Study on Paramagnetic Interactions of (CH3NH3)2CoBr4 Hybrid Perovskites Based on Nuclear Magnetic Resonance (NMR) Relaxation Time

The thermal properties of organic–inorganic (CH3NH3)2CoBr4 crystals were investigated using differential scanning calorimetry and thermogravimetric analysis. The phase transition and partial decomposition temperatures were observed at 460 K and 572 K. Nuclear magnetic resonance (NMR) chemical shifts depend on the local field at the site of the resonating nucleus. In addition, temperature-dependent spin–lattice relaxation times (T1ρ) were measured using 1H and 13C magic angle spinning NMR to elucidate the paramagnetic interactions of the (CH3NH3)+ cations. The shortening of 1H and 13C T1ρ of the (CH3NH3)2CoBr4 crystals are due to the paramagnetic Co2+ effect. Moreover, the physical properties of (CH3NH3)2CoBr4 with paramagnetic ions and those of (CH3NH3)2CdBr4 without paramagnetic ions are reported and compared.


Introduction
Hybrid organic-inorganic compounds based on perovskite structures are currently attracting an increased amount of interest owing to their potential as substitutes for perovskite solar cells [1][2][3][4][5][6][7][8][9][10]. However, the toxicity and chemical instability of perovskites continue to be the major problems associated with their use in solar cells. Compounds in the (CH 3 NH 3 ) 2 MX 4 family (where M is the transition metal and X is halide) exhibit a variety of physical properties [1,11]. Ions of the transition metal M are located in the tetrahedral structure formed by the halogen ions X, and lie in the planes bridged by the (CH 3 NH 3 ) + cations [12]. These crystals have a layered structure and exhibit quasi-, two-dimensional magnetic properties. Most recently, electrochemical oxygen evolution of (CH 3 NH 3 ) 2 CoBr 4 , a lead-free cobalt-based perovskite, has been reported by Babu et al. [13]. The (CH 3 NH 3 ) 2 CoBr 4 crystal belongs to the (CH 3 NH 3 ) 2 MX 4 series and the family of hybrid organic-inorganic compounds in which (CH 3 NH 3 ) + cations are connected via a bridge structure between the planes that contain the Co 2+ ions. At room temperature, the (CH 3 NH 3 ) 2 CoBr 4 crystal structure has monoclinic symmetry and belongs to the space group P2 1 /c, with lattice constants a = 7.9782 Å, b = 13.1673 Å, c = 11.2602 Å, and ß = 96.3260 • [14]. The unit cell contains four formula units and four magnetic Co 2+ ions. The (CoBr 4 ) 2− units are surrounded by seven (CH 3 NH 3 ) + cations, and two different crystallographic (CH 3 NH 3 ) + cations exist. Although the tetrahedral anion exhibits only C 1 symmetry, the deviation from an idealized tetrahedral symmetry is small. The NH 3 + polar heads of the chains connect the isolated (CoBr 4 ) 2− tetrahedral structure with weak N-H···Br hydrogen bonds. On the other hand, (CH 3 NH 3 ) 2 CdBr 4 crystals at room temperature have a monoclinic structure and belong to the space group P2 1 /c with lattice constants a = 8.1257 Å, b = 13.4317 Å, c = 11.4182 Å, ß = 96.1840 • , and Z = 4 [15,16]. The structure of this crystal is very similar to that of the (CH 3 NH 3 ) 2 CoBr 4 . Until now, the phase transition temperature, thermal property, and paramagnetic interactions of (CH 3 NH 3 ) 2 CoBr 4 have not been studied in full. The paramagnetic ions of the lead-free perovskite are eco-friendly, which is important for application to solar cells. The present study was conducted to investigate the thermodynamic properties of the (CH 3 NH 3 ) 2 CoBr 4 crystal using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and optical polarizing microscopy. Additionally, the nuclear magnetic resonance (NMR) chemical shifts and spin-lattice relaxation times T 1ρ in the rotating frame of (CH 3 NH 3 ) 2 CoBr 4 were obtained using 1 H magic angle spinning (MAS) NMR and 13 C cross-polarization (CP)/MAS NMR methods at several temperatures to probe the local environments and study the roles of the (CH 3 NH 3 ) + cations. Moreover, the physical properties of (CH 3 NH 3 ) 2 CoBr 4 including paramagnetic ions and (CH 3 NH 3 ) 2 CdBr 4 excluding paramagnetic ions were obtained from previous reports [17], and used as a comparison to understand the effects of Co 2+ and Cd 2+ ions.

