Thermodynamic Properties, Structural Characteristics, and Cation Dynamics of Perovskite-Type Layer Crystal [NH3(CH2)2NH3]ZnCl4.

In this study, we investigated the structural dynamic features of the [NH3(CH2)2NH3]ZnCl4 crystal as a function of temperature through magic angle spinning (MAS) 1H nuclear magnetic resonance (NMR), MAS 13C NMR, and static 14N NMR. From the chemical shifts, changes in the structural environments of 13C and 14N were evident. The 1H spin-lattice relaxation time (T 1ρ) values at high temperatures undergo molecular motion according to the Bloembergen-Purcell-Pound theory, and the 13C T 1ρ value also varied abruptly with increasing temperature. Although the phase-transition temperature was not detected from the differential scanning calorimetry result, the chemical shifts and T 1ρ results showed discontinuities around 300 K. Herein, the activation energies of molecular motion for 1H and 13C obtained from T 1ρ are discussed. In addition, we compare the structural dynamics of diammonium-type [NH3(CH2)2NH3]ZnCl4 obtained in this study and monoammonium-type [CH3NH3]2ZnCl4 previously reported. The findings reported herein can provide important insights for potential applications of [NH3(CH2)2NH3]ZnCl4 crystals.

Introduction

Recently, organic–inorganic compounds of two-dimensional (2D) hybrid perovskites have been extensively investigated. Compounds with chemical formulas of [(CH2NH3)]2BX4 (n = 1, 2, 3,...; B = Mn, Co, Cu, Zn, Cd,...; X = Cl, Br) and [NH3(CH2)NH3]BX4 (n = 2, 3,...) are examples of organic–inorganic hybrid perovskite-type layer compounds,[1−5] and they have been attracting considerable attention over recent years. Several studies on the monoammonium series [(CH2NH3)]2BX4 have been conducted by researchers worldwide, and the diammonium series [NH3(CH2)NH3]BX4 have been studied because of the relative stability and the H-bonds in the compounds.[6−11] The structure of a 2D hybrid perovskite is composed of alternately stacked organic layers and metal halide layers.[12]

The physical and chemical properties of the organic–inorganic hybrid perovskite [NH3(CH2)NH3]BX4 type are of significant scientific interest, and they depend on the hybrid perovskite characteristics, such as those of organic cations, inorganic anion geometry of the metal halide ions, (BX6)2– or (BX4)2–, and the reaction stoichiometric ratio.[1−5,13−19] In the cases of B = Mn, Cu, and Cd, the crystal structures consist of alternate octahedron (BX6)2– and organic chains, whereas for B = Co and Zn, isolated tetrahedral structures are formed, where an inorganic layer of (BX4)2– is sandwiched between the organic cation layers. The ammonium ions bonded at both ends of the organic chain combine with halide ions in the inorganic layer to stabilize the layered structure by forming N–H···Cl hydrogen bonds. This property enables these materials to be used as proton conductors[20,21] and renders them suitable for potential application in UV detection.[22] These compounds are of notable scientific interest owing to the multiplicity of their crystal structures, which is correlated to the structural dynamics of the cations and anions.

[NH3(CH2)2NH3]ZnCl4 (1,2-ethylenediammonium tetrachlorozincate(II)) with n = 2, B = Zn, and X = Cl in [NH3(CH2)NH3]BX4 has an orthorhombic structure with space group P212121 at 298 K. The unit cell dimensions are a = 8.832 Å, b = 9.811 Å, c = 11.089 Å, and Z = 4.[23] The (ZnCl4)2– anion forms an isolated tetrahedron, and the tetrahedron is interconnected via N–H···Cl hydrogen bonds to the [NH3(CH2)2NH3] cation. Each NH3 group is linked by N–H···Cl hydrogen bonds. The H atoms were located geometrically with N–H = 0.86 Å and C–H = 0.97 Å. The compounds with n = 2 do not exhibit any structural phase transition up to the decomposition temperature.[14] Thus far, the structure of the [NH3(CH2)2NH3]ZnCl4 crystal has only been investigated through X-ray diffraction.[23] In addition, although this compound containing Zn has applications in numerous areas, no study has been reported on it to date. A detailed study on the thermal properties and structural dynamics of the [NH3(CH2)2NH3]ZnCl4 crystal has also not been reported.

