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

Abstract

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 T_1ρ. From the ¹H T_1ρ 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. T_1ρ provides insight into the changes in the cation reorientation rates induced by heating at high temperatures.

---

1. 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.^1–6 Metal complexes with the formula (CH₃NH₃)₂MCl₄ (M = Mn, Fe, Cu, Zn, Cd) can be classified into two groups from a crystal structure point of view.^7–14 One group (CH₃NH₃)₂MCl₄ (M = 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.^15–18 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, Cl₆. The other group, to which (CH₃NH₃)₂MCl₄ (M = Cu, Zn) belongs, consists of discrete CH₃NH₃⁺ and MCl₄²⁻ ions packed in an arrangement similar to orthorhombic K₂SO₄-like members.^19,20 In these crystals, unassociated Cl₄ tetrahedra are presented instead of corner-sharing layers of chlorine octahedra. Hydrogen-bonding takes place between the hydrogens of CH₃NH₃⁺ and Cl⁻, and the several different possible hydrogen-bond configurations can give rise to structural phase transitions.

The (CH₃NH₃)₂CuCl₄ compound with M = Cu undergoes a structural phase transition at 348 K (= 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 (CH₃NH₃)₂ZnCl₄ with M = Zn, the existence of a phase transition at 483 K (= T_C) was reported by calorimetric, dielectric, thermal expansion, and optical measurements.²³ However, a transition at 426 K was reported from Raman and IR spectra but not by differential scanning calorimetry (DSC), differential thermal analysis (DTA), and ¹H nuclear magnetic resonance (NMR) measurements. The structure of (CH₃NH₃)₂ZnCl₄ 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 T₁ of ¹H in the CH₃ and NH₃ groups of (CH₃NH₃)₂CuCl₄ 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 T₁ at low temperatures exhibits a strong temperature dependence. Moreover, the self-diffusion and reorientation of the methylammonium ions in (CH₃NH₃)₂ZnCl₄ was reported by ¹H NMR.²⁶ In addition, the spin–spin relaxation time T₂ of ⁶³Cu and ³⁵Cl in (CH₃NH₃)₂CuCl₄ has been reported at 1.75 K.^27,28 In the case of (CH₃NH₃)₂ZnCl₄, ¹H T₁ 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 (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄ 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 CH₃NH₃⁺ cations and to also obtain information of the mechanism of the phase transition in (CH₃NH₃)₂MCl₄ (M = Cu, Zn), the chemical shifts and spin–lattice relaxation time in the rotating coordinate frame T_1ρ were measured as a function of temperature using ¹H magic-angle spinning (MAS) NMR and ¹³C cross-polarization (CP) MAS NMR. In addition, the ¹⁴N NMR spectra in (CH₃NH₃)₂ZnCl₄ 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 (CH₃NH₃)₂MCl₄. We use these results to analyze the behavior of CH₃ and NH₃ near the phase transition temperature from the results of ¹H MAS NMR, ¹³C CP MAS NMR, and ¹⁴N NMR. In addition, we compare the structural geometries of (CH₃NH₃)₂MCl₄ (M = Cu, Zn) obtained here and (CH₃NH₃)₂MCl₄ (M = Mn, Cd) previously reported.

2. Materials and methods

2.1. Crystal structure

The (CH₃NH₃)₂CuCl₄ undergoes a phase transition at 348 K. At temperatures below T_C = 348 K, the structure is monoclinic, the space group is P2₁/c, and the lattice constants are a = 7.155 Å, b = 7.424 Å, c = 9.814 Å, and β = 109.18°.⁶ The crystal structure at 363 K is orthorhombic, the space group is Ccmb, and the lattice constants are a = 7.34 Å, b = 18.71 Å, and c = 7.33 Å.^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⁻ ions. Further, (CH₃NH₃)₂ZnCl₄ undergoes a phase transition at 483 K. At room temperature, the crystal is monoclinic with the space group P2₁/c, and the lattice constants are a = 10.873 Å, b = 12.655 Å, c = 7.648 Å, β = 96.71°, and Z = 4.^19,23,26,32 Here, the two inequivalent sites, CH₃(1) and CH₃(2), and NH₃(1) and NH₃(2), in (CH₃NH₃)₂ZnCl₄ were reported by Morosin et al.¹⁹

Fig. 1 The structure of (CH₃NH₃)₂CuCl₄ at room temperature.