Results and Discussion
TGA and DSC measurements were obtained to understand the thermal stability, structural phase transitions, and melting temperatures. The TGA and DSC curves of (CH 3 NH 3 ) 2 CoBr 4 are plotted within the temperature range of 300-770 K, as shown in Figures 1 and 2. The transformation anomaly at 460 K (=T C ) in the DSC curve is related to the phase transition. The mass loss of 3.89% occurs at approximately 572 K (=T d ), and is ascribed to the onset of partial thermal decomposition. The compound (CH 3 NH 3 ) 2 CoBr 4 loses its crystallization at increased temperatures. When comparing the experimental TGA results and possible chemical reactions, the solid residue is calculated on the basis of Equations (1) and (2): phase transition temperature, thermal property, and paramagnetic interactions of (CH3NH3)2CoBr4 have not been studied in full. The paramagnetic ions of the lead-free perovskite are eco-friendly, which is important for application to solar cells. The present study was conducted to investigate the thermodynamic properties of the (CH3NH3)2CoBr4 crystal using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and optical polarizing microscopy. Additionally, the nuclear magnetic resonance (NMR) chemical shifts and spin-lattice relaxation times T1ρ in the rotating frame of (CH3NH3)2CoBr4 were obtained using 1 H magic angle spinning (MAS) NMR and 13 C cross-polarization (CP)/MAS NMR methods at several temperatures to probe the local environments and study the roles of the (CH3NH3) + cations. Moreover, the physical properties of (CH3NH3)2CoBr4 including paramagnetic ions and (CH3NH3)2CdBr4 excluding paramagnetic ions were obtained from previous reports [17], and used as a comparison to understand the effects of Co 2+ and Cd 2+ ions.

Results and Discussion
TGA and DSC measurements were obtained to understand the thermal stability, structural phase transitions, and melting temperatures. The TGA and DSC curves of (CH3NH3)2CoBr4 are plotted within the temperature range of 300-770 K, as shown in Fig. 1 and Fig. 2. The transformation anomaly at 460 K (=TC) in the DSC curve is related to the phase transition. The mass loss of 3.89% occurs at approximately 572 K (=Td), and is ascribed to the onset of partial thermal decomposition. The compound (CH3NH3)2CoBr4 loses its crystallization at increased temperatures. When comparing the experimental TGA results and possible chemical reactions, the solid residue is calculated on the basis of Eq. (1) and Eq. (2):  The mass loss of 37% near 669 K is likely attributable to the decomposition of the 2HBr moieties. Moreover, the mass loss near 700 K reaches 48.81%. These results are consistent with the TGA results reported by Babu et al. [13]. By the end, only CoBr2 remains. The solid-state decomposition is essentially one of the chemical reactions that occur at the surface. The second stage is associated with the thermal decomposition of (CH3NH3)2CoBr4 to CoBr2. Optical polarizing microscopy showed that the crystals have a seagrass color at room temperature. The color of the crystal does not vary as the temperature increases, and the crystal starts to melt at temperatures above Td, as indicated at the surface. From the TGA and DSC results, the phase transition temperature is 460 K, and the partial decomposition temperature is at 572 K. The high-temperature phenomenon above Td is not related to a physical change, such as structural phase transitions, but is instead related to chemical changes, such as thermal decomposition.
