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. 2021 Oct 5;6(41):27568–27577. doi: 10.1021/acsomega.1c04671

Physicochemical Property Investigations of Perovskite-Type Layer Crystals [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a Function of Length n of CH2

Ae Ran Lim †,‡,*, Sun Ha Kim §,∥
PMCID: PMC8529888  PMID: 34693178

Abstract

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Hybrid perovskites have potential applications in several electrochemical devices such as supercapacitors, batteries, and fuel cells. Here, the thermal stabilities as a function of the length n of the CH2 groups in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) crystals were considered by TGA and DTA. The structural characteristics and molecular dynamics were studied by MAS and static NMR experiments. A comparison of spin–lattice relaxation times indicated that the organic cation containing 1H and 13C was significantly more flexible than the inorganic anion containing 113Cd. The flexibility of 1H increased with an increase in the length of CH2 in the carbon chain, resulting in a decrease in the activation energy (Ea) of 1H. The Ea of 13C at n = 3 and 4 was more flexible at high temperatures than at low temperatures. In contrast, the Ea of 13C at n = 2 was more flexible at low temperatures. These results provide insight into the thermal stability and molecular dynamics of these crystals as a function of the length n of CH2 groups in the carbon chain and are expected to facilitate applications.

1. Introduction

Halide perovskites have been reported as one of the most promising materials for photovoltaic and light-emitting devices.1−3 In recent years, the development of novel functional materials has resulted in considerable progress in the synthesis of several hybrid organic–inorganic perovskites. The organic–inorganic hybrid perovskite crystals [NH3(CH2)nNH3]MX4 (n = 2, 3, 4,...; M = Mn, Fe, Co, Cu, Zn, Cd,...; X = Cl and Br) have drawn significant research interest. Their physicochemical properties depend on factors such as the characteristics of the organic cations and geometry of the inorganic metal halide anions constituting the crystal.4−13 The organic cation of the hybrid complex contributes to properties such as structural flexibility and optical properties, whereas the inorganic anion is responsible for the thermal and mechanical properties. In the case of M = Mn, Cu, and Cd, the crystal structures consist of an alternate octahedron (MX6)2– and organic chains. In the case of M = Co and Zn, isolated tetrahedral structures are formed, where an inorganic layer of (MX4)2– is sandwiched between the layers of an organic cation.14−18 Perovskite compounds composed of [NH3(CH2)nNH3] cations and MX4 anions are zero-dimensional, whereas those containing MX6 anions are two-dimensional. Furthermore, several studies focusing on their molecular structure have reported even–odd effects as the number of carbon atoms in the diammonium chain changes, which affects the structural properties of these materials.19 Structural phase transitions have also been reported for [NH3(CH2)nNH3]MX4 types, in which the link between adjacent octahedral or tetrahedral planes is realized by methylene chains bearing NH3 groups on both ends. The [NH3(CH2)nNH3] organic chains extend along the longest c-axis. The perovskite [NH3(CH2)nNH3]CdCl4 (M = Cd, X = Cl) consists of puckered layers of CdCl6 separated by nearly perpendicular layers of [NH3(CH2)nNH3] chains. The distance between the two neighboring inorganic layers depends on the length of the organic chain.20 These compounds have attracted considerable interest owing to the multiplicity of their crystal structures, which governs their thermodynamic properties and structural dynamics. These hybrid perovskites have potential applications in several electrochemical devices such as supercapacitors, batteries, and fuel cells.21−26