2.2. Experimental method

Single crystals of (CH₃NH₃)₂MCl₄ (M = Cu, Zn) were prepared by the slow evaporation of an aqueous solution of stoichiometric amounts of CH₃NH₃·HCl and MCl₂ (M = Cu, Zn) at room temperature. The color of (CH₃NH₃)₂CuCl₄ single crystals is brown with flat parallelepipeds. In addition, (CH₃NH₃)₂ZnCl₄ single crystals are colorless and transparent with a square shape. The phase transition temperatures were determined using DSC (Dupont, 2010) measurements at a heating rate of 10 K min⁻¹.

¹H MAS NMR and ¹³C CP MAS NMR spectra of (CH₃NH₃)₂MCl₄ (M = Cu, Zn) in the rotating coordinate frame were obtained at the Larmor frequencies of ω₀/2π = 400.13 and 100.61 MHz, respectively, using a Bruker 400 MHz NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. Powdered samples were placed in a 4 mm CP MAS probe, and the MAS rate was set to 10 kHz for both ¹H MAS and ¹³C CP MAS measurements to minimize the spinning sideband overlap. The chemical shifts were referred with respect to tetramethylsilane (TMS). The spin–lattice relaxation times for ¹H and ¹³C of (CH₃NH₃)₂MCl₄ in the rotating coordinate frame were determined using a π/2 − t sequence by varying the duration of the spin-locking pulses. In the case of (CH₃NH₃)₂CuCl₄, the width of the π/2 pulse used for measuring the T_1ρ values of ¹H and ¹³C was 3.9 μs, with a spin-locking field of 64.1 kHz. In the case of (CH₃NH₃)₂ZnCl₄, the width of the π/2 pulse used for measuring the T_1ρ values of ¹H and ¹³C was 4.5 and 5.6 μs, with the spin-locking field of 55.55 kHz and 44.64 kHz, respectively. The power level for ¹H and ¹³C was 4 db and 6.5 db, respectively. The ¹³C T_1ρ values were measured by varying the duration of the ¹³C spin-locking pulse applied after the CP preparation period.

In addition, the ¹⁴N NMR spectra of the (CH₃NH₃)₂ZnCl₄ single crystals in the laboratory frame were measured using a Unity INOVA 600 NMR spectrometer at the same facility. The static magnetic field was 14.1 T and the Larmor frequency was set to ω₀/2π = 43.342 MHz. The ¹⁴N NMR experiments were conducted using a solid-echo pulse sequence.

Temperature-dependent NMR spectra were recorded at 180–430 K as the chemical shift 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 ±0.5 K by controlling the nitrogen gas flow and heater current.

3. Results and discussion

The DSC analysis in (CH₃NH₃)₂CuCl₄ revealed two endothermic peaks at 347 K (= T_C) and 517 K (= T_m) related to the phase 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 (CH₃NH₃)₂ZnCl₄, two endothermic peaks are obtained at 475 K (= T_C) and 525 K (= 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 (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄, 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.

Fig. 2 Differential scanning calorimetry thermogram of (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄ single crystals.