The temperature-dependent 1 H-NMR spectrum of (CH3NH3)2CoBr4 is obtained to understand and analyze its structure. All recorded spectra contain only one resonance line, and Fig. 3 shows the spectrum at 410 K. The spinning sideband for 1 H in CH3 is marked with open circles, and that for 1 H in NH3 is marked with asterisks. The 1 H resonance line has an asymmetric shape, and the full-width at half maximum (FWHM) values on the left and right sides are not equal. The asymmetric line shape is attributed to the overlapping lines of the two 1 H in the (CH3NH3) + cations. The 1 H-NMR chemical shift of δ = −0.3 ppm is due to the CH3, while the 1 H-NMR chemical shift of δ = 4.2 ppm is due to the NH3. The 1 H-NMR chemical shifts for the two 1 H in the (CH3NH3) + cations are temperatureindependent. They remain quasi-constant with increasing temperature, indicating that the structural environment of 1 H in the CH3 and NH3 groups does not change. Fig. 4 shows the recovery traces for the 1 H resonance lines for delay times that range from 1 μs to 20 ms at 300 K. Herein, the arrows mark the resonance lines at each delay time, while the other resonance lines are the sidebands. The T1ρ values are obtained from the intensities of the magnetization recovery curves with respect to the delay time. The recovery traces are described by a simple mono-exponential function [18][19][20].
where P(τ) is the NMR signal intensity measured after recovery time τ, and P(0) is the total nuclear magnetization of the protons at thermal equilibrium. This analysis method is used to obtain The mass loss of 37% near 669 K is likely attributable to the decomposition of the 2HBr moieties. Moreover, the mass loss near 700 K reaches 48.81%. These results are consistent with the TGA results reported by Babu et al. [13]. By the end, only CoBr 2 remains. The solid-state decomposition is essentially one of the chemical reactions that occur at the surface. The second stage is associated with the thermal decomposition of (CH 3 NH 3 ) 2 CoBr 4 to CoBr 2 . Optical polarizing microscopy showed that the crystals have a seagrass color at room temperature. The color of the crystal does not vary as the temperature increases, and the crystal starts to melt at temperatures above T d , as indicated at the surface. From the TGA and DSC results, the phase transition temperature is 460 K, and the partial decomposition temperature is at 572 K. The high-temperature phenomenon above T d is not related to a physical change, such as structural phase transitions, but is instead related to chemical changes, such as thermal decomposition.
The temperature-dependent 1 H-NMR spectrum of (CH 3 NH 3 ) 2 CoBr 4 is obtained to understand and analyze its structure. All recorded spectra contain only one resonance line, and Figure 3 shows the spectrum at 410 K. The spinning sideband for 1 H in CH 3 is marked with open circles, and that for 1 H in NH 3 is marked with asterisks. The 1 H resonance line has an asymmetric shape, and the full-width at half maximum (FWHM) values on the left and right sides are not equal. The asymmetric line shape is attributed to the overlapping lines of the two 1 H in the (CH 3 NH 3 ) + cations. The 1 H-NMR chemical shift of δ = −0.3 ppm is due to the CH 3 , while the 1 H-NMR chemical shift of δ = 4.2 ppm is due to the NH 3 . The 1 H-NMR chemical shifts for the two 1 H in the (CH 3 NH 3 ) + cations are temperature-independent. They remain quasi-constant with increasing temperature, indicating that the structural environment of 1 H in the CH 3 and NH 3 groups does not change. Figure 4 shows the recovery traces for the 1 H resonance lines for delay times that range from 1 µs to 20 ms at 300 K. Herein, the arrows mark the resonance lines at each delay time, while the other resonance lines are the sidebands. The T 1ρ values are obtained from the intensities of the magnetization recovery curves with respect to the delay time. The recovery traces are described by a simple mono-exponential function [18][19][20].