In this study, the phase transitions and thermodynamic properties of the [NH3(CH2)2NH3]ZnCl4 crystal are investigated through differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). 1H, 13C, and 14N nuclear magnetic resonance (NMR) spectra are used to detect changes in the chemical shifts accompanying changes in the crystallographic environments. The chemical shift by NMR depends on the local field at the site of the resonating nucleus in the crystals. The spin–lattice relaxation times (T1ρ) are discussed in terms of the role of the [NH3(CH2)2NH3] cation on the molecular dynamics as well as by performing magic angle spinning (MAS) 1H NMR and MAS 13C NMR. Based on these results, the thermodynamic properties, crystallographic environments, and structural molecular dynamics of [NH3(CH2)2NH3]ZnCl4 crystals are discussed for [NH3(CH2)2NH3] cations. In addition, we compare the physical properties of diammonium-type [NH3(CH2)2NH3]ZnCl4 obtained in this study and monoammonium-type (CH3NH3)2ZnCl4 with n = 2 previously reported. The results aid in gaining an understanding of the thermodynamic properties and the structural dynamics of [NH3(CH2)2NH3]ZnCl4 crystals, which is expected to subsequently facilitate their potential applications.

Results and Discussion

Thermodynamic Properties

The DSC analysis of the [NH3(CH2)2NH3]ZnCl4 crystal with a heating rate of 10 °C/min under a nitrogen atmosphere exhibits an endothermic peak at 536 K, as shown in Figure . To verify whether the endothermic peak at 536 K corresponds to the structural phase transition or melting, TGA was performed with the same heating rate. The TGA result, also displayed in Figure , shows the crystal to be almost stable up to approximately 525 K. Above 525 K, the compound [NH3(CH2)2NH3]ZnCl4 (Mw = 269.31 mg) experiences a molecular weight loss with increasing temperature. From the total molecular weights, the amounts of residue were obtained using eqs and (11,24)

Figure 1

Differential scanning calorimetry (DSC) thermogram and thermogravimetric analysis (TGA) curves of [NH3(CH2)2NH3]ZnCl4.

Residue:

Residue:

Near 536 K, the molecular weight loss begins to mark the onset of partial thermal decomposition (= Td). Weight losses of approximately 13 and 27% near 589 and 618 K may be attributed to the thermal decomposition accompanied by the partial escape of the HCl and 2HCl moieties, respectively, as shown in Figure . The molecular weight sharply decreased between 530 and 650 K with a corresponding weight loss of 44% near 667 K. Therefore, from the DSC and TGA results, the endothermic peak near 536 K is due to the partial thermal decomposition, which caused a sudden weight loss above 536 K corresponding to HCl and 2HCl evaporations.

MAS 1H NMR

The 1H NMR spectrum in [NH3(CH2)2NH3]ZnCl4 crystals recorded by MAS NMR at different temperatures is shown in Figure . The observed resonance lines at 180 K have an asymmetric shape; the line widths at the full-width at half-maximum on the left and right sides are not the same. This asymmetry is due to the overlapping lines of 1H for CH2 and NH3. The spinning sidebands are marked with “+” in Figure . As observed in Figure , one overlapping signal begins to separate into two signals at temperatures above 380 K. The resonance lines at 430 K are split into two resonance lines with chemical shifts of 3.83 and 7.31 ppm for CH2 and NH3, respectively. The chemical shifts remain quasi-constant with respect to temperature variation, indicating that the structural environments of 1H in the [NH3(CH2)2NH3] cation do not change.

Figure 2

MAS 1H NMR spectrum of [NH3(CH2)2NH3]ZnCl4 at several temperatures.

The MAS 1H NMR spectrum was measured for several delay times at each temperature, and the relation between the intensity of the NMR spectrum and delay time was found to follow a single exponential function. The decay rate of the spin-locked proton magnetization is characterized by T1ρ as[25−27]

