Recent research has focused on exploring new and improved functional materials within the organic–inorganic hybrid perovskite materials. The wide application scope and rapid development speed of various studies on organic–inorganic hybrid compounds have garnered significant attention1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. The optical properties and structural flexibility of these compounds are determined by organic cations, while their thermal and mechanical properties are governed by inorganic anions19. Physicochemical characteristics are influenced by factors such as the properties of organic cations and the coordination geometry of inorganic metal halide anions constituting the crystal6,7,18,20,21,22,23,24,25. These materials are gaining prominence for their unique ability to create diverse and excellent materials by selectively leveraging the advantages of both organic and inorganic components. Furthermore, ferroelasticity is commonly observed in compounds with perovskite crystal structures, and the ferroelastic twin domains in organic–inorganic hybrid perovskites are drawing much attention26,27. The development of ferroelastic semiconductors associated with hybrid perovskite fabrication presents significant challenges7. Moreover, the successful integration of ferroelectric properties in hybrid perovskites positions them ideally for potential applications in flexible wearable devices28,29.

Initially, thin-film photovoltaic devices based on organic–inorganic hybrid CH3NH3PbX3 (X = Cl, Br, and I) were utilized as solar cells. The power conversion efficiency of lead halide perovskite CH3NH3PbX3 has enhanced from 3.8% to more than 20%. Despite advancements in employing 3-dimensional (3D) CH3NH3PbX3 as hybrid solar cells, these perovskites are highly susceptible to humid air and are toxic due to the presence of Pb, necessitating their replacement with environmentally friendly hybrid perovskite solar cells26,27,30,31,32,33,34,35. The primary drawbacks for their use in solar cells are the toxicity and chemical instability of halide perovskites. A lead-free hybrid perovskite based on methylammonium tin iodide was developed and showed initial efficiencies of up to 6.4%. And, the band gap energy relevant for photovoltaic applications of the 2D perovskite hybrid was found to be 1.75–2.65 eV. In terms of stability and safety, Bi alkyl ammonium has also been reported as a promising example of a lead-free and eco-friendly hybrid perovskite material for solar cell applications3,8. Additionally, novel groups of perovskite materials, such as [(CH3)2NH2]Zn(HCOO)3, composed of an organic cation and a metal ion, have been discussed36,37,38,39,40,41,42. These materials show potential for application in memory manipulation devices and next-generation memory storage technology.

Lead-free organic–inorganic hybrid compounds, [NH3(CH2)nNH3]MX4 (n = 2, 3, 4, …; M = transition metal (Mn, Co, Cu, Zn, and Cd); X = halogen ion (Cl, Br, I)), with one-dimensional (1D) and two-dimensional (2D) structures, has emerged1,2,3,4,5,6,7,8,9,23,24,34,43,44,45,46,47. The toxicity of heavy metals such as Co and Cd has long been known but accidental exposures of large populations to these elements remain unfortunately a topical issue48,49,50,51,52. These compounds consist of organic [NH3(CH2)nNH3] cations located between inorganic anions along the longest axis of the single crystal. Recently, research on [NH3(CH2)nNH3]MX2X’2, composed of different halogen ions, was conducted by Abdel-Aal et al.53,54,55. The physical and chemical properties of the organic–inorganic hybrid perovskite compounds depend on the characteristics of the organic cations and the coordination geometry of the inorganic anions; (MX4)2− or (MX6)2−6,18,24,56,57,58,59,60,61,62. When the transition metal M = Mn, Cu, or Cd, the structure consists of alternating corner-shared octahedral (MX6)2− units (as exemplified by the [NH3(CH2)5NH3]MnCl4 crystal shown in Fig. 1a). The organic and inorganic layers form infinite 2D structures, connected by N − H∙∙∙Cl hydrogen bonds. In the case of M = Co or Zn, the structure comprises tetrahedral (MX4)2− units positioned between layers of organic cations (as exemplified by the [NH3(CH2)6NH3]ZnCl4 crystal shown in Fig. 1b). The organic and inorganic layers form infinite 1D structures, connected by N − H∙∙∙Cl hydrogen bonds. The NH3 ions bonded at both ends of the organic chain are consistent with the halide ions in the inorganic layer. When the methylene length in the cation is greater than 4, structural rearrangement due to conformational changes in the chains becomes an important factor63. The distance between two inorganic layers varies greatly depending on the length of the organic chain, and, in particular, structural rearrangement and geometry due to changes in the length of the cation are also very important factors.

Figure 1
figure 1

The atomic numbering scheme of (a) [NH3(CH2)5NH3] cation and MnCl6 anion in [NH3(CH2)5NH3]MnCl4 at 300 K and (b) [NH3(CH2)6NH3] cation and ZnCl4 anion in [NH3(CH2)6NH3]ZnCl4 at 300 K.

Full size image

Previous studies on [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) single crystals have been reported as follows: the research related to [NH3(CH2)nNH3]MnCl4 with n values of 2, 3, 4, and 5, and M = Mn, was discussed primarily in research reports focusing on the phase transition temperature (TC) and crystal structures63,64,65,66,67,68. Their crystallographic characteristics, dielectric properties, and photoluminescence properties were reported9,69,70,71. For M = Co, previous studies by Abdel-Aal et al.72 and Criado et al.73 determined the crystal structures for [NH3(CH2)3NH3]CoCl4 and [NH3(CH2)5NH3]CoCl4, respectively. Additionally, Abdel-Aal et al.62 discussed the synthesis, structure, and lattice energy of the hybrid organic–inorganic perovskite [NH3(CH2)4NH3]CoCl4. Regarding M = Cu, investigations into the structure, magnetic, and optical properties of the layer-type [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, and 5) were conducted23,25,64,74,75,76,77,78,79. Iqbal et al.80 reported a study on Raman scattering at temperatures above and below the magnetic ordering temperature of 149 K. To date, the structures of [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) have been reported by X-ray diffraction analysis60,81,82,83. Finally, in the case of M = Cd, several studies on [NH3(CH2)nNH3]CdCl4 crystals have been reported in the past84,85,86,87,88,89,90, focusing on structural, thermal, and vibrational properties. Kind et al.63 discussed the phase transitions of [NH3(CH2)5NH3]CdCl4 using 35Cl nuclear magnetic resonance (NMR), dilatation and birefringence measurements, and domain investigations. In particular, the electric and optical characteristics of [NH3(CH2)nNH3]MCl4 perovskites, as these properties are crucial for solar cell applications, have been discussed by several researchers22,23,25,44,63,71,91,92,93. However, better optoelectronic research on lead-free metal halide materials for photovoltaic applications is still in its infancy and not much research has been done94,95,96. And, our groups discussed the crystal growth, crystal structure, thermal stabilities, structural geometries, and molecular dynamics of [NH3(CH2)nNH3]MCl4 single crystals97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113. Despite the numerous applications of these compounds, there has been limited discussion about their structural dynamics and thermal properties due to variations in transition metal ions and methylene chain length.

In recent times, the potential of perovskites has spurred heightened research into analyzing their structural and mechanical properties. It has been noted that the commonly used CH3NH3PbI3 undergoes a phase transition at 329 K, falling within the operational temperature range of solar cells, and exhibits poor photostability. Consequently, ensuring the stability of perovskite solar cell devices emerges as a paramount concern. Moreover, despite their intriguing attributes, materials like perovskites decompose in humid air and pose toxicity risks due to lead. Hence, the imperative lies in developing hybrid perovskites that can be substituted with environmentally friendly alternatives.

