Thermal property and structural molecular dynamics of organic–inorganic hybrid perovskite 1,4-butanediammonium tetrachlorocuprate

Abstract

We investigate the thermal behaviour and physical properties of the crystals of the organic inorganic hybrid perovskite [(NH₃)(CH₂)₄(NH₃)]CuCl₄. The compound's thermal stability curve as per thermogravimetric analysis exhibits a stable state up to ∼495 K, while the weight loss observed near 538 K corresponds to partial thermal decomposition. The ¹H nuclear magnetic resonance (NMR) chemical shifts for NH₃ change more significantly with temperature than those for CH₂, because the organic cation motion is enhanced at both ends of the organic chain. The ¹³C NMR chemical shifts for the ‘CH₂-1’ units of the chain show an anomalous change, and those for ‘CH₂-2’ (units closer to NH₃) are shifted sharply. Additionally, the ¹⁴N NMR spectra reflect the changes of local symmetry near T_C (=323 K). Moreover, the ¹³C T_1ρ values for CH₂-2 are smaller than those for CH₂-1, and the ¹³C T_1ρ data curve for CH₂-1 exhibits an anomalous behaviour between 260 and 310 K. These smaller T_1ρ values at lower temperatures indicate that ¹H and ¹³C in the organic chains are more flexible at these temperatures. The NH₃ group is attached to both ends of the organic chain, and NH₃ forms a N–H⋯Cl hydrogen bond with the Cl ion of inorganic CuCl₄. When H and C are located close to the paramagnetic Cu²⁺ ion, the T_1ρ value is smaller than when these are located far from the paramagnetic ion.

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I. Introduction

The search for new and improved functional materials in recent years has resulted in considerable progress in the synthesis of many families of organic–inorganic compounds. The properties and structural phase transition of these compounds are related to their structures and the interaction of the cationic units with complex anionic sublattices. One such group of hybrid compounds, whose structure can be expressed by the general formula [NH₃(CH₂)₄NH₃]MX₄ (M = divalent metal ion and X = Cl, Br) is known to crystallise in a 2D perovskite-like structure, and these compounds are usually referred to as organic–inorganic hybrid perovskites or organic–metal-halide composites.^1–6 These perovskites combine the advantages of both organic and inorganic materials in a single molecular scale.^1,7,8 In particular, in the diammonium hybrid perovskite with its formula of [NH₃(CH₂)₄NH₃]MX₄, the NH₃ group is attached to both ends of the organic chain.^3,7,8 At the end of the organic part of the chain, the ammonium ion forms a N–H⋯X hydrogen bond with the halide ion of the metallic inorganic layer.⁹ These perovskite hybrids tend to exhibit a number of phase transitions such as order-disorder transitions. Here, we note that the properties of organic–inorganic hybrid perovskites depend on the organic cation, divalent metal, and halogen ion, and thus, it is necessary to investigate the ‘structure-directing’ properties of these new materials. In general, 2D hybrid perovskites can find use in the fields of energy, optoelectronics, photonics, and catalysis in green chemistry applications.^9–13

The compound [(NH₃)(CH₂)₄(NH₃)]CuCl₄, or 1,4-butanediammonium tetrachlorocuprate, with M = Cu and X = Cl, undergoes a reversible phase transition at 325 K (=T_C)¹⁴ between the two monoclinic phases II and I. The transition can be explained by order-disorder mechanisms involving a model of twisted conformation chains, which was introduced to explain the decrease in interlayer distance with increasing temperature from X-ray diffraction experiment. From structural considerations, these results can be explained by the conformational change of organic chains from the left-handed conformation in phase II to an all-trans conformation in phase I.¹⁴ The structural geometry of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ in the room-temperature phase II and high-temperature phase I are represented in Fig. 1(a) and (b), respectively. The crystal structure at room temperature is monoclinic, corresponding to space group P2₁/c. The unit cell dimensions are a = 9.270 Å, b = 7.600 Å, c = 7.592 Å, β = 103.14°, and Z = 2.^14,15 Structural cohesion is achieved via N–H⋯Cl hydrogen bonds. [(NH₃) (CH₂)₄(NH₃)]CuCl₄ is composed of alternating inorganic CuCl₄²⁻ layers and organic [NH₃(CH₂)₄NH₃]²⁺ sheets. The CuCl₄²⁻ layers are sandwiched by [NH₃(CH₂)₄NH₃]²⁺ cations, which possess centrosymmetrical chains with left-handed conformations at both ends, thereby forming the organic sheets. Above 325 K, the symmetry changes to that of a monoclinic structure with space group P2₁/c, and the corresponding lattice constants are a = 10.420 Å, b = 7.442 Å, c = 7.225 Å, β = 93.46°, and Z = 2.¹⁵

