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

Abstract

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 T_1ρ in the rotating frames of CH₃CH₂ and NH₃, and for the ¹³C T_1ρ 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 T_1ρ values and activation energies E_a 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 E_a 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 E_a values for ¹H.

1. 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_nH_2n+1NH₃)₂MCl₄ compounds, where n = 1, 2, 3… and M represents a divalent metal (M = Co²⁺, Cu²⁺, Zn²⁺, and Cd²⁺), has been described as a sequence of alternating organic-inorganic layers [2,3,12]. The structures of (C₂H₅NH₃)₂MCl₄ 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 = Cu²⁺ and Cd²⁺ are corner-sharing MCl₆ octahedra, while those of M = Co²⁺ and Zn²⁺ are simple MCl₄ tetrahedra [13]. In addition, the organic chains are joined by weak hydrogen bonds between the NH₃ groups and the Cl ions. Indeed, the structural geometries and molecular dynamics of the organic molecules within the layered hybrid structures are important for determining the influence of temperature on the structural phase transitions.

As an example, (C₂H₅NH₃)₂CoCl₄ crystallizes as an orthorhombic structure, which undergoes a reversible phase transition at 235 K [14]. In addition, (C₂H₅NH₃)₂CuCl₄ undergoes phase transitions at 236, 330, 357, and 371 K [7,15,16,17,18,19], its crystal structure at room temperature is orthorhombic [20]. In contrast, (C₂H₅NH₃)₂ZnCl₄ undergoes five phase transitions at 231, 234, 237, 247, and 312 K [21], crystallizing as an orthorhombic system at room temperature [22]. Finally, (C₂H₅NH₃)₂CdCl₄ undergoes structural phase transitions at 114, 216, 358, and 470 K [9,23,24], whereby the room temperature orthorhombic phase has the Abma space group [23]. The structure of the organic component consists of a double layer of alkylammonium ions with the charged nitrogen atoms oriented to the nearest MCl₄ tetrahedra or MCl₆ octahedra. The phase transition temperatures, lattice constants, structures, and space groups for the four crystals are summarized in

Table 1. The phase transition temperatures, T_C, lattice constants, structures, and space groups of (C₂H₅NH₃)₂MCl₄ (M = Co, Cu, Zn, and Cd) crystals at room temperature.

M	TC (K)	Lattice Constant (Å)			Structure	Space Group
Co	235	a = 10.025	b = 7.403	c = 17.603	orthorhombic	Pnma
Cu	236, 330, 357, 371	a = 7.47	b = 7.35	c = 21.18	orthorhombic	Pbca
Zn	231, 234, 237, 247, 312	a = 10.043	b = 7.397	c = 17.594	orthorhombic	Pna21
Cd	114, 216, 358, 470	a = 7.354	b = 7.478	c = 22.11	orthorhombic	Abma

.

Based on our previously reported nuclear magnetic resonance (NMR) results, the molecular dynamics of the cation present in (C₂H₅NH₃)₂MCl₄ (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 ¹H and ¹³C nuclei [25,26,27].

Thus, to better elucidate the thermal stability in (C₂H₅NH₃)₂CoCl₄ 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 ¹H magic angle spinning (MAS) NMR spectroscopy and ¹³C cross-polarization (CP/MAS) NMR spectroscopy. Furthermore, the spin-lattice relaxation times T_1ρ in the rotating frames are measured for the ¹H and ¹³C nuclei to better understand the physical and structural properties of (C₂H₅NH₃)₂CoCl₄. The obtained results are compared with those of the previously reported (C₂H₅NH₃)₂CuCl₄, (C₂H₅NH₃)₂ZnCl₄, and (C₂H₅NH₃)₂CdCl₄, and the properties dependent on the characteristics of the metal anion and the organic cation are identified.

