Ionic dynamics of the cation in organic–inorganic hybrid compound (CH3NH3)2MCl4 (M = Cu and Zn) by 1H MAS NMR, 13C CP MAS NMR, and 14N NMR

The ionic dynamics of (CH3NH3)2MCl4 (M = Cu, Zn) by 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR), 13C cross-polarization (CP) MAS NMR, and 14N NMR are investigated as a function of temperature with a focus on the role of the CH3NH3+ cation. The molecular motions in (CH3NH3)2MCl4 are also discussed based on the 1H spin–lattice relaxation time in the rotating coordinate frame T1ρ. From the 1H T1ρ results, the activation energies for the tumbling motion of 1H for CH3 and NH3 were similar, and the uniaxial rotations occurred within a large temperature range. The molecular motions for 13C and 14N of the main chain in the CH3NH3+ cation were rigid, whereas those for 1H of the side chain in the CH3NH3+ cation were very free at high temperatures. T1ρ provides insight into the changes in the cation reorientation rates induced by heating at high temperatures.


Introduction
Hybrid organic-inorganic compounds have been known since 1976 but recently they have been revisited due to their potential use as substitute materials for perovskites. [1][2][3][4][5][6] Metal complexes with the formula (CH 3 NH 3 ) 2 MCl 4 (M ¼ Mn, Fe, Cu, Zn, Cd) can be classied into two groups from a crystal structure point of view. [7][8][9][10][11][12][13][14] One group (CH 3 NH 3 ) 2 MCl 4 (M ¼ Mn, Fe, Cd) has a perovskite-type layer structure consisting of cationic layers and layers of corner-sharing chlorine octahedra with a divalent metal ion at the center. [15][16][17][18] These compounds are characterized by a two-dimensional metal-chlorine network widely separated from one another by methyl ammonium groups. The metal ions are surrounded by a slightly distorted chlorine octahedron, Cl 6 . The other group, to which (CH 3 NH 3 ) 2 MCl 4 (M ¼ Cu, Zn) belongs, consists of discrete CH 3 NH 3 + and MCl 4 2À ions packed in an arrangement similar to orthorhombic K 2 SO 4 -like members. 19,20 In these crystals, unassociated Cl 4 tetrahedra are presented instead of corner-sharing layers of chlorine octahedra. Hydrogen-bonding takes place between the hydrogens of CH 3 NH 3 + and Cl À , and the several different possible hydrogenbond congurations can give rise to structural phase transitions. The (CH 3 NH 3 ) 2 CuCl 4 compound with M ¼ Cu undergoes a structural phase transition at 348 K (¼ T C ), with the respective phases denoted as orthorhombic structure at high temperature and monoclinic structure at room temperature. 21 A sharp peak at 230 K from a thermal capacity experiment was also reported by White and Staveley. 22 In the case of (CH 3 NH 3 ) 2 ZnCl 4 with M ¼ Zn, the existence of a phase transition at 483 K (¼ T C ) was reported by calorimetric, dielectric, thermal expansion, and optical measurements. 23 However, a transition at 426 K ð¼ T 0 C Þ was reported from Raman and IR spectra but not by differential scanning calorimetry (DSC), differential thermal analysis (DTA), and 1 H nuclear magnetic resonance (NMR) measurements. The structure of (CH 3 NH 3 ) 2 ZnCl 4 is orthorhombic at high temperatures and monoclinic at low temperatures. In addition, it has been reported from low-temperature DSC that a phase transition exists at 265 K during heating. 24,25 Following previous NMR investigations, the spin-lattice relaxation time T 1 of 1 H in the CH 3 and NH 3 groups of (CH 3 -NH 3 ) 2 CuCl 4 at the Larmor frequencies of 12 and 26 MHz was reported. The spectra of the two groups overlap at high temperatures and separate at low temperatures. 25 The T 1 at low temperatures exhibits a strong temperature dependence. Moreover, the self-diffusion and reorientation of the methylammonium ions in (CH 3 NH 3 ) 2 ZnCl 4 was reported by 1 H NMR. 26 In addition, the spin-spin relaxation time T 2 of 63 Cu and 35 Cl in (CH 3 NH 3 ) 2 CuCl 4 has been reported at 1.75 K. 27,28 In the case of (CH 3 NH 3 ) 2 ZnCl 4 , 1 H T 1 NMR studies at the Larmor frequency of 20 MHz revealed that the cation in the highest-temperature phase performs isotropic rotation and self-diffusion. The cation in the low-temperature phase undergoes reorientation about its C-N bond axis. 29 Although the structural phase transitions in (CH 3 NH 3 ) 2 CuCl 4 and (CH 3 NH 3 ) 2 ZnCl 4 have been performed by several research groups, the corresponding molecular motions and structural geometry changes have not been fully studied by NMR in the rotating frame.
