Effect of Methylene Chain Length on the Thermodynamic Properties, Ferroelastic Properties, and Molecular Dynamics of the Perovskite-type Layer Crystal [NH3(CH2) n NH3]MnCl4 (n = 2, 3, and 4).

The structures and phase transitions of [NH3(CH2) n NH3]MnCl4 (n = 2, 3, and 4) crystals were confirmed through X-ray diffraction and differential scanning calorimetry (DSC) experiments. Thermodynamic properties, ferroelastic properties, and molecular dynamics of three crystals were studied as a function of the number (n) of CH2 groups in the alkylene chains. The loss in molecular weight due to a decrease in n marked the onset of the partial thermal decomposition. The thermal decomposition temperature, T d, increased with increasing length of the CH2 chain. While the ferroelastic twin domain walls for n = 2 and 4 were in the same direction at all temperatures, the domain walls for n = 3 were rotated by 45°, and the direction of extinction in phase II was rotated by 45° with respect to phases I and III. The 1H and 13C MAS NMR spectra of the three crystals were recorded as a function of temperature. With increasing length of the CH2 chain, the 1H spin relaxation time decreased, indicating that molecular motions were activated. These results provide insights into the thermodynamic properties and structural dynamics of the [NH3(CH2) n NH3]MnCl4 crystals and are expected to facilitate their potential applications.

Introduction

The physical and chemical properties of the organic–inorganic hybrid perovskite [NH3(CH2)NH3]MX4 (n = 2, 3, 4, 5; M = Mn, Fe, Co, Cu, Zn, Cd,...; X = Cl and Br) have triggered significant scientific interest. The properties depend on factors such as the characteristics of organic cations and geometry of the inorganic metal halide anions ((MX6)2– or (MX4)2–) constituting the crystal.[1−13] When M = Mn, Cu, and Cd, the crystal structures consist of alternate octahedron (MX6)2– and organic chains. In contrast, when M = Co and Zn, isolated tetrahedral structures are formed, where an inorganic layer of (MX4)2– is sandwiched between layers of an organic cation.[14−22] The ammonium ions bonded at both ends of the organic chain combine with the halide ions in the inorganic layer and stabilize the layered structure by forming N–H···X hydrogen bonds. The organic chains extend along the a-axis. Perovskite [NH3(CH2)NH3]MnCl4 consists of puckered layers of MnCl4 separated by layers of NH3(CH2)NH3 chains that are nearly perpendicular to the layers. The distance between the two neighboring inorganic layers strongly depends on the length of the organic chain.[23] In addition, the ferroelasticity is commonly observed in materials with a perovskite crystal structure. Recently, the ferroelastic twin domain observed in organic–inorganic hybrid perovskite has also garnered much attention.[24−26] These compounds are of notable scientific interest owing to the multiplicity of their crystal structures, which govern their thermodynamic properties, ferroelastic properties, and structural dynamics.

1,2-Ethylenediammonium manganese tetrachloride, [NH3(CH2)2NH3]MnCl4, (n = 2), crystallizes in the monoclinic space group P21/b and is twinned at room temperature.[27] This crystal does not exhibit any structural phase transition up to the decomposition temperature. The unit cell parameters are a = 8.609 Å, b = 7.130 Å, c = 7.192 Å, γ = 92.69°, and Z = 2. 1,3-Propylenediammonium manganese tetrachloride, [NH3(CH2)3NH3]MnCl4, (n = 3), undergoes two structural phase transitions at 305 and 336 K.[28,29] All the three phases of [NH3(CH2)3NH3]MnCl4 belong to the orthorhombic space group. The crystal structure in phase III at room temperature has the orthorhombic space group Imma, with lattice constants a = 19.00 Å, b = 7.172 Å, c = 7.361 Å, and Z = 4.[30] The crystal structures in phases I and II are the same as that in phase III. The compound crystallizes in the Fmmm space group in phase I, with lattice constants a = 18.891 Å, b = 7.198 Å, c = 7.398 Å, and Z = 4. It crystallizes in the Pnma space group in phase II, with lattice constants a = 18.867 Å, b = 10.254 Å, c = 10.382 Å, and Z = 8.[29] 1,4-Butanediyldiammonium manganese tetrachloride, [NH3(CH2)4NH3]MnCl4, (n = 4), undergoes structural phase transition near 382 K.[31] At 295 K, it is monoclinic, with the space group P21/b and Z = 2. The unit cell parameters are a = 10.770 Å, b = 7.177 Å, c = 7.307 Å, and γ = 92.67°. The high-temperature phase is orthorhombic, with the space group Pnmb and Z = 2. The unit cell parameters are a = 10.690 Å, b = 7.218 Å, and c = 7.37 Å.[32] All the phases have one long axis lying perpendicular to the MnCl4 plane and parallel to the axis of the NH3(CH2)NH3 chains and two shorter axes lying within the MnCl4 plane.

