomposition of HX and 2HX (X = Cl and Br) moieties, respectively, as shown in Fig. 4a,b. Based on the DSC and TGA results, the phase transition temperatures for [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ were 268 K and 272 K, respectively. In addition, it was found that the endotherm peaks of 597 K for [NH₃(CH₂)₃NH₃]ZnCl₄ and 589 K for [NH₃(CH₂)₃NH₃]ZnBr₄ were related to T_d.

MAS ¹H NMR results

The temperature-induced ¹H NMR chemical shifts for [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ crystals were recorded through MAS NMR spectroscopy. A single resonance signal was observed in each of the NMR spectra of both the compounds. The ¹H NMR spectrum of [NH₃(CH₂)₃NH₃]ZnBr₄ recorded at 300 K is shown in Fig. 5. The observed resonance signal was asymmetric. The line width represented as symbol 1 and that represented as symbol 2 at the full-width at half-maximum (FWHM) value are not the same as those at 3.54 and 6.11 ppm for 1 and 2, respectively. This asymmetric signal is attributed to the overlapping lines of the two ¹H for CH₂ and NH₃ present in the [NH₃(CH₂)₃NH₃] cations. The spinning sidebands were marked with open circles. At 300 K, the ¹H line width of the spectrum recorded for [NH₃(CH₂)₃NH₃]ZnCl₄ was 10.57 ppm, whereas that of the spectrum recorded for [NH₃(CH₂)₃NH₃]ZnBr₄ was 9.65 ppm. The ¹H NMR chemical shifts for two ¹H in the [NH₃(CH₂)₃NH₃] cations are temperature independent. The chemical shift values remained practically constant over a wide range of temperature, indicating that the structural environment of the protons present in the [NH₃(CH₂)₃NH₃] cation does not change when the temperature is increased.

Figure 5

MAS ¹H NMR line shapes of [NH₃(CH₂)₃NH₃]ZnBr₄ at 300 K.

The ¹H T_1ρ was measured at each temperature by applying a spin lock of variable duration, t. The relationship between the intensities of the NMR signals and the duration of the spin lock is as follows^29,30,31:

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

(5)

where I(t) and I(0) are the signal intensities at times t and t = 0, respectively. The ¹H NMR signals were plotted as a function of the delay times at each temperature. The ¹H NMR signals for [NH₃(CH₂)₃NH₃]ZnCl₄ at 300 K were recorded by varying the delay times (in the range of 1–150 ms, Fig. 6). The decay curves were fit to an exponential function, as shown in Eq. (5). The T_1ρ values of the protons in [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ were obtained as a function of inverse temperature, as shown in Fig. 6. The T_1ρ values of the atoms in the two compounds did not change near T_C (268 K for [NH₃(CH₂)₃NH₃]ZnCl₄ and 272 K for [NH₃(CH₂)₃NH₃]ZnBr₄). The T_1ρ values initially increased and then decreased when the temperature was increased. In addition, the trend of changes in the ¹H T_1ρ values of [NH₃(CH₂)₃NH₃]ZnBr₄ and [NH₃(CH₂)₃NH₃]ZnCl₄ with temperature was similar. The T_1ρ values of the protons in [NH₃(CH₂)₃NH₃] in both the crystals were 10–1000 ms. The ¹H T_1ρ values of [NH₃(CH₂)₃NH₃]ZnCl₄ decreased abruptly at 360 K and that of [NH₃(CH₂)₃NH₃]ZnBr₄ decreased abruptly at 300 K (as represented by lines). The T_1ρ value of the protons in [NH₃(CH₂)₃NH₃]ZnCl₄ was higher than that in [NH₃(CH₂)₃NH₃]ZnBr₄ at all studied temperatures. For both the compounds, lower T_1ρ values were recorded at higher temperatures. On the slow side of T_1ρ, a decrease in T_1ρ results in smaller valued of the correlation time τ_C. The T_1ρ values for Arrhenius-type random motions with τ_C are described in terms of slow motions. When τ_C <  < ω₁, T_1ρ ∝ τ_C = τ₀ exp(-E_a/k_BT), where ω₁ denotes the spin-lock frequency and E_a represents the activation energy. The decrease in T_1ρ values with temperature indicates an increase in proton mobility at higher temperatures.

Figure 6

MAS ¹H NMR spin–lattice relaxation times (T_1ρ) of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ as a function of inverse temperature (Inset: recovery curves for delay times of MAS ¹H NMR spectrum in [NH₃(CH₂)₃NH₃]ZnCl₄ at 300 K).

