Thermal and structural properties, and molecular dynamics in organic–inorganic hybrid perovskite (C2H5NH3)2ZnCl4

The thermal and structural properties and molecular dynamics of layered perovskite-type (C2H5NH3)2ZnCl4 are investigated by differential scanning calorimetry, thermogravimetric analysis, and magic angle spinning nuclear magnetic resonance spectroscopy. The thermal properties and phase transitions are studied. Additionally, the Bloembergen–Purcell–Pound (BPP) curves for the 1H spin–lattice relaxation time T1ρ in the C2H5NH3 cation and for the 13C T1ρ in C2H5 are shown to have minima as a function of inverse temperature. This observation implies that these curves represent the rotational motions of 1H and 13C in the C2H5NH3 cation. The activation energies for 1H and 13C in the C2H5NH3 cation are obtained; the molecular motion of 1H is enhanced at the C-end and N-end of the organic cation, and that at the carbons of the main chain is not as free as that for protons at the C-end and N-end.


I. Introduction
Hybrid organic-inorganic compounds allow for the possibility of combining the properties of organic and inorganic materials at the molecular level. [1][2][3][4] This class of hybrid materials is very broad and offers a wide set of different structures, properties, and potential applications. [5][6][7][8][9][10][11][12] A new type of layered perovskite multiferroic, (C 2 H 5 NH 3 ) 2 CuCl 4 , as a metal organic compound was found by Kundys et al. 6 Multiferroics refer to materials that simultaneously have two or more of the following properties: spontaneous ferroelectricity, ferromagnetism, or ferroelasticity. 13 (C 2 H 5 NH 3 ) 2 CuCl 4 crystallizes in a layered perovskite structure consisting of nearly isolated layers of corner-sharing ZnCl 6 octahedra, and the interlayer distance is approximately 10 A, where the layers are separated by two layers of ethylammonium cations (C 2 H 5 NH 3 ) + . 14 The NH 3 polar heads of the chains are linked to the chlorine ions of the ZnCl 6 octahedra by three hydrogen bonds N-H/Cl. The organic chains are joined by weak hydrogen bonds from the NH 3 groups to the Cl ions. One interesting series is the metal-organic hybrids of chemical formula such as (C 2 H 5 NH 3 ) 2 ZnCl 4 with perovskite-type transition metal salts. A study of the electrical, dielectric, and optical properties of (C 2 H 5 NH 3 ) 2 ZnCl 4 was reported by Mohamed et al. 15 They found that (C 2 H 5 NH 3 ) 2 ZnCl 4 is a layered perovskitetype compound that undergoes ve phase transitions at 231 K, 234 K, 237 K, 247 K, and 312 K as determined by differential scanning calorimetry (DSC). 15 The intensities of the endotherm peaks at 231 K, 237 K, and 312 K are very weak and perhaps correspond to second-order transformations. Tello 16 reported a ferroelastoelectric phase transition at 243.3 K by optical and Xray measurements with a group theoretical analysis. The phase transitions in this crystal are mostly connected to changes in the arrangement of the alkylammonium chains. The crystal structure of (C 2 H 5 NH 3 ) 2 ZnCl 4 at room temperature is orthorhombic. Fig. 1 reveals that its atomic arrangement can be described by an alternation of the organic and inorganic entities in the bc plane. This compound is characterized by two Fig. 1 Orthorhombic structure of a (C 2 H 5 NH 3 ) 2 ZnCl 4 crystal for bcplane at room temperature. 18 simple hydrogen bonds N-H/Cl linking the organic (C 2 H 5 NH 3 ) + cation to the (ZnCl 4 ) 2À tetrahedral anions. 17 This compound is crystallized in the orthorhombic system with a space group of Pna2 1 at room temperature, and the lattice parameters are a ¼ 10.043 A, b ¼ 17.594 A, c ¼ 7.397 A, and molecules per unit cell, Z ¼ 4. 18 The Zn atoms in (C 2 H 5 NH 3 ) 2 -ZnCl 4 are tetrahedrally coordinated, while the Cu atoms in (C 2 H 5 NH 3 ) 2 CuCl 4 are octahedrally coordinated. Although the chemical composition of the Zn and Cu compounds is similar, the coordination of the metal atom is different.