This review delves into the potential applications of lead-free organic–inorganic perovskite-type crystals, specifically [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd). We seek organic–inorganic hybrid perovskite materials characterized by high or no phase transition temperature, and thermal stability. Through examining the characteristics related to methylene length and the variation of transition metals, our aim is to identify enhanced materials meeting the aforementioned criteria. In this study, single crystals of organic–inorganic hybrid [NH3(CH2)nNH3]MCl4 were grown via the aqueous solution method. We discuss their crystal structure, phase transition temperature (TC), and thermal decomposition temperature (Td). The 1H and 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) method plays a crucial role in understanding local dynamics. Determining the spin–lattice relaxation times T1ρ for 1H and 13C is essential for studying dynamical processes. By analyzing the relaxation times of nuclei in different cationic environments, we gain detailed insights into their motion, particularly in the low- to mid-kHz frequency range. Furthermore, we consider the NMR spin–lattice relaxation times T1ρ, which reflect the energy transfer surrounding 1H and 13C atoms. Our results provide a comparative overview of the thermal stability of [NH3(CH2)nNH3]MCl4, varying methylene length and metal ion as parameters. This review aims to advance the development of lead-free mixtures and the creation of a next-generation predictive model that combines both thermal stability and dynamic metal interactions.

Methods

Crystal growth

[NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) single crystals were grown using the aqueous solution method. NH2(CH2)nNH2∙2HCl (Sigma-Aldrich) and MCl2 (Sigma-Aldrich) were dissolved in distilled water. The mixture was stirred and heated, and the resulting solution was filtered. After a few weeks in a constant-temperature bath at 300 K, the single crystals were obtained.

Characterization

At 300 K, the lattice constants were determined via a single-crystal X-ray diffraction (SCXRD) experiment conducted at the Korea Basic Science Institute (KBSI) Seoul Western Center. The experiment utilized a Bruker diffractometer equipped with a graphite-monochromated Mo-Kα target (D8 Venture PHOTON III M14) and a nitrogen cold flow (− 50 °C). Data collection was performed using SMART APEX3 (Bruker 2016) and analyzed with SAINT software (Bruker, 2016). The structure was refined using full-matrix least-squares on F2 with SHELXTL114. Hydrogen atoms were positioned according to the geometric arrangement within the single crystal structure. Additionally, powder X-ray diffraction (PXRD) patterns were obtained at various temperatures using an XRD system with a Mo-Kα radiation source, the same as that used in SCXRD.

Differential scanning calorimetry (DSC) experiments were conducted over the temperature range of 193 to 573 K, employing a heating rate of 10 °C/min under N2 gas. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments, utilizing a thermogravimetric analyzer (TA Instrument), were performed within the temperature interval of 300 to 873 K, employing a heating rate of 10 °C/min under N2 gas as well.

The NMR spectra of the twenty-one crystals were acquired using a solid-state NMR spectrometer (AVANCE III, Bruker) operating at 400 MHz, located at the Seoul Western Seoul Center of the KBSI. The samples, placed in cylindrical zirconia rotors, were spun at a rate of 10 kHz for MAS NMR experiments. To determine the chemical shift, adamantane and tetramethylsilane (TMS) were used as standard materials for 1H and 13C, respectively, ensuring accurate NMR chemical shift measurements. MAS spin–lattice relaxation time T1ρ values were measured using a π/2 − τ pulse with a spin-lock pulse of duration τ, and the π/2 pulse width was determined using a previously reported method97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113.

Experimental results

Crystal growth

To obtain single crystals of [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd), NH2(CH2)nNH2∙2HCl (Sigma-Aldrich) and MCl2 (Sigma-Aldrich) were mixed in distilled water according to the molar ratio. The mixture was completely dissolved by stirring and heating to create a saturated solution. These prepared saturated solutions were then placed in a constant-temperature bath at 300 K and slowly evaporated to facilitate the growth of single crystals. Among them, crystals of (a) [NH3(CH2)3NH3]MnCl4, (b) [NH3(CH2)3NH3]CoCl4, (c) [NH3(CH2)3NH3]CuCl4, (d) [NH3(CH2)3NH3]ZnCl4 with n = 3, and (e) [NH3(CH2)2NH3]CdCl4 with n = 2 are depicted in Fig. 2. Mn crystals appear light brown, Co crystals are dark blue, Cu crystals are dark brown, and Zn and Cd crystals are colorless and transparent.

Figure 2
figure 2

Morphology of (a) [NH3(CH2)3NH3]MnCl4, (b) [NH3(CH2)3NH3]CoCl4, (c) [NH3(CH2)3NH3]CuCl4, (d) [NH3(CH2)3NH3]ZnCl4, and (e) [NH3(CH2)2NH3]CdCl4 single crystals.

Full size image

On the other hand, in the [NH3(CH2)nNH3]MCl4 crystals (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd), the positions of carbons along the lengths of n in the [NH3(CH2)nNH3] cation are indicated, as shown in Fig. 3. For n = 2, 3, and 4, the CH2 close to NH3 at both ends of the organic chain is denoted as CH2-3, the CH2 in the middle of two CH2-3 is designated as CH2-2. For n = 5 and 6, the CH2 close to NH3 is represented as CH2-3, the CH2 at the center is designated as CH2-1, and the CH2 in the middle of two CH2-3 and CH2-1 is denoted as CH2-2.

Figure 3
figure 3

Notations of carbons according to the length in the cation structure of [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals.

Full size image

[NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5)

Crystal structures

The SCXRD and PXRD experiments for the [NH3(CH3)nNH3]MnCl4 crystals (n = 2, 3, 4, and 5) were conducted at 300 K. The PXRD patterns for the four crystals are depicted in Fig. 4. Based on the SCXRD results, the space group and lattice parameters for [NH3(CH3)2NH3]MnCl4 (n = 2) were determined as P21/c with a = 8.614 (5) Å, b = 7.127 (3) Å, c = 7.188 (3) Å, β = 92.772 (28)°, and Z = 264,97. Similarly, for [NH3(CH3)3NH3] MnCl4 (n = 3), the space group was identified as Imma, and the lattice parameters were determined as a = 19.009 (3) Å, b = 7.169 (1) Å, c = 7.357 (1) Å, with Z = 49,67. In the case of [NH3(CH3)4NH3]MnCl4 (n = 4), the space group and lattice parameters were found to be P21/c, with a = 10.826 (4) Å, b = 7.178 (3) Å, c = 7.312 (3) Å, β = 92.605 (14)°, and Z = 263,97. Additionally, the crystal structure of [NH3(CH3)5NH3]MnCl4 adopts an orthorhombic structure, with lattice constants a = 24.004 Å, b = 7.190 Å, c = 7.395 Å, and Z = 4 in the space group Imma9,98. As shown in Fig. 1a, the Mn atom is coordinated to six Cl atoms, forming an almost regular octahedron, MnCl6. Furthermore, the six N-linked hydrogen atoms in one formula unit form N − H∙∙∙Cl hydrogen bonds. The structures, space groups, and lattice constants of [NH3(CH3)nNH3]MnCl4 (n = 2, 3, 4, and 5) crystals are summarized in Table 1. It is noteworthy that the crystal structures are monoclinic when n is even and orthorhombic when n is odd.

Figure 4
figure 4

The powder X-ray diffraction patterns of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5) at 300 K97.