where P(τ) and P(0) are the signal intensities at times τ and τ = 0, respectively. At 300 K, plots of the MAS 1H NMR signals in [NH3(CH2)2NH3]ZnCl4 for several delay times in the range of 1–160 ms are shown in Figure . All the decay curves were explained by a single exponential function represented by eq . From the slope of their recovery traces, the 1H T1ρ values for CH2 and NH3 in [NH3(CH2)2NH3]ZnCl4 were obtained as a function of 1000/temperature, as shown in Figure . As the temperature rises, T1ρ gradually increases and then reaches a maximum value of 570 ms at 270 K; however, T1ρ rapidly decreases above 300 K. Further, the T1ρ values for CH2 at 410, 420, and 430 K were found to be the same as those for NH3 within the error range. In Figure , the T1ρ versus 1000/temperature curve exhibits a minimum of 2.39 ms at 380 K, which is attributed to the molecular motions, and T1ρ again increases with a further increase in temperature above 380 K. Based on the theory of Bloembergen–Purcell–Pound (BPP), the T1ρ values are related to the rotational correlation time τC.[28,29] The T1ρ value for the molecular motion is given bywhere ga = τC/[1 + ω12τC2], gb = τC/[1 + (ωH – ωC)2τC2], gc = τC/[1 + ωH2τC2], gd = τC/[1 + (ωH + ωC)2τC2], and ge = τC/[1 + ωH2τC2].

Figure 3

Magnetization recovery curves for delay times from 1 to 160 ms of the MAS 1H NMR spectrum in [NH3(CH2)2NH3]ZnCl4 at 300 K.

Figure 4

1H NMR spin–lattice relaxation times T1ρ of [NH3(CH2)2NH3]ZnCl4 as a function of inverse temperature (solid square, NH3; solid circle, CH2) (inset: correlation times for T1ρ as a function of inverse temperature displayed in the purple box). Solid lines represent the fits to eq , yielding the activation energy, Ea.

Here, C is a coefficient, γH and γC are the gyromagnetic ratios for 1H and 13C nuclei, respectively, r is the H–C internuclear distance, ℏ = h/2π is the Planck constant, ωH and ωC are the Larmor frequencies of 1H and 13C, respectively, and finally, ω1 is a spin-lock field of 59.52 kHz. When ωCτC = 1, T1ρ is at its minimum; therefore, this relationship between T1ρ and ω1 was applied to obtain the coefficient C in eq . Using this coefficient, we calculated the correlation time τC as a function of temperature. According to the BPP theory, the local field fluctuation is governed by the thermal motion of CH2NH3, which is activated by thermal energy. In this case, correlation time τC is described by Arrhenius behavior, as follows[25,29,30]

where Ea and kB are the activation energy of the molecular motions and Boltzmann constant, respectively. As the magnitude of Ea depends on the molecular dynamics, we plotted the relation between τC and 1000/temperature on a logarithmic scale. Here, correlation times for the T1ρ values obtained in the temperature range of 300–430 K are shown in the inset of Figure . The Ea values for 1H above and below 300 K were estimated to be 78.17 ± 4.39 and 4.91 ± 0.62 kJ/mol, respectively. Here, it should be noted that T1ρ and Ea for 1H are the averaged values for all the hydrogen in the [NH3(CH2)2NH3] cation. Furthermore, the molecular motion for 1H at the ends of the organic cations was higher at a higher temperature.

MAS 13C NMR

The MAS 13C NMR chemical shifts in [NH3(CH2)2NH3]ZnCl4 were recorded as a function of temperature. The MAS 13C NMR spectrum for TMS was obtained at 38.3 ppm at 300 K, and this value was accurately calibrated to 0 ppm for the 13C chemical shift measurement of our sample. The MAS 13C NMR spectra at 180, 220, 260, 300, and 340 K are displayed in the inset in Figure . In addition, Figure shows the 13C NMR chemical shifts with increasing temperature, and the 13C chemical shifts can be found to have split into two lines; below 300 K, the 13C NMR chemical shift of CH2 splits into two distinguishable inequivalent lines of a-CH2 and b-CH2 in [NH3(CH2)2NH3]ZnCl4. This splitting indicates the existence of two different a-CH2 and b-CH2 below 300 K in the cation structure, as shown in Figure . It is thought that a-CH2 and b-CH2 having different environments exist because the bond lengths of N–H···Cl connected to both ends of NH3 are different. Further, the MAS 13C NMR spectrum consists of one resonance line for CH2 in the temperature range of 300–370 K. In addition, the 13C chemical shifts slowly move upward with increasing temperature. We will discuss this again in the 14N NMR experiment.

Figure 5

Chemical shifts of a-CH2 and b-CH2 by the MAS 13C NMR spectrum in [NH3(CH2)2NH3]ZnCl4 as a function of temperature (inset: in situ MAS 13C NMR spectrum at several temperatures).