where P(τ) is the NMR signal intensity measured after recovery time τ, and P(0) is the total nuclear magnetization of the protons at thermal equilibrium. This analysis method is used to obtain the T 1ρ values for the proton in the (CH 3 NH 3 ) + cations. However, the 1 H T 1ρ values for CH 3 and NH 3  [17] are shown in Figure 5 as a function of the inverse temperature. In the case of (CH 3 NH 3 ) 2 CoBr 4 , the 1 H T 1ρ values increased rapidly near 210 K, and those at high temperatures are almost continuous; the T 1ρ value at 180 K is 76 µs and that at 300 K is 10 times longer than that at 180 K. The T 1ρ value is very short at low temperatures, and thus indicates rapid energy transfer from the nuclear spin system to the surrounding environment. On the other hand, the 1 H T 1ρ values are obtained for each proton in CH 3 and NH 3 in the case of (CH 3 NH 3 ) 2 CdBr 4 as a function of reciprocal temperature. Herein, the T 1ρ values for the two protons of the (CH 3 NH 3 ) + cations are nearly the same within experimental error. The T 1ρ values of 1 H in the CH 3 and NH 3 ions abruptly decrease at approximately 360 K. The 1 H T 1ρ value of (CH 3 NH 3 ) 2 CoBr 4 including the paramagnetic ions is very short, whereas that of (CH 3 NH 3 ) 2 CdBr 4 excluding paramagnetic ions is very long.     [17] are shown in Fig. 5 as a function of the inverse temperature. In the case of (CH3NH3)2CoBr4, the 1 H T1ρ values increased rapidly near 210 K, and those at high temperatures are almost continuous; the T1ρ value at 180 K is 76 μs and that at 300 K is 10 times longer than that at 180     [17] are shown in Fig. 5 as a function of the inverse temperature. In the case of (CH3NH3)2CoBr4, the 1 H T1ρ values increased rapidly near 210 K, and those at high temperatures are approximately 360 K. The 1 H T1ρ value of (CH3NH3)2CoBr4 including the paramagnetic ions is very short, whereas that of (CH3NH3)2CdBr4 excluding paramagnetic ions is very long. The local environment of the carbons in (CH3NH3)2CoBr4 was studied by 13 C MAS NMR, and the corresponding 13 C-NMR chemical shifts are shown in Fig. 6. Attention was paid to 13 C-NMR, which should be a sensitive probe of the local environment and of the cation dynamics.  The 13 C-NMR spectrum at 300 K in (CH3NH3)2CoBr4 shows two signals at the chemical shifts of δ = 68.3 ppm and δ = 117.9 ppm with respect to TMS [21]. The 13 C-NMR spectrum consists of two lines that correspond to a-CH3 and b-CH3. The signals respectively represent the methyl carbons in the two The local environment of the carbons in (CH 3 NH 3 ) 2 CoBr 4 was studied by 13 C MAS NMR, and the corresponding 13 C-NMR chemical shifts are shown in Figure 6. Attention was paid to 13 C-NMR, which should be a sensitive probe of the local environment and of the cation dynamics. approximately 360 K. The 1 H T1ρ value of (CH3NH3)2CoBr4 including the paramagnetic ions is very short, whereas that of (CH3NH3)2CdBr4 excluding paramagnetic ions is very long. The local environment of the carbons in (CH3NH3)2CoBr4 was studied by 13 C MAS NMR, and the corresponding 13 C-NMR chemical shifts are shown in Fig. 6. Attention was paid to 13 C-NMR, which should be a sensitive probe of the local environment and of the cation dynamics.  The 13 C-NMR spectrum at 300 K in (CH3NH3)2CoBr4 shows two signals at the chemical shifts of δ = 68.3 ppm and δ = 117.9 ppm with respect to TMS [21]. The 13 C-NMR spectrum consists of two lines that correspond to a-CH3 and b-CH3. The signals respectively represent the methyl carbons in the two The 13 C-NMR spectrum at 300 K in (CH 3 NH 3 ) 2 CoBr 4 shows two signals at the chemical shifts of δ = 68.3 ppm and δ = 117.9 ppm with respect to TMS [21]. The 13 C-NMR spectrum consists of two lines that correspond to a-CH 3 and b-CH 3 . The signals respectively represent the methyl carbons in the two crystallographically different a-CH 3 and b-CH 3 . The 13 C-NMR chemical shifts of the two compounds of (CH 3 NH 3 ) 