Full size image
Table 1 The structures, space groups, and lattice constants (Å) of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5) crystals.
Full size table

Phase transition temperatures

The DSC curves in Fig. 5 depict the thermal behavior of four crystals under a heating rate of 10 °C/min. When n = 2, no peaks were observed97. However, for n = 3, two distinct endothermic peaks indicative of phase transition temperatures were observed at 308 K and 338 K. In the case of n = 4, a minor endothermic peak was observed at 378 K. Conversely, for n = 5, an endothermic peak was observed at 298 K98. The identified endothermic peaks from the DSC curves were further confirmed as phase transition temperatures through PXRD patterns and polarizing microscopy experiments in response to temperature changes97,98.

Figure 5
figure 5

The differential scanning calorimetry curves of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5) crystals97,98.

Full size image

Thermodynamic properties

To investigate the thermal stabilities, TGA experiments were conducted with a heating rate of 10 °C/min, mirroring the conditions of the DSC experiment. The TGA curves presented in Fig. 6 reveal that crystals with n = 2, 3, 4, and 5 exhibit thermal stability up to approximately 593, 600, 598, and 589 K, respectively. This stability is defined as the thermal decomposition temperature (Td) corresponding to a 2% weight loss97,98. It is noteworthy that these crystals initiate weight loss at higher temperatures. For n = 2, weight losses of 14 and 31% around 628 and 654 K are attributed to the partial thermal decomposition of HCl and 2HCl moieties, respectively. The temperatures at which partial decomposition occurs for HCl and 2HCl in n = 3 and n = 4 are comparable to those observed for n = 2. In the case of n = 5, mass losses of approximately 12 and 24% around 617 and 630 K may be attributed to the loss of HCl and 2HCl, respectively. For methylene lengths of 3, 4, and 5, weight losses of 55% are observed above 700 K, whereas a weight loss of about 48% is seen for n = 2. In summary, these four crystals exhibit relatively high stability at elevated temperatures.

Figure 6
figure 6

The thermogravimetry analysis curves of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5)97,98.

Full size image

MAS NMR chemical shifts and spin–lattice relaxation times

The 1H NMR spectra of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, 4, and 5) crystals were measured using NMR spectroscopy at various temperatures. Figure 7a displays the 1H NMR chemical shifts for [NH3(CH2)5NH3]MnCl4 with n = 5. The resonance lines observed at lower temperatures are asymmetric due to the overlap of signals representing NH3 and CH2. The line widths denoted by A and B on the left and right sides of the half-maximum in Fig. 7a are not equal. Above 300 K, the NH3 and CH2 signals are resolved, with chemical shifts of 9.29 and 2.89 ppm, respectively. Spinning sidebands are marked with + and o to represent 1H in NH3 and CH2, respectively. The 1H chemical shifts of CH2 show minimal variation near TC (= 298 K), while changes in the 1H chemical shifts of NH3 are observed at approximately TC. The more significant changes in the 1H NMR chemical shifts of NH3 compared to those in the 1H NMR chemical shifts of CH2 near TC suggest a modification in the N − H⋯Cl hydrogen bonding between Cl around Mn and H of NH3.

Figure 7
figure 7

(a) 1H NMR chemical shifts of [NH3(CH2)5NH3]MnCl4 at 200, 250, 300, 310, 320, and 350 K. + and o are the spinning sidebands for NH3 and CH2, respectively98. (b) 13C NMR chemical shifts of [NH3(CH2)5NH3]MnCl4 at 250, 280, 290, 300, and 350 K. o and * are the spinning sidebands for CH2-1 and CH2-2, respectively98.

Full size image

In addition, the 13C NMR chemical shifts in the MAS NMR spectra of CH2 in [NH3(CH2)5NH3]MnCl4 were recorded near TC. The 13C signal of TMS was observed at 38.3 ppm at 300 K; thus, 38.3 ppm was set as the standard for the 13C chemical shifts. Here, CH2-1 in the [NH3(CH2)5NH3] cation is located at the center of the cation, CH2-3 is positioned adjacent to the NH3 in the cation, and CH2-2 is situated between CH2-1 and CH2-3, as shown in the inset of Fig. 7b. The respective chemical shifts of CH2-1, CH2-2, and CH2-3 at 300 K are 80.96, 88.92, and 113.44 ppm, as illustrated in Fig. 7b. The o and * symbols represent the spinning sidebands for CH2-1 and CH2-2. The 13C chemical shifts of CH2-3 do not vary significantly near TC, whereas those of CH2-1 and CH2-2 show variations near TC.

The 1H and 13C MAS NMR spectra were acquired at various delay times for each temperature. The signal intensities in the NMR spectrum, corresponding to different delay times, are modeled as an exponential function. The magnetization decay rates for protons and carbon are characterized by T1ρ as shown below115:

$${\text{P}}\left( \tau \right)/{\text{P}}\left( 0 \right) \, = {\text{ exp}}\left( { - \tau /{\text{T}}_{{{1}\rho }} } \right),$$
(1)

Here, P(τ) and P(0) represent the NMR signal intensities at delay time τ and τ = 0, respectively. The 1H and 13C NMR spectra were recorded with various time delays, and the decay curves can be represented by a single exponential function as described in Eq. (1). The 1H T1ρ values for NH3 and CH2 at 300 K were notably short, measuring 20.8, 15.4, and 14.4 ms for 2, 3, and 4, respectively97. For a methylene length of n = 5, the 1H T1ρ values at 300 K were 94 μs for NH3 and 8.86 μs for CH2, respectively. These 1H T1ρ values exhibit a strong dependence on temperature, as illustrated in Fig. 8. The 1H T1ρ values of CH2 and NH2 undergo significant changes near TC, suggesting a substantial alteration in the 1H energy transfer of CH2 and NH3.

Figure 8
figure 8

The 1H and 13C spin–lattice relaxation times in [NH3(CH2)5NH3]MnCl4 with increasing temperatures98.

Full size image

Additionally, the 13C T1ρ values of CH2-1, CH2-2, and CH2-3 in the case of n = 5 are determined from the slopes of their recovery traces. The 13C T1ρ values near TC exhibit virtual continuity98. Notably, the T1ρ values of CH2-3, located adjacent to NH3, are the shortest. The relatively small T1ρ values of CH2-3, in proximity to Mn2+ ions, are associated with the magnetic moments of the paramagnetic Mn2+ ions.

[NH3(CH2)nNH3]CoCl4 (n = 3 and 5)

Crystal structures

The structures of [NH3(CH2)nNH3]CoCl4 crystals (n = 3 and 5) were determined through SCXRD and PXRD experiments conducted at room temperature. Additionally, the PXRD patterns for n = 3 and 5 at 300 K are presented in Fig. 9. And, the space group and lattice constants of the [NH3(CH2)3NH3]CoCl4 crystal with n = 3 were determined as P21/c with a = 10.738 (40) Å, b = 10.712 (36) Å, c = 10.926 (39) Å, β = 119.216 (266)°, and Z = 472,99. Similarly, the space group and lattice constants of [NH3(CH2)5NH3]CoCl4 with n = 5 were identified as P21/c, with a = 7.167 (1) Å, b = 15.949 (2) Å, c = 11.145 (1) Å, β = 98.497 (5)°, and Z = 473,96. The structures and space groups of the two crystals resembled monoclinic (P21/c), as detailed in Table 2. For odd values of n, the crystal structure of [NH3(CH2)nNH3]MnCl4 is orthorhombic, while the crystal structure of [NH3(CH2)nNH3]CoCl4 is monoclinic.

Figure 9
figure 9

The powder X-ray diffraction patterns of [NH3(CH2)nNH3]CoCl4 (n = 3 and 5) at 300 K99.

