Thermal property and structural molecular dynamics of organic–inorganic hybrid perovskite 1,4-butanediammonium tetrachlorocuprate

We investigate the thermal behaviour and physical properties of the crystals of the organic inorganic hybrid perovskite [(NH3)(CH2)4(NH3)]CuCl4. The compound's thermal stability curve as per thermogravimetric analysis exhibits a stable state up to ∼495 K, while the weight loss observed near 538 K corresponds to partial thermal decomposition. The 1H nuclear magnetic resonance (NMR) chemical shifts for NH3 change more significantly with temperature than those for CH2, because the organic cation motion is enhanced at both ends of the organic chain. The 13C NMR chemical shifts for the ‘CH2-1’ units of the chain show an anomalous change, and those for ‘CH2-2’ (units closer to NH3) are shifted sharply. Additionally, the 14N NMR spectra reflect the changes of local symmetry near TC (=323 K). Moreover, the 13C T1ρ values for CH2-2 are smaller than those for CH2-1, and the 13C T1ρ data curve for CH2-1 exhibits an anomalous behaviour between 260 and 310 K. These smaller T1ρ values at lower temperatures indicate that 1H and 13C in the organic chains are more flexible at these temperatures. The NH3 group is attached to both ends of the organic chain, and NH3 forms a N–H⋯Cl hydrogen bond with the Cl ion of inorganic CuCl4. When H and C are located close to the paramagnetic Cu2+ ion, the T1ρ value is smaller than when these are located far from the paramagnetic ion.


I. Introduction
The search for new and improved functional materials in recent years has resulted in considerable progress in the synthesis of many families of organic-inorganic compounds. The properties and structural phase transition of these compounds are related to their structures and the interaction of the cationic units with complex anionic sublattices. One such group of hybrid compounds, whose structure can be expressed by the general formula [NH 3 (CH 2 ) 4 NH 3 ]MX 4 (M ¼ divalent metal ion and X ¼ Cl, Br) is known to crystallise in a 2D perovskite-like structure, and these compounds are usually referred to as organic-inorganic hybrid perovskites or organic-metal-halide composites. [1][2][3][4][5][6] These perovskites combine the advantages of both organic and inorganic materials in a single molecular scale. 1,7,8 In particular, in the diammonium hybrid perovskite with its formula of [NH 3 (CH 2 ) 4 NH 3 ]MX 4 , the NH 3 group is attached to both ends of the organic chain. 3,7,8 At the end of the organic part of the chain, the ammonium ion forms a N-H/X hydrogen bond with the halide ion of the metallic inorganic layer. 9 These perovskite hybrids tend to exhibit a number of phase transitions such as order-disorder transitions. Here, we note that the properties of organic-inorganic hybrid perovskites depend on the organic cation, divalent metal, and halogen ion, and thus, it is necessary to investigate the 'structure-directing' properties of these new materials. In general, 2D hybrid perovskites can nd use in the elds of energy, optoelectronics, photonics, and catalysis in green chemistry applications. [9][10][11][12][13] The compound [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 , or 1,4-butanediammonium tetrachlorocuprate, with M ¼ Cu and X ¼ Cl, undergoes a reversible phase transition at 325 K (¼T C ) 14 between the two monoclinic phases II and I. The transition can be explained by order-disorder mechanisms involving a model of twisted conformation chains, which was introduced to explain the decrease in interlayer distance with increasing temperature from X-ray diffraction experiment. From structural considerations, these results can be explained by the conformational change of organic chains from the le-handed conformation in phase II to an all-trans conformation in phase I. 14 The structural geometry of [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 in the room-temperature phase II and high-temperature phase I are represented in Fig. 1(a) and (b), respectively. The crystal structure at room temperature is monoclinic, corresponding to space group P2 1 /c. The unit cell dimensions are a ¼ 9.270  16,17 They reported that the interplanar superexchange interaction along the linear Cu-Cl-Cl-Cu path exhibits a signicantly stronger Cu-Cu-distance dependence than that along the Cu-Cl-Cu path. This crystal structure has been reported in phases I and II by Garland et al. 18 Subsequently, the phase transitions occurring in the perovskite-type 2D molecular composite [(NH 3 )(-CH 2 ) 4 (NH 3 )]CuCl 4 have been studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), 11 and electron paramagnetic resonance (EPR). 14,19,20 Understanding the structural dynamics of organic-inorganic hybrid perovskite [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 is essential for their advanced use as new materials. Here, we study the structural dynamics of the organic-inorganic hybrid perovskite [(NH 3 ) (CH 2 ) 4 (NH 3 )]CuCl 4 via magic angle spinning (MAS) nuclear magnetic resonance (NMR) and static NMR experiments. The chemical shis and spin-lattice relaxation times in the rotating frame T 1r in the low-and high-temperature phases are measured by means of MAS 1 H NMR and cross-polarisation (CP)/MAS 13 C NMR to understand the role of the organic cation in this crystal. The 14 N NMR spectra of the compound in the laboratory frame are also obtained as a function of temperature. We use these results to discuss the structural dynamics of the NH 3 -CH 2 -CH 2 -CH 2 -CH 2 -NH 3 chain below and above the phase transition temperature T C . In particular, an examination of the hydrogen bonding of N-H/Cl between the Cu-Cl layer and the alkylammonium chain within [(NH 3 )(-CH 2 ) 4 (NH 3 )]CuCl 4 can provide important insights into the operational mechanism as regards potential applications.