2 CoBr 4 and (CH 3 NH 3 ) 2 CdBr 4 are shown in Figure 7 as a function of temperature. The 13 C-NMR chemical shifts vary significantly with temperature. Specifically, the 13 C-NMR chemical shifts in the case of (CH 3 NH 3 ) 2 CoBr 4 decrease slowly and monotonically as a function of temperature. Conversely, the 13 C-NMR spectrum at 300 K in (CH 3 NH 3 ) 2 CdBr 4 shows two signals at chemical shifts of δ = 27.9 ppm and δ = 29.3 ppm. The 13 C-NMR chemical shifts of the crystallographically different a-CH 3 and b-CH 3 slowly and monotonously increase as a function of temperature. The 13 C chemical shifts of the CH 3 groups differ between the two compounds. Generally, the paramagnetic contribution to the NMR shift is responsible for the NMR spectra. The 13 C chemical shift of (CH 3 NH 3 ) 2 CoBr 4 , which contains paramagnetic ions, was significantly different to that of (CH 3 NH 3 ) 2 CdBr 4 , which does not contain paramagnetic ions. The differences in the 13 C chemical shifts could potentially be due to differences in the electron structures of the metal ions. crystallographically different a-CH3 and b-CH3. The 13 C-NMR chemical shifts of the two compounds of (CH3NH3)2CoBr4 and (CH3NH3)2CdBr4 are shown in Fig. 7 as a function of temperature. The 13 C-NMR chemical shifts vary significantly with temperature. Specifically, the 13 C-NMR chemical shifts in the case of (CH3NH3)2CoBr4 decrease slowly and monotonically as a function of temperature. Conversely, the 13 C-NMR spectrum at 300 K in (CH3NH3)2CdBr4 shows two signals at chemical shifts of δ = 27.9 ppm and δ = 29.3 ppm. The 13 C-NMR chemical shifts of the crystallographically different a-CH3 and b-CH3 slowly and monotonously increase as a function of temperature. The 13 C chemical shifts of the CH3 groups differ between the two compounds. Generally, the paramagnetic contribution to the NMR shift is responsible for the NMR spectra. The 13 C chemical shift of (CH3NH3)2CoBr4, which contains paramagnetic ions, was significantly different to that of (CH3NH3)2CdBr4, which does not contain paramagnetic ions. The differences in the 13 C chemical shifts could potentially be due to differences in the electron structures of the metal ions. To determine the 13 C T1ρ, nuclear magnetization was measured as a function of the delay time. The signal intensities of the nuclear magnetization recovery curves are fitted by the monoexponential function of Eq. (3). From these results, T1ρ values were obtained for the carbons in (CH3NH3)2CoBr4 and (CH3NH3)2CdBr4 as a function of the inverse temperature, as shown in Fig. 8. In the case of (CH3NH3)2CoBr4, the T1ρ values of 13 C show a minimum value near 330 K, while the T1ρ value abruptly decreases above 410 K. The T1ρ values for a-CH3 and b-CH3 are also very similar and of the order of 10 ms. Conversely, the variation of T1ρ with temperature in the case of (CH3NH3)2CdBr4 exhibits a minimum near 250 K for a-CH3 and b-CH3, respectively, and T1ρ decreases abruptly above 360 K. The presence of these minima are attributed to the effects of the reorientation of (CH3NH3) + cations. From the 13 C T1ρ curves, the relaxation processes of (CH3NH3)2CdBr4 are affected by molecular motion described by the Bloembergen-Purcell-Pound (BPP) theory [22]. The experimental values of T1ρ are explained by the correlation time τC for molecular motion based on the BPP theory [22,23]  To determine the 13 C T 1ρ , nuclear magnetization was measured as a function of the delay time. The signal intensities of the nuclear magnetization recovery curves are fitted by the mono-exponential function of Equation (3). From these results, T 1ρ values were obtained for the carbons in (CH 3 NH 3 ) 2 CoBr 4 and (CH 3 NH 3 ) 2 CdBr 4 as a function of the inverse temperature, as shown in Figure 8. In the case of (CH 3 NH 3 ) 2 CoBr 4 , the T 1ρ values of 13 C show a minimum value near 330 K, while the T 1ρ value abruptly decreases above 410 K. The T 1ρ values for