Study on structural geometry and dynamic property of [NH₃(CH₂)₅NH₃]CdCl₄ crystal at phases I, II, and III

Abstract

Organic–inorganic hybrid perovskites can potentially be used in electrochemical devices, such as batteries and fuel cells. In this study, the structure and phase transition temperatures of the organic–inorganic material [NH₃(CH₂)₅NH₃]CdCl₄ crystal were confirmed by X-ray diffraction and differential scanning calorimetry. From the nuclear magnetic resonance results, the crystallographic configurations of ¹H, ¹³C, and ¹⁴N in the cation changed at temperatures close to T_C1 (336 K), whereas that of ¹¹³Cd in the anion shows significant changes at temperatures close to T_C1 and T_C2 (417 K). The activation energy, E_a, values for ¹H and ¹³C obtained from the spin–lattice relaxation time, T_1ρ, below and above T_C1 were evaluated, where the E_a value for ¹³C was more flexible at low temperatures than at high temperatures. In addition, the effect on molecular motion was effective at high temperatures. The phase transition at 336 K was associated with the change in the N–H···Cl bond due to the change in the coordination geometry of Cl around Cd in the CdCl₆ anion. On the other hand, the phase transition at 417 K was related to the ferroelastic phase transition attributed to the twin domains.

Introduction

Recently, many studies with the development of functional materials are being conducted on organic–inorganic hybrid perovskite materials. The organic–inorganic hybrid crystal [NH₃(CH₂)_nNH₃]BX₄ (n = 2, 3, 4, …), where B is a transition metal, such as Mn, Cd, Fe, Cu …, and X is a halogen ion, crystallizes perovskite-type layer structures^1–12. The organic part of the hybrid complex determines the optical properties and structural flexibility, whereas the inorganic part affects the mechanical and thermal properties¹³. The properties and structural phase transitions of organic–inorganic hybrid compounds are affected by their structures and the interactions between cation and anion¹². For chains in which n $\gg$ 4, structural rearrangement by conformational changes in the chains becomes important. An interesting group of hybrid compound is the perovskite-type layer [NH₃(CH₂)₅NH₃]CdCl₄ (pentylenediammonium cadmium tetrachloride) containing a [NH₃(CH₂)₅NH₃] cation and a two-dimensional (2D) layered CdCl₆ anion. [NH₃(CH₂)₅NH₃]CdCl₄ has two structural phase transitions at temperatures near 337 K (T_C1) and 417 K (T_C2)^14,15. It exhibits an unusual phase sequence, in which the phase that is stable at high temperatures exhibits the lowest symmetry.

The phase sequence in the following way¹⁶

Phases III (below 337 K) and II (above 337 K) are orthorhombic with the space groups Pnam and Imam, respectively. The lattice constants in phase III (at 293 K) are a = 7.330 Å, b = 7.504 Å, c = 23.862 Å, and Z = 4, while the unit cell parameters in phase II (at 353 K) are a = 7.376 Å, b = 7.561 Å, c = 23.555 Å, and Z = 4. The high-temperature phase I is monoclinic, and the unit cell parameters at 433 K are a = 7.516 Å, b = 7.563 Å, c = 11.22 Å, and β = 98.15° with the space group C12/m₁. The [NH₃(CH₂)₅NH₃] organic chains are arranged along the longest c-axis. The Cd octahedra is located the edge to form a 2D network, and the diammonium cations are connected to CdCl₆ octahedra by hydrogen bonds. In the inorganic layers, the structural geometries around the Cd atoms are described as distorted octahedra. These hybrid perovskite materials have potential applications in various electrochemical devices, such as batteries and fuel cells^17–25.

The synthesis and characterization of [NH₃(CH₂)₅NH₃]CdCl₄ were first discussed by Kind et al.²⁶, where the structural phase transitions were studied using ³⁵Cl and ²D nuclear magnetic resonance (NMR), birefringence, dilatation measurements, and optical domain investigations. Negrier et al.¹⁵ evaluated the crystal structures via X-ray diffraction (XRD) and Raman scattering experiments at 293 K and 353 K. Our group has also recently reported the effects of ¹³C length in the cation of [NH₃(CH₂)₂NH₃]CdCl₄, [NH₃(CH₂)₃NH₃]CdCl₄, and [NH₃(CH₂)₄NH₃]CdCl₄ crystals on the thermal and structural dynamic properties¹³. Meanwhile, a lot of research has been done on the electrical and conductive properties of this type of compound^16,27–30.

