Preparation, Thermal, and Physical Properties of Perovskite-Type (C₃H₇NH₃)₂CdCl₄ Crystals

Abstract

To investigate the thermal and physical properties of perovskite-type (C₃H₇NH₃)₂CdCl₄, its temperature-dependent chemical shifts and spin–lattice relaxation times are measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), magic angle spinning nuclear magnetic resonance (MAS NMR), and static NMR methods. Above 300 K, two phase transitions are observed at 398 K and 538 K. Each proton and carbon in the (C₃H₇NH₃) cation is distinguished in the MAS NMR results. The environments around ¹H, ¹³C, and ¹⁴N do not change with temperature according to the NMR spectra. In contrast, the resonance frequency of ¹¹³Cd in the CdCl₆ octahedra decreases with increasing temperature, indicating an environmental change. The uniaxial rotations for ¹H and ¹³C have high mobility at both high and low temperatures, and these are related to the phase transitions. In addition, the molecular motion of ¹¹³Cd in the anion becomes activated upon raising the temperature.

1. Introduction

Organic–metal hybrid compounds provide many opportunities for potential applications [1,2,3,4,5,6]. In these materials, a large number of organic and metal moieties with different properties can be combined within a single structure. Hybrid organic–metal compounds based on the perovskite structures were first reported in 1976, and recently there is increasing interest in them due to their potential as substitutes for perovskite [7,8]. Especially, layered perovskite-type compounds with the formula (C_nH_2n+1NH₃)₂MX₄ (M = divalent metal, X = halogen ion) have interesting physical properties [9,10,11,12,13,14,15,16]. These compounds consist of inorganic layers of corner-sharing MX₆ octahedra and organic layers of alkylammonium ions. N–H···X hydrogen bonds are formed between the NH₃⁺ polar heads of the alkylammonium and the halogen atoms, and the polar heads occupy cavities among the octahedra [17]. In these materials, the cation dynamics and ion–ion interactions through hydrogen bonds affect the physical properties and thermal properties of structural phase transitions.

Previous studies have shown that the compound, when n = 3 and X = Cl, (C₃H₇NH₃)₂CdCl₄, undergoes three phase transitions below 300 K [16,17,18]. At 183 K, 156 K, and 105 K, they move from normal (N) to incommensurate (IC) to commensurate (C), and to another commensurate (C’) phases respectively with the decreasing temperature. The N phase at room temperature has an orthorhombic structure, and its space group is Abma (Z = 4). In addition, at a higher temperature (535 K), a ferroelastic transition was reported to occur from the tetragonal phase (point group 4/mmm) to the ferroelastic orthorhombic phase (point group mmm) [19,20]. The lattice constants of the latter were a = 7.585 Å, b = 7.359 Å, and c = 25.053 Å [21,22]. The NH₃ polar head of the alkylammonium chain points toward the layer of CdCl₆ to form hydrogen bonds with Cl⁻ in CdCl₆, while the alkyl chains point away from the CdCl₆ layer [23].

Using nuclear magnetic resonance (NMR) techniques, Blinc et al. [24] and Suzuki et al. [17] reported the spin–lattice relaxation times in the laboratory frame T₁ for ¹H and ²H in (C₃H₇NH₃)₂CdCl₄ at low temperatures. However, the protons in C₃H₇ and NH₃ were not distinguished from each other. The structural phase transition at low temperature was reported to be connected with a change in the motion of the alkyl groups. The cation motion associated with the molecular axis was found to become activated at higher temperatures.

Solid state ¹³C magic angle spinning (MAS) NMR, has proven to be a very powerful tool for studying the difference of the C–H vectors at different sites. The ¹³C spin–lattice relaxation time in the rotating frame T_1ρ is not influenced by spin diffusion due to the weak dipolar coupling, which results from the low natural abundance and large separation of the nuclei [25]. The T_1ρ measured by MAS NMR allows a probing molecular motion in the kHz range, whereas that in the laboratory frame T₁ measured by static NMR reflects motion in the MHz range. Although the structural phase transitions of (C₃H₇NH₃)₂CdCl₄ at low temperature have been examined by a few research groups [17,24], the corresponding phenomena at high temperatures have not been fully studied.