1,2-Ethylenediammonium tetrachlorocadmate, [NH3(CH2)2NH3]CdCl4 (n = 2), crystallizes in the monoclinic form with the space group P21/a, and this crystal does not exhibit any structural phase transitions. The unit cell parameters at 293 K are a = 7.292 Å, b = 7.344 Å, c = 8.609 Å, β = 92.74°, and Z = 2.27 [NH3(CH2)2NH3] cations are situated between the layers and are linked to the layers via an N–H···Cl hydrogen-bonding network. The Cd atom is located on an inversion center, and the coordination environment is described as a highly distorted octahedral.28 1,3-Propylenediammonium tetrachlorocadmate, [NH3(CH2)3NH3]CdCl4 (n = 3), is known to undergo a structural phase transition at TC = 375 K.9,28 At room temperature, the crystals are orthorhombic with the space group Pman. The unit cell at 299 K has the following parameters: a = 7.373 Å, b = 7.523 Å, c = 19.111 Å, and Z = 4. When the temperature is above TC, the crystal is still orthorhombic; however, the space group becomes Imma, and the lattice constants at 403 K become a = 7.38577 Å, b = 7.56974 Å, c = 18.7300 Å, and Z = 4.9Figure 1 shows the structure of the [NH3(CH2)3NH3]CdCl4 crystal at 300 K. In each formula unit, the six hydrogen atoms in ammonium form hydrogen bonds N–H···Cl. The Cd atom is surrounded by six Cl atoms to form nearly regular CdCl6 octahedra. 1,4-Butanediyldiammonium tetrachlorocadmate, [NH3(CH2)4NH3]CdCl4 (n = 4), undergoes two structural phase transitions near 338 K (=TC2) and 367 K (=TC1).19 The phases III and II are monoclinic, whereas the high-temperature phase I is orthorhombic. At phases III and II, it is monoclinic, with space group P21/a and Z = 2. The unit cell parameters in phase III (at 293 K) are a = 7.657 Å, b = 7.585 Å, c = 9.541 Å, and β = 101.56°.19 Further, the lattice parameters in phase II (at 350 K) are a = 7.48 Å, b = 7.53 Å, c = 10.18 Å, and β = 97.5°. The high-temperature phase I is orthorhombic, with space group Pman and Z = 2. The unit cell parameters at 373 K are a = 7.377 Å, b = 7.538 Å, and c = 10.600 Å.

Figure 1.

Figure 1

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Structure of the [NH3(CH2)3NH3]CdCl4 crystal at room temperature.

The synthesis and characterization of [NH3(CH2)2NH3]CdCl4 with n = 2 were first discussed by Battaglia et al.28 Subsequently, the X-ray results for this crystal structure at 298 K were reported by Lamhamdi et al.27 The phase transition at 375 K for the [NH3(CH2)3NH3]CdCl4 crystal with n = 3 is reversible and continuous, according to previous dielectric and optical studies.29,30 In addition, the structural, thermal, and vibrational properties as well as molecular motions have been characterized by Staskieqicz et al.9 Recently, the physicochemical properties of this crystal were reported by our group.31 In the case of n = 4, the thermodynamic and crystallographic characters of the phase transitions for NH3(CH2)4NH3CdCl4 were studied, and the crystal structure in each phase was discussed from X-ray diffraction measurements.19,32 However, despite various potential applications, there have been limited studies on compounds containing Cd. In particular, the thermal properties, structural phase transitions, and structural dynamics resulting from differences in the methylene chain length of [NH3(CH2)nNH3]CdCl4 crystals have not been discussed in detail.

This study aims to investigate the thermodynamic properties and molecular dynamics of [NH3(CH2)nNH3]CdCl4 as a function of the length n of CH2 groups in the carbon chain. The crystal structures, phase transition temperatures, and thermodynamic properties of the crystals with n = 2, 3, and 4 are investigated using X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and differential thermal analysis (DTA). In addition, the chemical shifts and spin–lattice relaxation time T1ρ are probed using 1H magic angle spinning nuclear magnetic resonance (MAS NMR), 13C MAS NMR, and static 14N NMR as a function of temperature to elucidate the characteristics of the [NH3(CH2)nNH3] cation. Furthermore, the chemical shifts for 113Cd MAS NMR are recorded as a function of the temperature to understand the geometry of octahedral CdCl6. The results provide insights into the thermodynamic properties and structural dynamics of [NH3(CH2)nNH3]CdCl4 crystals based on the length of CH2 and even–odd effects of the organic chain and are expected to facilitate potential applications in the future.

2. Results

2.1. Crystal Structures

The X-ray powder diffraction patterns of the [NH3(CH2)nNH3]CdCl4 crystals (n = 2, 3, and 4) at 298 K are shown in Figure 2. The lattice constants for the [NH3(CH2)2NH3]CdCl4 crystal with n = 2 are determined to be a = 7.297 ± 0.002 Å, b = 7.336 ± 0.002 Å, c = 8.623 ± 0.002 Å, and β = 92.810 ± 0.011°, and those for [NH3(CH2)3NH3]CdCl4 with n = 3 are determined to be a = 7.351 ± 0.006 Å, b = 7.486 ± 0.005 Å, and c = 19.031 ± 0.014 Å. In the case of [NH3(CH2)4NH3]CdCl4 with n = 4, the lattice parameters are determined to be a = 7.663 ± 0.003 Å, b = 7.593 ± 0.002 Å, c = 9.514 ± 0.002 Å, and β = 101.616 ± 0.016°. These results are consistent with those reported previously.9,19,27−29

Figure 2.