Full size image
Table 2 The structures, space groups, and lattice constants (Å) of [NH3(CH2)nNH3]CoCl4 (n = 3 and 5) crystals.
Full size table

Phase transition temperatures

In the DSC thermogram of [NH3(CH2)3NH3]CoCl4 with n=3, only one weak endothermic peak was observed at 483 K, as shown in Fig. 1099. Meanwhile, in [NH3(CH2)5NH3]CoCl4 with n=5, a peak was observed at 494 K. To accurately confirm whether the observed peaks in the two crystals were melting or phase transition temperatures, PXRD results were obtained according to temperature changes.99 These peaks were confirmed to be phase transition temperatures.

Figure 10
figure 10

The differential scanning calorimetry curves of [NH3(CH2)nNH3]CoCl4 (n = 3 and 5) crystals99.

Full size image

Thermodynamic properties

To understand the thermodynamic properties of these crystals, TGA was performed at a heating rate of 10 °C/min, similar to the procedure used in the DSC experiments. The TGA curves for crystals with n=3 and 5 are presented in Fig. 11. The molecular weights of both crystals decreased as the temperature increased. The amount of residue at higher temperatures was calculated based on the total molecular weight. By considering the number of CH2 units in the methylene length, the molecular weight loss at approximately 583 and 589 K for n=3 and 5, respectively, marked the onset of partial thermal decomposition, with 2 % weight loss set as Td. For n=3, weight losses of approximately 13 and 26 % around 614 and 633 K could be attributed to thermal decomposition and the partial escape of HCl and 2HCl moieties. The temperatures corresponding to the partial escape of HCl and 2HCl moieties for n=5 were almost similar to those for n = 3, with weight losses of approximately 12 and 24 % near 614 and 632 K due to thermal decomposition and the partial escape of HCl and 2HCl moieties, respectively99. The molecular weights of both crystals sharply decreased between 600 and 700 K, with a 50~55% weight loss occurring at approximately 700 K.

Figure 11
figure 11

The thermogravimetry analysis curves of [NH3(CH2)nNH3]CoCl4 (n = 3 and 5)99.

Full size image

MAS NMR chemical shifts and spin-lattice relaxation times

The 1H NMR spectra of [NH3(CH2)3NH3]CoCl4 crystals recorded by MAS NMR experiment at 300 K were represented in Fig. 12a. The chemical shift of the resonance peak was observed at 6.40 ppm as a single resonance line. The spinning sideband is marked with open circles. The observed resonance line was symmetric, corresponding to the overlapping lines of NH3 and CH2; the line widths denoted by A and B on the left and right sides of the half-maximum are identical. Thus, the chemical shifts of NH3 and CH2 did not separate and completely overlapped.

Figure 12
figure 12

(a) The 1H NMR chemical shifts of [NH3(CH2)3NH3]CoCl4 at 300 K. The open circles are sidebands99. (b) The 13C NMR chemical shifts of [NH3(CH2)3NH3]CoCl4 at 340 K. The crosses and open circles are sidebands for CH2-2 and CH2-3, respectively99.

Full size image

Moreover, the MAS 13C NMR chemical shifts of CH2 in [NH3(CH2)3NH3]CoCl4 were recorded at 300 K. The NMR signal at 115.38 ppm shows the 13C baseline signal in the absence of a sample, and it was not considered. The chemical shifts at 340 K were observed at 91.65 and 188.25 ppm for CH2-2 and CH2-3, as shown in Fig. 12b. As shown in Fig. 3, CH2-1 is located in the center of the [NH3(CH2)3NH3] cation, while CH2-3 is located near NH3 in the cation. The crosses and open circles indicate the spinning sidebands for CH2-2 and CH2-3, respectively.

The MAS 1H NMR spectra of NH3(CH2)nNH3CoCl4 (n=3 and 5) were recorded, capturing signal intensities at various delay times ranging from 1 μs to 5 ms. The 1H T1ρ values, determined from the slopes of the decay rate of proton magnetization, were found to be short, specifically 15.89–19.45 μs for n=3 and 16.76–17.22 μs for n=5, with respect to changes in temperature. Notably, these values for n=3 and 5 were nearly identical and temperature-independent, as illustrated in Fig. 13. Here, the 1H T1ρ values for NH3 and CH2 could not be distinguished due to the overlapping 1H signals of NH3 and CH2.

Figure 13
figure 13

The 1H and 13C spin–lattice relaxation times in [NH3(CH2)nNH3]CoCl4 (n = 3 and 5) with increasing temperatures99.

Full size image

In contrast, the intensities of the 13C NMR spectra were measured at various delay times, ranging from 1 μs to 10 ms. The 13C T1ρ values for CH2-2 and CH2-3 were obtained from the slope of their recovery traces. Due to the 13C baseline being closer to CH2-2 for n=3 and CH2-3 for n=5, obtaining accurate T1ρ values was challenging. The 13C T1ρ values were determined to be 38–19 μs for CH2-3 in the case of n=3 and 25–15 μs for CH2-2 in the case of n=5, considering the change in temperature. In both crystals, the 13C T1ρ values exhibited a slightly shorter value as the temperature increased, suggesting the activation of molecular motion. Notably, the 1H and 13C T1ρ values of the two crystals with paramagnetic Co2+ ions were much shorter than the values of crystals without paramagnetic ions. These results align well with the observation that T1ρ is closely related to paramagnetic ions, being inversely proportional to the square of the magnetic moment99.

[NH3(CH2)nNH3]CuCl4 (n=2, 3, 4, 5 and 6)

Crystal structures

The PXRD patterns for [NH3(CH2)nNH3]CuCl4 (n=2, 3, 4, 5 and 6) were obtained at 300 K, as shown in Fig. 14. According to the previously reported case for n=2, the crystal at room temperature exhibits a monoclinic structure with the space group P21/b and Z=2. The unit cell parameters are a=8.109 Å, b=7.158 Å, c=7.363 Å, and γ=92.37°64,74. Based on our XRD experimental results, the [NH3(CH2)3NH3]CuCl4 crystal with n=3 has an orthorhombic structure with the space group Pnma. The cell parameters are a=7.202 Å, b=18.260 Å, c=7.515 Å, and Z=478,101. Additionally, the crystal structure of [NH3(CH2)4NH3]CuCl4 with n=4 is monoclinic, corresponding to space group P21/c. The unit cell parameters are a=9.270 Å, b=7.600 Å, c=7.592 Å, β=103.14˚, and Z=275,76,102. Furthermore, the crystal structure with n=5 is monoclinic, and the lattice constants analyzed from the SCXRD result were a=7.7385 Å, b=7.2010 Å, c=21.5308 Å, β=98.493°, and Z=4, with the space group P21/c79,103. Finally, the SCXRD result for the [NH3(CH2)6NH3]CuCl4 crystal was obtained at 300 K; the hybrid was crystallized as a monoclinic structure with the space group P21/n and lattice constants a=7.2224 Å, b=7.6112 Å, c=23.3315 Å, β=91.930°, and Z=4104. These structures consist of parallel 2D sheets of perovskite-type layers of corner-sharing CuCl6 octahedra interleaved by layers of [NH3(CH2)6NH3] cations that are nearly perpendicular to the layers. The structures, space groups, and lattice constants of [NH3(CH2)nNH3]CuCl4 (n=2, 3, 4, 5, and 6) crystals are summarized in Table 3.

Figure 14
figure 14

The powder X-ray diffraction patterns of [NH3(CH2)nNH3]CuCl4 (n = 3, 4, 5, and 6) at 300 K101,103,104.

