Comprehensive study of organic–inorganic hybrid [N(C₂H₅)₄]₂CdBr₄: crystal structure, phase transitions, and structural geometry

Abstract

The compound [N(C₂H₅)₄]₂CdBr₄, an organic–inorganic hybrid compound, has garnered attention for its potential applications across various fields. In this study, single crystals of [N(C₂H₅)₄]₂CdBr₄ were grown, and two distinct phase transition temperatures were identified: approximately 232 K (T_C1) and 476 K (T_C2). Single-crystal X-ray diffraction analysis at 300 K revealed a tetragonal structure belonging to the space group P4̄2₁m. The thermal properties of the material were also briefly examined. Interestingly, while the ¹H NMR chemical shifts exhibited minimal variation around T_C1, the ¹³C NMR spectra showed a noticeable change in the number of peaks, suggesting a structural phase transition. These observations indicate that the crystal retains a tetragonal structure above T_C1 but transitions to a phase with lower symmetry below this temperature. Furthermore, ¹¹³Cd NMR chemical shifts showed a significantly more pronounced change near T_C1 compared to the shifts observed for ¹H and ¹³C. This behavior reflects alterations in the local electronic environment around the Cd²⁺ ions, implying reconfiguration of the surrounding atomic framework. Additionally, the spin–lattice relaxation times for ¹H and ¹³C, which are related to molecular motion, were nearly identical for both CH₂ and CH₃ groups in [N(C₂H₅)₄] cations. The phase transition observed at T_C1 is thus attributed to substantial structural rearrangements, particularly involving changes in atomic coordinates and the rotation of the CdBr₄²⁻ tetrahedra.

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1. Introduction

Hybrid organic–inorganic metal halides have been extensively studied due to their promising applications in photovoltaic and optoelectronic devices. The broad range of applications and the rapid development of these materials have attracted significant attention from researchers.^1–12 In these compounds, the optical properties and structural flexibility are primarily governed by the organic cations, whereas the inorganic anions mainly determine their thermal and mechanical behavior. Their unique ability to combine the advantages of both organic and inorganic components makes them highly versatile for designing advanced functional materials.¹³ Among these, the three-dimensional (3D) perovskite CH₃NH₃PbI₃ has recently received considerable interest due to its exceptional photovoltaic performance. However, its practical application is limited by its sensitivity to humidity and the toxicity associated with lead.^14–18 As a result, significant efforts have been directed toward the development of zero-dimensional (0D) hybrid organic–inorganic metal halides as more stable and less toxic alternatives. 0D hybrid metal halides with the general formula A₂MeX₄ have emerged as excellent candidates for phase-transition materials. In particular, tetraethylammonium-based compounds, [N(C₂H₅)₄]₂MeX₄ (where Me = Mn, Co, Cu, Zn, Cd and X = Cl, Br), belong to a large family of A₂MeX₄ crystals known to exhibit complex phase transitions influenced by both the metal (Me) and halide (X) ions.^19–53 These transitions are highly sensitive to external conditions due to hydrogen bonding between the molecular cations and the metal halide anions. Each [N(C₂H₅)₄]⁺ cation resides in a cavity formed by surrounding MeX₄ anions, and many of these crystals undergo phase transitions involving rearrangements of both the MeX₄ units and the organic groups. The tetraethylammonium group itself can adopt two distinct stable conformations, with its ethyl chains offering greater configurational freedom compared to methyl-substituted analogs. While the nitrogen and terminal carbon atoms of the [N(C₂H₅)₄]⁺ ion occupy well-defined positions, the central carbon atom can adopt two orientations, suggesting a disordered ionic configuration. More recently, attention has also turned to compounds that incorporate either mixed inorganic or organic components, such as [N(C₂H₅)₄]₂CoCl₂Br₂,^54–60 which contains mixed halides, and [N(CH₃)₄][N(C₂H₅)₄]MeX₄,^61–64 which features different organic cations. These systems offer further opportunities to explore structure–property relationships and design novel hybrid materials.