Figure 2

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X-ray diffraction patterns of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) at 298 K.

2.2. Phase Transition Temperatures and Thermal Properties

The DSC curves of [NH3(CH2)nNH3]CdCl4 crystals at a heating rate of 10 K/min under a nitrogen atmosphere are shown in Figure 3. No peak was observed for the case of n = 2, whereas only one endothermic peak for n = 3 was observed at 374 K (=TC). Finally, in the case of n = 4, two endothermic peaks were observed at 341 K (=TC2) and 366 K (=TC1). These phase transition temperatures are consistent with those reported previously.9,19,30

Figure 3.

Figure 3

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Differential scanning calorimetry (DSC) curves of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4).

To verify whether the endothermic peaks correspond to phase transition or decomposition, TGA and DTA experiments were performed at the same heating rate. The TGA and DTA curves displayed in Figure 4–6 show that the crystals with n = 2, 3, and 4 are almost stable up to approximately 493, 539, and 536 K, respectively; according to the number n of CH2 groups in the carbon chain, the molecular weight loss near 493, 539, and 536 K marks the onset of partial thermal decomposition (at temperature Td). [NH3(CH2)nNH3]CdCl4 undergoes loss in the molecular weight with increasing temperature. The amount remaining as solid residues can be calculated from the molecular weights. When n = 2, the loss of 12 and 23% of its weight at temperatures of about 622 and 804 K was due to the decomposition of HCl and 2HCl, respectively (see Figure 4). The small endothermic peak at 374 K on the DTA curve for [NH3(CH2)3NH3]CdCl4 is assigned to the phase transition detected in the DSC experiment. Additionally, weight losses of 11% and 22% occurred at temperatures of 613 and 623 K, respectively (see Figure 5). Finally, in the case of [NH3(CH2)4NH3]CdCl4 crystals with n = 4, the two small endothermic peaks at 341 and 366 K on the DTA curve are attributed to the phase transition seen in the DSC result. At temperatures of 612 and 623 K, 11 and 21% of their weight were lost, respectively (see Figure 6). The molecular weight of the three crystals decreased sharply between 550 and 650 K. In the case of n = 3 and 4 near 800 K, weight losses of 45% occurred, whereas when n = 2, the weight loss was the smallest at 23%.

Figure 4.

Figure 4

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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results of [NH3(CH2)2NH3]CdCl4.

Figure 6.

Figure 6

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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results of [NH3(CH2)4NH3]CdCl4.

Figure 5.

Figure 5

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Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results of [NH3(CH2)3NH3]CdCl4.

To support the TGA results, the appearance of single crystals with changing temperature was observed with an optical polarizing microscope (Figure 7). In the case where n = 2, the crystal formed at 300 K was colorless and transparent, while it appeared slightly opaque above 547 K. Upon increasing the temperature to 622 K, HCl was eliminated, and the crystal turned orange. Finally, the surface near 633 K appeared to melt slightly. In the case where n = 3, the crystal had an opaque white color at room temperature. Upon increasing the temperature to 673 K, it remained opaque even though the 2HCl was blown away. For the case where n = 4, the crystal was transparent at 300 K, and it turned opaque white with increasing temperature, likely indicating the elimination of HCl. Finally, it turned bright brown as 2HCl was lost near 670 K.

Figure 7.

Figure 7

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Changes in crystal by optical polarizing microscopy: at (a) 300 K, (b) 547 K, (c) 573 K, (d) 622 K, and (e) 633 K for [NH3(CH2)2NH3]CdCl4 with n = 2; (a) 296 K, (b) 563 K, (c) 613 K, (d) 653 K, and (e) 673 K for [NH3(CH2)3NH3]CdCl4 with n = 3; and (a) 295 K, (b) 353 K, (c) 573 K, (d) 659 K, and (e) 670 K for [NH3(CH2)4NH3]CdCl4 with n = 4 (the photos were taken by the authors A. R. Lim).