Here, we discuss the organic–metal hybrid (C₃H₇NH₃)₂CdCl₄ crystal that was grown. The thermal and physical properties are discussed using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and MAS NMR. The chemical shifts and T_1ρ of (C₃H₇NH₃)₂CdCl₄ were measured using ¹H MAS NMR, ¹³C cross-polarization (CP)/MAS NMR, and ¹⁴N MAS NMR in order to clarify the structural geometry and dynamics of the cation. Each proton and carbon in the (C₃H₇NH₃) cation was distinguished by MAS NMR. Additionally, the chemical shift and T₁ value for ¹¹³Cd were obtained as a function of temperature. The dynamics of the cation and anion near the phase transition temperatures are discussed. In addition, the ferroelastic domain walls were observed by optical polarizing microscopy.

2. Experimental Section

Single crystals of (C₃H₇NH₃)₂CdCl₄ were grown by slowly evaporating an aqueous solution containing C₃H₇NH₃Cl and CdCl₂ in 2:1 molar ratio at 27 °C. The obtained single crystals were transparent with a thin-plate shape. In addition, to determine the phase transition temperatures, DSC was carried out on the crystals with a Dupont 2010 DSC instrument under a heating rate of 10 K/min.

The NMR signals and the T_1ρ values of (C₃H₇NH₃)₂CdCl₄ were obtained for powder materials by solid state ¹H MAS NMR, ¹³C CP/MAS NMR, and ¹⁴N MAS NMR at the respective Larmor frequencies of ω₀/2π = 400.13, 100.61, and 43.34 MHz. The instruments were the Bruker 400 MHz NMR and Unity INOVA 600 MHz NMR spectrometers at the Korea Basic Science Institute, Western Seoul Center. The chemical shifts were referenced to tetramethylsilane (TMS) and NH₄Cl. For all ¹H, ¹³C, and ¹⁴N measurements, the powder sample was loaded into a 4-mm CP/MAS probe, and the MAS rate was set to 10 kHz to minimize spinning sideband overlap. The ¹H T_1ρ values were obtained using a π/2−t sequence by varying the spin-locking pulse durations. The ¹³C T_1ρ values were obtained by changing the duration of the spin-locking pulse applied after the CP preparation period. The width of the π/2 pulse used for measuring T_1ρ of ¹H and ¹³C was 3.7 μs, and the spin-locking field was 67.56 kHz. The ¹⁴N MAS NMR signals were recorded using a π/2−t sequence, and the pulse width was 4 µs. The ¹¹³Cd NMR spectra and the T₁ values of the single crystals were measured using a Bruker 400 MHz NMR spectrometer. The static magnetic field was 9.4 T, and the central radio frequency was set to ω₀/2π = 88.73 MHz. The T₁ value of ¹¹³Cd was measured using saturation recovery pulse sequence sat−t−π/2. Nuclear magnetizations of ¹¹³Cd nuclei at time t after the sat pulse were determined following the π/2 excitation pulse, and the π/2 pulse was 4.2 µs wide. The NMR spectra were recorded at temperatures of 180–430 K. Relaxation time outside this range could not be measured because of the limitations of the spectrometer used.

3. Results and Discussion

DSC analysis was used to determine phase transition temperatures of the (C₃H₇NH₃)₂CdCl₄ crystal. Three endothermic peaks related to phase transitions were observed at 178 K (T_C3), 398 K (T_C2), and 538 K (T_C1), as shown in Figure 1. Here, the transition temperatures of 178 K and 538 K are consistent with those reported by White et al. [26] and Ecolivet and Kusto [19], respectively. Meanwhile, the phase transition corresponding to the endothermic peak at 398 K has not been reported on before. On the other hand, DSC analysis at above 300 K was used to understand the temperature dependence. The curves reveal two endothermic peaks (T_C2 and T_C1) during heating, and one exothermic peak (T_C2) during cooling, as shown in the inset of Figure 1. This change is irreversible with respect to temperature.