a-CH 3 and b-CH 3 are also very similar and of the order of 10 ms. Conversely, the variation of T 1ρ with temperature in the case of (CH 3 NH 3 ) 2 CdBr 4 exhibits a minimum near 250 K for a-CH 3 and b-CH 3 , respectively, and T 1ρ decreases abruptly above 360 K. The presence of these minima are attributed to the effects of the reorientation of (CH 3 NH 3 ) + cations. From the 13 C T 1ρ curves, the relaxation processes of (CH 3 NH 3 ) 2 CdBr 4 are affected by molecular motion described by the Bloembergen-Purcell-Pound (BPP) theory [22]. The experimental values of T 1ρ are explained by the correlation time τ C for molecular motion based on the BPP theory [22,23], where where µ o is the permeability, γ H and γ C are the respective gyromagnetic ratios for the 1 H and 13 C nuclei, r is the distance of H-C,h = h/2π, and ω H and ω C are the respective Larmor frequencies of 1 H and 13 C. where μo is the permeability, γH and γC are the respective gyromagnetic ratios for the 1 H and 13 C nuclei, r is the distance of H-C, ħ = h/2π, and ωH and ωC are the respective Larmor frequencies of 1 H and 13 C.
On the other hand, the relaxation processes of (CH3NH3)2CoBr4 with the paramagnetic Co 2+ ions are affected by the molecular motion described by the Solomon equation [24]. When paramagnetic ions exist, the T1ρ are represented by τC, as presented in [24] (1/T1ρ) = (1/15)(μo/4π) 2 [γI 2 γe 2 μB 2 S(S+1)/r 6 ][4Ga + Gb + 3Gc + 6Gd + 6Ge] where Here, γe is the gyromagnetic ratio of the electron, S is the total spin quantum number of the paramagnetic ion, and ωe is the Larmor frequency of the electron. Additionally, ω1 is the angular frequency at the spin-lock field; 59.52 kHz for (CH3NH3)2CoBr4 and 67.56 kHz for (CH3NH3)2CdBr4. The T1ρ exhibits a minimum when ω1τC = 1. Based on this condition, the coefficients of Eq. (4) and Eq. (5) which are dependent on ω1, ωH, and ωC, can be obtained. Furthermore, the value of τC can be calculated, and its temperature dependence follows a simple Arrhenius expression [22] according to, On the other hand, the relaxation processes of (CH 3 NH 3 ) 2 CoBr 4 with the paramagnetic Co 2+ ions are affected by the molecular motion described by the Solomon equation [24]. When paramagnetic ions exist, the T 1ρ are represented by τ C , as presented in [24] (1/T 1ρ ) = (1/15)(µ o /4π) 2 where Here, γ e is the gyromagnetic ratio of the electron, S is the total spin quantum number of the paramagnetic ion, and ω e is the Larmor frequency of the electron. Additionally, ω 1 is the angular frequency at the spin-lock field; 59.52 kHz for (CH 3 NH 3 ) 2 CoBr 4 and 67.56 kHz for (CH 3 NH 3 ) 2 CdBr 4 . The T 1ρ exhibits a minimum when ω 1 τ C = 1. Based on this condition, the coefficients of Equations (4) and (5) which are dependent on ω 1 , ω H , and ω C , can be obtained. Furthermore, the value of τ C can be calculated, and its temperature dependence follows a simple Arrhenius expression [22] according to, where τ o is the preexponential factor, T is the temperature, R is the gas constant, and E a is the activation energy. The activation energies for the tumbling motion of a-CH 3 and b-CH 3 in the case of (CH 3 NH 3 ) 2 CoBr 4 are obtained from the log τ C vs. 1000/T curve, and are respectively equal to 24.51 ± 0.99 kJ/mol and 23.25 ± 1.30 kJ/mol, whereas the corresponding values in the case of (CH 3 NH 3 ) 2 CdBr 4 are 8.18 ± 0.37 kJ/mol and 7.65 ± 0.21 kJ/mol (see Figure 9). When paramagnetic Co 2+ ions exist, 1/τ C = 1/τ r + 1/τ M + 1/τ e , where τ r , τ M , and τ e , are the rotational correlation time, exchange correlation time, and electronic relaxation correlation time, respectively. The τ r can represent molecular motion. For (CH 3 NH 3 ) 2 CdBr 4 , there is no chemical exchange or paramagnetic terms, and so τ C can directly reflect the molecular motion. In the case of (CH 3 NH 3 ) 2 CoBr 4 , τ e dominates the total correlation time, and thus, τ C is not directly related to molecular motion.