2.3. 1H MAS NMR

The 1H MAS NMR spectra of [NH3(CH2)nNH3]CdCl4 crystals according to the length of the carbon chain were recorded as a function of temperature. The results of 1H chemical shifts for n = 2, 3, and 4 are shown in Figure 8. At 300 K, the 1H chemical shifts for NH3 and CH2 were obtained at 7.96 and 4.78 ppm, respectively, in the case of n = 2 and at 7.57 and 3.23 ppm, respectively, in the case of n = 3. Finally, for n = 4, they were obtained at 6.90 and 4.69 ppm, respectively. It can be seen that the 1H chemical shifts for NH3 when n = 2, 3, and 4 are similar, while the 1H chemical shifts for CH2 are very different. In the case where n = 3, no change was observed near TC. In particular, in the case where n = 4, the 1H chemical shift for CH2 shows discontinuity near TC2 and continuity near TC1, as represented by the circle in Figure 8. The chemical shifts for NH3 of all three crystals are nearly independent of temperature, which means that the surrounding environment of the 1H in NH3 does not significantly change with temperature. However, in the case where n = 4, the surrounding environments of 1H in CH3 do significantly change with temperature.

Figure 8.

Figure 8

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1H NMR chemical shifts for NH3 and CH2 in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a function of temperature.

The 1H MAS NMR spectra were measured with several delay times at each given temperature, and the plot of spectral intensities vs delay times was found to follow a single exponential function. The decay rate of the spin-locked proton magnetization is characterized by the spin–lattice relaxation time T1ρ as33−35

2.3. 1

where P(τ) and P(0) are the signal intensities at time τ and τ = 0, respectively. From the slope of the logarithm of intensities vs delay times plot, the 1H T1ρ values were determined for NH3 and CH2 at several temperatures. The 1H T1ρ results are shown in Figure 9 for the three compounds as a function of inverse temperature. In the cases where n = 2 and 3, as the temperature increases, the T1ρ values increase rapidly from 2 to 700 ms and then rapidly reduce at temperatures above 350 K. At 350 K, 1H T1ρ has maximum values when n = 2, and the values for NH3 and CH2 are 352 and 388 ms, respectively. The values for NH3 and CH2 when n = 3 are 496 and 683 ms. In the case where n = 4, the 1H T1ρ value tends to increase gradually as the temperature increases. The three compounds show a similar tendency at temperatures below 350 K, but their values at temperatures above 350 K show different trends depending on the length of the carbon chain. It can be seen that the 1H T1ρ values at high temperatures are different according to the n value. In the case where n = 3, there was no significant change in the vicinity of TC, but in the case of n = 4, it was found to be slightly discontinuous in the vicinity of TC2. The discontinuous changes of 1H chemical shifts and 1H T1ρ near TC2 are thought to be due to the rapid changes in the lattice constants c and β.19 The activation energy (Ea) values for 1H in NH3 for the three crystals were evaluated based on the slopes (represented by the solid lines in Figure 9) of their log T1ρ vs 1000/T plot. For n = 2, Ea was 35.04 ± 4.10 kJ/mol at high temperatures and 12.81 ± 0.42 kJ/mol at low temperatures. For n = 3, Ea was 25.36 ± 2.97 kJ/mol above TC and 8.37 ± 0.34 kJ/mol below TC, and for n = 4, Ea was 10.06 ± 2.24 kJ/mol above TC1 and 5.18 ± 0.28 kJ/mol below TC2. The Ea values for 1H in CH2 for the three crystals were the same as those for 1H for NH3 in the error range.

Figure 9.

Figure 9

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1H MAS NMR spin–lattice relaxation times T1ρ for CH2 and NH3 in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a function of inverse temperature. Solid lines represent the activation energies.

2.4. 13C MAS NMR

The 13C MAS NMR chemical shifts for CH2 in [NH3(CH2)nNH3]CdCl4 were recorded at several temperatures. The signal for TMS reference was measured at 38.3 ppm at 300 K, and this value was set to 0 ppm for the 13C chemical shift. Here, it is seen that CH2-1 in the [NH3(CH2)nNH3] cation is far from NH3 and that CH2-2 is located near NH3. The results of 13C chemical shifts according to the methylene chain length at 300 K are shown in the Supplementary Information. In the case where n = 2, only one 13C resonance line was obtained for CH2-2. In the case where n = 3 and 4, two 13C resonance lines were obtained for CH2-1 and CH2-2, respectively. At 300 K, the 13C chemical shift for n = 2 was recorded at 37.36 ppm, and those for n = 3 were observed at 25.00 and 39.09 ppm for CH2-1 and CH2-2, respectively. The 13C chemical shifts for n = 4 were observed at 25.30 and 42.85 ppm, respectively. Here, the chemical shifts for CH2-1 are similar for these crystals, but those for CH2-2 are different between n = 2, 3, and 4. The full width at the half-maximum (FWHM) for 13C NMR at 300 K is relatively narrow, ranging from 1.2 to 1.6 ppm.