Fig. 1 Crystal arrangement on the bc-plane in [(NH₃)(CH₂)₄(NH₃)]CuCl₄ for (a) room temperature phase II and (b) high temperature phase I. Here, CH₂-1 represents two CH₂ between four CH₂, and CH₂-2 represents two CH₂ close to NH₃.

In the context of the property measurements of such compounds, Snively et al. conducted magnetic susceptibility measurements of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ powdered and single-crystals in the temperature range of 4 to 200 K.^16,17 They reported that the interplanar superexchange interaction along the linear Cu–Cl–Cl–Cu path exhibits a significantly stronger Cu–Cu-distance dependence than that along the Cu–Cl–Cu path. This crystal structure has been reported in phases I and II by Garland et al.¹⁸ Subsequently, the phase transitions occurring in the perovskite-type 2D molecular composite [(NH₃)(CH₂)₄(NH₃)]CuCl₄ have been studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD),¹¹ and electron paramagnetic resonance (EPR).^14,19,20

Understanding the structural dynamics of organic–inorganic hybrid perovskite [(NH₃)(CH₂)₄(NH₃)]CuCl₄ is essential for their advanced use as new materials. Here, we study the structural dynamics of the organic–inorganic hybrid perovskite [(NH₃) (CH₂)₄(NH₃)]CuCl₄ via magic angle spinning (MAS) nuclear magnetic resonance (NMR) and static NMR experiments. The chemical shifts and spin–lattice relaxation times in the rotating frame T_1ρ in the low- and high-temperature phases are measured by means of MAS ¹H NMR and cross-polarisation (CP)/MAS ¹³C NMR to understand the role of the organic cation in this crystal. The ¹⁴N NMR spectra of the compound in the laboratory frame are also obtained as a function of temperature. We use these results to discuss the structural dynamics of the NH₃–CH₂–CH₂–CH₂–CH₂–NH₃ chain below and above the phase transition temperature T_C. In particular, an examination of the hydrogen bonding of N–H⋯Cl between the Cu–Cl layer and the alkylammonium chain within [(NH₃)(CH₂)₄(NH₃)]CuCl₄ can provide important insights into the operational mechanism as regards potential applications.

II. Experimental method

Crystals of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ were prepared by mixing equimolar amounts of NH₂(CH₂)₄NH₂·2HCl and CuCl₂ in aqueous solution and allowing the resulting mixture to slowly evaporate. The light-green-coloured crystals grew as rectangular parallelepipeds with dimensions of 5 mm × 5 mm × 1 mm.

The crystal structure of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ was determined with a X-ray diffraction system, using a Cu-Kα radiation source at the KBSI, Seoul Western Center. DSC (TA, DSC 25) experiments were carried out at a heating rate of 10°C min⁻¹ in the temperature range of 190 to 600 K in a nitrogen-gas atmosphere. Thermogravimetry analysis (TGA) experiments were conducted using a thermogravimetric analyser (TA Instruments) under conditions identical to those of DSC over a temperature range of 300 to 680 K. The DSC and TGA experiments were performed by using crystal sample quantities of 6.23 and 7.53 mg, respectively.