Full size image
Table 3 The structures, space groups, and lattice constants (Å) of [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, 5, and 6) crystals.
Full size table

Phase transition temperatures

The results of the DSC analysis of [NH3(CH2)nNH3]CuCl4 (n=2, 3, 4, 5, and 6) under a nitrogen atmosphere are presented in Fig. 15. The crystals [NH3(CH2)2NH3]CuCl4 and [NH3(CH2)5NH3]CuCl4 with n=2 and 5 do not undergo a phase transition100,103. However, an endothermic peak at 434 K was observed in the case of n=3, consistent with the structural phase transition101. Additionally, an endothermic peak corresponding to the phase transition of the [NH3(CH2)4NH3]CuCl4 crystal was detected at 323 K101. Finally, upon heating, an endothermic peak was observed for the DSC experiment on the [NH3(CH2)6NH3]CuCl4 crystal at 363 K104.

Figure 15
figure 15

The differential scanning calorimetry curves of [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, 5, and 6) crystals100,101,102,103,104.

Full size image

Thermodynamic properties

The measured TGA curves for [NH3(CH2)nNH3]CuCl4 (n=2, 3, 4, 5, and 6) are depicted in Fig. 16. The TGA curves reveal that the crystals with n=2, 3, 4, 5, and 6 exhibit thermal stability up to approximately 520, 518, 518, 506, and 488 K, respectively100,101,102,103,104. For n=2, weight losses of 14 and 27 % near 552 and 594 K, respectively, were attributed to the thermal decomposition of HCl and 2HCl. In the case of n=3, thermal stability was observed up to around 518 K; however, signs of weight loss above this temperature Indicated partial thermal decomposition. The compound [NH3(CH2)3NH3]CuCl4 with n=3 undergoes decomposition at high temperatures, with a 13 % weight loss near 539 K attributed to the decomposition of HCl moieties. The weight loss decreases rapidly between 500 and 600 K, with a 65 % weight loss occurring around 650 K. Similarly, for n=4, a stable state is observed up to 518 K, but partial thermal decomposition is observed at higher temperatures, resulting in a 12% weight loss around 538 K. In the case of [NH3(CH2)5NH3]CuCl4, the initiation of partial thermal decomposition, indicated by the first occurrence of molecular weight loss, occurs at approximately 506 K. As the temperature increases, the molecular weight decreases, with losses of 12 and 24% calculated from the total molecular weight attributed to the decomposition of HCl and 2HCl, respectively, and a weight loss of 80% at ~900 K. Finally, for n=6, mass loss begins at approximately 488 K, representing the Td. The molecular weight loss of [NH3(CH2)6NH3]CuCl4 occurs with increasing temperature, with losses of 11 and 23% calculated from the molecular weight attributed to the decomposition of HCl and 2HCl, respectively, and a weight loss of 70% occurs near 900 K.

Figure 16
figure 16

The thermogravimetry analysis curves of [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, 5, and 6)100,101,102,103,104.

Full size image

MAS NMR chemical shifts and spin-lattice relaxation times

The chemical shift of the 1H NMR spectrum of [NH3(CH2)3NH3]CuCl4 crystals with n=3 was obtained, as illustrated in Fig. 17a. Two peaks in the NMR spectra are designated as NH3 and CH2, and the 1H signals from NH3 and CH2 were observed to slightly overlap. The spinning sidebands for CH2 are marked with crosses, and those for NH3 are denoted with open circles. At 300 K, the NMR chemical shift of 1H for CH2 was recorded at δ=2.76 ppm, whereas that for NH3 was observed at δ=11.48 ppm. According to the previous results from our group, the 1H peak for CH2 did not significantly change as the temperature increased, while for NH3, the chemical shift was temperature-dependent101.

Figure 17
figure 17

(a) The 1H NMR chemical shifts of [NH3(CH2)3NH3]CuCl4 at 300 K. The open circles and crosses are sidebands for NH3 and CH2, respectively101. (b) The 13C NMR chemical shifts of [NH3(CH2)3NH3]CuCl4 at 300 K101.

Full size image

The 13C NMR chemical shifts for CH2 in the crystals with n=2, 3, 4, 5, and 6 were measured as the temperature increased, and the 13C chemical shift at 300 K in the case of n=3 is shown in Fig. 17b. At 300 K, two resonance peaks were obtained at chemical shifts of δ=28.78 for CH2-2 and δ=124.97 ppm for CH2-3. The 13C chemical shifts for CH2-2 were different, being far away from NH3, while CH2-3 was close to NH3.

The 1H NMR spectra were also obtained with several delay times at each temperature, and the intensities of NMR spectra as a function of delay time were represented by a single exponential function. From the slope of the intensity vs. delay times curve, 1H T1ρ values were obtained for the CH2 and NH3 peaks. For methylene lengths n=2, 3, 4, 5, and 6, 1H T1ρ values at 300 K showed similar values in the range of 7–15 ms. The 1H T1ρ values for n=2, 3, 4, 5, and 6 are shown in Fig. 18 as a function of inverse temperature. The 1H T1ρ values were almost temperature-independent and were in the order of 10 ms. Here, the T1ρ values were compared according to the cation length from n=2~6, and they exhibited similar trends for different methylene chain lengths, with n=2 exhibiting slightly shorter values than n=3, 4, 5, and 6.

Figure 18
figure 18

The 1H spin–lattice relaxation times in [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature100,101,102,103,104.

Full size image

The 13C T1ρ values for CH2-1, CH2-2, and CH2-3 in five crystals were obtained as a function of 1000/temperature from the slope of the logarithm of intensity versus the delay time plot. The decay curves for each carbon were represented by a single exponential function. When n=2, 3, 4, 5, and 6, by methylene length, the 13C T1ρ at 300 K had values within the range of 1, 39, 32, 60-150, and 37-100 ms, respectively100,101,102,103,104. It is interesting to compare the results for 13C T1ρ according to the alkyl chain lengths. The 13C T1ρ values exhibited similar trends for n=3, 4, 5, and 6, with a very short value for n=2, as shown in Fig. 19. Unlike n=3, 4, 5, and 6, energy transfer was easier for the short alkyl chain length (n=2).

Figure 19
figure 19

The 13C spin–lattice relaxation times in [NH3(CH2)nNH3]CuCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature100,101,102,103,104.

Full size image

The 1H and 13C T1ρ values exhibited a similar trend upon increasing the methylene chain length, with n=2 exhibiting shorter T1ρ values than n=3, 4, 5, and 6. T1ρ increased according to the length of the CH2 chain, indicating that energy transfer became more difficult. 1H T1ρ values are short for [NH3(CH2)nNH3]CuCl4, including the paramagnetic ions. The paramagnetic Cu2+ ions bonded with the inorganic layer through N–H···Cl hydrogen bonds in [NH3(CH2)nNH3]CuCl4 directly affected the 1H environment.