Here, the crystal structures, thermodynamic properties, and ferroelastic domain walls of [NH₃(CH₂)₅NH₃]CdCl₄ were investigated. The roles of cations and anions in the [NH₃(CH₂)₅NH₃]CdCl₄ single crystal were discussed, and the chemical shifts and spin-lattice relaxation time, T_1ρ, with increasing temperature were measured using ¹H magic angle spinning (MAS) NMR, ¹³C MAS NMR, and static ¹⁴N NMR to identify the roles of the [NH₃(CH₂)₅NH₃] cation. Furthermore, the ¹¹³Cd MAS NMR chemical shifts were recorded to evaluate the coordination geometry of the CdCl₆ anion. The results would provide insights into the physicochemical properties of [NH₃(CH₂)₅NH₃]CdCl₄ crystals, facilitating their various applications in the future.

Methods

A saturated aqueous solution containing NH₂(CH₂)₅NH₂·2HCl and CdCl₂ was gradually evaporated at 300 K to grow single crystals of [NH₃(CH₂)₅NH₃]CdCl₄. Colorless single crystals measuring approximately 7 mm × 3 mm × 2 mm were grown for approximately 2–3 weeks in the thermostat.

The structures of the [NH₃(CH₂)₅NH₃]CdCl₄ crystals at 298 K were analyzed using an XRD system. The lattice parameter and space group was considered by single-crystal XRD at the Seoul Western Center of the Korea Basic Science Institute. Experiments were performed in the same manner as before³¹.

Differential scanning calorimetry (DSC) (TA, DSC 25) experiments were carried out at a heating rate of 10 K/min from 190 to 550 K in N₂ gas. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves were obtained using a thermogravimetric analyzer (TA Instrument) with the same heating rate as in DSC from 300 to 973 K in N₂ gas. In addition, the domain patterns were observed using an optical polarizing microscope within the temperature range of 300 to 450 K, where the prepared single crystals were placed on the plate with the temperature sensor of a Linkam THM-600.

NMR spectra of the [NH₃(CH₂)₅NH₃]CdCl₄ crystals were performed using a Bruker 400 MHz Avance II+ solid-state NMR spectrometer in the same facility. The Larmor frequencies for ¹H and ¹³C MAS NMR experiments were 400.13 and 100.61 MHz, respectively. In MAS NMR experiment, the spinning speed was set to 10 kHz to minimize sideband. And tetramethylsilane (TMS) was used as a standard material to obtain accurate NMR chemical shift. The experimental method to obtain the T_1ρ values for ¹H and ¹³C was used in the same way as the previously reported method¹³. And, static ¹⁴N NMR and ¹¹³Cd MAS NMR spectra were recorded at Larmor frequencies of 28.90 and 88.75 MHz, respectively. ¹⁴N and ¹¹³Cd chemical shift measurements were performed using NH₄NO₃ and CdCl₂O₈·6H₂O as standard materials.

Experimental results

Crystal structure

The powder XRD pattern of the [NH₃(CH₂)₅NH₃]CdCl₄ crystal at 298 K is shown in Fig. 1. And, the lattice constants analysized from the X-ray crystal diffraction were determined to be a = 7.3292 ± 0.002 Å, b = 7.5058 ± 0.002 Å, and c = 23.9376 ± 0.006 Å with the space group Pnam; this is consistent with the previously reported results^14,15.

Figure 1.

XRD powder pattern of the [NH₃(CH₂)₅NH₃]CdCl₄ crystal at 298 K.

Phase transition temperature, thermal property, and ferroelastic twin domain

The DSC curves of the [NH₃(CH₂)₅NH₃]CdCl₄ crystal at a heating and cooling rate of 10 K/min in N₂ gas are presented in Fig. 2. Two endothermic peaks were observed at 336 K (T_C1) and 418 K (T_C2) during heating, whereas two exothermic peaks were recorded at 327 K (T_C1′) and 407 K (T_C2′) during cooling. The phase transition enthalpy on heating is 3.17 kJ/mol at 337 K and 0.55 kJ/mol at 417 K, respectively. On the other hand, previous studies reported endothermic peaks at 337 K and 417 K during heating and at 336 K and 407 K during cooling^14,15.

Figure 2.

DSC curves of [NH₃(CH₂)₅NH₃]CdCl₄ during heating and cooling.