Solid-state MAS NMR investigations of the [(NH₃)(CH₂)₄(NH₃)]CuCl₄ crystals were conducted by using a 400 MHz Avance II+ Bruker NMR spectrometer at the same facility. The MAS ¹H NMR and CP/MAS ¹³C NMR experiments were performed at the Larmor frequencies of ω₀/2π = 400.13 and 100.61 MHz, respectively. Solid samples were packed into 4 mm-diameter zirconia rotors and closed off using Vespel caps. The samples were spun at 10 kHz MAS by using dry nitrogen gas. The ¹H and ¹³C NMR chemical shifts were obtained with the use of tetramethylsilane (TMS) as a standard. The T_1ρ data for ¹H and ¹³C were obtained by applying a π/2 pulse, immediately followed by a long spin-locking pulse phase-shifted by π/2 with respect to the π/2 pulse. The width of the π/2 pulse used for T_1ρ measurements was 3.3 μs, which yields the frequency of the rotating frame as ω₁ = 75.75 kHz. The T_1ρ data were obtained by varying the length of the spin-locking pulse. In addition, the ¹⁴N NMR spectra of a [(NH₃)(CH₂)₄(NH₃)]CuCl₄ single-crystal were obtained at the Larmor frequency of ω₀/2π = 28.90 MHz in the laboratory frame. The ¹⁴N resonance frequency was referenced with NH₃NO₃ as the standard sample. The ¹⁴N NMR spectrum was obtained by the application of the following solid-state echo sequence: 8 μs–tau (16 μs)–8 μs–tau (16 μs). The temperature change was maintained within the error range of ±0.5 K by adjusting the nitrogen gas flow and heater current.

III. Results and discussion

The powder X-ray diffraction pattern of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ at 300 K is described in the ESI,† and this data is consistent with previously reported results.¹⁴ Fig. 2 shows the DSC curve of the [(NH₃)(CH₂)₄(NH₃)]CuCl₄ crystals obtained with the heating rate of 10°C min⁻¹. An endothermic signal corresponding to the previously reported¹⁴ II–I phase transition is detected at 323 K. In addition, a very large exothermic peak is observed at 538 K. To understand the origins of this peak, we performed TGA experiments; these results are also shown in Fig. 2. In the TGA curve, a stable state is observed up to ∼495 K, whereas a weight loss is observed at higher temperatures, which represents partial thermal decomposition. Here, we note that [(NH₃)(CH₂)₄(NH₃)]CuCl₄ crystals show the weight loss with temperature increase. From the TGA experimental results and possible chemical reactions, we compared the weight loss. The weight loss of 12% around 538 K obtained from the DSC experiment is consistent with the calculated decomposition of HCl moieties. From the figure, we note that the weight sharply decreases between 500 and 650 K, with a corresponding weight loss of 67% near 650 K.

Fig. 2 Differential Scanning Calorimetry (DSC) and thermogravimetric analysis (TGA) curves of [(NH₃)(CH₂)₄(NH₃)]CuCl₄.

Next, we acquired the MAS ¹H NMR spectrum of the [(NH₃)(CH₂)₄(NH₃)]CuCl₄ crystals at various temperatures (Fig. 3). In the figure, we can observe two resonance signals for ¹H. The spinning sidebands corresponding to CH₂ are indicated by open circles, and those of NH₃ are indicated by crosses. At 300 K, the ¹H NMR chemical shifts for CH₂ and NH₃ are observed at δ = 2.73 ppm and δ = 11.86 ppm, respectively. Below 300 K, the ¹H resonance signal bonded to CH₂ mostly merges with the ¹H resonance signal bonded to NH₃, which makes it difficult to distinguish the two signals. In addition, the ¹H resonance signal for CH₂ is related to the number of bonded protons, which means that the signal exhibits a stronger intensity and wider linewidth than the corresponding ones for NH₃. The ¹H NMR chemical shifts according to the temperature exhibit a greater change for NH₃ than CH₂. These results indicate that NH₃ is temperature-sensitive.

Fig. 3 MAS ¹H NMR spectra for CH₂ and NH₃ of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ at various temperatures (spinning sidebands for CH₂ and NH₃ are indicated by open circles and crosses, respectively).