This study aimed to investigate the thermodynamic properties, ferroelastic properties, and structural dynamics of [NH3(CH2)NH3]MnCl4 as a function of the number of CH2 groups in the carbon chain. The phase transition temperatures and thermodynamic properties of the crystals with n = 2, 3, and 4 are investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, the ferroelastic domain walls of the three crystals were observed by optical polarizing microscopy. Finally, we study the structural dynamics of the [NH3(CH2)NH3] cation near the phase transition temperatures by performing magic angle spinning (MAS), 1H nuclear magnetic resonance (NMR), and MAS 13C NMR spectroscopy. The results provide insights into the thermodynamic properties and structural dynamics of the [NH3(CH2)NH3]MnCl4 crystals and are expected to facilitate their potential applications.

Results and Discussion

Crystal Structures

The X-ray powder diffraction patterns of the [NH3(CH2)NH3]MnCl4 crystals (n = 2, 3, and 4) at 298 K are shown in Figure and Table . The lattice constants for the [NH3(CH2)2NH3]MnCl4 crystal with n = 2 were determined as a = 8.614 ± 0.005 Å, b = 7.127 ± 0.003 Å, c = 7.188 ± 0.003 Å, and β = 92.772 ± 0.028° and those for [NH3(CH2)3NH3]MnCl4 with n = 3 as a = 19.009 ± 0.003 Å, b = 7.169 ± 0.001 Å, and c = 7.357 ± 0.001 Å. For [NH3(CH2)4NH3]MnCl4 with n = 4, the lattice parameters were determined as a = 10.826 ± 0.004 Å, b = 7.178 ± 0.003 Å, c = 7.312 ± 0.003 Å, and β = 92.605 ± 0.014°. These results are consistent with those reported previously.[27,30,32]

Figure 1

X-ray diffraction pattern of the [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) crystals at 300 K.

Table 1

Structure, Lattice Constant, Phase Transition Temperature (TC), Thermal Decomposition Temperature (Td), and 1H Spin–Lattice Relaxation Time T1ρ in the [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) Crystals as a Function of the Lengths of the CH2 Chain

	structure	lattice constants (Å)	TC (K)	Td (K)	1H T1ρ (ms)
n = 2	monoclinic	a = 8.614		564	20.8
		b = 7.127
		c = 7.188
		γ = 92.772°
		Z = 2
n = 3	orthorhombic	a = 19.009	305,336	582	15.4
		b = 7.169
		c = 7.357
		Z = 4
n = 4	monoclinic	a = 10.826	382	589	14.4
		b = 7.178
		c = 7.312
		γ = 92.605°
		Z = 2

Phase Transition Temperatures and Thermodynamic Properties

The DSC curves of the three crystals at a heating rate of 10 K/min under a nitrogen atmosphere are shown in Figure . No peak was observed for the case of n = 2, whereas two endothermic peaks at 308 K (TC2) and 338 K (TC1) were observed for n = 3. For the case of n = 4, a small endothermic peak was observed at ∼378 K. These phase transition temperatures are consistent with those reported previously.[28,29,31]

Figure 2

DSC thermogram of [NH3(CH2)NH3] MnCl4 (n = 2, 3, and 4).

To verify whether the endothermic peaks correspond to phase transition or decomposition, TGA and differential thermal analysis (DTA) were performed at the same heating rate. The TGA and DTG curves displayed in Figures , 4, and 5 show that the crystals with n = 2, 3, and 4 are almost stable up to approximately 564, 582, and 589 K, respectively. [NH3(CH2)NH3]MnCl4 undergoes loss in the molecular weight with increasing temperature. Based on the total molecular weight, the amount of residue was obtained using eqs , 2, and 3:[33−35]

Figure 3

TGA and DTA curves of [NH3(CH2)2NH3]MnCl4.

