1. 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
3NH
3)
2MX4 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
3NH
3)
+ cations [
12]. These crystals have a layered structure and exhibit quasi-, two-dimensional magnetic properties. Most recently, electrochemical oxygen evolution of (CH
3NH
3)
2CoBr
4, a lead-free cobalt-based perovskite, has been reported by Babu et al. [
13]. The (CH
3NH
3)
2CoBr
4 crystal belongs to the (CH
3NH
3)
2MX4 series and the family of hybrid organic–inorganic compounds in which (CH
3NH
3)
+ cations are connected via a bridge structure between the planes that contain the Co
2+ ions. At room temperature, the (CH
3NH
3)
2CoBr
4 crystal structure has monoclinic symmetry and belongs to the space group
P21/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
3NH
3)
+ cations, and two different crystallographic (CH
3NH
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
3NH
3)
2CdBr
4 crystals at room temperature have a monoclinic structure and belong to the space group
P21/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
3NH
3)
2CoBr
4. Until now, the phase transition temperature, thermal property, and paramagnetic interactions of (CH
3NH
3)
2CoBr
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
3NH
3)
2CoBr
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
3NH
3)
2CoBr
4 were obtained using
1H magic angle spinning (MAS) NMR and
13C cross-polarization (CP)/MAS NMR methods at several temperatures to probe the local environments and study the roles of the (CH
3NH
3)
+ cations. Moreover, the physical properties of (CH
3NH
3)
2CoBr
4 including paramagnetic ions and (CH
3NH
3)
2CdBr
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.
2. 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
3NH
3)
2CoBr
4 are plotted within the temperature range of 300–770 K, as shown in
Figure 1 and
Figure 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
3NH
3)
2CoBr
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):
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
3NH
3)
2CoBr
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
1H-NMR spectrum of (CH
3NH
3)
2CoBr
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
1H in CH
3 is marked with open circles, and that for
1H in NH
3 is marked with asterisks. The
1H 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
1H in the (CH
3NH
3)
+ cations. The
1H-NMR chemical shift of δ = −0.3 ppm is due to the CH
3, while the
1H-NMR chemical shift of δ = 4.2 ppm is due to the NH
3. The
1H-NMR chemical shifts for the two
1H in the (CH
3NH
3)
+ cations are temperature-independent. They remain quasi-constant with increasing temperature, indicating that the structural environment of
1H in the CH
3 and NH
3 groups does not change.
Figure 4 shows the recovery traces for the
1H 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
3NH
3)
+ cations. However, the
1H T
1ρ values for CH
3 and NH
3 are indistinguishable owing to the overlapping responses of the two protons. The
1H T
1ρ values for (CH
3NH
3)
2CoBr
4 obtained herein and the corresponding values for (CH
3NH
3)
2CdBr
4 reported previously [
17] are shown in
Figure 5 as a function of the inverse temperature. In the case of (CH
3NH
3)
2CoBr
4, the
1H 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
1H T
1ρ values are obtained for each proton in CH
3 and NH
3 in the case of (CH
3NH
3)
2CdBr
4 as a function of reciprocal temperature. Herein, the T
1ρ values for the two protons of the (CH
3NH
3)
+ cations are nearly the same within experimental error. The T
1ρ values of
1H in the CH
3 and NH
3 ions abruptly decrease at approximately 360 K. The
1H T
1ρ value of (CH
3NH
3)
2CoBr
4 including the paramagnetic ions is very short, whereas that of (CH
3NH
3)
2CdBr
4 excluding paramagnetic ions is very long.
The local environment of the carbons in (CH
3NH
3)
2CoBr
4 was studied by
13C MAS NMR, and the corresponding
13C-NMR chemical shifts are shown in
Figure 6. Attention was paid to
13C-NMR, which should be a sensitive probe of the local environment and of the cation dynamics.