The structural geometry and molecular dynamics of the organic molecules within the layered hybrid structure are important for determining the inuence of temperature on the evolution of structural phase transitions in the perovskite structure. Physical properties in particular depend on the characteristics of the metallic anion and the organic cation. Until now, the physical properties of (C 2 H 5 NH 3 ) 2 ZnCl 4 have been reported by a few research groups, whereas the thermal properties and molecular dynamics have not been studied.
The goal of this work is to analyze the crystal growth, thermodynamic properties, and structural dynamics of a (C 2 H 5 -NH 3 ) 2 ZnCl 4 single crystal. The analysis is based on DSC, thermogravimetric analysis (TGA), and magic angle spinning nuclear magnetic resonance (MAS NMR). We measured the line widths, chemical shis, and spin-lattice relaxation times (T 1r ) in the rotating frame using 1 H MAS NMR, 13 C cross-polarization (CP)/MAS NMR, and 14 N static NMR as a function of temperature. The molecular dynamics of the (C 2 H 5 NH 3 ) cation were investigated near the phase transition temperatures, and we discussed the activation energies for the molecular dynamics of C 2 H 5 and NH 3 in the (C 2 H 5 NH 3 ) + cation. Furthermore, we have compared the molecular motions of (C 2 H 5 NH 3 ) 2 ZnCl 4 obtained here and those of the previously reported (C 2 H 5 NH 3 ) 2 CuCl 4 .

II. Experimental methods
Crystals of (C 2 H 5 NH 3 ) 2 ZnCl 4 were prepared by dissolving stoichiometric amounts of the starting materials, commercial CH 3 CH 2 NH 2 $HCl (ethylamine hydrochloride, Aldrich 98%) and ZnCl 2 , in water. Single crystals were grown by a slow evaporation of the aqueous solution at room temperature. The obtained crystals were colorless and hexagonal in shape.
The thermal stability was checked by means of TGA and optical polarizing microscopy. The TGA curve at a heating rate of 10 C min À1 was measured under N 2 atmosphere, and the mass of the powdered sample used in the TGA experiment was 6.63 mg. The phase transitions were performed by DSC in the temperature range of 300-670 K with 10 C min À1 heating rates.
The line widths, chemical shis, and T 1r values for (C 2 H 5 -NH 3 ) 2 ZnCl 4 were obtained by 1 H MAS NMR and 13 C CP/MAS NMR at Larmor frequencies of u 0 /2p ¼ 400.13 and 100.62 MHz, respectively, using a Bruker 400 MHz NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. Powdered samples were placed within a 4 mm CP/MAS probe, and the MAS rate for 1 H and 13 C measurements, to minimize spinning sideband overlap, was set to 10 kHz. The 1 H T 1r values were determined using a p/2 À t sequence by varying the duration of spin-locking pulses. 13 C T 1r values were measured by varying the duration of the spin-locking pulse applied aer the CP preparation period. The width of the p/2 pulse used for measuring T 1r for 1 H and 13 C was 3.7 ms, with the spin-locking eld set at 67.56 kHz. The chemical shis and T 1r were measured over a temperature range of 180-430 K.
In addition, the 14 N NMR spectra of the (C 2 H 5 NH 3 ) 2 ZnCl 4 single crystals in the laboratory frame were measured using a Unity INOVA 600 NMR spectrometer at the same facility. The static magnetic eld was 14.1 T and the Larmor frequency was set to u 0 /2p ¼ 43.345 MHz. The 14 N NMR experiments were conducted using a solid-echo (p/2-t-p/2-t) pulse sequence. The width of the p/2 pulse was 4 ms. Fig. 2 shows the simultaneous TGA and DSC curves for the (C 2 H 5 NH 3 ) 2 ZnCl 4 single crystal. A drastic weight loss onset occurred at 460 K (¼T d ), which is attributed to the beginning of the evaporation of C 2 H 5 NH 2 and HCl due to partial thermal decomposition. The sample lost weight sharply between 500 K and 670 K, with a corresponding weight loss of 48%, which agrees almost exactly with the content of the organic component (45.53%) of C 2 H 5 NH 3 Cl in (C 2 H 5 NH 3 ) 2 ZnCl 4 . DSC studies on the perovskite were undertaken to demonstrate structural transitions below the melting/decomposition point. In the DSC curve, the endotherm peaks at 320 K, 376 K, and 438 K correspond to the phase transitions. The (C 2 H 5 NH 3 ) 2 ZnCl 4 undergoes phase transitions at 438 K, 376 K, and 320 K, which are denoted as the T C1 , T C2 , and T C3 with decreasing temperature. As the temperature increases, the crystal is constantly colorless and transparent (300 K, 400 K, 450 K), and then it starts melting near 460 K. The crystal is melted near 475 K, as shown in the inset in Fig. 2.