In the present study, to clarify the ionic dynamics of CH 3 NH 3 + cations and to also obtain information of the mechanism of the phase transition in (CH 3 NH 3 ) 2 MCl 4 (M ¼ Cu, Zn), the chemical shis and spin-lattice relaxation time in the rotating coordinate frame T 1r were measured as a function of temperature using 1 H magic-angle spinning (MAS) NMR and 13 C cross-polarization (CP) MAS NMR. In addition, the 14 N NMR spectra in (CH 3 NH 3 ) 2 ZnCl 4 single crystals in the laboratory frame were discussed in order to elucidate the structural geometry. We focus on the structural phase transitions of compounds with the formula (CH 3 NH 3 ) 2 MCl 4 . We use these results to analyze the behavior of CH 3 and NH 3 near the phase transition temperature from the results of 1 H MAS NMR, 13

Crystal structure
The (CH 3 NH 3 ) 2 CuCl 4 undergoes a phase transition at 348 K. At temperatures below T C ¼ 348 K, the structure is monoclinic, the space group is P2 1 /c, and the lattice constants are a ¼ 7.155Å, b ¼ 7.424Å, c ¼ 9.814Å, and b ¼ 109. 18 . 6 The crystal structure at 363 K is orthorhombic, the space group is Ccmb, and the lattice constants are a ¼ 7.34Å, b ¼ 18.71Å, and c ¼ 7.33Å. 30,31 The monoclinic structure at room temperature is shown in Fig. 1. 3 Here, the methylammonium moieties are located between the layers and are connected by hydrogen bonds to the Cl À ions. Powdered samples were placed in a 4 mm CP MAS probe, and the MAS rate was set to 10 kHz for both 1 H MAS and 13 C CP MAS measurements to minimize the spinning sideband overlap. The chemical shis were referred with respect to tetramethylsilane (TMS). The spin-lattice relaxation times for 1 H and 13 C of (CH 3 NH 3 ) 2 MCl 4 in the rotating coordinate frame were determined using a p/2 À t sequence by varying the duration of the spin-locking pulses. In the case of (CH 3 NH 3 ) 2 CuCl 4 , the width of the p/2 pulse used for measuring the T 1r values of 1 H and 13 C was 3.9 ms, with a spin-locking eld of 64.1 kHz. In the case of (CH 3 NH 3 ) 2 ZnCl 4 , the width of the p/2 pulse used for measuring the T 1r values of 1 H and 13 C was 4.5 and 5.6 ms, with the spinlocking eld of 55.55 kHz and 44.64 kHz, respectively. The power level for 1 H and 13 C was 4 db and 6.5 db, respectively. The 13 C T 1r values were measured by varying the duration of the 13 C spin-locking pulse applied aer the CP preparation period. In addition, the 14 N NMR spectra of the (CH 3 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 43.342 MHz. The 14 N NMR experiments were conducted using a solid-echo pulse sequence.
Temperature-dependent NMR spectra were recorded at 180-430 K as the chemical shi and relaxation time could not be determined outside this temperature range, because of the limitations of the spectrometer used. The sample temperatures were maintained within AE0.5 K by controlling the nitrogen gas ow and heater current.

Results and discussion
The DSC analysis in (CH 3 NH 3 ) 2 CuCl 4 revealed two endothermic peaks at 347 K (¼ T C ) and 517 K (¼ T m ) related to the phase Fig. 1 The structure of (CH 3 NH 3 ) 2 CuCl 4 at room temperature. transition and melting point, respectively, as shown in Fig. 2. The enlarged peak near 347 K in Fig. 2 is very small relative to the other endothermic peak. In the case of (CH 3 NH 3 ) 2 ZnCl 4 , two endothermic peaks are obtained at 475 K (¼ T C ) and 525 K (¼ T m ), which are due to the phase transition and melting point. In order to understand the additional endothermic peaks at high temperature, we conduct optical polarizing microscopy. The peaks of 517 and 525 K in (CH 3 NH 3 ) 2 CuCl 4 and (CH 3 NH 3 ) 2 -ZnCl 4 , respectively, are not related to physical changes such as structural phase transitions; they are instead related to the melting point. The phase transition temperatures obtained here are consistent with previous results. 21,23 This suggests that the differences in the chemical properties of Cu and Zn are responsible for the variations of the phase transition temperatures T C in the two crystals.