where τo is the preexponential factor, T is the temperature, R is the gas constant, and Ea is the activation energy. The activation energies for the tumbling motion of a-CH3 and b-CH3 in the case of (CH3NH3)2CoBr4 are obtained from the log τC vs. 1000/T curve, and are respectively equal to 24.51 ± 0.99 kJ/mol and 23.25 ± 1.30 kJ/mol, whereas the corresponding values in the case of (CH3NH3)2CdBr4 are 8.18 ± 0.37 kJ/mol and 7.65 ± 0.21 kJ/mol (see Fig. 9). When paramagnetic Co 2+ ions exist, 1/τC = 1/τr + 1/τM + 1/τe, where τr, τM, and τe, are the rotational correlation time, exchange correlation time, and electronic relaxation correlation time, respectively. The τr can represent molecular motion. For (CH3NH3)2CdBr4, there is no chemical exchange or paramagnetic terms, and so τC can directly reflect the molecular motion. In the case of (CH3NH3)2CoBr4, τe dominates the total correlation time, and thus, τC is not directly related to molecular motion.

Materials and Methods
The (CH3NH3)2CoBr4 single crystals were grown based on the slow evaporation of an aqueous solution with a 2:1 ratio of CH3NH2·HBr and CoBr2 at 300 K. Single crystals have a diamond shape and seagrass color.
The thermal properties and phase transition temperature were measured using a TGA (TA, DSC 25) instrument at a heating rate of 10 °C/min. The TGA and DSC curves were measured in an N2 atmosphere, and the mass of the powder sample used in the experiment was 9.22 mg.
The solid-state MAS NMR spectra and the spin-lattice relaxation time T1ρ in the rotating frame of (CH3NH3)2CoBr4 crystals were recorded on a Bruker 400 DSX NMR spectrometer (Bruker, Leipzig, Germany) at the Korean Basic Science Institute at the Western Seoul Center. Solid samples were inserted into 4 mm diameter zirconia rotors. The samples were spun at a sufficient speed to avoid spinning sidebands overlapping. The chemical shifts were defined with respect to tetramethylsilane (TMS). The 1 H T1ρ values were measured using a π/2-τ pulse sequence by varying the duration of the spin-locking pulses. The 13 C T1ρ values were measured based on the variation of the duration of the 13 C spin-locking pulse. The usual experimental approach assumes the use of cross-polarization from protons to enhance the 13 C sensitivity. The widths of the π/2 pulses for 1 H and 13 C were 4.1 μs and 4.2 μs, respectively. The T1ρ values were measured in the temperature range of 180-430 K due to limitations of the experimental equipment associated with the measurements of the spectra and T1ρ

Materials and Methods
The (CH 3 NH 3 ) 2 CoBr 4 single crystals were grown based on the slow evaporation of an aqueous solution with a 2:1 ratio of CH 3 NH 2 ·HBr and CoBr 2 at 300 K. Single crystals have a diamond shape and seagrass color.
The thermal properties and phase transition temperature were measured using a TGA (TA, DSC 25) instrument at a heating rate of 10 • C/min. The TGA and DSC curves were measured in an N 2 atmosphere, and the mass of the powder sample used in the experiment was 9.22 mg.