The NMR spectra for ¹H in (CH₃NH₃)₂MCl₄ (M = 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 ¹H in CH₃ and NH₃. One of them, the spectrum of the two peaks at chemical shifts of δ = 3.82 and 12.52 in (CH₃NH₃)₂CuCl₄ 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 CH₃ are marked with open circles and those for NH₃ are marked with crosses. The line component of the lower chemical shift is attributed to the ¹H in CH₃, and that of the higher chemical shift is attributed to the ¹H in NH₃. The protons of CH₃ and NH₃ are distinguished from the ¹H chemical shifts. In the case of (CH₃NH₃)₂CuCl₄ across the phase transition temperature of T_C, the chemical shift slowly and monotonously decreases with temperature, indicating that the environments of the surrounding ¹H in the CH₃ and NH₃ groups change continuously (see Fig. 4(a)). However, the proton spectrum of the two peaks in (CH₃NH₃)₂ZnCl₄ at room temperature is recorded at chemical shifts of δ = 2.88 and 6.75. The ¹H chemical shifts in (CH₃NH₃)₂ZnCl₄ are almost constant with temperature, as shown in Fig. 4(b).

Fig. 3 ¹H MAS NMR spectra of (CH₃NH₃)₂CuCl₄ at 300 K (the spinning sidebands are marked with crosses and open circles).

Fig. 4 (a) ¹H chemical shifts for CH₃ and NH₃ groups in (CH₃NH₃)₂CuCl₄ as a function of temperature. (b) ¹H chemical shifts for CH₃ and NH₃ groups in (CH₃NH₃)₂ZnCl₄ as a function of temperature.

The ¹H spin–lattice relaxation times in the rotating coordinate frame of (CH₃NH₃)₂MCl₄ (M = Cu, Zn) were obtained for the CH₃ and NH₃ at several temperatures. The nuclear magnetization decay of ¹H follows a single exponential function. Thus, T_1ρ can be determined by fitting the traces with the following equation:³⁵

S(t)/S(∞) = exp(−t/T_1ρ), (1)

where S(t) is the magnetization with the spin-locking pulse duration t and S(∞) is the total nuclear magnetization of ¹H at thermal equilibrium. The values of ¹H T_1ρ 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_1ρ values for the methyl protons and ammonium protons in the CH₃NH₃⁺ cations exhibit similar trends with temperature. The T_1ρ values of ¹H in the CH₃ and NH₃ groups of (CH₃NH₃)₂CuCl₄ are almost continuous near T_C, and these values are of the order of milliseconds. Above 400 K, the two T_1ρ values abruptly decrease, and the ¹H T_1ρ values for CH₃ are longer than those for NH₃. In contrast, the significant change in the ¹³C T_1ρ values of (CH₃NH₃)₂ZnCl₄ is strongly affected, which is primarily considered the result of molecular motions. Further, the variation of T_1ρ with temperature exhibits a minimum of 16.3 and 12.8 ms for CH₃ and NH₃ near 400 K, respectively. This behavior of T_1ρ indicates that distinct molecular motions are present. It is clear that the minimum T_1ρ is attributable to the uniaxial rotation of CH₃ and NH₃ ions. The experimental value of T_1ρ is expressed in terms of the isotropic correlation time τ_C for molecular motion using the Bloembergen–Purcell–Pound (BPP) theory,³⁶ according to which the T_1ρ value for a spin–lattice interaction of molecular motion is given by^37–39

(nT_1ρ⁻¹) = 0.05(μ_o/4π)² (γ_Hγ_Cħ/r_H–C³)²[4a + b + 3c + 6d + 6e], (2)