The
13C-NMR spectrum at 300 K in (CH
3NH
3)
2CoBr
4 shows two signals at the chemical shifts of δ = 68.3 ppm and δ = 117.9 ppm with respect to TMS [
21]. The
13C-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
13C-NMR chemical shifts of the two compounds of (CH
3NH
3)
2CoBr
4 and (CH
3NH
3)
2CdBr
4 are shown in
Figure 7 as a function of temperature. The
13C-NMR chemical shifts vary significantly with temperature. Specifically, the
13C-NMR chemical shifts in the case of (CH
3NH
3)
2CoBr
4 decrease slowly and monotonically as a function of temperature. Conversely, the
13C-NMR spectrum at 300 K in (CH
3NH
3)
2CdBr
4 shows two signals at chemical shifts of δ = 27.9 ppm and δ = 29.3 ppm. The
13C-NMR chemical shifts of the crystallographically different a-CH
3 and b-CH
3 slowly and monotonously increase as a function of temperature. The
13C 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
13C chemical shift of (CH
3NH
3)
2CoBr
4, which contains paramagnetic ions, was significantly different to that of (CH
3NH
3)
2CdBr
4, which does not contain paramagnetic ions. The differences in the
13C chemical shifts could potentially be due to differences in the electron structures of the metal ions.
To determine the
13C 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
3NH
3)
2CoBr
4 and (CH
3NH
3)
2CdBr
4 as a function of the inverse temperature, as shown in
Figure 8. In the case of (CH
3NH
3)
2CoBr
4, the T
1ρ values of
13C 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
3NH
3)
2CdBr
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
3NH
3)
+ cations. From the
13C T
1ρ curves, the relaxation processes of (CH
3NH
3)
2CdBr
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 μo is the permeability, γH and γC are the respective gyromagnetic ratios for the 1H and 13C nuclei, r is the distance of H–C, ħ = h/2π, and ωH and ωC are the respective Larmor frequencies of 1H and 13C.
On the other hand, the relaxation processes of (CH
3NH
3)
2CoBr
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]
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
3NH
3)
2CoBr
4 and 67.56 kHz for (CH
3NH
3)
2CdBr
4. The T
1ρ exhibits a minimum when ω
1τ
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
3NH
3)
2CoBr
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
3NH
3)
2CdBr
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
3NH
3)
2CdBr
4, there is no chemical exchange or paramagnetic terms, and so τ
C can directly reflect the molecular motion. In the case of (CH
3NH
3)
2CoBr
4, τ
e dominates the total correlation time, and thus, τ
C is not directly related to molecular motion.
4. Conclusions
The thermal properties and phase transition temperature of (CH3NH3)2CoBr4 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 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 (CH3NH3)2CoBr4, associated with the role of the (CH3NH3)+ cations were studied by 1H-NMR and 13C-NMR as a function of temperature. The 1H and 13C MAS NMR were used to probe the dynamics of cations in (CH3NH3)2CoBr4 and (CH3NH3)2CdBr4. 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 1H and 13C-NMR chemical shifts was investigated using temperature-dependent NMR experiments. The contributions to the 13C-NMR chemical shifts are correlated with the distribution of spin density in the ligand moiety.
The temperature dependence of the T
1ρ values for
1H reflect the modulation of the inter-NH
3 and inter-CH
3 dipolar interactions due to the (CH
3NH
3)
+ cations. The variation of T
1ρ for
13C yielded a minimum, and it is apparent that the T
1ρ values for
13C 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
1H and
13C of the (CH
3NH
3)
2CoBr
4 crystals, which contain paramagnetic ions, are much shorter than those of the (CH
3NH
3)
2CdBr
4 crystals, which do not contain paramagnetic ions.
The (CH3NH3)2CoBr4 and (CH3NH3)2CdBr4 crystals are of the (CH3NH3)2MX4 type, whereas their individual dynamics differ significantly from the dynamic of the cations. The differences between the T1ρ of the (CH3NH3)2MBr4 crystals (M = Co and Cd) are due to the differences between the electron structures of their Co2+ and Cd2+ ions. These ions screen the nuclear charge from the motion of the outer electrons. The Co2+ has unpaired d electrons, whereas Cd2+ has filled d shells. Their NMR properties stem from the differences between the chemical properties of paramagnetic Co2+ and non-paramagnetic Cd2+ ions. Furthermore, the NMR relaxation of diamagnetic Cd2+ ions is most probably driven by dipolar interactions, whereas the relaxation of paramagnetic Co2+ ions is mostly driven by interactions with the paramagnetic center.