The solid-state MAS NMR spectra and the spin-lattice relaxation time T 1ρ in the rotating frame of (CH 3 NH 3 ) 2 CoBr 4 crystals were recorded on a Bruker 400 DSX NMR spectrometer (Bruker, Leipzig, Germany) at the Korean Basic Science Institute at the Western Seoul Center. Solid samples were inserted into 4 mm diameter zirconia rotors. The samples were spun at a sufficient speed to avoid spinning sidebands overlapping. The chemical shifts were defined with respect to tetramethylsilane (TMS). The 1 H T 1ρ values were measured using a π/2-τpulse sequence by varying the duration of the spin-locking pulses. The 13 C T 1ρ values were measured based on the variation of the duration of the 13 C spin-locking pulse. The usual experimental approach assumes the use of cross-polarization from protons to enhance the 13 C sensitivity. The widths of the π/2 pulses for 1 H and 13 C were 4.1 µs and 4.2 µs, respectively. The T 1ρ values were measured in the temperature range of 180-430 K due to limitations of the experimental equipment associated with the measurements of the spectra and T 1ρ outside of this range. The sample temperatures were held constant by controlling the helium gas flow and the heater current [25,26].

Conclusions
The thermal properties and phase transition temperature of (CH 3 NH 3 ) 2 CoBr 4 crystals grown based on the slow evaporation method were investigated with TGA, DSC, and optical polarizing microscopy. The phase transition and partial decomposition temperatures were observed at 460 K and Molecules 2019, 24, 2895 9 of 10 572 K, respectively. The high-temperature phenomenon above 572 K was not related to a physical change like the structural phase transition. Instead, it was related to a chemical change, such as thermal decomposition.
The paramagnetic interactions of (CH 3 NH 3 ) 2 CoBr 4 , associated with the role of the (CH 3 NH 3 ) + cations were studied by 1 H-NMR and 13 C-NMR as a function of temperature. The 1 H and 13 C MAS NMR were used to probe the dynamics of cations in (CH 3 NH 3 ) 2 CoBr 4 and (CH 3 NH 3 ) 2 CdBr 4 . The chemical shift by the MAS NMR depended on the local field at the site of the resonating nucleus in crystals. The effect of these crystals on the 1 H and 13 C-NMR chemical shifts was investigated using temperature-dependent NMR experiments. The contributions to the 13 C-NMR chemical shifts are correlated with the distribution of spin density in the ligand moiety.
The temperature dependence of the T 1ρ values for 1 H reflect the modulation of the inter-NH 3 and inter-CH 3 dipolar interactions due to the (CH 3 NH 3 ) + cations. The variation of T 1ρ for 13 C yielded a minimum, and it is apparent that the T 1ρ values for 13 C are governed by tumbling motions. Moreover, the paramagnetic dopant led to the shortening of their T 1ρ values. Accordingly, it has been shown that the T 1ρ value is inversely proportional to the square of the magnetic moment of the paramagnetic ion [27]. The T 1ρ values of 1 H and 13 C of the (CH 3 NH 3 ) 2 CoBr 4 crystals, which contain paramagnetic ions, are much shorter than those of the (CH 3 NH 3 ) 2 CdBr 4 crystals, which do not contain paramagnetic ions.
The (CH 3 NH 3 ) 2 CoBr 4 and (CH 3 NH 3 ) 2 CdBr 4 crystals are of the (CH 3 NH 3 ) 2 MX 4 type, whereas their individual dynamics differ significantly from the dynamic of the cations. The differences between the T 1ρ of the (CH 3 NH 3 ) 2 MBr 4 crystals (M = Co and Cd) are due to the differences between the electron structures of their Co 2+ and Cd 2+ ions. These ions screen the nuclear charge from the motion of the outer electrons. The Co 2+ has unpaired d electrons, whereas Cd 2+ has filled d shells. Their NMR properties stem from the differences between the chemical properties of paramagnetic Co 2+ and non-paramagnetic Cd 2+ ions. Furthermore, the NMR relaxation of diamagnetic Cd 2+ ions is most probably driven by dipolar interactions, whereas the relaxation of paramagnetic Co 2+ ions is mostly driven by interactions with the paramagnetic center.