Study on structural geometry and dynamic property of [NH3(CH2)5NH3]CdCl4 crystal at phases I, II, and III

Organic–inorganic hybrid perovskites can potentially be used in electrochemical devices, such as batteries and fuel cells. In this study, the structure and phase transition temperatures of the organic–inorganic material [NH3(CH2)5NH3]CdCl4 crystal were confirmed by X-ray diffraction and differential scanning calorimetry. From the nuclear magnetic resonance results, the crystallographic configurations of 1H, 13C, and 14N in the cation changed at temperatures close to TC1 (336 K), whereas that of 113Cd in the anion shows significant changes at temperatures close to TC1 and TC2 (417 K). The activation energy, Ea, values for 1H and 13C obtained from the spin–lattice relaxation time, T1ρ, below and above TC1 were evaluated, where the Ea value for 13C was more flexible at low temperatures than at high temperatures. In addition, the effect on molecular motion was effective at high temperatures. The phase transition at 336 K was associated with the change in the N–H···Cl bond due to the change in the coordination geometry of Cl around Cd in the CdCl6 anion. On the other hand, the phase transition at 417 K was related to the ferroelastic phase transition attributed to the twin domains.

The synthesis and characterization of [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 were first discussed by Kind et al. 26 , where the structural phase transitions were studied using 35 Cl and 2 D nuclear magnetic resonance (NMR), birefringence, dilatation measurements, and optical domain investigations. Negrier et al. 15 evaluated the crystal structures via X-ray diffraction (XRD) and Raman scattering experiments at 293 K and 353 K. Our group has also recently reported the effects of 13 C length in the cation of [NH 3 (CH 2 ) 2 NH 3 ]CdCl 4 , [NH 3 (CH 2 ) 3 NH 3 ]CdCl 4 , and [NH 3 (CH 2 ) 4 NH 3 ]CdCl 4 crystals on the thermal and structural dynamic properties 13 . Meanwhile, a lot of research has been done on the electrical and conductive properties of this type of compound 16,[27][28][29][30] .
Here, the crystal structures, thermodynamic properties, and ferroelastic domain walls of [NH 3 (CH 2 ) 5 NH 3 ] CdCl 4 were investigated. The roles of cations and anions in the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 single crystal were discussed, and the chemical shifts and spin-lattice relaxation time, T 1ρ , with increasing temperature were measured using 1 H magic angle spinning (MAS) NMR, 13 C MAS NMR, and static 14 N NMR to identify the roles of the [NH 3 (CH 2 ) 5 NH 3 ] cation. Furthermore, the 113 Cd MAS NMR chemical shifts were recorded to evaluate the coordination geometry of the CdCl 6 anion. The results would provide insights into the physicochemical properties of [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystals, facilitating their various applications in the future.
The structures of the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystals at 298 K were analyzed using an XRD system. The lattice parameter and space group was considered by single-crystal XRD at the Seoul Western Center of the Korea Basic Science Institute. Experiments were performed in the same manner as before 31 .
Differential scanning calorimetry (DSC) (TA, DSC 25) experiments were carried out at a heating rate of 10 K/min from 190 to 550 K in N 2 gas. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves were obtained using a thermogravimetric analyzer (TA Instrument) with the same heating rate as in DSC from 300 to 973 K in N 2 gas. In addition, the domain patterns were observed using an optical polarizing microscope within the temperature range of 300 to 450 K, where the prepared single crystals were placed on the plate with the temperature sensor of a Linkam THM-600.
NMR spectra of the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystals were performed using a Bruker 400 MHz Avance II+ solid-state NMR spectrometer in the same facility. The Larmor frequencies for 1 H and 13 C MAS NMR experiments were 400.13 and 100.61 MHz, respectively. In MAS NMR experiment, the spinning speed was set to 10 kHz to minimize sideband. And tetramethylsilane (TMS) was used as a standard material to obtain accurate NMR chemical shift. The experimental method to obtain the T 1ρ values for 1 H and 13 C was used in the same way as the previously reported method 13 . And, static 14

Experimental results
Crystal structure. The powder XRD pattern of the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystal at 298 K is shown in Fig. 1.
And, the lattice constants analysized from the X-ray crystal diffraction were determined to be a = 7.3292 ± 0.002 Å, b = 7.5058 ± 0.002 Å, and c = 23.9376 ± 0.006 Å with the space group Pnam; this is consistent with the previously reported results 14,15 . Phase transition temperature, thermal property, and ferroelastic twin domain. The DSC curves of the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystal at a heating and cooling rate of 10 K/min in N 2 gas are presented www.nature.com/scientificreports/ in Fig. 2. Two endothermic peaks were observed at 336 K (T C1 ) and 418 K (T C2 ) during heating, whereas two exothermic peaks were recorded at 327 K (T C1 ′) and 407 K (T C2 ′) during cooling. The phase transition enthalpy on heating is 3.17 kJ/mol at 337 K and 0.55 kJ/mol at 417 K, respectively. On the other hand, previous studies reported endothermic peaks at 337 K and 417 K during heating and at 336 K and 407 K during cooling 14,15 .