where a = τ_C/[1 + ω₁²τ_C²], b = τ_C/[1 + (ω_H − ω_C)²τ_C²], c = τ_C/[1 + ω_C²τ_C²], d = τ_C/[1 + (ω_H + ω_C)²τ_C²], and e = τ_C/[1 + ω_H²τ_C²]. Here, μ_o is the permeability constant, γ_H and γ_C are the gyromagnetic ratios for the ¹H and ¹³C nuclei, respectively, n is the number of directly bound protons, r is the H–C internuclear distance, ħ = h/2π (where h is Planck's constant), ω_H and ω_C are the Larmor frequencies of ¹H and ¹³C, respectively, and ω₁ is the spin-lock field of 55.55 kHz. Our data are analyzed assuming T_1ρ shows a minimum when ω_Cτ_C = 1 and the BPP relation between T_1ρ and ω₁ is applicable. As the T_1ρ curves are found to exhibit minima, it was possible to determine the coefficient, 0.05(μ_o/4π)² (γ_Hγ_Cħ/r_H–C³)², in the BPP formula. With this coefficient determined, we were then able to calculate the parameter τ_C as a function of temperature. The temperature dependence of τ_C follows a simple Arrhenius expression, τ_C = τ_Co exp(−E_a/RT), where τ_Co is the pre-exponential 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 CH₃ and NH₃, obtained from the log τ_C vs. 1000/T curve shown in the inset of Fig. 5(b), is 19.72 ± 1.10 and 19.88 ± 0.89 kJ mol⁻¹, respectively, and is the same within the error range. In addition, the E_a value for CH₃ and NH₃ at temperatures below 200 K is 6.59 ± 0.51 and 5.92 ± 0.40 kJ mol⁻¹, respectively.

Fig. 5 (a) ¹H spin–lattice relaxation times in the rotating coordinate frame for CH₃ and NH₃ groups in (CH₃NH₃)₂CuCl₄ as a function of inverse temperature. (b) ¹H spin–lattice relaxation times in the rotating coordinate frame for CH₃ and NH₃ groups in (CH₃NH₃)₂ZnCl₄ as a function of inverse temperature (inset: Arrhenius plots of the natural logarithm of the correlation time for each proton of CH₃ and NH₃ in (CH₃NH₃)₂ZnCl₄ as a function of inverse temperature).

The chemical shifts for ¹³C in (CH₃NH₃)₂CuCl₄ were measured as a function of temperature, as shown in Fig. 6. At room temperature, the ¹³C CP MAS NMR spectrum shows a signal at a chemical shift of δ = 190.50 with respect to TMS. The ¹³C chemical shift slowly and monotonously decreases with temperature. In contrast, the chemical shifts for ¹³C in (CH₃NH₃)₂ZnCl₄ were also measured over the temperature range of 180 to 430 K, as shown in the inset of Fig. 6. At room temperature, the ¹³C CP MAS NMR spectrum possesses two signals at chemical shifts of δ = 27.82 and 29.02. These signals are attributed to the methyl carbons of the two inequivalent sites CH₃ (1) and CH₃ (2), and these results are consistent with the X-ray result previously reported:¹⁸ there exist two kinds of crystallographically inequivalent cations. The ¹³C chemical shifts near 426 K decrease by only one line; the change near 426 K was measured from the ¹³C chemical shift but not by the DSC result. Although the anomaly was not found around 426 K in the present DSC experiment, the existence of the ¹³C chemical shift and ¹³C T_1ρ was obtained. This anomaly near 426 K is consistent with that obtained from Raman and IR spectra previously reported. There exist two kinds of inequivalent CH₃ in (CH₃NH₃)₂ZnCl₄, whereas only one kind of equivalent CH₃ in (CH₃NH₃)₂CuCl₄ exists. On the other hand, the chemical shifts of the CH₃ groups in the ¹³C NMR spectra were very different between the two compounds. Generally, the paramagnetic contribution to the NMR shift is responsible for the NMR spectra.⁴⁰ Thus, the ¹³C NMR chemical shift of (CH₃NH₃)₂CuCl₄, which contain paramagnetic ions, was significantly different from that of (CH₃NH₃)₂ZnCl₄, which does not contain paramagnetic ions. The differences in the ¹³C NMR chemical shifts 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. Zn²⁺ has a filled d shell, whereas Cu²⁺ has one s electron outside the closed d shell.

Fig. 6 ¹³C chemical shift in CH₃ groups in (CH₃NH₃)₂CuCl₄ as a function of temperature (inset: that in (CH₃NH₃)₂ZnCl₄ as a function of temperature).