Figure 4

TGA and DTG curves of [NH3(CH2)3NH3]MnCl4.

Figure 5

TGA and DTG curves of [NH3(CH2)4NH3]MnCl4.

First step for n = 2

Residue

Second step

Residue

First step for n = 3

Residue

Second step

Residue

First step for n = 4

Residue

Second step

Residue

According to the number of CH2 groups in the carbon chain, the molecular weight loss near 564, 582, and 589 K marks the onset of partial thermal decomposition (at temperature Td). For n = 2, weight losses of about 14 and 31% near 628 and 654 K may be attributed to the thermal decomposition and the partial escape of the HCl and 2HCl moieties, respectively, as shown in Figure . The temperatures corresponding to the partial escape of the HCl and 2HCl moieties for n = 3 and 4 are nearly same as those for n = 2. The molecular weight of the three crystals decreased sharply between 600 and 700 K, with 50% weight loss at around 700 K.

To support the TGA results, the appearance of single crystals with changing temperature was observed using an optical polarizing microscope (Figure ). For the case of n = 2, the crystal formed at 300 K was transparent and dark orange, whereas it appeared slightly opaque at 533 K. Upon further increasing the temperature to 653 K, 2HCl was eliminated and the crystal turned dark brown; the surface also appeared to melt slightly. For the case of n = 3, the crystal was light brown at room temperature. Upon increasing the temperature to 643 K, it turned dark brown because of the elimination of 2HCl, although no melting was observed. For the case of n = 4, the crystal was light yellow at 300 K, and it turned white with increasing temperature, indicating the elimination of HCl.

Figure 6

Photographs of [NH3(CH2)2NH3]MnCl4 [(a) 303 K, (b) 533 K, (c) 573 K, (d) 653 K], [NH3(CH2)3NH3]MnCl4 [(e) 303 K, (f) 561 K, (g) 613 K, (h) 643 K], and [NH3(CH2)4NH3]MnCl4 [(i) 303 K, (j) 418 K, (k) 603 K, (l) 623 K] (the photos were taken by the authors S. J. Lee and M. Y. Choi).

Ferroelastic Domain Walls

Optical polarizing microscopy was used to examine the ferroelastic properties of the three crystals. The domain patterns of [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) were examined under an optical polarizing microscope (Figures , 8, and 9). The parallel lines represent the twin domain walls, and each photograph was taken after waiting for about 1 min for temperature stabilization and attainment of the equilibrium domain structure. The domain walls were observed at 200× magnification. The twin domain walls in the lamellar type structure were observed perpendicular to the plane of the principal growth face of [NH3(CH2)NH3]MnCl4.

Figure 7

Optical polarizing microscopy images of [NH3(CH2)2NH3]MnCl4 at (a) 303 K, (b) 343 K, (c) 373, and 403 K. The visible lines represent the ferroelastic twin domain patterns.

Figure 8

Optical polarizing microscopy images of [NH3(CH2)3NH3]MnCl4 at (a) 300 K, (b) 313 K, (c) 323 K, and (d) 345 K. The visible lines represent the ferroelastic twin domain patterns.

Figure 9

Optical polarizing microscopy images of [NH3(CH2)4NH3]MnCl4 at (a) 300 K, (b) 353 K, (c) 393 K, and (d) 403 K. The visible lines represent the ferroelastic twin domain patterns.

For the case of n = 2 (Figure ), several parallel lines representing the ferroelastic twin domain walls were observed at 303 K (Figure a). With increasing temperature, the domain walls remained intact, without the occurrence of any unusual phenomenon (Figure b–d). The twin boundary existed in the same direction at all temperatures. For the case of n = 3, the parallel lines representing the ferroelastic domains were observed at 300 K (Figure a). With increasing temperature, the parallel lines corresponding to the domain walls at 313 and 323 K (Figure b,c), which are above TC2 (305 K), were rotated by approximately 45° with respect to those at 300 K (Figure a). When the temperature was increased beyond TC1 (336 K), the direction of the domain walls was found to revert to that observed at room temperature, which was lower than TC2. Extinction axes of phase II are rotated by 45° from the axis perpendicular to the layers of phases I and III. The direction of the domain walls when n = 4 is constant at all temperatures, as shown in Figure . Observations of domain walls signify the interchange of the b and c axes. At 300 K, the distance between the domain walls is remarkably broad (Figure a), whereas at 403 K, above TC (383 K), the domain wall spacing is very narrow (Figure d).