To determine the preliminary thermal characteristics, including the structural phase transitions, TGA and DTA results were conducted at the same heating rate as the DSC experiment. Based on the TGA and DTA curves shown in Fig. 3, the crystal exhibited excellent stability up to approximately 600 K. The small inflection points observed at temperatures near 336 K and 417 K in the DTA curve were coincides with the two phase transition temperatures obtained from the DSC results, suggesting that the molecular weight of [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 decreased at increasing temperatures. The amount of crystal remaining in the solid state was evaluated from the molecular weights. The 10% and 20% weight losses of the crystal at temperatures of about 617 K and 626 K were attributed to the decomposition of HCl and 2HCl, respectively. On the other hand, the weight loss at approximately 800 K and 900 K shown in Fig. 3 was observed 46% and 87%, respectively.
A single crystal with ferroelastic properties exhibits two or more orientation states even if mechanical stress does not exist since mechanical stress can change the existing orientation state of the single crystal. Polarized microscopy observations revealed the ferroelastic domain structures of the crystal and their changes at the phase transition temperatures, as shown in Fig. 4. The domain pattern represented by parallel lines was not observed in phases III (300 K, Fig. 4a) and II (403 K, Fig. 4b). No change in the behavior of the crystal was observed at T C1 . However, in phase I, twinning occurred in the crystal at temperatures above T C2 , resulting in a highly dense domain pattern indicated by the red circle (Fig. 4c). At 433 K, new domain walls indicated by the blue circles were formed next to the parallel domain walls (Fig. 4d). The phase transition at T C2 occurred due to the ferroelastic twin domain. The [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystal existed in two crystallographic phases: monoclinic (2/m) at temperatures above 417 K, orthorhombic (mmm) at temperatures between 417 and 337 K, and orthorhombic (mmm) at temperatures below 337 K. According to Aizu 32 and Sapriel 33 , for the transition from the mmm space group of the orthorhombic phase II to the 2/m space group of the monoclinic phase I, the domain wall directions   Fig. 5 as a function of temperature. At low temperatures, only one resonance signal was observed. These resonance signals were asymmetric due to the overlapping 1 H lines of NH 3 and CH 2 in [NH 3 (CH 2 ) 5 NH 3 ] cations. At 180 K, a single resonance line was present at a chemical shift of 9.04 ppm. The line width and full-width at half-maximum (FWHM) at this temperature were also different from those represented as symbol "1" at 2.97 ppm and as symbol "2" at 6.07 ppm, respectively. At 330 K, which was close to T C1 , the NMR spectrum was divided into two resonance lines, showing chemical shifts of 7.56 and 2.58 ppm for NH 3 and CH 2 , respectively. The spinning sidebands were marked with crosses and open circles. Here, phases I, II, and III were plotted in olive, red, and black, respectively. The 1 H chemical shifts of NH 3 and CH 2 , presented by dotted lines in Fig. 5, were almost independent of temperature. These results suggested that the surrounding environments of 1 H of NH 3 and CH 2 did not change with temperature. 13 C MAS NMR spectrum. The 13 C chemical shifts at increasing temperature for the in situ MAS NMR spectra are shown in Fig. 6. The TMS reference signal at 300 K recorded at 38.3 ppm was used as the standard for the 13 C chemical shift. In the [NH 3 (CH 2 ) 5 NH 3 ] cation, CH 2 located close to NH 3 was designated as C-3, CH 2 located at the center was designated as C-1, and CH 2 located between C-3 and C-1 was designated as C-2. The structure of the cation for this crystal is shown in the inset of Fig. 6. At 300 K, the 13 C chemical shifts were recorded at 28.26, 25.90, and 41.67 ppm for C-1, C-2, and C-3, respectively. The FWHM for 13 C NMR at 300 K were 6.20, 5.72, and 9.06 ppm for C-1, C-2, and C-3, respectively. The line width of C-3 located close to N was wider than those of C-1 and C-2. The chemical shifts changed at temperatures close to T C1 (336 K), but not at temperatures close to T C2 (417 K). Below T C1 , all 13 C positions showed positive chemical shifts with increasing temperatures. Above T C1 , the chemical shift of C-2 was almost independent of temperature, while the shifts in C-1 and C-3 progressed in a negative and positive direction, respectively. The results proved that below T C1 , the surrounding environments of all 13 C ions would change with temperature. At temperatures above T C1 , the surrounding environments of C-2 did not change. However, the chemical shifts of C-1 and C-3 continuously changed in all temperature ranges, including T C1 and T C2 .    