Thermodynamic, Physical, and Structural Characteristics in Layered Hybrid Type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) Crystals.

The thermal, physical, and molecular dynamics of layered hybrid type (C2H5NH3)2MCl4 (M = 59Co, 63Cu, 65Zn, and 113Cd) crystals were investigated by thermogravimetric analysis (TGA) and magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The temperatures of the onset of partial thermal decomposition were found to depend on the identity of M. In addition, the Bloembergen–Purcell–Pound curves for the 1H spin-lattice relaxation time T1ρ in the rotating frames of CH3CH2 and NH3, and for the 13C T1ρ of CH3 and CH2 were shown to exhibit minima as a function of the inverse temperature. These results confirmed the rotational motion of 1H and 13C in the C2H5NH3 cation. Finally, the T1ρ values and activation energies Ea obtained from the 1H measurements for the H‒Cl···M (M = Zn and Cd) bond in the absence of paramagnetic ions were larger than those obtained for the H‒Cl···M (M = Co and Cu) bond in the presence of paramagnetic ions. Moreover, the Ea value for 13C, which is distant from the M ions, was found to decrease upon increasing the mass of the M ion, unlike in the case of the Ea values for 1H.


Introduction
Layered hybrid compounds have drawn great attention as a new generation of high performance materials due to their interesting physical and chemical properties obtained through the combination of organic and inorganic materials at the molecular level [1][2][3]. They consist of a wide range of inorganic anion chains, alternating with a large variety of organic cations as building blocks. The organic component of the hybrid complex provides several useful properties, such as structural flexibility and optical properties, while the inorganic part is responsible for the mechanical and thermal stabilities, in addition to interesting magnetic and dielectric transitions [4,5]. The diversity of such hybrid materials is therefore large, and so offers a wide range of structures, properties, and potential applications [6][7][8][9][10][11]. More specifically, hybrid layered compounds based on the perovskite structure are interesting materials due to their potential application in solar cells [2,3]. However, the toxicity and chemical instability of halide perovskites limit their use. As a result, the replacement of the lead in present in the perovskite structure with alternative cost-effective materials that are environmentally friendly, less-toxic, and more readily available (e.g., transition metals) is necessary for the extended application of perovskites in solar cells [3]. The structure of (C n H 2n+1 NH 3 ) 2 MCl 4 compounds, where n = 1, 2, 3 . . . and M represents a divalent metal (M = Co 2+ , Cu 2+ , Zn 2+ , and Cd 2+ ), has been described as a sequence of alternating organic-inorganic layers [2,3,12]. The structures of (C 2 H 5 NH 3 ) 2 MCl 4 crystals with n = 2 are similar within each group but dissimilar between groups due to differences between either the inorganic or organic components. For example, the inorganic frames where M = Cu 2+ and Cd 2+ are Based on our previously reported nuclear magnetic resonance (NMR) results, the molecular dynamics of the cation present in (C 2 H 5 NH 3 ) 2 MCl 4 (M = Cu, Zn, and Cd) crystals were discussed in terms of temperature-dependent chemical shifts and spin-lattice relaxation times T 1ρ in the rotating frames for the 1 H and 13 C nuclei [25][26][27].
Thus, to better elucidate the thermal stability in (C 2 H 5 NH 3 ) 2 CoCl 4 single crystals grown by the slow evaporation method, we herein describe the use of thermogravimetric analysis (TGA), in addition to structural analysis by variable-temperature 1 H magic angle spinning (MAS) NMR spectroscopy and 13 C cross-polarization (CP/MAS) NMR spectroscopy. Furthermore, the spin-lattice relaxation times T 1ρ in the rotating frames are measured for the 1 H and 13 C nuclei to better understand the physical and structural properties of (C 2 H 5 NH 3 ) 2 CoCl 4 . The obtained results are compared with those of the previously reported (C 2 H 5 NH 3 ) 2 CuCl 4 , (C 2 H 5 NH 3 ) 2 ZnCl 4 , and (C 2 H 5 NH 3 ) 2 CdCl 4 , and the properties dependent on the characteristics of the metal anion and the organic cation are identified.