This observation suggests that the structural phase transition occurs at phase transition temperatures. For the transition from the mmm point group (space group Pnma) of the orthorhombic system to the 2/m point group (space group P21/b) of the monoclinic system, the domain wall orientations are expressed as x = 0 and z = 0, as suggested by Aizu[36] and Sapriel.[37] These equations of the twin domain walls reflect the ferroelasticity of the mmmF2/m species, where mmm and 2/m indicate the symmetries at a high and low temperatures, and F represents the ferroelastic phase.

MAS 1H NMR Results

The 1H NMR spectra of the [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) crystals recorded by MAS NMR spectroscopy at 300 K are shown in Figure . For the case of n = 2, 3, and 4, the chemical shift of the resonance line was observed at 8.95, 8.72, and 9.79 ppm as one resonance line, respectively. The spinning sidebands are marked with “+”. The observed resonance lines are asymmetric, corresponding to the overlap of NH3 and CH2; the line widths a and b on the left and right sides of the half-maximum are not same. In addition, the in situ MAS 1H NMR spectra were recorded as a function of temperature according to the length of the carbon chain (Supporting Information 1, 2, and 3). The temperature-dependence of 1H chemical shifts for n = 2, 3, and 4 are shown in Figure .

Figure 10

MAS 1H NMR spectra of [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) at 300 K.

Figure 11

MAS 1H NMR spectra of [NH3(CH2)NH3]MnCl4 as a function of the length of the CH2 chain as a function of temperature.

The 1H chemical shifts show similar trends for n = 2, 3, and 4. The chemical shifts of all three crystals depend on temperature, implying that the surrounding environments of the 1H change continuously with temperature. For n = 2 and 4, no change was observed with the increasing temperature. In particular, for n = 3, the 1H chemical shift showed discontinuity near TC2 and continuity near TC1. At this time, the difference in chemical shifts is unlikely to be caused by the different N–H and C–H bond lengths for the three crystals. It was thought to be related to the N–H···Cl bond length.

The MAS 1H NMR spectrum was acquired for several delay times at room temperature. The relation between the intensity of NMR signals and delay time can be represented by an exponential function. The magnetization decay rate of the spin-locked proton is characterized by T1ρ as follows[38,39]

Here, P(τ) and P(0) are the signal intensities at times τ and τ = 0, respectively. The MAS 1H NMR spectra of [NH3(CH2)NH3]MnCl4 at 300 K were recorded for several delay times, ranging from 1 μs to 5 ms. All the decay curves could be explained by the exponential function represented by eq . Slopes of their recovery traces indicate that the 1H T1ρ values of [NH3(CH2)NH3]MnCl4 were very short, being 20.8, 15.4, and 14.4 ms for n = 2, 3, and 4, respectively. However, 1H T1ρ for NH3 and CH2 could not be distinguished because of the overlapping of the 1H signals of NH3 and CH2. The trend in their values indicates that T1ρ decreases with increasing length of the CH2 chain.

MAS 13C NMR Results

The MAS 13C NMR chemical shifts for CH2 in [NH3(CH2)NH3]MnCl4 were recorded at several temperatures. The signal for the 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)NH3] cation is far from NH3, and CH2-2 is located near NH3. The results of the 13C chemical shifts according to the methylene chain length at 300 K are shown in Figure . For n = 2, one 13C NMR signal is predicted as CH2-2 with the same surrounding environment; one 13C NMR signal was observed at 152.66 ppm for CH2-2. The chemical shifts for n = 3 were observed at 89.95 and 121.26 ppm for CH2-1 and CH2-2, respectively. The 13C chemical shifts for n = 4 were observed at 90.27 and 111.49 ppm, respectively.

Figure 12

MAS 13C NMR spectra of [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) at 300 K.