2. Results and Discussion

2.1. Thermal Stability

The thermal stabilities of the various (C₂H₅NH₃)₂MCl₄ 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 (C₂H₅NH₃)₂MCl₄ were calculated based on Equations (1)–(4) [28]:

(C₂H₅NH₃)₂CoCl₄ → (C₂H₅NH₂)₂CoCl₂ (s) + 2HCl (g)
Residue: (C₂H₅NH₂)₂CoCl₂/(C₂H₅NH₃)₂CoCl₄ = 74.03%
(C₂H₅NH₃)₂CoCl₄ → CoCl₂ (s) + 2HCl (g) + 2(C₂H₅NH₂) (g)
Residue: CoCl₂/(C₂H₅NH₃)₂CoCl₄ = 46.24%

(1)

(C₂H₅NH₃)₂CuCl₄ → (C₂H₅NH₂)₂CuCl₂ (s) + 2HCl (g)
Residue: (C₂H₅NH₂)₂CuCl₂/(C₂H₅NH₃)₂CuCl₄ = 75.49%
(C₂H₅NH₃)₂CuCl₄ → CuCl₂ (s) + 2HCl (g) + 2(C₂H₅NH₂) (g)
Residue: CuCl₂/(C₂H₅NH₃)₂CuCl₄ = 45.19%

(2)

(C₂H₅NH₃)₂ZnCl₄ → (C₂H₅NH₂)₂ZnCl₂ (s) + 2HCl (g)
Residue: (C₂H₅NH₂)₂ZnCl₂/(C₂H₅NH₃)₂ZnCl₄ = 75.64%
(C₂H₅NH₃)₂ZnCl₄ → ZnCl₂ (s) + 2HCl (g) + 2(C₂H₅NH₂) (g)
Residue: ZnCl₂/(C₂H₅NH₃)₂ZnCl₄ = 45.53%

(3)

(C₂H₅NH₃)₂CdCl₄ → (C₂H₅NH₂)₂CdCl₂ (s) + 2HCl (g)
Residue: (C₂H₅NH₂)₂CdCl₂/(C₂H₅NH₃)₂CdCl₄ = 78.95%
(C₂H₅NH₃)₂CdCl₄ → CdCl₂ (s) + 2HCl (g) + 2(C₂H₅NH₂) (g)
Residue: CdCl₂/(C₂H₅NH₃)₂CdCl₄ = 52.92%

(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, 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 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, T_d, and mass loss of 2HCl, and final decomposition product for four crystals are summarized in

Table 2. The decomposition temperature, T_d, mass loss of 2HCl, and final decomposition product MCl₂ for four crystals.

M	Td (K)	Weight Loss of 2HCl (%) (cal. Value)	Weight Loss of 2HCl (%) (exp. Value)	Final Decomposition Product (%) (cal. Value)
Co	378	25.97	26.09	53.76
Cu	430	24.51	24.84	54.81
Zn	460	24.36	23.17	54.47
Cd	550	21.05	21.00	47.08

.

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 T_d was not related to any structural phase transitions, but rather to a thermal decomposition, suggested by Lee [29].

2.2. Investigation of the Structural Properties and Molecular Dynamics by ¹H MAS NMR

The ¹H 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 ¹H 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 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 C₂H₅ and NH₃ protons in (C₂H₅NH₃)₂CoCl₄ can be described by a single exponential function [30,31]

P(t)/P₀ = exp(‒t/T_1ρ)

(5)

where P(t) is the magnetization as a function of the spin-locking pulse duration t, and P₀ is the total nuclear magnetization of the proton at thermal equilibrium. The recovery traces of the ¹H 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 C₂H₅ and NH₃ 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 ¹H T_1ρ 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 ¹H T_1ρ values in the presence of the paramagnetic Co²⁺ and Cu²⁺ ions are particularly short, i.e., 0.01–20 ms, while those of the non-paramagnetic Zn²⁺ and Cd²⁺ ions are longer, i.e., 2–200 ms. In addition, the ¹H T_1ρ values for C₂H₅ are longer than those for NH₃. In contrast, the relaxation times for the ¹H nuclei in the presence of M = Cu, Zn, and Cd reach minimum values, unlike in the case of Co²⁺. For (C₂H₅NH₃)₂CuCl₄, the T_1ρ for the ¹H 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 T_1ρ shows a minimum value at 270 K. It is therefore apparent that the ¹H 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].