The MAS 13C NMR signals of CH2 in [NH3(CH2)2NH3]ZnCl4 were obtained for different delay times at each temperature. The decay curves for a-CH2 and b-CH2 were analyzed by a single exponential function represented by eq . From the slope of their recovery traces, the 13C T1ρ values were obtained as a function of 1000/temperature, as shown in Figure . Similar to the 1H T1ρ value dropping sharply above 300 K, the 13C T1ρ value decreases sharply above 300 K with increasing temperature. In addition, the 13C T1ρ values for a-CH2 and b-CH2 are similar, as can be seen in Figure . At 300 K, the 13C T1ρ value is 8.35 ms, and it drops significantly to 0.16 ms at 430 K. The activation energies (Ea) obtained from the slopes of the log T1ρ versus 1000/temperature plot (Figure ) below and above 300 K are estimated to be 2.23 ± 0.45 and 38.88 ± 7.33 kJ/mol, respectively. In addition, the Ea values for b-CH2 below 300 K are the same with those of a-CH2 within the experimental error range. Further, the molecular motion of carbons was higher at a higher temperature.

Figure 6

13C NMR spin–lattice relaxation times T1ρ of [NH3(CH2)2NH3]ZnCl4 as a function of inverse temperature. Solid lines represent the activation energy, Ea.

Static 14N NMR

The NMR spectra of 14N were measured at a Larmor frequency of 28.90 MHz using the static NMR method. To obtain information concerning possible changes in the surroundings of the 14N ion, static NMR spectra of 14N were obtained. Here, the measurement was performed, keeping the c axis of the single crystal and direction of the magnetic field parallel. Temperature-dependent changes of the 14N chemical shift are attributable to alterations in the structural geometry.[31] Two resonance lines are obtained by the quadrupole interaction of the 14N (I = 1) nucleus. The static 14N NMR spectra at 220, 260, and 300 K are plotted in the inset in Figure , and the chemical shifts are referenced with respect to NH3NO3. The four signals, as shown in Figure , are attributed to the two inequivalent a′-NH3 and b′-NH3 ions. The two types of 14N resonance lines due to a′-NH3 and b′-NH3 are thought to be related to the two types of 13C resonance lines due to a-CH2 and b-CH2 measured earlier. The 14N chemical shift for a′-NH3 increases with increasing temperature until 300 K, whereas that for b′-NH3 decreases with increasing temperature until 300 K. It should be noted that a′-NH3 and b′-NH3 are arbitrarily determined for NH3. Above 300 K, the 14N chemical shifts for a′-NH3 and b′-NH3 increase sharply with increasing temperature. The temperature-dependent changes in the 14N chemical shifts are attributed to the changes in the structural geometry of N–H···Cl hydrogen bonds, reflecting changes in atomic configurations around the 14N nuclei near 300 K.

Figure 7

Chemical shifts of a′-NH3 and b′-NH3 by the static 14N NMR spectrum in [NH3(CH2)2NH3]ZnCl4 as a function of temperature (inset: static 14N NMR spectrum in the [NH3(CH2)2NH3]ZnCl4 single crystal at 220, 260, and 300 K).

Conclusions

To investigate the structural dynamic features of [NH3(CH2)2NH3]ZnCl4 crystals as a function of temperature, we performed MAS 1H NMR, MAS 13C NMR, and static 14N NMR. From these results, the 1H chemical shifts for the [NH3(CH2)2NH3] cation were found to be nearly temperature-independent. The two inequivalent a-CH2 and b-CH2 were distinguished by 13C NMR experiments, and two inequivalent a′-NH3 and b′-NH3 were distinguished by 14N NMR experiments. NH3 in the structure is correlated to CH2, and the main factor is a change in the surroundings of C–N groups in the [NH3(CH2)2NH3] cation. Remarkably, the changes in the 13C and 14N chemical shifts occurred around 300 K, and 1H and 13C T1ρ exhibited maximum values near 300 K. From the DSC results, no phenomenon was observed at 300 K; however, the chemical shifts and T1ρ results of 1H, 13C, and 14N nuclei showed discontinuous changes around 300 K. From these results, we can infer that the surrounding environments of 1H, 13C, and 14N sites change around 300 K.