MAS ¹³C NMR results

The MAS ¹³C NMR chemical shifts in [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ were measured according to the change in temperature. Two resonance signals were observed in the MAS ¹³C NMR spectra of the compounds. The chemical shift values of the other carbon atoms were determined relative to the TMS signal. Here, the CH₂ group sandwiched between two other CH₂ groups is labeled as CH₂-1. The CH₂ group close to NH₃ is labeled CH₂-2. At 300 K, the signals corresponding to the CH₂-1 and CH₂-2 carbon atoms in [NH₃(CH₂)₃NH₃]ZnCl₄ appear at δ = 26.41 and δ = 38.53 ppm, respectively. The chemical shift values of the carbon atoms in [NH₃(CH₂)₃NH₃]ZnBr₄ are similar to the chemical shift values of the carbon atoms in [NH₃(CH₂)₃NH₃]ZnCl₄ (Fig. 7, inset). It was observed that for both the crystals, the ¹³C chemical shift values remained almost constant with changes in temperature (Supplementary Fig. 1).

Figure 7

Line widths of ¹³C for CH₂-1 and CH₂-2 in [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ as a function of temperature (Inset: MAS ¹³C NMR spectra of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃] ZnBr₄ at 300 K).

The change in FWHM for ¹³C NMR spectra with temperature for both crystals is shown in Fig. 7. In the case of [NH₃(CH₂)₃NH₃]ZnCl₄, the ¹³C line widths (CH₂-1 and CH₂-2) decreased monotonically with an increase in temperature. Significant anomaly due to phase transition was not observed. The ¹³C line widths in [NH₃(CH₂)₃NH₃]ZnBr₄ changed with the increase in temperature. A transformation from a Gaussian to Lorentzian shape was observed. The line width reduced because of internal molecular motion. The line width of CH₂-1 was lower than that of CH₂-2. The decrease in line width with an increase in temperature can be attributed to the internal molecular motion.

The intensities of the ¹³C NMR signals were determined by varying the delay times at each temperature. The ¹³C NMR signals (CH₂-1 and CH₂-2) recorded for [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ were plotted as a function of delay time. The decay curves for CH₂-1 and CH₂-2 were fitted to an exponential equation of Eq. (5). From the slopes of the recovery traces, the ¹³C T_1ρ values were obtained (CH₂-1 and CH₂-2) as a function of inverse temperature, as shown in Fig. 8. The ¹³C T_1ρ values of [NH₃(CH₂)₃NH₃]ZnCl₄ were higher than those of [NH₃(CH₂)₃NH₃]ZnBr₄. The ¹³C T_1ρ value of CH₂-1 was slightly higher than the ¹³C T_1ρ value of CH₂-2. The T_1ρ values of the atoms in both the compounds were temperature independent at > 200 K.

Figure 8

T_1ρ for CH₂-1 and CH₂-2 of [NH₃(CH₂)₃NH₃]ZnCl₄ and [NH₃(CH₂)₃NH₃]ZnBr₄ as a function of inverse temperature.

Conclusion

The physical properties of organic–inorganic hybrid crystals [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl, Br) were investigated through DSC, TGA, and NMR spectroscopy. The TGA results revealed that the [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl, Br) complex exhibited good thermal stability. When the temperature was increased, the ¹H and ¹³C chemical shifts remained almost constant. Although the T_C was obtained from the DSC experiments, in particular, changes in the chemical shift values were not observed at temperatures near T_C. This indicated that the chemical environment surrounding the protons and the carbon atoms in the [NH₃(CH₂)₃NH₃] cation remained unaltered.

The structural dynamics were discussed in terms of the ¹H T_1ρ and ¹³C T_1ρ values. The ¹H T_1ρ values changed with the temperature, whereas the T_1ρ value of the carbon atom located in the middle of the N–C–C–C–N chain did not change significantly with the temperature. The ¹H T_1ρ and ¹³C T_1ρ values of the atoms in [NH₃(CH₂)₃NH₃]ZnCl₄ were higher than those in [NH₃(CH₂)₃NH₃]ZnBr₄ (Table 1). The reason why ¹³C T_1ρ values in [NH₃(CH₂)₃NH₃]ZnCl₄ are longer than ¹³C T_1ρ values in [NH₃(CH₂)₃NH₃]ZnBr₄ are as follows. The ¹³C T_1ρ is most likely driven by ¹H-¹³C interactions, and hence, the H-C bond lengths are important.

Table 1 The lattice constants (Å), phase transition temperatures T_C (K), thermal decomposition temperatures T_d (K), and spin–lattice relaxation times T_1ρ (ms) in [NH₃(CH₂)₃NH₃]ZnX₄ (X = Cl and Br) crystals at 300 K.

However, the H-C bond lengths of the two materials are unlikely to be different, and also cannot be accurately measured from X-ray results. We assume that the differences in the dynamics between the materials contribute more to the difference in T_1ρ. Although the structures and lattice constants of two crystals are similar, the differences between the local environments and structural dynamics obtained from the chemical shifts and T_1ρ values of the two compounds can be attributed to the different bond lengths and halogen atoms. Thus, the results of this study elucidate, the physical properties of [NH₃(CH₂)₃NH₃]ZnX₄, which will expand the application scope of this organic–inorganic hybrid crystals.