The [N(C₂H₅)₄]₂CdBr₄ compounds, the case where Me = Cd and X = Br, i.e., has been reported in the literature only with respect to a few aspects. The transition temperature at 230 K was first reported by Kahrizi et al.²³ through capacitance dilatometry measurements. Vlokh et al.²⁵ investigated the optical birefringence of this single crystal and revealed a second-order phase transition at 118 K. Additionally, Sveleba et al.²⁹ reported a second-order phase transition at 311 K based on birefringence and dielectric experiments. Furthermore, changes in lattice parameters observed via X-ray diffraction suggested first-order phase transitions at 174 and 226 K, while an anomalous change was detected at 293 K.⁴⁴ In addition, Dekola et al.⁴⁵ demonstrated, through temperature-dependent heat capacity measurements, the occurrence of a first-order phase transition at 226.5 K. However, as shown in Table 1, these results are inconsistent with one another, and few detailed studies have been published on this compound to date. Although [N(C₂H₅)₄]₂CdBr₄ crystals have been relatively understudied among A₂MeX₄-type compounds, they have recently attracted attention due to their promising potential for practical applications.

Table 1 The phase transition temperatures for previously reported [N(C₂H₅)₄]₂CdBr₄ crystals according to the various experimental methods

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In the present work, single crystals of [N(C₂H₅)₄]₂CdBr₄ were grown, and their phase transition behavior was investigated using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). The thermodynamic properties of the compound were briefly examined using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). In addition, the crystal structure and lattice parameters at 300 K were determined by single-crystal X-ray diffraction (SCXRD) experiments. To examine the roles of [N(C₂H₅)₄]⁺ cations and CdBr₄²⁻ anions near the phase transition temperature, temperature-dependent chemical shifts in the ¹H, ¹³C, and ¹¹³Cd magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were analyzed. Moreover, spin–lattice relaxation times (T_1ρ) obtained from the NMR measurements were used to investigate molecular motion rates.

2. Experimental

2.1. Crystal growth

High-quality single crystals of [N(C₂H₅)₄]₂CdBr₄ were successfully grown using a slow evaporation method from supersaturated aqueous solutions. The crystals were obtained by dissolving tetraethylammonium bromide (N(C₂H₅)₄Br, Sigma-Aldrich, 99%) and cadmium bromide (CdBr₂, Sigma-Aldrich, 97%) in distilled water at a molar ratio of 2 : 1. The solution was thoroughly stirred and gently heated to achieve saturation, followed by gradual evaporation over several weeks in a constant-temperature bath maintained at 300 K. Several colorless and transparent single crystals with dimensions of approximately 5 × 4 × 3 mm were obtained. The resulting single crystals were carefully stored in a desiccator to prevent degradation caused by moisture exposure.

2.2. Characterization

Fourier-transform infrared (FT-IR) spectra were recorded in the range of 4000–500 cm⁻¹ using a PerkinElmer L1600300 spectrometer (Waltham, MA, USA), with samples prepared as compressed KBr pellets. Differential scanning calorimetry (DSC) analyses were performed using a TA Instruments DSC system (Model 25) over the temperature range of 200–550 K, employing a heating rate of 10 K min⁻¹ under a dry nitrogen atmosphere. The morphology of the crystals and their thermal evolution were monitored using a Carl Zeiss optical polarizing microscope equipped with a Linkam THM-600 heating stage. Thermogravimetric analysis (TGA) was carried out from 300 to 873 K at a heating rate of 10 K min⁻¹ under a continuous flow of nitrogen gas.

The crystal structure and lattice parameters at 300 K were determined by single-crystal X-ray diffraction (SCXRD) analysis, conducted at Sookmyung Women's University using the Sookmyung Research Facilities. SCXRD data were obtained using a Bruker SMART CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation source. Data acquisition and integration were carried out using the SMART APEX3 and SAINT software suites, and absorption corrections were applied via the multiscan method implemented in SADABS.⁶⁵ Additionally, powder X-ray diffraction (PXRD) analysis was conducted using an Expert Pro-MPD X-ray diffractometer (PANalytical) fitted with Cu-Kα radiation and operated at 40 kV and 40 mA at the Korea Basic Science Institute (KBSI), Metropolitan Seoul Center. This instrumentation enabled the examination of the crystalline structure and phase composition of the samples. Furthermore, high temperature XRD analysis was performed utilizing an Anton Paar HTK 1200N stage coupled with the aforementioned XRD system, allowing for the investigation of structural changes in the samples at elevated temperatures.