The crystal structure of [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 was determined with a X-ray diffraction system, using a Cu-Ka radiation source at the KBSI, Seoul Western Center. DSC (TA, DSC 25) experiments were carried out at a heating rate of 10 C min À1 in the temperature range of 190 to 600 K in a nitrogen-gas atmosphere. Thermogravimetry analysis (TGA) experiments were conducted using a thermogravimetric analyser (TA Instruments) under conditions identical to those of DSC over a temperature range of 300 to 680 K. The DSC and TGA experiments were performed by using crystal sample quantities of 6.23 and 7.53 mg, respectively.
Solid-state MAS NMR investigations of the [(NH 3 )(CH 2 ) 4 (-NH 3 )]CuCl 4 crystals were conducted by using a 400 MHz Avance II+ Bruker NMR spectrometer at the same facility. The MAS 1 H NMR and CP/MAS 13 C NMR experiments were performed at the Larmor frequencies of u 0 /2p ¼ 400.13 and 100.61 MHz, respectively. Solid samples were packed into 4 mm-diameter zirconia rotors and closed off using Vespel caps. The samples were spun at 10 kHz MAS by using dry nitrogen gas. The 1 H and 13 C NMR chemical shis were obtained with the use of tetramethylsilane (TMS) as a standard. The T 1r data for 1 H and 13 C were obtained by applying a p/2 pulse, immediately followed by a long spin-locking pulse phase-shied by p/2 with respect to the p/2 pulse. The width of the p/2 pulse used for T 1r measurements was 3.3 ms, which yields the frequency of the rotating frame as u 1 ¼ 75.75 kHz. The T 1r data were obtained by varying the length of the spin-locking pulse. In addition, the 14 N NMR spectra of a [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 single-crystal were obtained at the Larmor frequency of u 0 /2p ¼ 28.90 MHz in the laboratory frame. The 14 N resonance frequency was referenced with NH 3 NO 3 as the standard sample. The 14 N NMR spectrum was obtained by the application of the following solid-state echo sequence: 8 ms-tau (16 ms)-8 ms-tau (16 ms). The temperature change was maintained within the error range of AE0.5 K by adjusting the nitrogen gas ow and heater current.

III. Results and discussion
The powder X-ray diffraction pattern of [(NH 3 )(CH 2 ) 4 (NH 3 )] CuCl 4 at 300 K is described in the ESI, † and this data is consistent with previously reported results. 14 Fig. 2 shows the DSC curve of the [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 crystals obtained with the heating rate of 10 C min À1 . An endothermic signal corresponding to the previously reported 14 II-I phase transition is detected at 323 K. In addition, a very large exothermic peak is CuCl 4 for (a) room temperature phase II and (b) high temperature phase I. Here, CH 2 -1 represents two CH 2 between four CH 2 , and CH 2 -2 represents two CH 2 close to NH 3 .