Thermal Stability
The thermal stabilities of the various (C 2 H 5 NH 3 ) 2 MCl 4 were examined by TGA, and the results are presented in Figure 1. Upon comparison of the TGA results with the possible chemical reactions taking place, the solid residues formed for (C 2 H 5 NH 3 ) 2 MCl 4 were calculated based on Equations (1)-(4) [  For the M = Co, Cu, Zn, and Cd species, the first mass losses were observed at approximately 378, 430, 460, and 550 K, respectively, which represent the onset of partial thermal decomposition, Td. From the results calculated using the molecular weights, mass losses of 25.97, 24.51, 24.36, and 21.05% for the different M ions were attributed to decomposition of the 2HCl moieties. These results are consistent with the TGA experiment results shown by dotted lines in Figure 1. Moreover, the final decomposition product is MCl2, which corresponds to mass losses of 53.76, 54.81, 54.47, and 47.08%. These results indicate some differences between the calculated and experimental values. The difference between the calculation and experimental value of the final decomposition product is presumably dependent on the heating rate in the TGA experiment. Another difference is thought to be due to experimental conditions in air or N2 atmosphere. The decomposition temperature, Td, and mass loss of 2HCl, and final decomposition product for four crystals are summarized in Table 2. For the M = Co, Cu, Zn, and Cd species, the first mass losses were observed at approximately 378, 430, 460, and 550 K, respectively, which represent the onset of partial thermal decomposition, T d . From the results calculated using the molecular weights, mass losses of 25.97, 24.51, 24.36, and 21.05% for the different M ions were attributed to decomposition of the 2HCl moieties. These results are consistent with the TGA experiment results shown by dotted lines in Figure 1. Moreover, the final decomposition product is MCl 2 , which corresponds to mass losses of 53.76, 54.81, 54.47, and 47.08%. These results indicate some differences between the calculated and experimental values. The difference between the calculation and experimental value of the final decomposition product is presumably dependent on the heating rate in the TGA experiment. Another difference is thought to be due to experimental conditions in air or N 2 atmosphere. The decomposition temperature, T d , and mass loss of 2HCl, and final decomposition product for four crystals are summarized in Table 2. Optical polarizing microscopy was used in order to determine whether these transformations are structural phase transitions or chemical reactions, as presented in Figure 2. In the case of (C 2 H 5 NH 3 ) 2 CoCl 4 , the crystals are blue at room temperature, and no change in the crystal state was observed upon increasing temperature to 360 or 460 K, although melting was observed to commence at 465 K. In contrast, the (C 2 H 5 NH 3 ) 2 CuCl 4 crystals are dark yellow at room temperature, although they present a slightly inhomogeneous hue due to surface roughness. Upon increasing the temperature, the crystal color changed from dark yellow (300 K), to brown (380 K), to dark brown (450 and 500 K), and start melting was observed at 530 K. Interestingly, the crystals of (C 2 H 5 NH 3 ) 2 ZnCl 4 remained colorless and transparent (300, 450, and 460 K), and melting was observed between 470 and 475 K. Similarly, in the case of (C 2 H 5 NH 3 ) 2 CdCl 4 , the crystals remained colorless and transparent between 300 and 480 K, although they became slightly opaque at approximately 540 K, prior to becoming fully opaque close to 570 K. Here, the sample temperatures shown in Figure 2 were kept constant during 2 min each temperature. For all four crystals, it was apparent that the phenomenon above T d was not related to any structural phase transitions, but rather to a thermal decomposition, suggested by Lee [29].
Molecules 2020, 25, x FOR PEER REVIEW 4 of 12 Optical polarizing microscopy was used in order to determine whether these transformations are structural phase transitions or chemical reactions, as presented in Figure 2. In the case of (C2H5NH3)2CoCl4, the crystals are blue at room temperature, and no change in the crystal state was observed upon increasing temperature to 360 or 460 K, although melting was observed to commence at 465 K. In contrast, the (C2H5NH3)2CuCl4 crystals are dark yellow at room temperature, although they present a slightly inhomogeneous hue due to surface roughness. Upon increasing the temperature, the crystal color changed from dark yellow (300 K), to brown (380 K), to dark brown (450 and 500 K), and start melting was observed at 530 K. Interestingly, the crystals of (C2H5NH3)2ZnCl4 remained colorless and transparent (300, 450, and 460 K), and melting was observed between 470 and 475 K. Similarly, in the case of (C2H5NH3)2CdCl4, the crystals remained colorless and transparent between 300 and 480 K, although they became slightly opaque at approximately 540 K, prior to becoming fully opaque close to 570 K. Here, the sample temperatures shown in Figure 2 were kept constant during 2 min each temperature. For all four crystals, it was apparent that the phenomenon above Td was not related to any structural phase transitions, but rather to a thermal decomposition, suggested by Lee [29].