III. Experimental results
The 1 H NMR spectra at a frequency of 400.13 MHz were obtained by MAS NMR. The 1 H spectrum recorded at 300 K is shown in the inset in Fig. 3. The observed resonance line has an asymmetric shape, as the full-width at half-maximum (FWHM) values on the le side (symbol 1) and right side (symbol 2) are not the same. The asymmetric line shape is attributed to the two overlapping lines for C 2 H 5 and NH 3 . Additionally, the FWHM line width narrows signicantly with increasing temperature, as shown in Fig. 3. Note that there is no abrupt change in the line width near T C3 and T C4 , whereas near T C2 it is abruptly decreased. Here, the T C4 (¼247 K) is the previously reported phase transition temperature. 16 The asymmetric line shape in the inset in Fig. 3 is attributed to the C 2 H 5 and NH 3 overlapping lines. Thus, the overlapping peak is very broad. However, the spinning sidebands of the two types are evident in Fig. 3. Therefore, the peaks for C 2 H 5 and NH 3 by distance of sidebands can be distinguished. The spinning sidebands for C 2 H 5 were marked with asterisks, while those for NH 3 were marked with crosses. The peak of the lower chemical shi was attributed to 1 H in C 2 H 5 , while that of the higher chemical shi was attributed to 1 H in NH 3 . The spectrum at 300 K consisted of two peaks at chemical shis of d ¼ 3.05 ppm and d ¼ 7 ppm, which were assigned to the 1 H in the ethyl group C 2 H 5 and the ammonium group NH 3 , respectively. The 1 H chemical shis for methyl and ammonium groups were nearly constant with temperature, as shown in Fig. 4.
In order to obtain the 1 H relaxation time values, the magnetization recovery curves as a function of delay time were measured at several temperatures for (C 2 H 5 NH 3 ) 2 ZnCl 4 . The magnetization recovery curves for various delay times for 1 H at 300 K are shown in Fig. 5. All traces obtained can be described by the single-exponential function 19,20 where P(t) is the magnetization as a function of the spin-locking pulse duration t, and P 0 is the total nuclear magnetization of the proton at thermal equilibrium. The recovery traces are shown for delay times ranging from 0.2 ms to 70 ms. The T 1r values were obtained from the slopes of the delay time vs. intensity. This analysis method was used to obtain the T 1r values for the protons, which are plotted as a function of inverse temperature in Fig. 6. From these results, the spin-lattice relaxation time in the rotating frame was obtained, and its temperature dependences are shown in Fig. 6.