The T_1ρ values were obtained for the carbon of (CH₃NH₃)₂MCl₄ (M = Cu, Zn) at several temperatures. ¹³C magnetization was generated by CP after spin-locking the protons. All magnetization traces obtained for the methyl carbon were described by a single exponential function S(t) = S(∞)exp(−t/T_1ρ) of eqn (1).³⁵ The recovery curves for various delay times of ¹³C in (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄ were measured at several temperatures. The saturation recovery traces for ¹³C 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_1ρ values were obtained for the carbon in the two compounds as a function of the inverse temperature. The temperature dependence of the ¹³C T_1ρ values in (CH₃NH₃)₂CuCl₄ is illustrated in Fig. 8, and these values are almost constant with temperature. The T_1ρ values around T_C are unchanged, in agreement with the conclusion drawn from the ¹³C chemical shifts. In the case of (CH₃NH₃)₂ZnCl₄, the phase transition occurring at (= 426 K) reported by Perez-Mato et al.²³ is not observed from our DSC results, whereas the changes near are observed by the ¹³C chemical shift and ¹³C T_1ρ results. Thus, is denoted by dotted lines in the inset of Fig. 5, 6, and 8. The T_1ρ values for the two ¹³C signals of CH₃ (1) and CH₃ (2) in (CH₃NH₃)₂ZnCl₄ are almost the same within the experimental error range.

Fig. 7 (a) Recovery spectra for delay times of ¹³C CP MAS NMR spectrum in (CH₃NH₃)₂CuCl₄ at room temperature. (b) Recovery spectra for delay times of ¹³C CP MAS NMR spectrum in (CH₃NH₃)₂ZnCl₄ at room temperature.

Fig. 8 ¹³C spin–lattice relaxation times in the rotating coordinate frame for CH₃ groups in (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄ as a function of temperature.

In order to obtain information concerning the possible distortion surrounding the ¹⁴N ion, the NMR spectrum of ¹⁴N (I = 1) in the laboratory frame was obtained using static NMR at a Larmor frequency of ω₀/2π = 43.342 MHz. Two resonance signals were expected from the quadrupole interactions of the ¹⁴N nucleus. A magnetic field was applied along the crystallographic axis. The in situ ¹⁴N NMR spectra and resonance frequency in (CH₃NH₃)₂ZnCl₄ single crystals are plotted in Fig. 9 as a function of temperature, respectively. The ¹⁴N NMR spectra of the two resonance signals for ¹⁴N are attributed to the NH₃, and this splitting of the ¹⁴N 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 ¹⁴N resonance frequency are generally attributed to changes in the structural geometry, indicating a change in the quadrupole parameter of the ¹⁴N nuclei. The electric field gradient tensors at the N sites vary, reflecting the changing atomic configurations around the nitrogen centers.

Fig. 9 The temperature-dependent resonance frequency of ¹⁴N NMR spectra in (CH₃NH₃)₂ZnCl₄ single crystal as a function of temperature (inset: ¹⁴N NMR spectra as a function of temperature).

4. Conclusions

The ionic dynamics of (CH₃NH₃)₂MCl₄ (M = Cu, Zn), focusing on the role of the CH₃NH₃⁺ cation, were investigated by ¹H MAS NMR, ¹³C CP MAS NMR, and ¹⁴N NMR as a function of temperature. We studied the molecular motions for ¹H and ¹³C in (CH₃NH₃)₂MCl₄ (M = Cu, Zn) based on the spin–lattice relaxation time in the rotating coordinate frame. From the ¹H T_1ρ results, we found that the molecular motions for ¹H in (CH₃NH₃)₂CuCl₄ are very active at high temperatures. In addition, the activation energies for the uniaxial rotation of ¹H for the CH₃ and NH₃ ions in (CH₃NH₃)₂ZnCl₄ have very similar values, and the uniaxial rotation occurs within the high-temperature range. The T_1ρ results reveal that the CH₃ and NH₃ cations exhibit high mobility at high temperatures. The T_1ρ provides insight into the changes in the cation reorientation rates induced by heating at high temperatures. On the other hand, the minima for (CH₃NH₃)₂ZnCl₄ are attributed to the uniaxial rotation of the CH₃NH₃ cations. However, minima such as T_1ρ for (CH₃NH₃)₂CuCl₄ were not reached for that compound. The lack of a minimum T_1ρ indicates that this motion is so slow that there was no detectable T_1ρ temperature dependence and also that the uniaxial rotation in (CH₃NH₃)₂CuCl₄ was slower than that in (CH₃NH₃)₂ZnCl₄. The motion of the CH₃NH₃ cations is slower than the C₃ internal rotation of CH₃ and NH₃; therefore, it reveals T_1ρ minima in the high temperature regime above liquid nitrogen temperature. The minima related to the C₃ rotation will appear in the low temperature regime.