Investigation of the Structural Properties and Molecular Dynamics by 1 H MAS NMR
The 1 H MAS NMR spectra of (C2H5NH3)2CoCl4 were recorded at a range of temperatures as shown in Figure 3. More specifically, at 300 and 370 K, the 1 H signals for C2H5 and NH3 could not be distinguished, and the superimposed peak was rather broad; at 300 and 370 K, single peaks were observed at δ = 1.68 and δ = 0.02 ppm, respectively. In Figure 3, the spinning sidebands for the protons of C2H5NH3 are marked with asterisks. At 420 and 430 K, signals with chemical shifts of δ = 1.76 and 4.36 ppm, and δ = 1.79 and 4.37 ppm, were observed, respectively, which represent the protons of the C2H5 and NH3 ions. In addition, at these higher temperatures, the obtained signals

Investigation of the Structural Properties and Molecular Dynamics by 1 H MAS NMR
The 1 H MAS NMR spectra of (C 2 H 5 NH 3 ) 2 CoCl 4 were recorded at a range of temperatures as shown in Figure 3. More specifically, at 300 and 370 K, the 1 H signals for C 2 H 5 and NH 3 could not be distinguished, and the superimposed peak was rather broad; at 300 and 370 K, single peaks were observed at δ = 1.68 and δ = 0.02 ppm, respectively. In Figure 3, the spinning sidebands for the protons of C 2 H 5 NH 3 are marked with asterisks. At 420 and 430 K, signals with chemical shifts of δ = 1.76 and 4.36 ppm, and δ = 1.79 and 4.37 ppm, were observed, respectively, which represent the protons of the C 2 H 5 and NH 3 ions. In addition, at these higher temperatures, the obtained signals became more intense, and the full-width at half-maximum (FWHM) values narrowed significantly, which were attributed to a high internal mobility.
Molecules 2020, 25, x FOR PEER REVIEW 5 of 12 became more intense, and the full-width at half-maximum (FWHM) values narrowed significantly, which were attributed to a high internal mobility.   The magnetization recovery traces for both the C 2 H 5 and NH 3 protons in (C 2 H 5 NH 3 ) 2 CoCl 4 can be described by a single exponential function [30,31] P(t)/P 0 = exp(-t/T 1ρ ) 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 of the 1 H nuclei for delay times ranging from 1 µs to 50 ms at 300 K are presented in the inset of Figure 4. Here, the asterisks represent spinning sidebands for the center peak. The T 1ρ values were obtained from the slopes of the delay time vs. the signal intensity, and were plotted as a function of the inverse temperature in Figure 4. As shown, the T 1ρ values sharply decrease close to 430 K, while near the phase-transition temperature T C , no changes are evident. At higher temperatures, the T 1ρ values for the C 2 H 5 and NH 3 protons were comparable within the range of error, and from the slope of T 1ρ vs. the inverse temperature, the activation energy E a for the rotational motion below 400 K was determined to be E a = 3.11 ± 0.15 kJ/mol.   The previously reported 1 H T 1ρ values for C 2 H 5 and NH 3 of (C 2 H 5 NH 3 ) 2 MCl 4 (M = Cu, Zn, and Cd) are shown in Figure 5 as a function of the inverse temperature. More specifically, the 1 H T 1ρ values in the presence of the paramagnetic Co 2+ and Cu 2+ ions are particularly short, i.e., 0.01-20 ms, while those of the non-paramagnetic Zn 2+ and Cd 2+ ions are longer, i.e., 2-200 ms. In addition, the 1 H T 1ρ values for C 2 H 5 are longer than those for NH 3 . In contrast, the relaxation times for the 1 H nuclei in the presence of M = Cu, Zn, and Cd reach minimum values, unlike in the case of Co 2+ . For (C 2 H 5 NH 3 ) 2 CuCl 4 , the T 1ρ for the 1 H nucleus reaches its minimum values at 190 and 200 K for C 2 H 5 and NH 3 , respectively, while for (C 2 H 5 NH 3 ) 2 ZnCl 4 , the minimum values of 2.17 and 2.48 ms were reached at 260 and 330 K, respectively. Moreover, in case of (C 2 H 5 NH 3 ) 2 CdCl 4 , the T 1ρ shows a minimum value at 270 K. It is therefore apparent that the 1 H T 1ρ values for (M = Cu, Zn, and Cd) vary due to molecular motion according to the Bloembergen-Purcell-Pound (BPP) theory [30], while no such molecular motion is observed for the (M = Co) species. Indeed, the T 1ρ values are related to the corresponding values of the rotational correlation time, τ C , which is a direct measure of the rate of molecular motion. The experimental value of T 1ρ can therefore be expressed in terms of τ C for the molecular motion as suggested by the BPP theory [26,29,[31][32][33].