Figure 2 shows the TGA and DSC curves under a N₂ atmosphere above 300 K. Minor weight loss of 1% is observed in the TGA for (C₃H₇NH₃)₂CdCl₄ at a temperature as low as 398 K, which is ascribed to the onset of partial thermal decomposition. There is a sudden weight loss at 538 K and it is attributed to the beginning of C₃H₇NH₂ and HCl evaporations. The weight loss was sharp between 540 and 617 K, with a corresponding weight loss of 36.5%. The DSC study results were used to confirm the presence of structural transitions below the melting/decomposition temperature. In the DSC curve, the endothermic peak at 398 K corresponds to a phase transition, while the endothermic peaks at 561 and 576 K (=T_d) indicate the decomposition of (C₃H₇NH₃)₂CdCl₄. Optical polarizing microscopic images (Figure 2a–c) showed that the crystals were colorless and transparent at 293–473 K.

The ¹H NMR spectra were obtained by MAS NMR, and the spectrum at 300 K shows two peaks (inset in Figure 3). The spinning sidebands are denoted with crosses and asterisks. The spectrum consists of two distinct peaks at δ = 2.77 and 7.18 ppm, which are assigned to the ¹H in C₃H₇ and NH₃, respectively. Both of these peaks have nearly constant chemical shift upon increasing the temperature; therefore, the local structures of the different ¹H atoms remained similar.

The nuclear magnetization decay of ¹H is single exponential S(t)/S₀ = exp(−t/T_1ρ), and the value of T_1ρ can be determined by fitting the traces [25,27,28]. The recovery curves for various delay times were measured at given temperatures, and the recovery traces have different slopes. This way, we could obtain the T_1ρ values for each proton in C₃H₇ and NH₃, and plot them vs. 1000/T in Figure 3. The ¹H T_1ρ data all show similar trends with temperature, although the values are smaller for NH₃ than for C₃H₇. The T_1ρ values abruptly decreases when the temperature approaches T_C2 = 398 K, and the values above T_C2 were short at hundreds of ms. Additionally, when the temperature decreases to near 180 K (T_C3), T_1ρ also becomes very short, like the case above 398 K, due to the impending phase transition. Therefore, although the local environments of ¹H suggested by the chemical shifts do not change with temperature, the molecular motion according to T_1ρ is very active at both high and low temperatures.

The ¹³C CP/MAS NMR spectrum of (C₃H₇NH₃)₂CdCl₄ at room temperature shows three peaks at 12.42, 21.46, and 43.41 ppm that are assignable to CH₃-3, CH₂-2, and CH₂-1, respectively, as shown in Figure 4. The crystal structure is confirmed from the chemical shifts obtained by the ¹³C MAS NMR method, and it is also consistent with the report by Suzuki et al. [17]. The peak of CH₃-3 at 12.42 ppm is relatively stronger, and all three ¹³C chemical shifts are temperature-independent. Therefore, the local environments of ¹³C do not change with the temperature based on the chemical shifts of (C₃H₇NH₃) cations. To obtain the ¹³C T_1ρ values, the nuclear magnetization is also measured as a function of delay time at several temperatures, and the ¹³C NMR spectrum at 300 K is shown in the inset of Figure 5. The signal intensity in the ¹³C nuclear magnetization recovery curves can be described by a single exponential function at all temperatures. For each carbon in (C₃H₇NH₃)₂CdCl₄, the calculated T_1ρ values are plotted vs. 1000/T in Figure 5. The T_1ρ values for CH₃-3, CH₂-2, and CH₂-1 all increase similarly with increasing temperature. At 398 K (T_C2), the spin–lattice relaxation time abruptly decreases with temperature like the case of ¹H T_1ρ, and this is related to the phase transition. On the other hand, the ¹³C T_1ρ values near T_C3 are more or less continuous. The T_1ρ values are 105.9, 38.5, and 40 ms for CH₃-3, CH₂-2, and CH₂-1 at 300 K, respectively. The much longer T_1ρ of CH₃-3 is consistent with the fact that the dipolar relaxation is made more efficient when there are more bound protons. The side chains have additional mobility because of the internal rotation.

Chemical shifts of ¹⁴N in (C₃H₇NH₃)₂CdCl₄ were also studied using ¹⁴N MAS NMR. The room-temperature spectrum shows one signal at δ = 6.71 ppm when referenced to NH₄Cl (inset in Figure 6). This signal shows no temperature dependence, indicating that the local environment surrounding ¹⁴N in the NH₃⁺ group hardly changes.