The NMR spectra for 1 H in (CH 3 NH 3 ) 2 MCl 4 (M ¼ Cu, Zn) were recorded by MAS NMR at a frequency of 400.13 MHz. In the case of the two compounds, the spectrum of the two peaks is assigned to the 1 H in CH 3 and NH 3 . One of them, the spectrum of the two peaks at chemical shis of d ¼ 3.82 and 12.52 in (CH 3 NH 3 ) 2 CuCl 4 at room temperature, is presented in Fig. 3.
Here, the unit of the NMR scale is represented according to the IUPAC convention. 33,34 The spinning sidebands for CH 3 are marked with open circles and those for NH 3 are marked with crosses. The line component of the lower chemical shi is attributed to the 1 H in CH 3 , and that of the higher chemical shi is attributed to the 1 H in NH 3 . The protons of CH 3 and NH 3 are distinguished from the 1 H chemical shis. In the case of (CH 3 NH 3 ) 2 CuCl 4 across the phase transition temperature of T C , the chemical shi slowly and monotonously decreases with temperature, indicating that the environments of the surrounding 1 H in the CH 3 and NH 3 groups change continuously (see Fig. 4(a)). However, the proton spectrum of the two peaks in (CH 3 NH 3 ) 2 ZnCl 4 at room temperature is recorded at chemical shis of d ¼ 2.88 and 6.75. The 1 H chemical shis in (CH 3 NH 3 ) 2 ZnCl 4 are almost constant with temperature, as shown in Fig. 4(b).
The 1 H spin-lattice relaxation times in the rotating coordinate frame of (CH 3 NH 3 ) 2 MCl 4 (M ¼ Cu, Zn) were obtained for the CH 3 and NH 3 at several temperatures. The nuclear magnetization decay of 1 H follows a single exponential function. Thus, T 1r can be determined by tting the traces with the following equation: 35 where S(t) is the magnetization with the spin-locking pulse duration t and S(N) is the total nuclear magnetization of 1 H at thermal equilibrium. The values of 1 H T 1r for two compounds in the rotating coordinate frame between 180 and 430 K are shown in Fig. 5(a) and (b) as a function of the inverse temperature. The T 1r values for the methyl protons and ammonium protons in the CH 3 NH 3 + cations exhibit similar trends with temperature. The T 1r values of 1 H in the CH 3 and NH 3 groups of (CH 3 NH 3 ) 2 CuCl 4 are almost continuous near T C , and these values are of the order of milliseconds. Above 400 K, the two T 1r values abruptly decrease, and the 1 H T 1r values for CH 3 are longer than those for NH 3 . In contrast, the signicant change in the 13 C T 1r values of (CH 3 NH 3 ) 2 ZnCl 4 is strongly affected, which is primarily considered the result of molecular motions. Further, the variation of T 1r with temperature exhibits a minimum of 16.3 and 12.8 ms for CH 3 and NH 3 near 400 K, respectively. This behavior of T 1r indicates that distinct molecular motions are present. It is clear that the minimum T 1r is attributable to the uniaxial rotation of CH 3 and NH 3 ions. The experimental value of T 1r is expressed in terms of the isotropic correlation time s C for molecular motion using the Bloembergen-Purcell-Pound (BPP) theory, 36 according to which the T 1r value for a spin-lattice interaction of molecular motion is given by [37][38][39] (nT 1r , and e ¼ s C /[1 + u H 2 s C 2 ]. Here, m o is the permeability constant, 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 is the H-C internuclear distance, ħ ¼ h/ 2p (where h is Planck's constant), u H and u C are the Larmor frequencies of 1 H and 13 C, respectively, and u 1 is the spin-lock eld of 55.55 kHz. Our data are analyzed assuming T 1r shows a minimum when u C s C ¼ 1 and the BPP relation between T 1r and u 1 is applicable. As the T 1r curves are found to exhibit minima, it was possible to determine the coefficient, 0.05(m o /4p) 2 (g H g C ħ/ r H-C 3 ) 2 , in the BPP formula. With this coefficient determined, we were then able to calculate the parameter s C as a function of temperature. The temperature dependence of s C follows a simple Arrhenius expression, s C ¼ s Co exp(ÀE a /RT), where s Co is the preexponential factor, T is the temperature, R is the gas constant, and E a is the activation energy. Thus, the slope of the straight-line portion of the semi-log plot can be used to determine E a . The activation energy for the uniaxial rotation of CH 3 and NH 3 , obtained from the log s C vs. 1000/T curve shown in the inset of Fig. 5(b), is 19.72 AE 1.10 and 19.88 AE 0.89 kJ mol À1 , respectively, and is the same within the error range. In addition, the E a value for CH 3 and NH 3 at temperatures below 200 K is 6.59 AE 0.51 and 5.92 AE 0.40 kJ mol À1 , respectively.