A comparison with other compounds of the (CH₃NH₃)₂MCl₄ (M = Cu, Zn) indicates a different phase sequence for (CH₃NH₃)₂MCl₄ (M = Cd, Mn). For M = 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 (CH₃NH₃)₂MCl₄ with those for the analogous compounds containing other metals. In the case of (CH₃NH₃)₂MnCl₄ and (CH₃NH₃)₂CdCl₄, there is an intermediate tetragonal phase between the monoclinic and orthorhombic phases.^16,31 In contrast, the phase transition sequence for (CH₃NH₃)₂CuCl₄ and (CH₃NH₃)₂ZnCl₄ changes to an orthorhombic to monoclinic structure with decreasing temperature.^22,31

The created magnetization decay for each proton in (CH₃NH₃)₂MCl₄ (M = Cu, Zn) was analyzed by a single exponential function S(t)/S(∞) = A exp(−t/T_1ρ), whereas that for each proton in (CH₃NH₃)₂MCl₄ (M = Mn, Cd) was analyzed by a double-exponential function S(t)/S(∞) = A exp(−t/T_1ρ(s)) + B exp(−t/T_1ρ(L)). These results are consistent with the interactions between the CH₃NH₃ cations and its surrounding MCl₄²⁻ anions. This difference of T_1ρ is possibly due to the difference between the electron structures of metal ions. Cu²⁺ and Zn²⁺ have one and two s electrons, respectively, outside the closed d shell; Mn²⁺ has two s electrons in the unfilled 3d orbital; Cd²⁺ has two electrons outside the closed d shell.

The T_1ρ values for ¹H of CH₃ and NH₃ indicate that the protons in the CH₃NH₃ cations that are involved in the hydrogen bonding exhibit large and small T_1ρ 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 applications. We will be examined the effect for lengths of alkyl chains as further study.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

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 (2016R1A6A1A03012069 and 2015R1A1A3A04001077).