¹¹³Cd has a spin of I = 1/2 and its natural abundance is 12.3%. ¹¹³Cd NMR experiments have been employed to examine the structure and dynamics of several organic and inorganic materials [29]. We did not perform the ¹¹³Cd MAS NMR experiment because the ¹¹³Cd NMR signals are well detected with single crystals despite the low natural abundance ratio. The ¹¹³Cd static NMR spectrum consists of one peak at all temperatures. As shown in the inset of Figure 7, from 180 K to 420 K this resonance frequency for ¹¹³Cd signal decreases almost linearly. This result indicates that the environment of chloride surrounding the Cd is changed. The chemical shift of the Cd resonance lines is due to the contributions of the Cl ions [22]. The ¹¹³Cd saturation recovery plots of (C₃H₇NH₃)₂CdCl₄ single crystals were obtained at several temperatures by using a static NMR method, and were fitted with a single exponential function S(∞)‒S(t)/S(∞)=exp(‒Wt)=exp(‒t/T₁) [30]. The T₁ value, given by T₁=1/W, decreases upon increasing the temperature, as shown in Figure 7. At low temperatures, T₁ is very long in the order of 1000 s, but shortened to approximately 400 s at high temperatures. This temperature dependence of T₁ follows a single Arrhenius relation, T₁=T₀exp(‒E_a/RT), where T₀ is a pre-exponential factor, T is the temperature, R is the gas constant, and E_a is the activation energy. From the log T₁ vs. 1000/T plot, the E_a value for the molecular motion was found to be 3.64 ± 0.14 kJ/mol.

A crystal is ferroelastic if (1) it has two or more stable orientation states in the absence of mechanical stress or an electric field and (2) transition between these orientation states can be induced by the application of mechanical stress [31]. Upon heating, the ferroelastic effect usually disappears at a well-defined temperature. The domain structures at several temperatures are observed by optical polarizing microscopy. Figure 8 shows the domain patterns at (a) 300 K, (b) 350 K, (c) 400 K, (d) 450 K, (e) 480 K, and (f) 500 K. The microscopic domain walls represented by parallel lines are due to the ferroelastic property [32]. A magnification of 200x was used to take pictures of the domain walls. The domain pattern at high temperature became dimmer and faded out. Additionally, ferroelastic domains exist in all ferroelastic crystals because of the reduction in symmetry between the paraelastic and ferroelastic phases [33,34]. To change from a tetragonal structure (point group 4/mmm) to an orthorhombic one (point group mmm), the domain wall orientations can be expressed as x = 0 and y = 0. These equations of twin boundaries indicate the ferroelasticity of the 4/mmmFmmm species, where 4/mmm is the symmetry above 538 K, mmm is the symmetry below 538 K, and F represents the ferroelastic phase.

4. Conclusions

The thermal and physical properties of perovskite-type (C₃H₇NH₃)₂CdCl₄ were investigated by TGA, DSC, MAS NMR, and static NMR methods. Above room temperature, two phase transitions were observed at 398 K and 538 K. The C₃H₇ and NH₃ in (C₃H₇NH₃) were separated in the ¹H MAS NMR spectra, and the CH₃, CH₂, and CH₂ units in CH₃CH₂CH₂NH₃⁺ were clearly distinguished in the ¹³C CP/MAS NMR spectra. The chemical shifts of ¹H, ¹³C, and ¹⁴N signals were constant at all temperatures, whereas those of ¹¹³Cd decreased upon increasing the temperature. From these results, it is evident that the crystal geometries surrounding ¹H, ¹³C, and ¹⁴N do not change with temperature, whereas that surrounding ¹¹³Cd does. Further information about the dynamics of this crystal can be derived from the T_1ρ values of ¹H and ¹³C, and the results show that these atoms have higher mobility at high temperatures. The ¹³C T_1ρ in CH₃ increased with temperature, and the same trend was observed in alkyl chains attached to the (C₃H₇NH₃) cation due to a greater mobility toward the free end. In addition, the T₁ value of ¹¹³Cd was obtained, and it was very active at high temperatures. Overall, the cationic motion is associated with fluctuation of the uniaxial rotation and becomes more excited with increasing temperature. The dynamics of ¹¹³Cd ions in Cd–Cl bonds of CdCl₆ octahedra were strongly activated at higher temperatures, and these results are consistent with the rotational disorder of the CdCl₄ planes found by X-ray diffraction [22].