We compared the phase-transition temperatures, crystal structures, space groups, lattice constants, spin–lattice relaxation times, and activation energies of the previously reported (CH3NH3)2ZnCl4[32] with those of [NH3(CH2)2NH3]ZnCl4 examined in this study, and the comparison results are summarized in Table . The difference between the two crystals lies only in the presence of organic cations. Although 1H T1ρ values are similar for both compounds, 1H Ea values at high temperatures are notably different. In addition, for (CH3NH3)ZnCl4, 13C T1ρ has a larger value, whereas Ea has a lower value than those for [NH3(CH2)2NH3]ZnCl4. For both compounds, Ea values for 1H and 13C are relatively similar at low temperatures; however, at high temperatures, the corresponding values for NH3(CH2)2NH3ZnCl4 are approximately 3.5 times larger than those for (CH3NH3)2ZnCl4. A significant difference between the two compounds is thought to be due to the bond lengths of (CH3NH3) cations in monoammonium-type (CH3NH3)2ZnCl4 and the bond lengths of [NH3(CH2)2NH3] cations in diammonium-type [NH3(CH2)2NH3]ZnCl4. These differences in molecular motions due to the bond length will prove to be beneficial for potential applications for the fields of energy and green chemistry.

Table 1

Phase-Transition Temperature TC, Crystal Structure, Space Group, Lattice Constant, Spin–Lattice Relaxation Time T1ρ, and Activation Energies Ea for (CH3NH3)2ZnCl4 and [NH3(CH2)2NH3]ZnCl4 Crystals

	(CH3NH3)2ZnCl4[32]	[NH3(CH2)2NH3]ZnCl4
TC (K)	265	483
structure	monoclinic	orthorhombic
space group	P21/c	P212121
lattice constants (Å)	a = 10.873	a = 8.832
	b = 12.655	b = 9.811
	c = 7.648	c = 11.089
1H T1ρ (ms)	10–500	2–600
13C T1ρ (ms)	10–200	0.1–20
1H Ea (kJ/mol)	19.72 ± 1.10 (CH3 above 250 K)	78.17 ± 4.39 (above 300 K)
	6.59 ± 0.51 (CH3 below 250 K)	4.91 ± 0.62 (below 300 K)
	19.88 ± 0.89 (NH3 above 250 K)
	5.92 ± 0.40 (CH3 below 250 K)
13C Ea (kJ/mol)	11.19 ± 1.33 (above 250 K)	38.88 ± 7.33 (above 300 K)
	1.02 ± 0.37 (below 250 K)	2.23 ± 0.45 (below 300 K)

Experimental Method

Single crystals of [NH3(CH2)2NH3]ZnCl4 were synthesized by slow evaporation at 300 K from an aqueous solution containing NH2(CH2)2NH2·2HCl and ZnCl2. Most of the colorless crystals grew in the shape of hexagonal thin plates.

DSC (thermogravimetric analyzer (TA), DSC 25) experimental results with a scanning speed of 10 °C/min were obtained in the temperature range of 190–600 K under nitrogen gas. TGA was performed on a TA (TA Instruments) in a temperature range of 300–680 K with the same heating rate as that in the DSC measurement. The amounts of samples used in DSC and TGA experiments were 5.96 and 7.98 mg, respectively.

NMR spectra of [NH3(CH2)2NH3]ZnCl4 crystal were obtained using a 400 MHz Avance II+ Bruker solid-state NMR spectrometer equipped with 4 mm MAS probes at KBSI, Seoul Western Center. MAS 1H NMR and MAS 13C NMR were measured at Larmor frequencies of 400.13 and 100.61 MHz, respectively. The MAS rate to minimize the spinning sideband was 10 kHz. The NMR chemical shifts were recorded by using tetramethylsilane (TMS) as the standard. The 1H and 13C T1ρ values were obtained using a π/2−τ sequence method by changing the spin-locking pulses, and the widths of the π/2 pulse for 1H and 13C were approximately 3.75 and 4.2 μs, respectively. In addition, the static 14N NMR spectra of the [NH3(CH2)2NH3]ZnCl4 crystals in the laboratory frame were measured using the same abovementioned NMR spectrometer at the same facility. The Larmor frequency for 14N NMR spectra was set at 28.90 MHz, and the static 14N NMR experiments were performed using a solid-echo pulse sequence. The temperature change was maintained within an error range of ±0.5 K by adjusting the nitrogen gas flow and heater current.