Meanwhile, the chemical shifts for the in situ 13C MAS NMR spectra for the three compounds are shown in Figures 10–12 with increasing temperature. In the case where n = 2 (Figure 10), the 13C chemical shifts for CH2-2 increase slightly as the temperature increases. Some chemical shift changes around 290 K can also be seen. In the case where n = 3 (Figure 11), the slopes of the chemical shifts marked by red dotted lines are temperature dependent, with more variation for CH2-2 than for CH2-1. In addition, there was no change in chemical shifts near TC. However, in the case where n = 4 (Figure 12), the chemical shifts show discontinuity near TC2, whereas they show continuity near TC1. It can be seen that the chemical shifts of CH2-2 near TC2 change more than those of CH2-1. In the three compounds, the chemical shifts of CH2-2 (more so than that of CH2-1) are thought to be affected by the N sites bonded to both ends of CH2-2.

Figure 10.

Figure 10

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In situ 13C MAS NMR chemical shifts of [NH3(CH2)2NH3]CdCl4 as a function of temperature.

Figure 12.

Figure 12

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In situ 13C MAS NMR chemical shifts of [NH3(CH2)4NH3]CdCl4 as a function of temperature.

Figure 11.

Figure 11

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In situ 13C MAS NMR chemical shifts of [NH3(CH2)3NH3]CdCl4 as a function of temperature.

The 13C MAS NMR spectrum showed a change in intensity with increasing delay time at each temperature. All these decay curves could be described by a single exponential function, and from the slope of their recovery traces, the 13C T1ρ values for CH2-1 and CH2-2 were obtained for the three compounds and plotted as a function of 1000/T, as shown in Figure 13. In the case where n = 2, the 13C T1ρ value first decreased slightly with increasing temperature and then decreased rapidly at higher temperatures. In the case where n = 3, it decreased slightly with an increase in temperature, then increased again, and finally decreased at temperatures above TC. Meanwhile, the minimum T1ρ values (34.74 and 28.40 ms for CH2-1 and CH2-2, respectively) occur at 280 K. In the case where n = 4, T1ρ decreases slightly as the temperature increases and then increases rapidly near TC2. Similar to that in the case of n = 3, this tendency results from molecular motion below the phase transition temperature. There are distinct molecular motions, and the minimum T1ρ is due to the molecular motion of CH2-1 and CH2-2 in the [NH3(CH2)nNH3] cations in the case where n = 3 and 4, respectively. These T1ρ values could be described by the correlation time τC for the molecular motion, and the T1ρ value for the molecular motion is given by36,37

2.4. 2

where fa = τC /[1 + ω12τC2], fb = τC /[1 + (ωH – ωC)2τC2], fc = τC /[1 + ωC2τC2], fd = τC /[1 + (ωH + ωC)2τC2], and fe = τC /[1 + ωH2τC2]. Here, C is a coefficient, γH and γC are the gyromagnetic ratios for 1H and 13C, respectively, ℏ is the reduced Planck constant, r is the internuclear distance, ωH and ωC are the Larmor frequencies of 1H and 13C, respectively, and ω1 is the frequency of the spin-lock field. Here, the 13C T1ρ values were measured using the spin-locking pulse sequence with a locking pulse of ω1 = 75.76 kHz for n = 3 and ω1 = 70.42 kHz for n = 4. When ω1τC = 1, T1ρ has the minimum value. Therefore, a relationship between T1ρ and ω1 was applied to obtain the coefficient C in eq 2. Using this coefficient, τC was calculated as a function of temperature. According to Bloembergen–Purcell–Pound (BPP) theory, the local field fluctuation is governed by the thermal motion of CH2-1 and CH2-2. The correlation time τC for molecular motion at several temperatures follows the Arrhenius equation33