This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 34800-34805 | 34801 observed at 538 K. To understand the origins of this peak, we performed TGA experiments; these results are also shown in Fig. 2. In the TGA curve, a stable state is observed up to $495 K, whereas a weight loss is observed at higher temperatures, which represents partial thermal decomposition. Here, we note that [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 crystals show the weight loss with temperature increase. From the TGA experimental results and possible chemical reactions, we compared the weight loss. The weight loss of 12% around 538 K obtained from the DSC experiment is consistent with the calculated decomposition of HCl moieties. From the gure, we note that the weight sharply decreases between 500 and 650 K, with a corresponding weight loss of 67% near 650 K.
Next, we acquired the MAS 1 H NMR spectrum of the [(NH 3 )(CH 2 ) 4 (NH 3 )]CuCl 4 crystals at various temperatures (Fig. 3). In the gure, we can observe two resonance signals for 1 H. The spinning sidebands corresponding to CH 2 are indicated by open circles, and those of NH 3 are indicated by crosses. At 300 K, the 1 H NMR chemical shis for CH 2 and NH 3 are observed at d ¼ 2.73 ppm and d ¼ 11.86 ppm, respectively. Below 300 K, the 1 H resonance signal bonded to CH 2 mostly merges with the 1 H resonance signal bonded to NH 3 , which makes it difficult to distinguish the two signals. In addition, the 1 H resonance signal for CH 2 is related to the number of bonded protons, which means that the signal exhibits a stronger intensity and wider linewidth than the corresponding ones for NH 3 . The 1 H NMR chemical shis according to the temperature exhibit a greater change for NH 3 than CH 2 . These results indicate that NH 3 is temperature-sensitive.
Next, we measured the MAS 1 H NMR spectrum at various temperatures, and the intensity change for the delay time was observed to obtain the spin-lattice relaxation time in the rotating frame (T 1r ) for 1 H at each temperature. Normally, the T 1r data can be obtained as the slope of the intensity or the ratio of the area of the resonance signal to the delay time. The change in the proton magnetisation intensity in terms of T 1r is expressed as below: 21-23 where P(s) and P(0) denote the signal intensities at time s and s ¼ 0, respectively. Next, at 300 K, the MAS 1 H NMR signals of CH 2 and NH 3 were plotted for various delay times in the range from 0.2 to 80 ms (Fig. 4)  possibly because NH 3 is closer to the inorganic CuCl 4 layer; the T 1r value becomes smaller as the distance between H and the paramagnetic Cu 2+ ion reduces. This is because T 1r is inversely proportional to the square of the magnetic moment of the paramagnetic ion. 21 The CP/MAS 13 C NMR chemical shis measured at various temperatures are shown in Fig. 5. In the study, the MAS 13 C NMR spectrum for TMS was recorded at 38.3 ppm at 300 K, and this value was calibrated to determine the chemical shi in 13 C. Here, the two inner CH 2 groups of the four CH 2 ones are together designated as CH 2 -1, and the two CH 2 units close to the NH 3 ones are designated as CH 2 -2. We note from the gure that the 13 C chemical shis for CH 2 -1 (far from NH 3 ) are different from those for CH 2 -2, which is closer to NH 3 . In the 13 C NMR spectra obtained for CH 2 -1 and CH 2 -2, two unusual resonance lines are observed between 260 and 310 K. At 300 K, the two resonance signals for CH 2 -1 are recorded at chemical shis of d ¼ 38.44 and 59.56 ppm. Furthermore, the signal of d ¼ 98.23 ppm corresponds to CH 2 -2. The 13 C chemical shis for CH 2 -1 exhibit an anomalous change with increase in temperature, whereas those for CH 2 -2 shi abruptly with increasing temperature, as shown in Fig. 6. The two resonance lines between 260 and 310 K correspond to CH 2 -1, and hitherto unreported anomalous phenomena are observed in this temperature range.
We next remark that line broadening in the MAS 13 C NMR spectra is inuenced by relaxation processes such as the motional modulations of the chemical shi anisotropy and dipolar carbon-proton coupling. Fig. 7 shows the 13 C full-width at half-maximum (FWHM) linewidth of [(NH 3 )(CH 2 ) 4 (NH 3 )] CuCl 4 . The 13 C NMR line shapes vary from the Gaussian type at lower temperatures to the Lorentzian shape at higher temperatures. The appearance of these two-component spectra is caused by difference in different molecular motions. The linewidth near the phase transition temperature T C shows    This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 34800-34805 | 34803 a monotonic decrease, thereby indicating the presence of motional narrowing at high temperatures.