The chemical shis for 13 C in (CH 3 NH 3 ) 2 CuCl 4 were measured as a function of temperature, as shown in Fig. 6. At room temperature, the 13 C CP MAS NMR spectrum shows a signal at a chemical shi of d ¼ 190.50 with respect to TMS. The 13 C chemical shi slowly and monotonously decreases with temperature. In contrast, the chemical shis for 13 C in (CH 3 -NH 3 ) 2 ZnCl 4 were also measured over the temperature range of 180 to 430 K, as shown in the inset of Fig. 6. At room temperature, the 13 C CP MAS NMR spectrum possesses two signals at chemical shis of d ¼ 27.82 and 29.02. These signals are attributed to the methyl carbons of the two inequivalent sites CH 3 (1) and CH 3 (2), and these results are consistent with the Xray result previously reported: 18 there exist two kinds of  crystallographically inequivalent cations. The 13 C chemical shis near 426 K decrease by only one line; the change near 426 K was measured from the 13 C chemical shi but not by the DSC result. Although the anomaly was not found around 426 K in the present DSC experiment, the existence of the 13 C chemical shi and 13 C T 1r was obtained. This anomaly near 426 K ð¼ T 0 C Þ is consistent with that obtained from Raman and IR spectra previously reported. There exist two kinds of inequivalent CH 3 in (CH 3 NH 3 ) 2 ZnCl 4 , whereas only one kind of equivalent CH 3 in (CH 3 NH 3 ) 2 CuCl 4 exists. On the other hand, the chemical shis of the CH 3 groups in the 13 C NMR spectra were very different between the two compounds. Generally, the paramagnetic contribution to the NMR shi is responsible for the NMR spectra. 40 Thus, the 13 C NMR chemical shi of (CH 3 NH 3 ) 2 CuCl 4 , which contain paramagnetic ions, was signicantly different from that of (CH 3 NH 3 ) 2 ZnCl 4 , which does not contain paramagnetic ions. The differences in the 13 C NMR chemical shis could potentially be due to differences in the electron structures of the metal ions, in particular, the structure of the d electrons, which screen the nuclear charge from the motion of the outer electrons. Zn 2+ has a lled d shell, whereas Cu 2+ has one s electron outside the closed d shell.
The T 1r values were obtained for the carbon of (CH 3 NH 3 ) 2 -MCl 4 (M ¼ Cu, Zn) at several temperatures. 13 C magnetization was generated by CP aer spin-locking the protons. All magnetization traces obtained for the methyl carbon were described by a single exponential function S(t) ¼ S(N)exp(Àt/T 1r ) of eqn (1). 35 The recovery curves for various delay times of 13 C in (CH 3 -NH 3 ) 2 CuCl 4 and (CH 3 NH 3 ) 2 ZnCl 4 were measured at several temperatures. The saturation recovery traces for 13 C were measured for delay times ranging from 0.2 to 150 ms at room temperature and are presented in Fig. 7(a) and (b). The recovery traces have different slopes at several temperatures. From these results, the T 1r values were obtained for the carbon in the two compounds as a function of the inverse temperature. The temperature dependence of the 13 C T 1r values in (CH 3 NH 3 ) 2 -CuCl 4 is illustrated in Fig. 8, and these values are almost constant with temperature. The T 1r values around T C are unchanged, in agreement with the conclusion drawn from the 13 C chemical shis. In the case of (CH 3 NH 3 ) 2 ZnCl 4 , the phase transition occurring at T 0 C (¼ 426 K) reported by Perez-Mato et al. 23 is not observed from our DSC results, whereas the changes near T 0 C are observed by the 13 C chemical shi and 13 C T 1r results. Thus, T 0 C is denoted by dotted lines in the inset of Fig. 5, 6, and 8. The T 1r values for the two 13 C signals of CH 3 (1) and CH 3 (2) in (CH 3 -NH 3 ) 2 ZnCl 4 are almost the same within the experimental error range.