T_1ρ⁻¹ = F{4f(ω₁) + f(ω_H − ω_C) + 3f(ω_C) + 6f(ω_H + ω_C) + 6f(ω_H)}

(6)

f(ω₁) = τ_C/(1 + ω₁²τ_C²),
f(ω_H − ω_C) = τ_C/[1 + (ω_H − ω_C)²τ_C²],
f(ω_C) = τ_C/(1 + ω_C²τ_C²),
f(ω_H + ω_C) = τ_C/[1 + (ω_H + ω_C)²τ_C²],
f(ω_H) = τ_C/(1 + ω_H²τ_C²).

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 ω₁ 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 C₂H₅ and NH₃ ions. F is defined as a relaxation constant:

F = (N/20)(γ_H γ_Cħ/r_H-C³)²

(7)

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, and ħ is the reduced Planck constant. The obtained data were analyzed assuming that T_1ρ has a minimum at ω₁τ_C = 1, and the BPP relationship was applied between T_1ρ and the characteristic frequency ω₁. 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 C₂H₅ and NH₃ were calculated from the F values. The temperature dependence of τ_C follows the simple Arrhenius equation:

τ_C = τ₀ exp(E_a/RT)

(8)

where E_a is the activation energy, τ₀ 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 spin-lattice relaxation times, T_1ρ, and activation energies, E_a, for ¹H and ¹³C of (C₂H₅NH₃)₂MCl₄ (M = Co, Cu, Zn, and Cd) crystals.

M	1H T1ρ (ms)	Ea (kJ/mol)	13C T1ρ (ms)	Ea (kJ/mol)
Co	0.01–2	3.11 (for C2H5NH3)	0.1–10	45.98 (for CH3)
Cu	7–20	12.19 (for C2H5 below 240 K)	1–100	21.35 (for CH3)
		8.33 (for NH3 below 240 K)		19.72 (for CH2)
Zn	2–200	39.41 (for C2H5NH3 above 290 K)	6–100	21.13 (for C2H5)
		57.59 (for C2H5NH3 below 290 K)
Cd	2–100	22.63 (for C2H5NH3)	5–100	18.05 (for CH3)

; the activation energies for molecular motion in the presence of paramagnetic Co²⁺ and Cu²⁺ ions were smaller than those for the species containing Zn²⁺ and Cd²⁺.

2.3. Investigation of the Structural Properties and Molecular Dynamics by ¹³C CP/MAS NMR

The structural analysis of (C₂H₅NH₃)₂CoCl₄ was also performed using ¹³C CP/MAS NMR over a range of increasing temperatures. Thus, the two peaks corresponding to the CH₃ and CH₂ 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 CH₃ and CH₂ results obtained by ¹³C MAS NMR were distinguished in that the signals corresponding to CH₂ could not be observed at low temperatures. In these experiments, the chemical shift of CH₃ remained relatively constant, while that of CH₂ decreased with increasing temperature, and a sharp decrease was observed close to 420 K.

