Advances in the physicochemical characterization of lead-free hybrid perovskite [NH3(CH2)3NH3]CuBr4 Crystals – Scientific Reports

crystal structure

Single crystal XRD analysis of [NH₃(CH₂)₃NH₃]CuBr₄ was performed at 298 K. The structure, lattice constants and space group of this crystal were monoclinic, with a = 8.052 ± 0.009 , b = 7.560 ± 0.009 , c = 17.611 ± 0.190 , β = 96.920 ± 0.05°, Z = 4 and p2₁†nand this result was in good agreement with an earlier report by Halvorson and Willett²⁸†

Thermodynamic property:

In the DSC experiment, there was no phase transition temperature in the range of 200-500 K; however, large exothermic peaks were observed at 546 and 577 K (Supplementary Information) 1† To confirm that the DSC peaks at 546 and 577 K are related to the phase transition, TGA and differential thermal analysis (DTA) experiments were performed; the results are shown in fig. 2† The TGA results showed that this crystal is thermally stable up to 504 K. The initial weight loss of [NH₃(CH₂)₃NH₃]CuBr₄ started at 504 K, and there was no weight loss before the decomposition temperature. In the TGA curve, [NH₃(CH₂)₃NH₃]CuBr₄ showed a two-phase decomposition at high temperatures. The initial weight loss (17%) occurred in the 500-550 K range, which may be due to the breakdown of HBr in [NH₃(CH₂)₃NH₃]CuBr₄† The decomposition in the second phase (63%) occurred due to the presence of an inorganic group in the range of 550-650 K. The amount remaining as a solid was calculated from the TGA data and chemical reactions. The weight losses of 17 and 35% at approximately 546 and 607 K are probably due to the decomposition of the HBr and 2HBr residues, respectively, which is consistent with the exothermic peak in the DSC curve. The molecular weight abruptly decreased between 550 and 650 K, with a corresponding weight loss of 63% at about 650 K.

Furthermore, optical polarization microscopy experiments were performed to understand the thermal decomposition and melting phenomena of the crystals. The crystal was brown at 300 K, as shown in the inset of FIG. 2† Although no changes were observed from 300 to 500 K, the crystal started to melt slightly and turned from brown to dark brown at about 543 K. The color change was probably due to decomposition due to the loss of HBr, as well as geometric changes in CuBr₄† Above 600 K, the surface and edges melted significantly. The thermogram clearly indicated that 543 K was the melting point of the crystal. Hence the [NH₃(CH₂)₃NH₃]CuBr₄ crystal is suitable for applications up to 504 K.

¹H NMR chemical shifts and spin-lattice relaxation times

The ¹H NMR chemical shifts of the [NH₃(CH₂)₃NH₃]CuBr₄ crystals were recorded while the temperature was raised as shown in Fig. 3† Below 270K, only one ¹H resonance signal was observed and the intensity and line width of the ¹The H signal was very small and wide, respectively, which made detection challenging. The resonance signal showed asymmetrical shapes due to the overlap of the two types NH₃ and CH₂ signals. The ¹H NMR chemical shift for CH₂ was recorded at δ = 5.51 ppm at 300 K, while that for NH₃ was recorded at δ = 10.94 ppm, which was then split into two resonance lines. The rotating side tires for CH₂ are represented by open circles and those for NH₃ are represented by crosses. The ¹H chemical shifts for CH₂represented by the dotted line in FIG. 3did not change significantly with increasing temperature, while the change in the ¹H chemical shifts for NH₃ to the lower chemical shift as the temperature increased. The greater shift in the ¹H NMR chemical shift of the NH₃ in the cation with changes in temperature, than that of the CH₂ is reason for the big change in the N−H∙∙∙Br hydrogen bonding between the Br around Cu and the H of NH₃†

The relation of the decay rate of proton magnetization is determined by T_1ρ and the Eq. †1†^{27†36†37†38†39}