where the quantities f (ω) are spectral density functions, i.e., Fourier transforms of the time correlation functions. ω H and ω C are the Larmor frequencies of proton and carbon, respectively, and ω 1 is the frequency of the spin-locking field. The parameter τ C is a characteristic correlation time, that is, the time scale of the motion of the C 2 H 5 and NH 3 ions. F is defined as a relaxation constant: where γ H and γ C are the proton and carbon gyromagnetic ratios, respectively, N is the number of directly bound protons, r H-C is the H-C internuclear distance, andh is the reduced Planck constant. The obtained data were analyzed assuming that T 1ρ has a minimum at ω 1 τ C = 1, and the BPP relationship was applied between T 1ρ and the characteristic frequency ω 1 . The value of the relaxation constant F was therefore obtained using Equation (7). From these results, the temperature dependences of the τ C values for the rotational motions of C 2 H 5 and NH 3 were calculated from the F values. The temperature dependence of τ C follows the simple Arrhenius equation: where E a is the activation energy, τ 0 is the high temperature limit of the correlation time, T is the temperature, and R is the gas constant. The slope of the linear portion of the semi-log plot represents the E a , and the E a for the rotational motion can be obtained from the log τ C vs. 1000/T curve. Thus, the calculated E a values for the four compounds are summarized in Table 3; the activation energies for molecular motion in the presence of paramagnetic Co 2+ and Cu 2+ ions were smaller than those for the species containing Zn 2+ and Cd 2+ .
for delay times ranging from 1 μs to 50 ms at 300 K are presented in the inset of Figure 4. Here, the asterisks represent spinning sidebands for the center peak. The T1ρ values were obtained from the slopes of the delay time vs. the signal intensity, and were plotted as a function of the inverse temperature in Figure 4. As shown, the T1ρ values sharply decrease close to 430 K, while near the phase-transition temperature TC, no changes are evident. At higher temperatures, the T1ρ values for the C2H5 and NH3 protons were comparable within the range of error, and from the slope of T1ρ vs. the inverse temperature, the activation energy Ea for the rotational motion below 400 K was determined to be Ea = 3.11 ± 0.15 kJ/mol. The previously reported 1 H T1ρ values for C2H5 and NH3 of (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) are shown in Figure 5 as a function of the inverse temperature. More specifically, the 1 H T1ρ values in the presence of the paramagnetic Co 2+ and Cu 2+ ions are particularly short, i.e., 0.01-20 ms, while those of the non-paramagnetic Zn 2+ and Cd 2+ ions are longer, i.e., 2-200 ms. In addition, the 1 H T1ρ values for C2H5 are longer than those for NH3. In contrast, the relaxation times for the 1 H nuclei in the presence of M = Cu, Zn, and Cd reach minimum values, unlike in the case of Co 2+ . For (C2H5NH3)2CuCl4, the T1ρ for the 1 H nucleus reaches its minimum values at 190 and 200 K for C2H5 and NH3, respectively, while for (C2H5NH3)2ZnCl4, the minimum values of 2.17 and 2.48 ms were reached at 260 and 330 K, respectively. Moreover, in case of (C2H5NH3)2CdCl4, the T1ρ shows a minimum value at 270 K. It is therefore apparent that the 1 H T1ρ values for (M = Cu, Zn, and Cd) vary due to molecular motion according to the Bloembergen-Purcell-Pound (BPP) theory [30], while no such molecular motion is observed for the (M = Co) species. Indeed, the T1ρ values are related to the corresponding values of the rotational correlation time, τC, which is a direct measure of the rate of molecular motion. The experimental value of T1ρ can therefore be expressed in terms of τC for the molecular motion as suggested by the BPP theory [26,29,[31][32][33].