[NH3(CH2)nNH3]ZnCl4 (n=2, 3, 4, 5, and 6)

Crystal structures

The SCXRD and PXRD experiments were conducted on [NH3(CH2)nNH3]ZnCl4 crystals, where n is 2, 3, 4, 5, and 6, at 300 K. The resulting PXRD patterns are presented in Fig. 20. Specifically, [NH3(CH2)2NH3]ZnCl4 with n=2 exhibits an orthorhombic structure with the P212121 space group. The unit cell parameters are a=8.832 Å, b=9.811 Å, c=11.089 Å, and Z=483. For [NH3(CH2)3NH3]ZnCl4, the lattice constants are determined as a=10.670 Å, b=10.576 Å, c=10.755 Å, and β=118.477°, with the space group being P21/n81,106. SCXRD experiments were carried out on [NH3(CH2)4NH3]ZnCl4, revealing a triclinic structure with the P1(bar) space group. The lattice parameters were found to be a=7.2839 (1) Å, b=8.1354 (1) Å, c=10.4592 (2) Å, and α=77.6527 (5)°, β=80.3358 (4)°, γ=82.8355 (5)°107. Moreover, [NH3(CH2)5NH3] ZnCl4 crystallized in a monoclinic structure with a C2/c space group and lattice constants a=21.4175 Å, b=7.3574 Å, c=19.1079 Å, β=120.5190°, and Z=8108. The SCXRD result of [NH3(CH2)6NH3]ZnCl4 revealed a triclinic system with the P1(bar) space group, and the cell constants were determined as a=7.2844 Å, b=10.1024 Å, c=10.1051 Å, α=74.3060°, β=85.9270°, γ=88.0170°, with Z=222,109. In Fig. 1b, the structure of the organic [NH3(CH2)6NH3] cation and inorganic ZnCl4 anion in the [NH3(CH2)6NH3]ZnCl4 crystal is illustrated. In ZnCl4, the Zn atoms are surrounded by four Cl atoms, forming a tetrahedron. The (ZnCl4)2− anion exists as an isolated tetrahedron, connected to the [NH3(CH2)6NH3] cation through N−H···Cl hydrogen bonds. Additionally, Table 4 provides information on the structures, space groups, and lattice constants of [NH3(CH2)nNH3]ZnCl4 (n=2, 3, 4, 5, and 6) crystals, revealing monoclinic structures for odd values of n, triclinic structures for even values of n, and orthorhombic structures for n=2.

Figure 20
figure 20

The powder X-ray diffraction patterns of [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) at 300 K.106,108,111.

Full size image
Table 4 The structures, space groups, and lattice constants (Å) of [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) crystals.
Full size table

Phase transition temperatures

The DSC curves for [NH3(CH2)nNH3]ZnCl4 crystals, where n ranges from 2 to 6, are presented in Fig. 21. In the case of [NH3(CH2)2NH3]ZnCl4 (n=2), the DSC curve did not show any structural phase transition even above the decomposition temperature of 534 K105. For [NH3(CH2)3NH3]ZnCl4, only one endothermic peak at 268 K was observed, indicating a phase transition at this temperature106. In the case of [NH3(CH2)4NH3]ZnCl4 (n=4), two endothermic peaks at 481 and 506 K were identified on the DSC curve, suggesting a phase transition106. The DSC result for [NH3(CH2)5NH3]ZnCl4 indicated a very strong endothermic peak at 481 K, while two weaker peaks at 256 and 390 K were observed. The phase-transition temperatures obtained from the DSC results were compared with those from SCXRD and PXRD patterns. The small peak at 256 K in the DSC curve was determined to be unrelated to the phase transition. The phase-transition temperature was defined as TC=390 K, and the melting temperature was determined as Tm=481 K108. In the case of n=6, the DSC thermogram revealed a weak endothermic peak at 408 K and a strong endothermic peak at 473 K. Further confirmation of the phase transition temperature at 408 K and the melting point at 473 K was obtained through PXRD and polarizing experiments, accounting for temperature changes109.

Figure 21
figure 21

The differential scanning calorimetry curves of [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) crystals105,106,108,109.

Full size image

Thermodynamic properties

TGA analysis was conducted on [NH3(CH2)nNH3]ZnCl4 crystals, where n ranges from 2 to 6, using the same DSC heating rate. The TGA results are depicted in Fig. 22, indicating that the crystals exhibit nearly stable behavior up to approximately 534, 549, 559, 583, and 581 K for n values of 2, 3, 4, 5, and 6, respectively108,109. For [NH3(CH2)2NH3]ZnCl4, above 534 K, a molecular weight loss is observed with increasing temperature. Weight losses of about 13 and 27 % occur near 589 and 618 K, respectively, attributed to the partial thermal decomposition of HCl and 2HCl moieties. TGA experiments with [NH3(CH2)3NH3]ZnCl4 reveal an onset of thermal decomposition (Td) at 549 K. In the case of n=4, the TGA result remains stable up to approximately 559 K. Weight losses of 12 and 25% due to the loss of HCl and 2HCl moieties occur at temperatures of 604 and 622 K, respectively, with a total weight loss of 95% near 900 K. For [NH3(CH2)5NH3]ZnCl4 crystals, molecular weight loss initiates at approximately 583 K, indicating partial thermal decomposition. Two-step decomposition processes are observed: first, a weight loss of 45% occurs at 685 K, and second, a weight loss of 95% occurs at 825 K. The 45% weight loss is attributed to organic decomposition, reaching 90% weight reduction, indicating almost complete decomposition of the organic component, leaving only Zn. Finally, the thermal characteristics of [NH3(CH2)6NH3]ZnCl4 crystals were assessed, and the DTA curve exhibited a peak around 473 K, consistent with the melting temperature determined from DSC and optical polarizing microscope analyses. The TGA and DTA curves indicate thermal stability up to about 581 K (Td). Weight loss rapidly decreases between 600 and 800 K, with approximately 22 % weight loss near 636 K due to the decomposition of 2HCl. Around 800 K, a 90 % weight loss occurs, resulting from the decomposition of NH2(CH2)nNH2·2HCl in all five compounds.

Figure 22
figure 22

The thermogravimetry analysis curves of [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6)105,106,107,108,109.

Full size image

MAS NMR chemical shifts and spin-lattice relaxation times

The 1H NMR chemical shifts of [NH3(CH2)nNH3]ZnCl4 crystals (n=2, 3, 4, 5, and 6) were recorded using MAS NMR spectroscopy. In the case of [NH3(CH2)3NH3]ZnCl4 with n=3, only one peak in the NMR spectra was observed, resulting from the overlap of NH3 and CH2 signals. The observed resonance signal was asymmetric, as illustrated in Fig. 23a. The line widths, represented as symbols A and B at the half-maximum value, differ from those at 3.54 and 6.11 ppm, respectively106. This asymmetry is attributed to the overlapping lines of the two 1H signals for CH2 and NH3 in the [NH3(CH2)3NH3] cations. At 300 K, the 1H NMR chemical shift was observed at δ=6.74 ppm. Spinning sidebands were marked with open circles and crosses.

Figure 23
figure 23

(a) The 1H NMR chemical shifts of [NH3(CH2)3NH3]ZnCl4 at 300 K. The open circles and crosses are sidebands for NH3 and CH2, respectively106. (b). The 13C NMR chemical shifts of [NH3(CH2)3NH3]ZnCl4 at 300 K106.

Full size image

In the case of n=3, MAS 13C NMR chemical shifts were measured with increasing temperature. Two resonance signals were observed in the MAS 13C NMR spectra of the compounds. The 13C chemical shifts for a spinning rate of 10 kHz were obtained, and the chemical shift for 13C was set as a standard reference for 13C in TMS. Specifically, the signals corresponding to the CH2-2 and CH2-3 carbon atoms in [NH3(CH2)3NH3]ZnCl4 at 300 K appeared at δ=26.41 and δ=38.53 ppm, respectively, as depicted in Fig. 23b.