The    which is a direct measure of the rate of molecular motion. For the spin-lattice relaxation time in the rotating frame, the experimental value of T 1r can be expressed in terms of the correlation time s C for the molecular motion as suggested by the Bloembergen-Purcell-Pound (BPP) theory: 21,22 Here, g H and g C are the gyromagnetic ratios for the 1 H and 13 C nuclei, respectively; N is the number of directly bound protons; r H-C is the H-C internuclear distance; ħ is the reduced Planck constant; u H and u C are the Larmor frequencies of 1 H and 13 C, respectively; and u 1 is the frequency of the spin-locking eld of 67.56 kHz. We analyzed our data assuming that T 1r would show a minimum when u 1 s C ¼ 1, and that the BPP relation between T 1r and the characteristic frequency u 1 could be applied. We sensitively controlled the minima in the T 1r temperature variations and the slopes around the minima. From these results, the value of (g H g C ħ/r H-C 3 ) 2 for the constant in eqn (2) was obtained. We then calculated the temperature dependences of the s C values for protons by using the obtained values of (g H g C ħ/r H- where s 0 is a pre-exponential factor, T is the temperature, R is the gas constant, and E a is the activation energy. Thus, the slope of the linear portion of a semi-log plot should yield E a . The E a value for the rotational motion can be obtained from the log s C vs. 1000/T curve shown in Fig. 6; we obtained E a ¼ 39.41 AE 1.56 kJ mol À1 and E a ¼ 57.59 AE 2.96 kJ mol À1 for high and low temperatures, respectively. Here, T 1r and E a for 1 H are averaged for all hydrogens in the (C 2 H 5 NH 3 ) cation. The rotational motion for protons at the end of the organic cation is more activated at the low temperature than at the high temperature. Structural analysis of the 13 C in C 2 H 5 was also performed using 13 C CP/MAS NMR. The 13 C MAS NMR spectrum for (C 2 -H 5 NH 3 ) 2 ZnCl 4 is shown in Fig. 7 as a function of temperature. The overlapped two signals in the spectrum for CH 3 and CH 2 in C 2 H 5 are shown in Fig. 7. The resonance line has an asymmetric shape, similar to the 1 H line shape. At 200 K, the chemical shis of d ¼ 36.51 ppm and d ¼ 37.12 ppm with respect to tetramethysilane (TMS) are assigned to CH 3 and CH 2 , respectively. The chemical shis above 250 K were only continuous changes, whereas there was an abrupt change near 250 K. The change in the chemical shi is associated with a structural phase transition occurring at this temperature.
The nuclear magnetization was also measured as a function of delay time in order to obtain the 13 C T 1r values. The signal intensity of the nuclear magnetization recovery curves for 13 C is described by a single exponential function of eqn (1) at all temperatures. The 13 C T 1r values for C 2 H 5 in (C 2 H 5 NH 3 ) 2 ZnCl 4 are plotted as a function of inverse temperature in Fig. 8. The 13 C T 1r values near the phase-transition temperatures T C3 and T C4 are approximately continuous, whereas the T 1r near T C2 is abruptly decreased, similar to the 1 H T 1r . The T 1r value for carbon at room temperature is 13.65 ms. The T 1r curve below T C2 can be reproduced by BPP theory, and the BPP curve shows a minimum of 6.30 ms at 260 K. The correlation time for the rotational motion of C 2 H 5 is obtained, and the activation energy from the log s C vs. 1000/T curve shown in Fig. 8; we obtained E a ¼ 21.13 AE 1.27 kJ mol À1 .
In order to obtain information concerning the phase transition near 247 K (¼T C4 ), the NMR spectrum of 14 N (I ¼ 1) in the laboratory frame was obtained. Two resonance signals with respect to NH 4 Cl were expected from the quadrupole interactions of the 14 N nucleus. The 14 N NMR spectra in (C 2 H 5 NH 3 ) 2 -ZnCl 4 single crystals between 220 K and 290 K are plotted in Fig. 6 1 H spin-lattice relaxation times T 1r in the rotating frame and correlation time of (C 2 H 5 NH 3 ) 2 ZnCl 4 as a function of inverse temperature. This journal is © The Royal Society of Chemistry 2019 Fig. 9. The number of resonance lines varies near 243 K; the 14 N signals below 240 K show two resonance lines denoted by symbol 1, whereas those above 240 K show four resonance lines denoted by symbols 1 and 2. These four signals are attributed to the N(1) and N(2) sites in the physically inequivalent NH 3 (1) and NH 3 (2) ions, respectively. The abrupt splitting of the 14 N NMR line is related to the phase transition at 247 K. This splitting of the 14 N resonance signals is nearly constant with temperature. However, the 14 N NMR spectrum above 300 K could not be detected due to a low intensity.