To determine the preliminary thermal characteristics, including the structural phase transitions, TGA and DTA results were conducted at the same heating rate as the DSC experiment. Based on the TGA and DTA curves shown in Fig. 3, the crystal exhibited excellent stability up to approximately 600 K. The small inflection points observed at temperatures near 336 K and 417 K in the DTA curve were coincides with the two phase transition temperatures obtained from the DSC results, suggesting that the molecular weight of [NH₃(CH₂)₅NH₃]CdCl₄ decreased at increasing temperatures. The amount of crystal remaining in the solid state was evaluated from the molecular weights. The 10% and 20% weight losses of the crystal at temperatures of about 617 K and 626 K were attributed to the decomposition of HCl and 2HCl, respectively. On the other hand, the weight loss at approximately 800 K and 900 K shown in Fig. 3 was observed 46% and 87%, respectively.

Figure 3.

TGA and DTA curves of [NH₃(CH₂)₅NH₃]CdCl₄.

A single crystal with ferroelastic properties exhibits two or more orientation states even if mechanical stress does not exist since mechanical stress can change the existing orientation state of the single crystal. Polarized microscopy observations revealed the ferroelastic domain structures of the crystal and their changes at the phase transition temperatures, as shown in Fig. 4. The domain pattern represented by parallel lines was not observed in phases III (300 K, Fig. 4a) and II (403 K, Fig. 4b). No change in the behavior of the crystal was observed at T_C1. However, in phase I, twinning occurred in the crystal at temperatures above T_C2, resulting in a highly dense domain pattern indicated by the red circle (Fig. 4c). At 433 K, new domain walls indicated by the blue circles were formed next to the parallel domain walls (Fig. 4d). The phase transition at T_C2 occurred due to the ferroelastic twin domain. The [NH₃(CH₂)₅NH₃]CdCl₄ crystal existed in two crystallographic phases: monoclinic (2/m) at temperatures above 417 K, orthorhombic (mmm) at temperatures between 417 and 337 K, and orthorhombic (mmm) at temperatures below 337 K. According to Aizu³² and Sapriel³³, for the transition from the mmm space group of the orthorhombic phase II to the 2/m space group of the monoclinic phase I, the domain wall directions were x = 0 and z = 0. The equations of the twin domain walls was expressed as 2/mFmmm, corresponding to the “inverted” mmmF2/m instead of mmmF2/m as reported by Sapriel³³.

Figure 4.

Optical polarizing microscopy images of [NH₃(CH₂)₅NH₃]CdCl₄ at (a) phase III (300 K), (b) phase II (403 K), (c) phase I (420 K), and (d) phase I (433 K). The parallel lines represent the ferroelastic twin domain walls.

¹H MAS NMR spectrum

The ¹H MAS NMR spectra of the [NH₃(CH₂)₅NH₃]CdCl₄ crystal were obtained, and the ¹H chemical shifts are shown in Fig. 5 as a function of temperature. At low temperatures, only one resonance signal was observed. These resonance signals were asymmetric due to the overlapping ¹H lines of NH₃ and CH₂ in [NH₃(CH₂)₅NH₃] cations. At 180 K, a single resonance line was present at a chemical shift of 9.04 ppm. The line width and full-width at half-maximum (FWHM) at this temperature were also different from those represented as symbol “1” at 2.97 ppm and as symbol “2” at 6.07 ppm, respectively. At 330 K, which was close to T_C1, the NMR spectrum was divided into two resonance lines, showing chemical shifts of 7.56 and 2.58 ppm for NH₃ and CH₂, respectively. The spinning sidebands were marked with crosses and open circles. Here, phases I, II, and III were plotted in olive, red, and black, respectively. The ¹H chemical shifts of NH₃ and CH₂, presented by dotted lines in Fig. 5, were almost independent of temperature. These results suggested that the surrounding environments of ¹H of NH₃ and CH₂ did not change with temperature.

Figure 5.

MAS ¹H NMR spectra of [NH₃(CH₂)₅NH₃]CdCl₄ at phases I, II, and III (olive areas: phase I, red areas: phase II, and black areas: phase III). The spinning sidebands are marked by crosses and open circles.