To obtain the corresponding ¹³C 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 ¹³C were analyzed by a single exponential function of Equation (5) at all temperatures, and the ¹³C T_1ρ values for CH₃ and CH₂ in (C₂H₅NH₃)₂CoCl₄ were plotted as a function of inverse temperature (see Figure 7). Indeed, the ¹³C T_1ρ curve for CH₃ 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 (C₂H₅NH₃)₂MCl₄ (M = Cu, Zn, and Cd) (see Figure 8) were compared with those of (C₂H₅NH₃)₂CoCl₄ determined herein. In addition, the molecular motions influenced by ¹³C T_1ρ in (C₂H₅NH₃)₂CoCl₄ were found to exhibit BPP trends, unlike in the case of the ¹H T_1ρ results. Furthermore, for (C₂H₅NH₃)₂CuCl₄, the temperature dependences of the ¹³C T_1ρ values for CH₂ and CH₃ appeared similar, and the BPP curves for CH₃ and CH₂ showed minima at 190 K. The T_1ρ curve for (C₂H₅NH₃)₂ZnCl₄ can be also represented by the BPP theory, with a minimum being observed at 260 K in the curve. Finally, in case of (C₂H₅NH₃)₂CdCl₄, the T_1ρ curves show minima at 260 and 250 K for CH₃ and CH₂, respectively. The ¹³C T_1ρ and E_a values obtained from the ¹³C results for the four compounds are summarized in Table 2, whereby it is apparent that the ¹³C 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 (C₂H₅NH₃)₂MCl₄ (M = Co and Cu) were driven by fluctuations of the magnetic dipoles of the paramagnetic Co²⁺ and Cu²⁺ species, and the E_a values for ¹³C decreased upon increasing the mass of the M²⁺ ion, unlike in the case of the ¹H E_a values. These differences are due to variations in the electronic structures of the M²⁺ ions, and in particular, the d electrons, which screen the nuclear charge from the motion of the outer electrons.

3. Materials and Methods

Single crystals of (C₂H₅NH₃)₂MCl₄ (M = Co, Cu, Zn, and Cd) were grown from CH₃CH₂NH₂∙HCl (ethylamine hydrochloride, Aldrich 98%), and CoCl₂ (cobalt chloride, Aldrich 97%), CuCl₂ (copper chloride, Aldrich 97%), ZnCl₂ (zinc chloride, Aldrich 98%), and CdCl₂ (cadmium chloride, Aldrich 99.99%), respectively, which were weighed in stoichiometric proportions at 300 K. These crystals were obtained by slow evaporating aqueous solutions containing of CH₃CH₂NH₂ HCl and MCl₂ in the molar ratio of 2:1.

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 N₂ atmosphere using a heating rate of 10 °C/min.

The ¹H MAS NMR and ¹³C CP/MAS NMR spectra for the rotating frame of (C₂H₅NH₃)₂MCl₄ 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 ¹H MAS and ¹³C 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 (C₂H₅NH₃)₂MCl₄ 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.

4. Conclusions

We herein discussed the thermodynamic, physical, and structural properties of (C₂H₅NH₃)₂MCl₄ (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 (C₂H₅NH₃)₂MCl₄ single crystals were investigated as a function of temperature by ¹H MAS NMR and ¹³C CP/MAS NMR experiments. To obtain detailed information regarding the cation dynamics of these crystals, the T_1ρ values for both ¹H and ¹³C were obtained, revealing that these atoms undergo rotational motion.

The reason why ¹H T_1ρ of C₂H₅ is longer than ¹H T_1ρ of NH₃ is as follows; the rotational motion for C₂H₅ is activated, and that for NH₃ at the end of the organic cation is less strongly activated. In addition, the reason why ¹³C T_1ρ of CH₂ is longer than ¹³C T_1ρ of CH₃ is as follows; the amplitude of the cation motion is enhanced at its CH₃ end, and the central CH₂ moiety is fixed to the NH₃ 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 Co²⁺ and Zn²⁺ ions differed from those of the crystals containing Cu²⁺ and Cd²⁺ ions. It was also found that the inorganic frames of the M = Cu²⁺ and Cd²⁺ species are corner-sharing MCl₆ octahedra, while those of M = Co²⁺ and Zn²⁺ are simple MCl₄ 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 MCl₄ tetrahedra and the MCl₆ octahedra. The presence of different paramagnetic ions and different lattice constants may also account for these differences.