The 1H MAS NMR spectra were measured with varying delay times, and the plot of spectral intensities vs. delay times was obtained using a single exponential function described by Eq. (1). The 1H T1ρ values at 300 K were determined from the overlapping CH2 and NH3 peaks by analyzing the slope of the intensities vs. delay times results. The T1ρ values for the 1H of NH3 and CH2 in [NH3(CH2)nNH3]ZnCl4 (n=2, 3, 4, 5, and 6) were obtained and are presented in Fig. 24 as a function of 1000/temperature. For methylene lengths n=2, 3, 4, 5, and 6, the 1H T1ρ values at 300 K showed values of 444, 849, 440, 320~410, and 261~510 ms, respectively105,106,107,108,109. As the temperature increased, the 1H T1ρ values also increased, and the values of T1ρ above 300 K exhibited variations, as illustrated in Fig. 24. The most notable case was n=2, where T1ρ gradually increased with rising temperature, reaching a maximum value at 270 K before rapidly decreasing above 300 K. Additionally, T1ρ reached its minimum value near 380 K and then tended to increase again. This trend between 300 and 430 K indicates the presence of molecular motion. The T1ρ values influenced by thermal motion are related to the correlation time (τC) for molecular motion, according to the Bloembergen-Purcell-Pound theory116. Unlike the case of Mn, Co, and Cu containing paramagnetic ions, T1ρ exhibited very long values in the case of Zn without paramagnetic ions.

Figure 24
figure 24

The 1H spin–lattice relaxation times in [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature105,106,107,108,109.

Full size image

The intensities of the 13C NMR signals were measured by varying the delay times, and the resulting 13C T1ρ values for CH2-1, CH2-2, and CH2-3 in the cases of n=2, 3, 4, 5, and 6 were depicted in Fig. 25 as a function of 1000/temperature. These values exhibited similar trends for n=3, 4, and 6. However, the 13C T1ρ values for n=2 displayed strong temperature dependence, sharply decreasing above 300 K with increasing temperature. Notably, there was a significant difference between the 13C T1ρ values (10 times less) and the 1H T1ρ values, indicating that energy transfer for 13C is more efficient. It is worth mentioning that when n=2, the 1H and 13C T1ρ values exhibited different characteristics compared to n=3, 4, 5, and 6.

Figure 25
figure 25

The 13C spin–lattice relaxation times in [NH3(CH2)nNH3]ZnCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature105,106,107,108,109.

Full size image

Furthermore, the length of the methylene did not distinctly affect the temperature dependence of 1H and 13C T1ρ. Instead, Td, the onset of thermal decomposition, showed temperature changes along the methylene length of the cation.

[NH3(CH2)nNH3]CdCl4 (n=2, 3, 4, 5, and 6)

Crystal structures

SCXRD and PXRD experiments were conducted on [NH3(CH2)nNH3]CdCl4 crystals (n = 2, 3, 4, 5, and 6) at room temperature, and their PXRD results are presented in Fig. 26. The space group and lattice constants for [NH3(CH2)2NH3]CdCl4 with n=2 were determined to be P21/a and a=7.297 (2) Å, b=7.336 (2) Å, c=8.623 (2) Å, β=92.810 (11)°, Z=289,110. For [NH3(CH2)3NH3]CdCl4 with n=3, the space group is Pman, and the lattice parameters are a=7.351 (6) Å, b=7.486 (5) Å, c=19.031 (14) Å, with Z=488,110. In the case of [NH3(CH2)4NH3]CdCl4 with n=4, the space group is P21/a, and the lattice parameters are a=7.663 (3) Å, b=7.593 (2) Å, c=9.514 (2) Å, β=101.616 (16)°, with Z=2110. The crystal structure of [NH3(CH2)5NH3]CdCl4 was determined to have a space group of Pnam, with lattice constants a=7.3292 (2) Å, b=7.5058 (2) Å, c=23.9376 (6) Å, and Z=487,112. Finally, the structure of the [NH3(CH2)6NH3]CdCl4 crystal is monoclinic, with unit cell parameters a=7.305 Å, b=7.587 Å, c=23.997 Å, and β=91.22°84,113. The crystal structure of [NH3(CH2)nNH3]CdCl4 features slightly distorted CdCl62− ions, with the six hydrogen atoms in the terminal ammonium of the [NH3(CH2)nNH3] cation connected by N−H∙∙∙Cl interactions. The Cd atom is surrounded by six Cl atoms, forming nearly regular CdCl6 octahedra. The structural details, space groups, and lattice constants of [NH3(CH2)nNH3]CdCl4 (n=2, 3, 4, 5, and 6) crystals are summarized in Table 5. Notably, the structures are monoclinic when n is even and orthorhombic when n is odd.

Figure 26
figure 26

The powder X-ray diffraction patterns of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) at 300 K110,112,113.

Full size image
Table 5 The structures, space groups, and lattice constants (Å) of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) crystals.
Full size table

Phase transition temperatures

The DSC curves of [NH3(CH2)nNH3]CdCl4 (n=2, 3, 4, 5, and 6) are presented in Fig. 27. No peak was observed for n=2, while only one endothermic peak at 374 K was observed for n=3. In the case of n=4, two endothermic peaks were observed at 341 K and 366 K110. For the [NH3(CH2)5NH3]CdCl4 crystal, two endothermic peaks were observed at 336 K and 418 K112. The enthalpy for the phase transition is 3.17 kJ/mol at 336 K and 0.55 kJ/mol at 418 K, respectively. Finally, in the case of [NH3(CH2)6NH3]CdCl4, two endothermic peaks with enthalpies of 3.27 and 0.93 kJ/mol were observed at 337 K and 472 K, respectively113. These two peaks correspond to the phase transition temperatures.

Figure 27
figure 27

The differential scanning calorimetry curves of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) crystals.110,111,112,113.

Full size image

Thermodynamic properties

The TGA curves presented in Fig. 28 reveal that the [NH3(CH2)nNH3]CdCl4 crystals with n=2, 3, 4, 5, and 6 are nearly stable up to approximately 587, 592, 595, 595, and 587 K, respectively110,111,112,113, marking the onset of partial thermal decomposition corresponding to the number n of CH2 groups in the carbon chain. These compounds undergo breakdown near 650 K, experiencing a loss in molecular weight as the temperature rises. The remaining amount as solid residues can be calculated based on the molecular weights. In the case of n=2, a loss of 12 and 23% of its weight at temperatures of about 622 and 804 K was attributed to the partial decomposition of HCl and 2HCl, respectively. Similarly, weight losses of 11 and 22% occurred at temperatures of 613 and 623 K in the case of n=3. For [NH3(CH2)4NH3]CdCl4 with n=4, at temperatures of 612 and 623 K, 11 and 21% of their weight were lost, respectively. In the case of n=5, the 10 and 20% weight losses at temperatures of about 617 and 626 K were attributed to the partial thermal decomposition of HCl and 2HCl, respectively. Furthermore, weight loss at approximately 800 and 900 K was observed to be 46 and 87%, respectively. Finally, in the case of n=6, the weight loss rapidly decreased between 600 and 800 K, with a weight loss of about 20% occurring near 625 K due to the decomposition of 2HCl. Notably, when Cd is included, there is a significant difference in weight loss at high temperatures. Specifically, a loss of 25% in the case of n=2 occurs around 800 K, while n=3, 4, and 5 show a loss of 45%, and n=6 exhibits a loss of 65 %.

Figure 28
figure 28

The thermogravimetry analysis curves of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6)110,111,112,113.

Full size image

MAS NMR chemical shifts and spin-lattice relaxation times

The MAS 1H NMR spectra of [NH3(CH2)3NH3]CdCl4 crystal with n=3 were recorded at 300 K, as shown in Fig. 29a. These resonance signals appeared asymmetric due to the overlapping of the 1H resonance lines of NH3 and CH2 in the [NH3(CH2)3NH3] cation. The spinning sidebands for NH3 and CH2 are denoted with open circles and crosses. At 300 K, the 1H chemical shift for CH2 was δ=3.23 ppm, whereas that for NH3 was δ=7.67 ppm111.