¹³C MAS NMR spectrum

The ¹³C chemical shifts at increasing temperature for the in situ MAS NMR spectra are shown in Fig. 6. The TMS reference signal at 300 K recorded at 38.3 ppm was used as the standard for the ¹³C chemical shift. In the [NH₃(CH₂)₅NH₃] cation, CH₂ located close to NH₃ was designated as C-3, CH₂ located at the center was designated as C-1, and CH₂ located between C-3 and C-1 was designated as C-2. The structure of the cation for this crystal is shown in the inset of Fig. 6. At 300 K, the ¹³C chemical shifts were recorded at 28.26, 25.90, and 41.67 ppm for C-1, C-2, and C-3, respectively. The FWHM for ¹³C NMR at 300 K were 6.20, 5.72, and 9.06 ppm for C-1, C-2, and C-3, respectively. The line width of C-3 located close to N was wider than those of C-1 and C-2. The chemical shifts changed at temperatures close to T_C1 (336 K), but not at temperatures close to T_C2 (417 K). Below T_C1, all ¹³C positions showed positive chemical shifts with increasing temperatures. Above T_C1, the chemical shift of C-2 was almost independent of temperature, while the shifts in C-1 and C-3 progressed in a negative and positive direction, respectively. The results proved that below T_C1, the surrounding environments of all ¹³C ions would change with temperature. At temperatures above T_C1, the surrounding environments of C-2 did not change. However, the chemical shifts of C-1 and C-3 continuously changed in all temperature ranges, including T_C1 and T_C2.

Figure 6.

MAS ¹³C NMR spectra of [NH₃(CH₂)₅NH₃]CdCl₄ at phases I, II, and III (olive areas: phase I, red areas: phase II, and black areas: phase III).

Static ¹⁴N NMR

The ¹⁴N NMR spectra of the [NH₃(CH₂)₅NH₃]CdCl₄ single crystal in the temperature range of 180–420 K were recorded by the solid-state echo method with static NMR. Since ¹⁴N has quadrupole interactions with spin number I = 1, two ¹⁴N NMR signals were expected³⁴. The ¹⁴N resonance frequency at increasing temperatures is shown in Fig. 7. Despite the presence of intense background noise due to the very low NMR frequency (28.90 MHz), the ¹⁴N spectrum was obtained without difficulty. Here, the crystal demonstrated an arbitrary direction with respect to the magnetic field. The six resonance lines of the three pairs at increasing temperatures were below T_C1. At temperatures close to 336 K (T_C1), the number of resonance lines and resonance frequencies of the NMR spectra showed abrupt changes. At T_C1, a reduction from three pairs to two pairs of NMR lines was observed. At T_C2, another pair of NMR lines reappeared. Below T_C1, as the temperature increased, the resonance frequencies increased, and above T_C1, as the temperature increased, the resonance frequencies decreased. At T_C2, only the number of resonance lines changed, and the resonance frequency showed almost continuous values. Symbols with the same color indicated the same pairs of ¹⁴N. Changes in the ¹⁴N resonance frequencies due to the change in temperature were related to the changes in the crystallographic configuration of the crystal.

Figure 7.

Static ¹⁴N NMR spectra of [NH₃(CH₂)₅NH₃]CdCl₄ at phases I, II, and III.

¹¹³Cd MAS NMR

The ¹¹³Cd MAS NMR experiments were measured to detect the structural environments around Cd when the temperature in the CdCl₆ anions of the [NH₃(CH₂)₅NH₃]CdCl₄ single crystal were varied. This information was crucial to demonstrate the anion coordination environments around Cd²⁺ in CdCl₆ using ¹¹³Cd NMR spectroscopy. The changes in the in situ ¹¹³Cd MAS NMR spectra are shown in Fig. 8. The ¹¹³Cd chemical shift at 300 K was 323.19 ppm. As the temperature increased, the ¹¹³Cd chemical shifts slightly moved in the negative direction, but these chemical shifts changed discontinuously near T_C1 and T_C2. In particular, more changes were observed at temperatures near T_C2 than at temperatures near T_C1, suggesting that temperature affected the environments around Cd. This proved that the coordination geometry of 6Cl around Cd ions in the CdCl₆ octahedra, as shown in the inset of Fig. 8, would change at the phase transition temperatures.

Figure 8.

MAS ¹¹³Cd NMR spectra of [NH₃(CH₂)₅NH₃]CdCl₄ at phases I, II, and III (olive areas: phase I, red areas: phase II, and black areas: phase III).