IV. Conclusion
We discussed the molecular motions of cations of Zn-based hybrid materials based on NMR studies. The present work is devoted to the crystal growth, DSC, TGA, and NMR spectroscopy of the (C 2 H 5 NH 3 ) 2 ZnCl 4 compound. The thermal stability at different temperatures was considered. The cation dynamics in a layered perovskite-type (C 2 H 5 NH 3 ) 2 ZnCl 4 single crystal were investigated as a function of temperature by 1 H MAS NMR, 13 C CP/MAS NMR, and 14 N static NMR experiments. There was no jump in T 1r across the phase transition at T C3 and T C4 , while T 1r showed a slight jump at T C2 . To obtain detailed information about the cation dynamics of this crystal, the spin-lattice relaxation time T 1r in the rotating frame for both 1 H and 13 C were obtained, revealing that these atoms undergo rotational motions. The BPP curves for the 1 H T 1r in C 2 H 5 NH 3 cation and for the 13 C T 1r in C 2 H 5 were shown to have minima as a function of inverse temperature. This implies that these curves represent the rotational motions of 1 H and 13 C. The activation energy for 1 H in the C 2 H 5 NH 3 cation is E a ¼ 39.41 kJ mol À1 above 290 K and 57.59 kJ mol À1 below 290 K, whereas that for 13 C in the C 2 H 5 NH 3 cation is 21.13 kJ mol À1 . Furthermore, the carbon dynamics of C 2 H 5 undergo rotation slower than protons. This implies that molecular motion is enhanced at the carbon-end and nitrogen-end of the organic cation, whereas molecular motion is not free at the main chain carbons of the organic cation.
Moreover, we compared the phase transition temperatures and molecular motions of the previously reported (C 2 H 5 NH 3 ) 2 -CuCl 4 (ref. 23) and those of (C 2 H 5 NH 3 ) 2 ZnCl 4 studied here. The difference between these compounds is only the inorganic cation. (C 2 H 5 NH 3 ) 2 ZnCl 4 and (C 2 H 5 NH 3 ) 2 CuCl 4 are characterized by ve (231, 234, 237, 247, and 312 K) and four (236, 330, 357, and 371 K) phase transitions, respectively. Furthermore, the molecular motions affecting the spin-lattice relaxation time T 1r in (C 2 H 5 NH 3 ) 2 ZnCl 4 are very different from those for (C 2 -H 5 NH 3 ) 2 CuCl 4 . The activation energies obtained from T 1r by the 1 H and 13 C measurements for the two compounds are summarized in Table 1. The E a for 1 H in (C 2 H 5 NH 3 ) 2 ZnCl 4 are the values at high temperatures above 290 K and the low temperatures below 290 K, respectively. In the case of (C 2 H 5 -NH 3 ) 2 CuCl 4 , E a for each C 2 H 5 and NH 3 is shown at a temperature range from 180 K to 240 K. 23 The values of E a obtained from the 1 H measurements of (C 2 H 5 NH 3 ) 2 ZnCl 4 are larger than those of (C 2 H 5 NH 3 ) 2 CuCl 4 , whereas those obtained from the 13 C measurments are similar. These results indicate that the activation energies obtained from the 1 H measurements for the H-Cl-Zn bond in (C 2 H 5 NH 3 ) 2 ZnCl 4 without the paramagnetic ions Fig. 8 13 C spin-lattice relaxation times T 1r in the rotating frame and correlation time of (C 2 H 5 NH 3 ) 2 ZnCl 4 as a function of inverse temperature. Fig. 9 The resonance frequency of 14 N NMR spectra in (C 2 H 5 NH 3 ) 2 -ZnCl 4 single crystal between 220 K and 290 K. is larger than that for the H-Cl-Cu bond in (C 2 H 5 NH 3 ) 2 CuCl 4 including paramagnetic ions. These differences are due to the differences in the electronic structure of the Zn 2+ and Cu 2+ ions, particularly, the d electrons, which screen the nuclear charge from the motion of the outer electrons. Zn 2+ has lled d shell, whereas Cu 2+ has one s electron outside the closed d shell. It is also likely due to several other factors, such as different coordination of the metal atom, different lattice constants, and hydrogen bonding strength. This suggests that the differences in the chemical properties of metal ions are responsible for the variations in the characteristics of the phase transitions and molecular motions in these crystals. This study can motivate us to nd a solution for improving the material features as well as solar cell performance using lead-free perovskites based on Zn or Cu in market-competitive optoelectronic materials for photovoltaics (PV) and light emitting diodes (LED) applications. 3,24,25

Conflicts of interest
There are no conicts to declare.