Figure 29
figure 29

(a) The 1H NMR chemical shifts of [NH3(CH2)3NH3]CdCl4 at 300 K. The open circles and crosses are sidebands for NH3 and CH2, respectively110. (b) The 13C NMR chemical shifts of [NH3(CH2)3NH3]CdCl4 at 300 K110.

Full size image

The 13C MAS NMR chemical shifts for CH2 in [NH3(CH2)3NH3]CdCl4 were recorded at different temperatures. At 300 K, the two resonance signals appeared at δ=25.04 and 39.07 ppm for CH2-2 and CH2-3, respectively, as shown in Fig. 29b. The 13C chemical shifts for CH2 were different for CH2-2, far away from NH3, and CH2-3, close to NH3. The line width of CH2-3 is wider than that of CH2-2.

The 1H NMR spectra were measured with various delay times at each temperature for five crystals, and the slopes of the intensities vs. delay times followed a single exponential function. From the slope of the logarithm of intensities vs. delay times, the 1H T1ρ values were obtained for NH3 and CH2. These values are shown in Fig. 30 for five crystals as a function of 1000/temperature. In the case of n=2, 3, 4, and 5, the 1H T1ρ values increase rapidly as the temperature rises, and those of n=2 and 3 rapidly reduce at high temperatures. The 1H T1ρ values of n=6 exhibited a slight dependence on temperature. It can be seen that 1H T1ρ values according to the n values are different at high temperatures110,111,112,113.

Figure 30
figure 30

The 1H spin–lattice relaxation times in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature110,111,112,113.

Full size image

The intensities of the 13C NMR spectrum showed changes due to various delay times. The 13C T1ρ values, obtained from the slope of their recovery traces, were determined for CH2-1, CH2-2, and CH2-3. Their results for five crystals are shown as a function of 1000/temperature in Fig. 31. In the case of n=2, 13C T1ρ decreased slightly with a rise in temperature and decreased rapidly at high temperatures. For n=3, 4, 5, and 6, the 13C T1ρ values decreased slightly with an increase in temperature, and increased again.

Figure 31
figure 31

The 13C spin–lattice relaxation times in [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) as a function of inverse temperature110,111,112,113.

Full size image

Conclusion

In the pursuit of applications for an improved lead-free organic-inorganic perovskite-type solar cell, our research focused on identifying conditions conducive to organic-inorganic hybrid perovskite materials with high or no phase transition temperature, and high thermal stability. Consequently, we sought improved organic-inorganic hybrid perovskite materials by exploring the characteristics related to methylene length and the variation of various transition metals.

Single crystals of the organic-inorganic hybrid [NH3(CH2)nNH3]MCl4 (n=2, 3, 4, 5, and 6; M=Mn, Co, Cu, Zn, and Cd) were grown using the aqueous solution method, and their crystal structures, phase transition temperatures (TC), and thermal decomposition temperatures (Td) were thoroughly examined. Additionally, our investigation delved into the impact of the even–odd number in the methylene length and various transition metals, holding potential implications for future applications.

From a structural standpoint, compounds involving Mn, Cu, and Cd, consisting of octahedral (MCl6)2− units, exhibited a monoclinic structure when n was even and an orthorhombic structure when n was odd. In contrast, for Co and Zn compounds with tetrahedral (MCl4)2− units, the crystal structure displayed an orthorhombic or triclinic arrangement when n was even and a monoclinic structure when n was odd.

Surprisingly, the phase transition temperatures did not exhibit a clear trend based on different transition metals (M=Mn, Co, Cu, Zn, and Cd), nor did they show a consistent pattern with varying methylene lengths of n=3, 4, 5, and 6. Remarkably, in the case of n=2, possessing the shortest methylene length, no phase transition temperature was observed. Notably, when the transition metal was Co, a relatively high phase transition temperature was recorded, as detailed in Table 6.

Table 6 The phase transition temperature by the methylene length of [NH3(CH2)nNH3]CdCl4 (n = 2, 3, 4, 5, and 6) crystals.
Full size table

The results of comparative analysis and explanations for research on the thermal stabilities of [NH3(CH2)nNH3]MCl4, where the methylene length varies as a parameter, are depicted in Fig. 32. In the case of transition metals Mn and Cd with an octahedral MCl62- structure, the thermal decomposition temperature (Td) remains almost constant with respect to the methylene length, whereas for Co and Zn with a tetrahedral MCl42- structure, Td increases with the methylene length. Notably, in the case of Cu, Td demonstrates a notable and rapid decrease. The changes in thermal decomposition temperature (Td) according to the transition metals can be explained by electronic configuration considerations. In the cases of Mn2+ and Co2+, where the 3d electron of the M shell is not filled and the valence electron is 4s2, as well as in the cases of Zn2+ and Cd2+, where the 3d electrons of the M shell and the 4d electrons of the N shell are filled, and the valence electrons are 4s2 and 5s2, respectively, Td remains almost constant or increases as the methylene length increases. However, in the case of Cu2+, where the 3d electrons of the M shell are filled and the valence electrons are 4s1, Td tends to decrease as the methylene length increases.

Figure 32
figure 32

The relation between the decomposition temperatures and the methylene chain lengths of [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals.

Full size image

The NMR chemical shifts were found to be associated with the local field around the location of the resonance nucleus in the single crystals. Upon including metal ions (Mn, Co, Cu, Zn, and Cd) in [NH3(CH2)nNH3]MCl4 (n=2, 3, 4, 5, and 6), the 1H and 13C chemical shifts exhibited the following trends: All 1H chemical shifts for the twenty-one compounds appeared at almost identical positions. However, the 13C chemical shifts displayed marked differences between the cases of paramagnetic ions (Mn, Co, and Cu) and the cases of Zn and Cd, which do not contain paramagnetic ions. Notably, 1H and 13C chemical shifts based on the methylene length n did not reveal any unusual patterns or trends.

The experiment results presented in Tables 7 and 8 highlight notable differences in 1H T1ρ values based on the presence or absence of paramagnetic ions. When paramagnetic ions such as Mn, Co, and Cu are included, 1H T1ρ exhibits very short values, whereas when Zn and Cd, which lack paramagnetic ions, are present, 1H T1ρ values become considerably longer. Interestingly, 13C T1ρ shows similar values regardless of the presence of paramagnetic ions. This suggests that 1H has a significant impact on T1ρ, while 13C has a minimal effect due to its distance from the paramagnetic ion. Examining T1ρ values across different methylene lengths (n) reveals no significant differences or unusual patterns.

Table 7 The spin–lattice relaxation times T1ρ (ms) for 1H of [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals.
Full size table
Table 8 The spin–lattice relaxation times T1ρ (ms) for 13C of [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals.
Full size table

Considering the overall trends of 1H and 13C T1ρ, n=3, 4, 5, and 6 exhibit almost similar trends as temperatures rise. In contrast, for n=2, a similar trend to n=3, 4, 5, and 6 is observed at low temperatures, but it undergoes a rapid shortening phenomenon at high temperatures. A longer T1ρ implies increased difficulty in energy transfer from the nuclear spin to the surrounding environment. Consequently, the physicochemical properties of organic-inorganic hybrid perovskite [NH3(CH2)nNH3]MCl4, influenced by various transition metal ions and the methylene length of the cation, hold potential applications as materials with lead-free, and high thermal stability attributes.