References

1. C. N. R. Rao, A. K. Cheetham and A. Thirumurugan, J. Phys.: Condens. Matter, 2008, 20, 83202.
2. A. H. Arkenbout, T. Uemura, J. Takeya and T. T. M. Palstra, Appl. Phys. Lett., 2009, 95, 173104.
3. P. Zolfaghari, G. A. de Wijs and R. A. de Groot, J. Phys.: Condens. Matter, 2013, 25, 295502.
4. R. Yadav, D. Swain, P. P. Kundu, H. S. Nair, C. Narayana and S. Elizabeth, Phys. Chem. Chem. Phys., 2015, 17, 12207.
5. A. M. Elseman, A. E. Shalan, S. Sajid, M. M. Rashad, A. M. Hassan and M. Li, ACS Appl. Mater. Interfaces, 2018, 10, 11699.
6. J. A. Aramburu, P. Garcia-Fernandez, N. R. Mathiesen, J. M. Garcia-Lastra and M. Moreno, J. Phys. Chem. C, 2018, 122, 5071.
7. R. Kind, Ferroelectrics, 1980, 24, 81.
8. A. Rahman, P. R. Clayton and L. A. K. Staveley, J. Chem. Thermodyn., 1981, 13, 735.
9. A. Levstik, C. Filipic, R. Blinc, H. Arend and R. Kind, Solid State Commun., 1976, 20, 127.
10. T. Yoshinari, T. Matsuyama, H. Yamaoka and K. Aoyagi, J. Phys. Soc. Jpn., 1989, 58, 4222.
11. I. R. Jahn, K. Knorr and J. Ihringer, J. Phys.: Condens. Matter, 1989, 1, 6005.
12. H. Manaka, I. Yamada, M. Nishi and T. Goto, J. Phys. Soc. Jpn., 2001, 70, 1390.
13. A. R. Lim, S. W. Kim and Y. L. Joo, J. Appl. Phys., 2017, 121, 215501.
14. A. R. Lim, Solid State Commun., 2017, 267, 18.
15. G. Chapuis, R. Kind and H. Arend, Phys. Status Solidi A, 1976, 36, 285.
16. G. Heger, D. Mullen and K. Knorr, Phys. Status Solidi A, 1975, 31, 455.
17. P. S. R. Prasad, Phys. Status Solidi A, 1995, 149, k13.
18. J. J. M. Steijger, E. Frikkee, L. J. de Jongh and W. J. Huiskamp, Physica. B, 1984, 123, 284.
19. B. Morosin and K. Emerson, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1976, 32, 294.
20. A. Ben Salah, J. W. Bats, R. Kalus, H. Fuess and A. Daoud, Z. Anorg. Allg. Chem., 1982, 493, 178.
21. G. Heygster and W. Kleemann, Physica. B, 1977, 89, 165.
22. M. A. White and L. A. K. Staveley, J. Phys. Chem. Solids, 1982, 43, 1019.
23. J. M. Perez-Mato, J. L. Manes, J. Fernandez, J. Zuniga, M. J. Tello, C. Socias and M. A. Arriandiaga, Phys. Status Solidi A, 1981, 68, 29.
24. G. Amirthaganesan, U. Binesh, M. A. Kandhaswamy, M. Dhandapani and V. Srinivasan, Cryst. Res. Technol., 2006, 41, 708.
25. H. Nishihara, T. Goto, Y. Kimishima and H. Kubo, J. Phys. Soc. Jpn., 1982, 51, 407.
26. H. Ishida, T. Iwachido, N. Hayama, R. Ikeda, M. Terashima and D. Nakamura, Zeitschrift für Naturforschung A, 1989, 44, 741.
27. H. Kubo, Y. Machida and N. Uryu, J. Phys. Soc. Jpn., 1976, 41, 1071.
28. H. Kubo, Y. Machida and N. Uryu, J. Phys. Soc. Jpn., 1977, 43, 459.
29. H. Ishida, Zeitschrift für Naturforschung A, 2000, 55, 412.
30. I. Pabst, J. Karolyi, H. Fuess and M. Couzi, Phys. Stat. Sol., 1996, 155, 341.
31. I. Pabst, H. Fuess and J. W. Bats, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1987, 43, 413.
32. A. Daoud, J. Appl. Crystallogr., 1977, 10, 133.
33. R. K. Harris, J. Kowalewski and S. C. de Menezes, Magn. Reson. Chem., 1998, 36, 145.
34. R. K. Harris, E. D. Becker, S. M. C. de Menezes, R. Goodfellow and P. Granger, Magn. Reson. Chem., 2002, 40, 489.
35. A. Abragam, The Principles of Nuclear Magnetism, Oxford University press, 1961.
36. N. Bloembergen, E. M. Purcell and R. V. Pound, Phys. Rev., 1948, 73, 679.
37. R. K. Harris, Nuclear Magnetic Resonance Spectroscopy, Pitman Pub., UK, 1983.
38. J. L. Koenig, Spectroscopy of Polymers, Elsevier, New York, 1999.
39. A. R. Lim, RSC Adv., 2017, 7, 55276.
40. J. Novotny, M. Sojka, S. Komorovsky, M. Necas and R. Marek, J. Am. Chem. Soc., 2016, 138, 8432.

---