¹H and ¹³C spin-lattice relaxation times

The ¹H MAS NMR and ¹³C MAS NMR spectra were obtained with increasing delay times, and the plot of spectral intensities against increasing delay times was expressed as an exponential function. The decay rates of the spin-locked proton and carbon magnetization are expressed as the spin-lattice relaxation time, T_1ρ, as^34,35:

$${\text{P}}_{\text{H}(\text{C})}(\tau )={\text{P}}_{\text{H}(\text{C})}(0\text{)exp}(-\tau /{\text{T}}_{1\mathrm{\rho }}),$$ 1

where P_H(C)(τ) and P_H(C)(0) are the signal intensities for the proton (carbon) at time τ and τ = 0, respectively. The ¹H T_1ρ values of NH₃ and CH₂ at several temperatures were determined by the slope of the logarithmic plots of intensities against delay times. From the slope of their recovery curves, the ¹³C T_1ρ values for C-1, C-2, and C-3 were determined. The ¹H T_1ρ and ¹³C T_1ρ values are shown in Fig. 9 as a function of the inversed temperature. The ¹H T_1ρ values increased rapidly from 100 to 1000 ms. While the slope of the T_1ρ values at temperatures near T_C1 changed, the slope at temperatures near T_C2 exhibited a rather continuous value. Above T_C1, the ¹H T_1ρ value for NH₃ showed a decreasing trend. The activation energy, E_a, values for ¹H in NH₃ were evaluated from the slopes (represented by the solid lines in Fig. 9) of their log T_1ρ versus 1000/T plots. The E_a values below T_C1 were 6.65 ± 0.40 kJ/mol and 8.60 ± 2.32 kJ/mol for NH₃ and CH₂, respectively, while the E_a values above T_C1 were 2.85 ± 0.96 kJ/mol and 3.49 ± 1.47 kJ/mol for NH₃ and CH₂, respectively. And, the ¹³C T_1ρ values below T_C1 increased gradually with increasing temperature and then increased rapidly above T_C1. Near T_C2, the T_1ρ values were almost continuous, showing no significant changes. The E_a values of C-1, C-2, and C-3 below T_C obtained from the plot of log T_1ρ versus 1000/T were 1.73 ± 0.58 kJ/mol, 1.33 ± 0.49 kJ/mol, and 1.36 ± 0.76 kJ/mol, respectively. The E_a values of C-1, C-2, and C-3 above T_C1 were 3.04 ± 1.38 kJ/mol, 5.57 ± 1.04 kJ/mol, and 0.97 ± 1.43 kJ/mol, respectively. The behavior of T_1ρ for random motions with a correlation time, τ_C, could be described as fast- and slow-motion zones. The ¹H and ¹³C T_1ρ values at low and high temperatures correspond to the fast-motion region, where ω₁τ_C $\ll$ 1 and T_1ρ⁻¹ α exp(E_a/k_BT). In contrast, the ¹H T_1ρ values in NH₃ at high temperatures were attributed to the slow-motion region, where ω₁τ_C $\gg$ 1 and T_1ρ⁻¹ α ω₁⁻²exp(E_a/k_BT).

Figure 9.

Temperature dependences of ¹H and ¹³C NMR spin–lattice relaxation times, T_1ρ, in [NH₃(CH₂)₅NH₃]CdCl₄ near phase transition temperatures. Solid lines represent the activation energies.

Conclusion

The structure and phase transition temperatures of the [NH₃(CH₂)₅NH₃]CdCl₄ crystal were confirmed using XRD and DSC. Based on the NMR analysis of the crystal, we deduced that the crystallographic surroundings of ¹H, ¹³C, and ¹⁴N in the cation at temperatures close to T_C1 changed, whereas that of ¹¹³Cd in the anion at temperatures close to T_C1 and T_C2 exhibited significant changes. The changes in the NMR chemical shifts near T_C1 and T_C2 also suggested that the N–H···Cl hydrogen bond was affected.

On the other hand, the T_1ρ values of ¹H in NH₃ changed from fast to slow motion near T_C1. The T_1ρ values of ¹³C in CH₂ increased rapidly at T_C1, and the E_a values for ¹³C were more flexible at low temperatures than at high temperatures. By evaluating the T_1ρ values, we deduced that the effect on the molecular motion was effective at high temperatures.

Consequently, the phase transition at 336 K was associated with the change in the N–H···Cl bond due to the change in the coordination geometry of Cl around Cd in the CdCl₆ anion. The phase transition at 417 K was related to the ferroelastic phase transition attributed to the twin domains.

The thermodynamic properties, ferroelastic domain walls, coordination geometries, and molecular motions of [NH₃(CH₂)₅NH₃]CdCl₄ in this study are thought to be helpful in the study of hybrid perovskite types for their various applications in batteries and fuel cells.

Author contributions

A.R.L. designed the project and wrote the manuscript. Y.L.J. prepared Figs. 1 and 3. All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.