In order to obtain information concerning the possible distortion surrounding the 14 N ion, the NMR spectrum of 14 N (I ¼ 1) in the laboratory frame was obtained using static NMR at a Larmor frequency of u 0 /2p ¼ 43.342 MHz. Two resonance signals were expected from the quadrupole interactions of the 14 N nucleus. A magnetic eld was applied along the crystallographic axis. The in situ 14 N NMR spectra and resonance frequency in (CH 3 NH 3 ) 2 ZnCl 4 single crystals are plotted  in Fig. 9 as a function of temperature, respectively. The 14 N NMR spectra of the two resonance signals for 14 N are attributed to the NH 3 , and this splitting of the 14 N resonance signals slightly decreases with temperature. The small change of the resonance frequency near 300 K is not related to the phase transition. Note that temperature-dependent changes in the 14 N resonance frequency are generally attributed to changes in the structural geometry, indicating a change in the quadrupole parameter of the 14 N nuclei. The electric eld gradient tensors at the N sites vary, reecting the changing atomic congurations around the nitrogen centers.

Conclusions
The ionic dynamics of (CH 3  The lack of a minimum T 1r indicates that this motion is so slow that there was no detectable T 1r temperature dependence and also that the uniaxial rotation in (CH 3 NH 3 ) 2 CuCl 4 was slower than that in (CH 3 NH 3 ) 2 ZnCl 4 . The motion of the CH 3 NH 3 cations is slower than the C 3 internal rotation of CH 3 and NH 3 ; therefore, it reveals T 1r minima in the high temperature regime above liquid nitrogen temperature. The minima related to the C 3 rotation will appear in the low temperature regime. A comparison with other compounds of the (CH 3 NH 3 ) 2 MCl 4 (M ¼ Cu, Zn) indicates a different phase sequence for (CH 3 -NH 3 ) 2 MCl 4 (M ¼ Cd, Mn). For M ¼ Cd, Mn, these systems at room temperature reveal orthorhombic symmetry followed by a tetragonal phase below room temperature. A phase with monoclinic symmetry is also reported at low temperatures. It is interesting to compare the results for (CH 3 NH 3 ) 2 MCl 4 with those for the analogous compounds containing other metals. In the case of (CH 3 NH 3 ) 2 MnCl 4 and (CH 3 NH 3 ) 2 CdCl 4 , there is an intermediate tetragonal phase between the monoclinic and orthorhombic phases. 16,31 In contrast, the phase transition sequence for (CH 3 NH 3 ) 2 CuCl 4 and (CH 3 NH 3 ) 2 ZnCl 4 changes to an orthorhombic to monoclinic structure with decreasing temperature. 22,31 The created magnetization decay for each proton in (CH 3 -NH 3 ) 2 MCl 4 (M ¼ Cu, Zn) was analyzed by a single exponential function S(t)/S(N) ¼ A exp(Àt/T 1r ), whereas that for each proton in (CH 3 NH 3 ) 2 MCl 4 (M ¼ Mn, Cd) was analyzed by a double-exponential function S(t)/S(N) ¼ A exp(Àt/T 1r (s)) + B exp(Àt/T 1r (L)). These results are consistent with the interactions between the CH 3 NH 3 cations and its surrounding MCl 4 2À anions. This difference of T 1r is possibly due to the difference between the electron structures of metal ions. Cu 2+ and Zn 2+ have one and two s electrons, respectively, outside the closed d shell; Mn 2+ has two s electrons in the unlled 3d orbital; Cd 2+ has two electrons outside the closed d shell.
The T 1r values for 1 H of CH 3 and NH 3 indicate that the protons in the CH 3 NH 3 cations that are involved in the hydrogen bonding exhibit large and small T 1r values corresponding to the long C-H and short N-H bonds, respectively. The molecular motion of the cation is induced by heating at high temperatures. The cation dynamics and interionic interactions through hydrogen bonds are expected to be closely related with the physical properties due to the potential  The temperature-dependent resonance frequency of 14 N NMR spectra in (CH 3 NH 3 ) 2 ZnCl 4 single crystal as a function of temperature (inset: 14 N NMR spectra as a function of temperature).
applications. We will be examined the effect for lengths of alkyl chains as further study.

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