Next, we measured the MAS ¹H NMR spectrum at various temperatures, and the intensity change for the delay time was observed to obtain the spin–lattice relaxation time in the rotating frame (T_1ρ) for ¹H at each temperature. Normally, the T_1ρ data can be obtained as the slope of the intensity or the ratio of the area of the resonance signal to the delay time. The change in the proton magnetisation intensity in terms of T_1ρ is expressed as below:^21–23

P(τ) = P(0) exp(−τ/T_1ρ), (1)

where P(τ) and P(0) denote the signal intensities at time τ and τ = 0, respectively. Next, at 300 K, the MAS ¹H NMR signals of CH₂ and NH₃ were plotted for various delay times in the range from 0.2 to 80 ms (Fig. 4); we note that the intensities of the ¹H NMR signal as a function of the delay times exhibit considerable variation. From the slope of the intensity vs. delay time curve, the ¹H T_1ρ data of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ were obtained for CH₂ and NH₃ in the low-temperature phase II and high-temperature phase I. From Fig. 4, we note that no changes are observed in the T_1ρ value near T_C, and T_1ρ slightly increases according to the temperature change. The ¹H T_1ρ values for CH₂ and NH₃ at 300 K are 14.37 and 12.88 ms, respectively. Here, the ¹H T_1ρ value for NH₃ is smaller than that for CH₂. This is possibly because NH₃ is closer to the inorganic CuCl₄ layer; the T_1ρ value becomes smaller as the distance between H and the paramagnetic Cu²⁺ ion reduces. This is because T_1ρ is inversely proportional to the square of the magnetic moment of the paramagnetic ion.²¹

Fig. 4 ¹H NMR spin-lattice relaxation times T_1ρ for CH₂ and NH₃ ions of [(NH₃) (CH₂)₄(NH₃)]CuCl₄ as a function of temperature (inset: ¹H NMR spectrum at several delay times at 300 K).

The CP/MAS ¹³C NMR chemical shifts measured at various temperatures are shown in Fig. 5. In the study, the MAS ¹³C NMR spectrum for TMS was recorded at 38.3 ppm at 300 K, and this value was calibrated to determine the chemical shift in ¹³C. Here, the two inner CH₂ groups of the four CH₂ ones are together designated as CH₂-1, and the two CH₂ units close to the NH₃ ones are designated as CH₂-2. We note from the figure that the ¹³C chemical shifts for CH₂-1 (far from NH₃) are different from those for CH₂-2, which is closer to NH₃. In the ¹³C NMR spectra obtained for CH₂-1 and CH₂-2, two unusual resonance lines are observed between 260 and 310 K. At 300 K, the two resonance signals for CH₂-1 are recorded at chemical shifts of δ = 38.44 and 59.56 ppm. Furthermore, the signal of δ = 98.23 ppm corresponds to CH₂-2. The ¹³C chemical shifts for CH₂-1 exhibit an anomalous change with increase in temperature, whereas those for CH₂-2 shift abruptly with increasing temperature, as shown in Fig. 6. The two resonance lines between 260 and 310 K correspond to CH₂-1, and hitherto unreported anomalous phenomena are observed in this temperature range.

Fig. 5 In situ MAS ¹³C NMR spectra for CH₂-1 and CH₂-2 of [(NH₃)(CH₂)₄ (NH₃)]CuCl₄ as a function of temperature.

Fig. 6 MAS ¹³C chemical shifts for CH₂-1 and CH₂-2 of [(NH₃)(CH₂)₄ (NH₃)]CuCl₄ at low temperature phase II and high temperature phase I.

We next remark that line broadening in the MAS ¹³C NMR spectra is influenced by relaxation processes such as the motional modulations of the chemical shift anisotropy and dipolar carbon-proton coupling. Fig. 7 shows the ¹³C full-width at half-maximum (FWHM) linewidth of [(NH₃)(CH₂)₄(NH₃)]CuCl₄. The ¹³C NMR line shapes vary from the Gaussian type at lower temperatures to the Lorentzian shape at higher temperatures. The appearance of these two-component spectra is caused by difference in different molecular motions. The linewidth near the phase transition temperature T_C shows a monotonic decrease, thereby indicating the presence of motional narrowing at high temperatures.