Investigation of the Structural Properties and Molecular Dynamics by 13 C CP/MAS NMR
The structural analysis of (C 2 H 5 NH 3 ) 2 CoCl 4 was also performed using 13 C CP/MAS NMR over a range of increasing temperatures. Thus, the two peaks corresponding to the CH 3 and CH 2 species at 360 K were observed at chemical shifts of δ = 49.65 and 176.55 ppm, respectively, as shown in the inset of Figure 6. The CH 3 and CH 2 results obtained by 13 C MAS NMR were distinguished in that the signals corresponding to CH 2 could not be observed at low temperatures. In these experiments, the chemical shift of CH 3 remained relatively constant, while that of CH 2 decreased with increasing temperature, and a sharp decrease was observed close to 420 K. theory [32], and the BPP curve shows a minimum of 0.57 ms at 260 K. This characteristic of T1ρ means that distinct molecular motions existed. The correlation time was then obtained using Equation (6), and the activation energy was obtained from these results. More specifically, the Ea for the rotational motion was determined to be 45.98 ± 1.78 kJ/mol from the log τC vs. 1000/T curve shown in Figure 7.  To obtain the corresponding 13 C T 1ρ values, the nuclear magnetization recovery traces were measured as a function of the delay time. The signal intensities of the magnetization recovery curves for 13 C were analyzed by a single exponential function of Equation (5) at all temperatures, and the 13 C T 1ρ values for CH 3 and CH 2 in (C 2 H 5 NH 3 ) 2 CoCl 4 were plotted as a function of inverse temperature (see Figure 7). Indeed, the 13 C T 1ρ curve for CH 3 at low temperatures can be reproduced by the BPP theory [32], and the BPP curve shows a minimum of 0.57 ms at 260 K. This characteristic of T 1ρ means that distinct molecular motions existed. The correlation time was then obtained using Equation (6), and the activation energy was obtained from these results. More specifically, the E a for the rotational motion was determined to be 45.98 ± 1.78 kJ/mol from the log τ C vs. 1000/T curve shown in Figure 7.
The T 1ρ values of the previously reported (C 2 H 5 NH 3 ) 2 MCl 4 (M = Cu, Zn, and Cd) (see Figure 8) were compared with those of (C 2 H 5 NH 3 ) 2 CoCl 4 determined herein. In addition, the molecular motions influenced by 13 C T 1ρ in (C 2 H 5 NH 3 ) 2 CoCl 4 were found to exhibit BPP trends, unlike in the case of the 1 H T 1ρ results. Furthermore, for (C 2 H 5 NH 3 ) 2 CuCl 4 , the temperature dependences of the 13 C T 1ρ values for CH 2 and CH 3 appeared similar, and the BPP curves for CH 3 and CH 2 showed minima at 190 K. The T 1ρ curve for (C 2 H 5 NH 3 ) 2 ZnCl 4 can be also represented by the BPP theory, with a minimum being observed at 260 K in the curve. Finally, in case of (C 2 H 5 NH 3 ) 2 CdCl 4 , the T 1ρ curves show minima at 260 and 250 K for CH 3 and CH 2 , respectively. The 13 C T 1ρ and E a values obtained from the 13 C results for the four compounds are summarized in Table 2, whereby it is apparent that the 13 C T 1ρ values for compounds containing paramagnetic ions are shorter than those without paramagnetic ions, since the relaxation time should be inversely proportional to the square of the magnetic moment of the paramagnetic ions. Therefore, the T 1ρ values of (C 2 H 5 NH 3 ) 2 MCl 4 (M = Co and Cu) were driven by fluctuations of the magnetic dipoles of the paramagnetic Co 2+ and Cu 2+ species, and the E a values for 13 C decreased upon increasing the mass of the M 2+ ion, unlike in the case of the 1 H E a values. These differences are due to variations in the electronic structures of the M 2+ ions, and in particular, the d electrons, which screen the nuclear charge from the motion of the outer electrons. theory [32], and the BPP curve shows a minimum of 0.57 ms at 260 K. This characteristic of T1ρ means that distinct molecular motions existed. The correlation time was then obtained using Equation (6), and the activation energy was obtained from these results. More specifically, the Ea for the rotational motion was determined to be 45.98 ± 1.78 kJ/mol from the log τC vs. 1000/T curve shown in Figure 7. The T1ρ values of the previously reported (C2H5NH3)2MCl4 (M = Cu, Zn, and Cd) (see Figure 8) were compared with those of (C2H5NH3)2CoCl4 determined herein. In addition, the molecular   Table 2, whereby it is apparent that the 13 C T1ρ values for compounds containing paramagnetic ions are shorter than those without paramagnetic ions, since the relaxation time should be inversely proportional to the square of the magnetic moment of the paramagnetic ions. Therefore, the T1ρ values of (C2H5NH3)2MCl4 (M = Co and Cu) were driven by fluctuations of the magnetic dipoles of the paramagnetic Co 2+ and Cu 2+ species, and the Ea values for 13 C decreased upon increasing the mass of the M 2+ ion, unlike in the case of the 1 H Ea values. These differences are due to variations in the electronic structures of the M 2+ ions, and in particular, the d electrons, which screen the nuclear charge from the motion of the outer electrons.