Fig. 7. Despite the presence of intense background noise due to the very low NMR frequency (28.90 MHz), the 14 N spectrum was obtained without difficulty. Here, the crystal demonstrated an arbitrary direction with respect to the magnetic field. The six resonance lines of the three pairs at increasing temperatures were below T C1 . At temperatures close to 336 K (T C1 ), the number of resonance lines and resonance frequencies of the NMR spectra showed abrupt changes. At T C1 , a reduction from three pairs to two pairs of NMR lines was observed. At T C2 , another pair of NMR lines reappeared. Below T C1 , as the temperature increased, the resonance frequencies increased, and above T C1 , as the temperature increased, the resonance frequencies decreased. At T C2 , only the number of resonance lines changed, and the resonance frequency showed almost continuous values. Symbols with the same color indicated the same pairs of 14 N. Changes in the 14 N resonance frequencies due to the change in temperature were related to the changes in the crystallographic configuration of the crystal. Cd NMR spectroscopy. The changes in the in situ 113 Cd MAS NMR spectra are shown in Fig. 8. The 113 Cd chemical shift at 300 K was 323.19 ppm. As the temperature increased, the 113 Cd chemical shifts slightly moved in the negative direction, but these chemical shifts changed discontinuously near T C1 and T C2 . In particular, more changes were observed at temperatures near T C2 than at temperatures near T C1 , suggesting that temperature affected the environments around Cd. This proved that the coordination geometry of 6Cl around Cd ions in the CdCl 6 octahedra, as shown in the inset of Fig. 8, would change at the phase transition temperatures. 1 H and 13 C spin-lattice relaxation times. The 1 H MAS NMR and 13 C MAS NMR spectra were obtained with increasing delay times, and the plot of spectral intensities against increasing delay times was expressed as an exponential function. The decay rates of the spin-locked proton and carbon magnetization are expressed as the spin-lattice relaxation time, T 1ρ , as 34,35 : where P H(C) (τ) and P H(C) (0) are the signal intensities for the proton (carbon) at time τ and τ = 0, respectively. The 1 H T 1ρ values of NH 3 and CH 2 at several temperatures were determined by the slope of the logarithmic plots of intensities against delay times. From the slope of their recovery curves, the 13 C T 1ρ values for C-1, C-2, and C-3 were determined. The 1 H T 1ρ and 13 C T 1ρ values are shown in Fig. 9 as a function of the inversed temperature. The 1 H T 1ρ values increased rapidly from 100 to 1000 ms. While the slope of the T 1ρ values at temperatures near T C1 changed, the slope at temperatures near T C2 exhibited a rather continuous value. Above T C1 , the 1 H T 1ρ value for NH 3 showed a decreasing trend. The activation energy, E a , values for 1 H in NH 3 were evaluated from the slopes (represented by the solid lines in Fig. 9) of their log T 1ρ versus 1000/T plots. The E a values below T C1 were 6.65 ± 0.40 kJ/mol and 8.60 ± 2.32 kJ/mol for NH 3 and CH 2 , respectively, while the E a values above T C1 were 2.85 ± 0.96 kJ/mol and 3.49 ± 1.47 kJ/mol for NH 3 and CH 2 , respectively. And, the 13 C T 1ρ values below T C1 increased gradually with increasing temperature and then increased rapidly above T C1 . Near T C2 , the T 1ρ values were almost continuous, showing no significant changes. The E a values of C-1, C-2, and C-3 below T C obtained from the plot of log T 1ρ versus 1000/T were 1.73 ± 0.58 kJ/mol, 1.33 ± 0.49 kJ/mol, and 1.36 ± 0.76 kJ/ mol, respectively. The E a values of C-1, C-2, and C-3 above T C1 were 3.04 ± 1.38 kJ/mol, 5.57 ± 1.04 kJ/mol, and 0.97 ± 1.43 kJ/mol, respectively. The behavior of T 1ρ for random motions with a correlation time, τ C , could be www.nature.com/scientificreports/ described as fast-and slow-motion zones. The 1 H and 13 C T 1ρ values at low and high temperatures correspond to the fast-motion region, where ω 1 τ C ≪ 1 and T 1ρ −1 α exp(E a /k B T). In contrast, the 1 H T 1ρ values in NH 3 at high temperatures were attributed to the slow-motion region, where ω 1 τ C ≫ 1 and T 1ρ −1 α ω 1 −2 exp(E a /k B T).

Conclusion
The structure and phase transition temperatures of the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystal were confirmed using XRD and DSC. Based on the NMR analysis of the crystal, we deduced that the crystallographic surroundings of 1 H, 13 C, and 14 N in the cation at temperatures close to T C1 changed, whereas that of 113 Cd in the anion at temperatures close to T C1 and T C2 exhibited significant changes. The changes in the NMR chemical shifts near T C1 and T C2 also suggested that the N-H···Cl hydrogen bond was affected.
On the other hand, the T 1ρ values of 1 H in NH 3 changed from fast to slow motion near T C1 . The T 1ρ values of 13 C in CH 2 increased rapidly at T C1 , and the E a values for 13 C were more flexible at low temperatures than at high temperatures. By evaluating the T 1ρ values, we deduced that the effect on the molecular motion was effective at high temperatures.