Meanwhile, the chemical shifts for the MAS 13C NMR spectra for the three compounds with increasing temperature are shown in Figure . In addition, the in situ MAS 13C NMR spectra according to the length of the carbon chain were measured with increasing temperature (Supporting Information 4, 5, and 6). For n = 2 and 3, 13C of CH2-2 decreased with increasing temperature, but for n = 4, it was almost independent of temperature. In the case of CH2-1, for n = 3 and 4 it, decreased slightly with temperature. These results are considered to be consistent with the decrease in the N–C and C–C bond lengths around 13C. As shown in Figure , the changes near TC for n = 3 and 4 could not be seen clearly; however, the changes in the chemicals shifts near TC were more evident in the in situ 13C NMR spectrum shown in the Supporting Information 2(b,c).

Figure 13

MAS 13C NMR spectra of [NH3(CH2)NH3]MnCl4 as a function of the length of the CH2 chain as a function of temperature.

Conclusions

The thermodynamic properties, ferroelastic properties, and molecular dynamics of the [NH3(CH2)NH3]MnCl4 crystals were investigated as a function of the number of CH2 groups in the carbon chain. The loss in molecular weight of these crystals marks the onset of their partial thermal decomposition near 564, 582, and 589 K, for n = 2, 3, and 4, respectively. Td increases with increasing length of the CH2 chain.

For the case of n = 2 and 4, several parallel lines representing the ferroelastic twin domain walls were observed in the same direction at all temperatures. In contrast, the twin boundaries for the case of n = 3 were rotated by 45° beyond TC2 (305 K) and again by 45° beyond TC1 (336 K); the directions of extinction in phase II were rotated by 45° with respect to phases I and III. Assuming the crystal to be orthorhombic in all the three phases, it can be said that the frame of crystallographic axes at the transition from phase I to phase II rotates by 45° around the axis perpendicular to the layers and reverts to the original direction during the transition into phase III.

Finally, the 1H NMR and 13C NMR spectra of the [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) crystals obtained by MAS NMR spectroscopy were acquired as a function of temperature. For n = 2, no anomaly was observed with a change in temperature. For n = 3 and 4, the 13C NMR chemical shifts changed slightly, and the direction of the ferroelastic twin domain wall changed, indicating that the phase transition was in the order–disorder type without significant changes near TC. Also, 1H T1ρ decreased with increasing length of the CH2 chain, indicating that the energy transfer became very easy. It is expected that these results will be used in potential applications.

Experimental Method

Single crystals of [NH3(CH2)NH3]MnCl4 (n = 2, 3, and 4) were grown by slow evaporation at 300 K in an aqueous solution containing NH2(CH2)NH2·2HCl (Aldrich, 98%) and MnCl2 (Aldrich, 98%).

The structures of the three crystals at 300 K were determined by X-ray diffraction (PANalytical, X’Pert PRO MPD) with a Cu Kα radiation source at the Korea Basic Science Institute (KBSI) Seoul Western Center. The lattice parameters were determined by single-crystal X-ray diffraction at the same facility. The crystals were mounted on a Bruker D8 VENTURE equipped with a 1 μS micro-focus sealed tube Mo Kα and a PHOTON III M14 detector.

DSC (DSC 25, TA Instruments) was performed at a scan rate of 10 K/min in the temperature range 200–480 K under nitrogen gas. TGA was performed on a thermogravimetric analyzer (TA Instruments) in the temperature range 300–870 K at a heating rate of 10 K/min.

Ferroelastic domain walls in the (001) plane were studied using an optical polarizing microscope. A hot stage that was capable of withstanding the temperature required for single crystals (Linkam THMS 600) was used.

1H NMR spectra of the [NH3(CH2)NH3]MnCl4 crystals were obtained at KBSI Seoul Western Center using a 400 MHz AVANCE II + Bruker solid-state NMR spectrometer equipped with 4 mm MAS probes. The MAS 1H NMR and MAS 13C NMR spectra were recorded at a Larmor frequency of 400.13 and 100.61 MHz, respectively. The MAS rate to minimize the spinning sideband overlap was 10–12 kHz. The NMR chemical shifts were recorded using tetramethylsilane (TMS) as the standard. The 1H T1ρ values were obtained using the π/2−τ sequence method by changing the spin-locking pulses; the width of the π/2 pulse for 1H was ∼3.75 μs. The temperature variation was obtained within an error range of ±0.5 K by adjusting the nitrogen gas flow and heater current.