Fig. 7 MAS ¹³C NMR spin-lattice relaxation times T_1ρ for CH₂-1 and CH₂-2 of [(NH₃) (CH₂)₄(NH₃)]CuCl₄ as a function of temperature (inset: line widths for CH₂-1 and CH₂-2 according to the temperature).

The spin–lattice relaxation time in the rate of relaxation is due to spin–lattice interactions in the rotating frame. The ¹³C T_1ρ relaxations are not influenced by spin diffusion because of the small dipolar coupling which arises from the low natural abundance and large separation of the nuclei. Under these conditions, we next analysed differences in the chain motions. The integration change of the ¹³C NMR spectrum obtained for various delay times was measured, and all the decay curves for CH₂-1 and CH₂-2 were plotted by using a single exponential function. From the slope of their recovery traces, the ¹³C T_1ρ data were obtained for CH₂-1 and CH₂-2 as a function of temperature, as shown in Fig. 7. It can be observed that although no change in the T_1ρ value is observed near T_C, T_1ρ above T_C abruptly increases with increasing temperature. The ¹³C T_1ρ values for CH₂-1 and CH₂-2 at lower temperatures (below T_C) are nearly identical; however, the ¹³C T_1ρ values for CH₂-2 close to NH₃ at high temperatures are smaller than that for CH₂-1. At high temperatures, smaller ¹³C T_1ρ values for CH₂-2 are more flexible than the CH₂-1. Just as the 13C resonance lines for CH₂-1 exhibited anomalies between 260 and 310 K, the 13C T_1ρ also exhibits two different sets of values. The relaxation time for Arrhenius-type random motions with correlation time τC is described in term of slow motions; for τC ≪ ω_L, T_1ρ ∼ τ_C = τ₀ exp(−E_a/k_BT), where ω_L denotes the Larmor frequency and E_a the activation energy.

The ¹⁴N NMR spectrum in the laboratory frame was next measured in the temperature range from 180 to 430 K by using the solid-state echo method at the Larmor frequency of 28.90 MHz by means of static NMR. The two resonance lines are obtained by spin number I = 1,^24,25 and the resonance frequency around T_C (=323 K) changes as shown in Fig. 8. The observed change in the ¹⁴N resonance frequency with temperature is due to structural geometry change, which means a change in the quadrupole coupling constant. The linewidth at 300 K is ∼44 ppm, and this spectrum is relatively broader than the ¹H and ¹³C NMR spectra. The ¹⁴N resonance frequency decreases almost continuously until 270 K, while that of the ¹⁴N signal between 280 and 310 K exhibits an anomalous pattern. Similar to the anomaly of the 13C resonance line observed between 260 and 310 K, an abnormal phenomenon is observed in the 14 N resonance line in this region. At 320 K near T_C, there is only one ¹⁴N resonance line, and at temperatures >320 K, no resonance lines are observed. At the transition point of 323 K, the ¹⁴N NMR lines merge into one line. This single ¹⁴N resonance line indicates that there is no electric field gradient (EFG) tensor at the N site in phase I because of site symmetry. The EFG tensor changes around the N site, which indicates a change in the structural configuration around the N site near T_C.

Fig. 8 ¹⁴N resonance frequency of [(NH₃) (CH₂)₄(NH₃)]CuCl₄ single crystal as a function of temperature (inset: in situ ¹⁴N resonance frequency at several temperatures).

IV. Conclusions

In this study, we investigated the thermal behaviour and physical properties of organic–inorganic hybrid perovskite [(NH₃)(CH₂)₄(NH₃)]CuCl₄ crystals. The structural dynamics of [(NH₃)(CH₂)₄(NH₃)]CuCl₄ with emphasis on the role of the [(NH₃)(CH₂)₄(NH₃)] cation, were discussed by MAS ¹H NMR, MAS ¹³C NMR, and static ¹⁴N NMR as a function of temperature. Firstly, we found that the TGA curve exhibited stability until 495 K, and the observed weight loss of 12% near 538 K was due to the partial thermal decomposition of HCl moieties.