The thermodynamic properties were measured by TGA (TA, Q600) and optical polarizing microscopy. The differential scanning calorimetry (DSC) and TGA data were recorded between 300 and 770 K under a N 2 atmosphere using a heating rate of 10 • C/min. The 1 H MAS NMR and 13 C CP/MAS NMR spectra for the rotating frame of (C 2 H 5 NH 3 ) 2 MCl 4 were measured at the Larmor frequencies of 400.13 and 100.61 MHz, respectively, using a Bruker 400 MHz Avance II+ NMR spectrometer (BRUKER, Germany) at the Korea Basic Science Institute, Western Seoul Center. The powder samples were placed in a 4 mm MAS probe, and the MAS rate was set at 10 kHz for the 1 H MAS and 13 C CP MAS measurements to minimize any overlap of the spinning sidebands with respect to the central peak. The chemical shifts are listed using tetramethylsilane (TMS) as an internal reference. The spin-lattice relaxation times T 1ρ for the rotating frame of (C 2 H 5 NH 3 ) 2 MCl 4 were determined using a π/2−t sequence by variation of the spin-locking pulses. The NMR spectra and T 1ρ values were recorded between 180 and 430 K.

Conclusions
We herein discussed the thermodynamic, physical, and structural properties of (C 2 H 5 NH 3 ) 2 MCl 4 (M = Co, Cu, Zn, and Cd) layered hybrid materials, where we replaced Pb with nontoxic M metals for the production of lead-free perovskite solar cells, and investigated their potential toward solar cell applications based on NMR studies.
The temperature of T d and the degree of mass loss for the decomposition of the 2HCl moieties were both found to depend on the M ion present in the structure. Furthermore, the cation dynamics in layered (C 2 H 5 NH 3 ) 2 MCl 4 single crystals were investigated as a function of temperature by 1 H MAS NMR and 13 C CP/MAS NMR experiments. To obtain detailed information regarding the cation dynamics of these crystals, the T 1ρ values for both 1 H and 13 C were obtained, revealing that these atoms undergo rotational motion.
The reason why 1 H T 1ρ of C 2 H 5 is longer than 1 H T 1ρ of NH 3 is as follows; the rotational motion for C 2 H 5 is activated, and that for NH 3 at the end of the organic cation is less strongly activated. In addition, the reason why 13 C T 1ρ of CH 2 is longer than 13 C T 1ρ of CH 3 is as follows; the amplitude of the cation motion is enhanced at its CH 3 end, and the central CH 2 moiety is fixed to the NH 3 group in the organic cation.
Overall, it was found that all components of this series exhibit an orthorhombic structure at room temperature. However, the lattice constants of the crystals containing Co 2+ and Zn 2+ ions differed from those of the crystals containing Cu 2+ and Cd 2+ ions. It was also found that the inorganic frames of the M = Cu 2+ and Cd 2+ species are corner-sharing MCl 6 octahedra, while those of M = Co 2+ and Zn 2+ are simple MCl 4 tetrahedra. Finally, it was concluded that the physical properties of these species depend on the characteristics of the organic cation and the inorganic metal ion, but are independent of the arrangements of the MCl 4 tetrahedra and the MCl 6 octahedra. The presence of different paramagnetic ions and different lattice constants may also account for these differences.