where P() and P(0) are sometimes the signal intensities and = 0, respectively. The intensity changes observed in the ¹H NMR spectra were recorded with changing delay times at a given temperature, and at 300 K, the ¹H NMR spectrum was plotted with a delay time ranging from 0.01 to 30 ms as shown in FIG. 4† From the slope of the intensities of the ¹H signal indicated by the arrow vs. delay times, the ¹HT_1ρ can be calculated using Eq. †1† As a result, the ¹HT_1ρ values ​​for CH₂ and NH₃ are shown in FIG. 5 as a function of the inverse temperature. The ¹HT_1ρ values ​​were on the order of a few milliseconds for CH₂ and NH₃, and their values ​​were temperature dependent. As shown in the cation structure in FIG. 5the ¹H from CH₂ is expressed in red, and the ¹H of NH₃ is expressed in black, which is the same as the T_1ρ values. the T_1ρ values ​​decreased as the temperature increased and then rose sharply again at 210 K. Below 300 K, only the ¹HT_1ρ value for NH₃ is displayed, and the ¹HT_1ρ values ​​of CH₂ above 300 K have a longer T_1ρ values ​​then ¹H of NH₃† the T_1ρ vs. inverse temperature curve showed minima of 5.80 ms at 210 K, indicating the existence of clear molecular movements. the T_1ρ values ​​can be explained by the correlation time τ_C for molecular motion. the T_1ρ value for the molecular motion based on the Bloembergen-Purcell-Pound (BPP) theory is given by^{35†36†37†38†39}†

where F_a †_C†[1 + ω₁²τ_C²]f_b †_C†[1 + (ω_C − ω_H)²τ_C²]f_c †_C†[1 + ω_C²τ_C²]f_d †_C†[1 + (ω_C + ω_H)²τ_C²]and F_e †_C†[1 + ω_H²τ_C²]† Here G is a coefficient, γ_huh and_C are the gyromagnetic ratios of the proton and carbon, respectively, ħ is Planck’s reduced constant, ω_huh and_C are the Larmor frequencies of ¹Hand ¹³C, respectively r is the distance between the proton and carbon, and ω₁ is the spin-locking pulse sequence with a 71.42 kHz locking pulse. In the rotating frame, τ_C can be obtained when_1C = 1, and the coefficient G in Eq. †2) can be obtained from T_1ρ†_{h, C}and₁† Using this G-value, τ_C was obtained as a function of inverse temperature. The local field fluctuation is determined by the thermal movement, which is activated by thermal energy. Therefore,_C is represented by the Arrhenius behavior: τ_C †_Oexp(− E_a/k_BT), where E_a and k_B are the activation energy and Boltzmann constant, respectively³⁶† the_C vs. 1000/T was plotted on a logarithmic scale (inset of Fig. 5), and the E_a by ¹H, depending on molecular dynamics, was obtained at 4.25 ± 0.25 kJ/mol per dot line.

¹³C NMR chemical shifts and spin-lattice relaxation times

The ¹³C NMR chemical shifts of [NH₃(CH₂)₃NH₃]CuBr₄ were measured as a function of temperature, as shown in Fig. 6† The ¹³CMAS NMR spectra showed two resonance signals at all temperatures. The ¹³C NMR spectrum for TMS was recorded at 38.3 ppm at 300 K and used to determine the exact chemical shift of . to decide ¹³C²⁷† Here, the CH₂ between the two CH₂ groups is labeled C1 and the CH₂ close to NH₃ is labeled C2, as shown in the inset of Fig. 6† The two resonance signals at 300 K were recorded at chemical shifts of = 33.54 and δ = 177.07 ppm for C1 and C2, respectively. The ¹³C chemical shifts for CH₂ were different for C1 far from those of NH₃ and C2 close to that of NH₃† The ¹³C chemical shift for C1 changed slowly and did not vary significantly with increasing temperature, while that for C2 moved abruptly to the lower chemical shift side with increasing temperature compared to that for C1.

The changes in the intensity of the ¹³C NMR spectral peaks with increasing delay times were measured at a given temperature in the same way as the ¹HT_1ρ measuring method. The ¹³CT_1ρ values ​​of the slope of the recovery traces are described by a single exponential function in Eq. †1† the T_1ρ values ​​for C1 and C2 as a function of 1000/temperature are shown in fig. 7† In the cation structure, C1 is shown in green, C2 in red and T_1ρ is displayed in the same way. ¹³CT_1ρ values ​​for C2 showed no changes in the temperature range measured in this study, and the T_1ρ for C1 according to the temperature change showed a similar trend as that for ¹HT_1ρ† The ¹³CT_1ρ values ​​were about 10 times longer than the ¹HT_1ρ values. The ¹³CT_1ρ values ​​were not affected by the spin diffusion due to the small dipolar coupling, which results from the low natural abundance. On the other hand, the T_1ρ values ​​decreased as the temperature increased and then rose again at 200 K. Below 300 K, only the ¹³CT_1ρ value for C1 vs. the inverse temperature showed a minimum of 28.58 ms at 200 K, implying the existence of active molecular movements at low temperatures._C values ​​on a logarithmic scale, as obtained by Eq. †2) versus 1000/T, were plotted (inset of Fig. 7† the E_adepending on the molecular dynamics of ¹³C, was measured as 8.59 ± 0.47 kJ/mol. The ¹³CE_a value was about double that of ¹HE_a†