Secondly, the ¹H NMR chemical shift of NH₃ for crystallographic environments changed more significantly with temperature than that for CH₂ because the [(NH₃)(CH₂)₄(NH₃)] cation motion is enhanced at both ends of the cation of the NH₃ group. The ¹³C NMR chemical shifts for CH₂-1 showed an anomalous change, and those for CH₂-2 shifted sharply to lower values when compared with that of CH₂-1. The 13C chemical shifts of the CH2-2 unit (closer to the N–H⋯Cl bond) sharply changed relative to those of CH₂-1. In addition, the ¹⁴N NMR spectra reflected the changes in the local symmetry of the crystal near T_C.

The ¹H T_1ρ values for CH₂ and NH₃ slightly increased with temperature increase. Moreover, the ¹³C T_1ρ value for CH₂-2 was smaller than that of CH₂-1, and the ¹³C T_1ρ value for CH₂-1 exhibited an anomalous trend between 260 and 310 K. At low temperatures, the ¹H and ¹³C T_1ρ values were smaller than at high temperatures. Smaller T_1ρ values at lower temperatures indicate that ¹H and ¹³C in the organic chains are more flexible at these temperatures. Moreover, the ¹³C of CH₂-2 close to NH₃ of the organic chain is more flexible than the ¹³C of CH₂-1 between the four CH₂ sites. The NH₃ group is attached to both ends of the organic chain, and it forms a N–H⋯Cl hydrogen bond with the Cl ion of the inorganic CuCl₄. The T_1ρ value is smaller when H and C are located close to the paramagnetic Cu²⁺ ion than when far away. Additionally, the NH₃ groups are coordinated by CuCl₄, and thus, atomic displacements in the environment of the ¹⁴N nuclei with temperature are correlated with CuCl₄. We also note here that detailed studies are required to examine the anomalies observed in the range of 260 to 310 K.

Here, we compared the phase transition temperatures, decomposition temperatures, crystal structures, space groups, lattice constants, and spin-lattice relaxation times of the previously reported [C₂H₅NH₃]₂CuCl₄ (ref. 26–35) and those of [NH₃(CH₂)₄NH₃]CuCl₄ examined in this study; this is summarised in Table 1. The difference between the two crystals is only the presence of organic cation. The two compounds have four and one phase transition temperatures, respectively. The decomposition temperature of [NH₃(CH₂)₄NH₃]CuCl₄ is higher than that of [C₂H₅NH₃]₂CuCl₄, and the [NH₃(CH₂)₄NH₃]CuCl₄ has a high thermal stability. Furthermore, the ¹H and ¹³C T_1ρ values in [C₂H₅NH₃]₂CuCl₄ are slightly different from those for [NH₃(CH₂)₄NH₃]CuCl₄. Although the two crystals have same anions, the molecular motions according to the ¹³C bond lengths of the (C₂H₅NH₃) cation in [C₂H₅NH₃]₂CuCl₄ and [NH₃(CH₂)₄NH₃] cation in [NH₃(CH₂)₄NH₃]CuCl₄ are different. The above results suggest that the molecular motions obtained from 1H and 13C T_1ρ will be considered as good examples of potential applicability.

Table 1 Phase transition temperature T_C, decomposition temperature T_d, structure, space group, lattice constant, spin-lattice relaxation time T_1ρ for [C₂H₅NH₃]₂CuCl₄ and [NH₃(CH₂)₄NH₃]CuCl₄ crystals

	[C2H5NH3]2CuCl4	[NH3(CH2)4NH3]CuCl4
TC (K)	236, 330, 357, 371 (ref. 26–31)	325 (ref. 14)
Td (K)	430	495
Structure	Orthorhombic^32–34	Monoclinic^14,15
Space group	Pbca	P21/c
Lattice constant	a = 7.47	a = 9.270
(Å)	b = 7.35	b = 7.600
	c = 21.18	c = 7.592
		β = 103.14°
^1H T1ρ (ms)	7–20 (ref. 35)	6–16
^13C T1ρ (ms)	2–200	10–120

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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, funded by the Ministry of Education, Science, and Technology (grant numbers 2018R1D1A1B07041593 and 2016R1A6A1A03012069).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra06551j

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