2.4. 3

where Ea and kB are the activation energy of the motions and Boltzmann constant, respectively. The magnitude of Ea depends on the molecular dynamics. The plot of log τC vs 1000/T provided the Ea values for CH2-1 and CH2-2, as shown in Figure 14; in the case where n = 3, the Ea values of CH2-1 and CH2-2 below TC were 26.96 ± 8.85 and 39.94 ± 10.45 kJ/mol, respectively. When n = 4, the Ea values of CH2-1 and CH2-2 below TC2 were 28.18 ± 2.05 and 23.18 ± 2.51 kJ/mol, respectively. Additionally, above TC when n = 3, the Ea values for CH2-1 and CH2-2 were 10.18 ± 2.68 and 8.45 ± 2.04 kJ/mol, respectively, determined using T1ρ = exp(±Ea/kBT) similar to the one expressed in eq 3. Unlike that in the cases of n = 2 and 3, the trend of 13C T1ρ at high temperatures is very different for n = 4. Above TC1, the values for n = 4 were 13.42 ± 2.01 and 14.35 ± 1.41 kJ/mol, respectively. When n = 3 and 4, the Ea values at low temperatures are greater than those at high temperatures, whereas when n = 2, their results are opposite (1.70 ± 0.24 kJ/mol below 330 K and 35.19 ± 4.32 kJ/mol above 330 K).

Figure 13.

Figure 13

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13C MAS NMR spin–lattice relaxation times T1ρ for CH2-1 and CH2-2 in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a function of inverse temperature. Solid lines represent the activation energies.

Figure 14.

Figure 14

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Arrhenius plots of the natural logarithm of the correlation times, τC, for 13C in [NH3(CH2)3NH3]CdCl4 and [NH3(CH2)4NH3]CdCl4 as a function of inverse temperature. Solid lines represent the activation energies.

2.5. Static 14N NMR

Static 14N NMR investigations of [NH3(CH2)nNH3]CdCl4 single crystals were conducted over the temperature range of 180–430 K. The 14N spectra were obtained using the solid-state echo method by static NMR. Two 14N NMR signals were expected from the quadrupole interactions due to the spin number I = 1.38 The static 14N chemical shifts for n = 2, 3, and 4 at 300 K are shown in Figure 15. Despite the presence of intense background noise in the spectra due to the extremely low NMR frequency (28.90 MHz) used for the experiment, the 14N signal was easily discerned.

Figure 15.

Figure 15

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In situ static 14N chemical shifts of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) single crystals at 300 K.

The FWHM of 14N NMR was similar for the three n values, which was approximately 33 ppm. The 14N NMR spectrum for n = 2, 3, and 4 differed with increasing temperature, as shown in Figure 16. Here, the measurements were performed by keeping the c-axis of the single crystals parallel to the direction of the magnetic field. In the case of n = 2, the two resonance lines of one pair decreased with increasing temperature, then decreased to a minimum near 400 K, and then increased again. In the case of n = 3, the chemical shifts for four resonance lines due to the two pairs caused a large change near TC. The symbols with the same color below TC indicate the same pairs for 14N. Near 374 K (=TC), the number of resonance lines and chemical shifts of the NMR spectrum showed abrupt changes; two pairs turned into just one pair. The changes in the 14N chemical shift as a function of temperature were attributed to the variations in the structural geometry. In addition, the chemical shifts of the 14N signals below TC changed almost continuously, and the chemical shifts for 14N above TC remained constant with temperature. The 14N NMR spectrum exhibits a reduction in the NMR lines from two to one pair of lines at the phase transition TC.

Figure 16.

Figure 16

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Static 14N NMR chemical shifts of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a function of temperature.

Finally, when n = 4, the two resonance lines showed no change in temperature below TC2; however, it was difficult to detect because the line width suddenly increased above TC2. In the case of n = 3, the different N spectra were explained as follows. None of the previously reported X-ray results9,19,27,30 include different N sites; therefore, two different N sites are thought to have a twin domain due to the ferroelastic property in materials with the organic–inorganic perovskite structure reported recently.39,40