The spin-lattice relaxation time in the rate of relaxation is due to spin-lattice interactions in the rotating frame. The 13 C T 1r relaxations are not inuenced by spin diffusion because of the small dipolar coupling which arises from the low natural abundance and large separation of the nuclei. Under these conditions, we next analysed differences in the chain motions. The integration change of the 13 C NMR spectrum obtained for various delay times was measured, and all the decay curves for CH 2 -1 and CH 2 -2 were plotted by using a single exponential function. From the slope of their recovery traces, the 13 C T 1r data were obtained for CH 2 -1 and CH 2 -2 as a function of temperature, as shown in Fig. 7. It can be observed that although no change in the T 1r value is observed near T C , T 1r above T C abruptly increases with increasing temperature. The 13 C T 1r values for CH 2 -1 and CH 2 -2 at lower temperatures (below T C ) are nearly identical; however, the 13 C T 1r values for CH 2 -2 close to NH 3 at high temperatures are smaller than that for CH 2 -1. At high temperatures, smaller 13 C T 1r values for CH 2 -2 are more exible than the CH 2 -1. Just as the 13C resonance lines for CH 2 -1 exhibited anomalies between 260 and 310 K, the 13C T 1r also exhibits two different sets of values. The relaxation time for Arrhenius-type random motions with correlation time sC is described in term of slow motions; for sC ( u L , T 1r $ s C ¼ s 0 exp(ÀE a /k B T), where u L denotes the Larmor frequency and E a the activation energy.
The 14 N NMR spectrum in the laboratory frame was next measured in the temperature range from 180 to 430 K by using the solid-state echo method at the Larmor frequency of 28.90 MHz by means of static NMR. The two resonance lines are obtained by spin number I ¼ 1, 24,25 and the resonance frequency around T C (¼323 K) changes as shown in Fig. 8. The observed change in the 14 N resonance frequency with temperature is due to structural geometry change, which means a change in the quadrupole coupling constant. The linewidth at 300 K is $44 ppm, and this spectrum is relatively broader than the 1 H and 13 C NMR spectra. The 14 N resonance frequency decreases almost continuously until 270 K, while that of the 14

IV. Conclusions
In this study, we investigated the thermal behaviour and physical properties of organic-inorganic hybrid perovskite [(NH 3 )(-CH 2 ) 4 (NH 3  Firstly, we found that the TGA curve exhibited stability until 495 K, and the observed weight loss of 12% near 538 K was due to the partial thermal decomposition of HCl moieties. Secondly, the 1 H NMR chemical shi of NH 3 for crystallographic environments changed more signicantly with temperature than that for CH 2 because the [(NH 3 )(CH 2 ) 4 (NH 3 )] cation motion is enhanced at both ends of the cation of the NH 3 group. The 13 C NMR chemical shis for CH 2 -1 showed an anomalous change, and those for CH 2 -2 shied sharply to lower values when compared with that of CH 2 -1. The 13C chemical shis of the CH2-2 unit (closer to the N-H/Cl bond) sharply changed relative to those of CH 2 -1. In addition, the 14 N NMR spectra reected the changes in the local symmetry of the crystal near T C .
The 1 H T 1r values for CH 2 and NH 3 slightly increased with temperature increase. Moreover, the 13 C T 1r value for CH 2 -2 was smaller than that of CH 2 -1, and the 13 C T 1r value for CH 2 -1 exhibited an anomalous trend between 260 and 310 K. At low temperatures, the 1 H and 13 C T 1r values were smaller than at high temperatures. Smaller T 1r values at lower temperatures indicate that 1 H and 13 C in the organic chains are more exible at these temperatures. Moreover, the 13 C of CH 2 -2 close to NH 3 of the organic chain is more exible than the 13 C of CH 2 -1 between the four CH 2 sites. The NH 3 group is attached to both ends of the organic chain, and it forms a N-H/Cl hydrogen bond with the Cl ion of the inorganic CuCl 4 . The T 1r value is smaller when H and C are located close to the paramagnetic Cu 2+ ion than when far away. Additionally, the NH 3 groups are coordinated by CuCl 4 , and thus, atomic displacements in the environment of the 14 N nuclei with temperature are correlated with CuCl 4 . We also note here that detailed studies are required to examine the anomalies observed in the range of 260 to 310 K.

Conflicts of interest
There are no conicts to declare.