2.6. 113Cd MAS NMR

113Cd MAS NMR experiments were performed to examine the structural dynamics in the anions of [NH3(CH2)nNH3]CdCl4 single crystals. 113Cd has an isotopic abundance of 12.3% and a spin of I = 1/2. This information is significant for the deduction of the anion coordination environments around Cd2+ in CdCl6 with unknown structures using 113Cd NMR spectroscopy.41,42 The 113Cd MAS NMR spectra for the three crystals were obtained at 300 K, as shown in Figure 17. Spinning sidebands were observed on the three spectra, which have been indicated in the figure by the symbol *. The spectrum for n = 3 is shown in a separate graph in Figure 17 as its peak intensity was larger than those for n = 2 and 4. The 113Cd chemical shift was 215.38 ppm for n = 2, 199.07 ppm for n = 3, and 192.42 ppm for n = 4. As n increased, the chemical shift decreased. The FWHM at 300 K was similar for the three crystals, which was approximately 3–3.5 ppm. The 113Cd chemical shifts for the three crystals shift slightly in the negative direction as the temperature increases, as shown in Figure 18. However, in the case where n = 4, it was discontinuous near TC2. This result suggests that in the cases when n = 2 and 3, there is no significant change in the environment near Cd depending on the temperature, whereas in the case of n = 4, the environment around Cd changes near the phase transition temperature.

Figure 17.

Figure 17

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In situ 133Cd MAS NMR chemical shifts of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) at 300 K.

Figure 18.

Figure 18

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113Cd MAS NMR chemical shifts of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) as a function of temperature.

The 113Cd MAS NMR spectrum for three crystals measured the change in intensity with various delay times at 300 K. The decay curves were described by a single exponential function, and the 113Cd T1ρ values were obtained from the slope of their recovery traces. In the cases where n = 2, 3, and 4, the T1ρ values were 2058, 1512, and 1101 ms, respectively. All of the T1ρ values for 1H, 13C, and 113Cd at 300 K are listed in Table 1. 113Cd T1ρ was very long compared to 1H T1ρ and 13C T1ρ. The long 113Cd T1ρ values with n = 2 were considered to be more rigid than the others (n = 3 and 4); a longer T1ρ indicates that the transfer of energy from the nuclear spin system to the surrounding environment is not very easy.

Table 1. Spin–Lattice Relaxation Times, T1ρ, Values for 1H, 13C, and 113Cd of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) at 300 K.

  1H T1ρ 13C T1ρ 113Cd T1ρ
n = 2 204.20 (NH3) 27.67 (CH2-2) 2058
211.92 (CH2)
n = 3 411.53 (NH3) 37.13 (CH2-1) 1512
551.12 (CH2) 29.13 (CH2-2)
n = 4 334.98 (NH3) 14.06 (CH2-1) 1101
357.83 (CH2) 13.90 (CH2-2)
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3. Discussion

The structures and phase transition temperatures of the [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) crystals were confirmed using X-ray diffraction and DSC. The thermal stability of the crystal improved as the length of the CH2 groups in the [NH3(CH2)nNH3] cations increased. Through the NMR analysis of the crystals, we deduced that n = 2 crystallographic environments of 1H, 13C, and 14N in the cation and 113Cd in the anion exhibited no significant changes with temperature. For n = 3, the results were similar to that of n = 2 excluding that of the 14N NMR, which indicated significant changes in the crystallographic environment of 14N in the cation. For n = 4, crystallographic environments of all atoms of the cation and anion exhibited changes near the phase transition temperature.

Through the evaluation of the T1ρ values for the various cation lengths, we deduced that the effect of the cation length on the molecular motion was evident only at high temperatures. The 1H and 13C T1ρ values for n = 2 and 3 decreased rapidly at high temperatures, whereas the values increased for n = 4. The Arrhenius-type behavior of T1ρ for random motions with a correlation time τC is described by two motion regimes: fast and slow motion regimes. The 1H T1ρ values at low temperatures for n = 2 and 3 were attributed to the fast motion regime, where ω1τC ≪ 1 and T1ρ–1α exp(Ea/kBT), and their values at high temperatures to the slow motion regime, where ω1τC ≫ 1 and T1ρ–1αω1–2 exp(Ea/kBT). The 1HT1ρ for n = 4 was associated with fast motion at all temperatures, whereas the 13C T1ρ for n = 2 and 3 to slow motion at all temperatures. However, for n = 4, the 13C T1ρ was associated to slow motion at low temperatures and fast motion at high temperatures.

4. Conclusions

Even–odd effects were significant on the structural properties of the single crystal but not on its physicochemical properties. In the case of n = 4, 1H and 13C T1ρ show the opposite nature at high temperatures, unlike the cases of n = 2 and 3. Furthermore, 113Cd T1ρ at 300 K was considerably longer than those of 1H and 13C, which indicated that the organic cation containing 1H and 13C was significantly flexible than the inorganic anion containing 113Cd. In addition, the flexibility of 1H increased with the length of the CH2 groups in the cation, which resulted in a decrease in Ea for 1H (see Table 2). Ea for 13C at n = 3 and 4 was more flexible at high temperatures than at low temperatures. On the contrary, Ea for 13C at n = 2 was more flexible at low temperatures. The effects of the length of CH2 in the cation on the molecular motion reported in this study will facilitate future research on hybrid perovskites for their potential applications in supercapacitors, batteries, and fuel cells.

Table 2. Activation Energies Ea Obtained from Spin–Lattice Relaxation Times T1ρ in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4).

  1H Ea (kJ/mol)
 13C Ea (kJ/mol)
  low temp. high temp. low temp. high temp.
n = 2 12.81 35.04 1.70 35.19
n = 3 8.37 25.36 26.96 (CH2-1) 10.18 (CH2-1)
39.94 (CH2-2) 8.45 (CH2-2)
n = 4 5.18 10.06 28.18 (CH2-1) 13.42 (CH2-1)
23.18 (CH2-2) 14.35 (CH2-2)
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5. Experimental Method

An aqueous solution containing NH2(CH2)nNH2·2HCl (Aldrich, 98%) and CdCl2 (Aldrich, 99.9%) was slowly evaporated in a thermostat at 300 K to produce single crystals of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4). The rectangular transparent single crystals were produced within 3–4 weeks, and these single crystals were stored in a desiccator to avoid moisture. The structures of the [NH3(CH2)nNH3]CdCl4 crystals at 298 K were analyzed using an X-ray diffraction system equipped with a Cu Kα radiation source.43 The lattice parameters were determined by single-crystal X-ray diffraction at the Seoul Western Center of the Korea Basic Science Institute (KBSI). The crystals were mounted on a Bruker D8 Venture equipped with a 1 μS microfocus sealed tube Mo Kα and a PHOTON III M14 detector.43

DSC (DSC 25, TA Instruments) measurements for the three crystals were carried out at a scanning speed of 10 K/min between 190 and 600 K under nitrogen gas. TGA and DTA experiments were performed on a thermogravimetric analyzer (TA Instrument) at the same heating rate between 300 and 873 K under N2 gas.44 In addition, optical observations were made using an optical polarizing microscope in the temperature range of 300–680 K, where the as-grown single crystals were placed on the heating stage of a Linkam THM-600.

NMR spectra of [NH3(CH2)nNH3]CdCl4 crystals were obtained using a Bruker 400 MHz Avance II+ solid-state NMR spectrometer equipped with 4 mm MAS probes at the Seoul Western Center, KBSI. The Larmor frequencies for 1H MAS NMR and 13C MAS NMR experiments were 400.13 and 100.61 MHz, respectively. The MAS rate to minimize the spinning sideband was 10 kHz, and the NMR chemical shifts were recorded using tetramethylsilane (TMS) as the standard.45 The T1ρ values were obtained using a π/2-τ pulse, followed by a spin-lock pulse of duration τ, and the width of the π/2 pulse for 1H and 13C was in the range of 3.4–3.62 μs. In addition, static 14N NMR and 113Cd MAS NMR spectra were measured with Larmor frequencies of 28.90 and 88.75 MHz, respectively. The 14N NMR experiments were performed using a solid-state echo sequence: 4 μs-τ-4 μs-τ; τ = 5 μs for n = 2, τ = 8 μs for n = 3 and 4. The 113Cd MAS NMR experiments were performed using a π/2-τ pulse, followed by a spin-lock pulse of duration τ, and the width of the π/2 pulse for 113Cd was 3.2 μs. The chemical shift measurements referenced NH4NO3 and CdCl2O8·6H2O as standard samples. The temperature was changed by adjusting the nitrogen gas flow and heater current, and it was maintained within ±0.5 K.

Acknowledgments

This research was supported by the Basic Science Research program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (2018R1D1A1B07041593 and 2016R1A6A1A03012069).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04671.

  • 13C NMR spectra for [NH3(CH2)nNH3]CdCl4 (n = 2, 3, and 4) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04671_si_001.pdf (169.8KB, pdf)

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