Solid-state ¹H and ¹³C NMR spectra of [N(C₂H₅)₄]₂CdBr₄ were acquired using Bruker NMR spectrometers operating at 400 MHz and 500 MHz (Germany) at the Metropolitan Seoul Center of the KBSI and the NCIRF at Seoul National University, respectively. The ¹H magic angle spinning (MAS) NMR experiments were conducted at a Larmor frequency of 400.13 MHz, while the ¹³C MAS spectra were recorded at 100.61 MHz and 125.77 MHz. Chemical shifts for both nuclei were referenced to tetramethylsilane (TMS). To minimize spinning sidebands, the MAS rate was set to 5 kHz. One-dimensional ¹H and ¹³C spectra were collected with delay times of 5 s and 10 s, respectively. Spin–lattice relaxation times in the rotating frame (T_1ρ) were measured using a π/2 pulse followed by a spin-lock pulse of varying duration t. T_1ρ values for ¹H and ¹³C were determined with variable delay times ranging from 10 ms to 15 s. Additionally, ¹¹³Cd MAS NMR chemical shifts were recorded at a Larmor frequency of 88.75 MHz, with CdCl₂O₈·6H₂O serving as the reference compound. One-dimensional ¹¹³Cd spectra were obtained using the one-pulse method with a delay time of 100 s. Temperature-dependent NMR measurements were carried out over the range of 180–430 K, with precise temperature control maintained by regulating the nitrogen gas flow and heating current.

3. Results and discussion

3.1. Fourier-transform infrared spectroscopy

The information related to FT-IR is presented in SI S1.

3.2. Phase transition temperature and thermodynamic properties

To determine the phase transition temperatures of [N(C₂H₅)₄]₂CdBr₄, single crystals were finely ground into powder using a mortar. Differential scanning calorimetry (DSC) measurements were conducted on approximately 8.5 mg of the sample at controlled heating and cooling rates of 10 K min⁻¹. As shown in Fig. 1, two endothermic peaks appeared during heating: a subtle peak near 232 K and a prominent peak around 476 K, corresponding to enthalpy changes of 246 J mol⁻¹ and 15.15 kJ mol⁻¹, respectively. Upon cooling, a strong exothermic peak was observed at approximately 424 K, with an associated enthalpy change of 18.17 kJ mol⁻¹. These thermal transitions were found to be irreversible.

Fig. 1 Differential scanning calorimetry curves of [N(C₂H₅)₄]₂CdBr₄ measured at a heating and cooling rate of 10 K min⁻¹ (inset: single crystal images at (a) 300 K, (b) 513 K, and (c) 583 K).

Complementary observations using optical polarizing microscopy revealed no significant morphological changes in the single crystals between 300 K and 513 K (see (a) and (b) within Fig. 1). At temperatures above this range, the initially colorless and transparent crystal became opaque (see (c) within Fig. 1), although no melting was observed up to approximately 600 K. This interpretation is further supported by subsequent powder X-ray diffraction (PXRD) analysis.

To investigate the thermal properties of [N(C₂H₅)₄]₂CdBr₄, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out. As with the DSC measurements, the sample was finely ground into powder, and approximately 13.6 mg was used for analysis. The measurements were performed over the temperature range of 300–873 K at a heating rate of 10 K min⁻¹, consistent with the DSC conditions. As illustrated in Fig. 2, the TGA results indicated that the crystal remained thermally stable up to approximately 560 K, showing only a slight weight loss of about 2%. Partial thermal decomposition began at this point. Above the decomposition temperature (T_d) of 560 K, a significant weight loss was observed. The DTA curve exhibited two endothermic peaks, one near 476 K and another around 581 K. The peak at 476 K aligns well with that observed in the DSC results. In contrast, the peak at 581 K does not correspond to melting, as confirmed by polarizing microscopy, but rather indicates the onset of substantial sample decomposition. An inflection point observed at 662 K is attributed to the release of 2 equivalents of [N(C₂H₅)₄]Br, accounting for approximately 60% of the initial molecular weight, suggesting major thermal degradation. At temperatures exceeding 854 K, the sample showed nearly complete decomposition, with a total weight loss approaching 100%. These thermal analyses offer valuable insights into the compound's thermal stability, phase transition behavior, and decomposition profile.

Fig. 2 Thermogravimetry and differential thermal analysis curves of [N(C₂H₅)₄]₂CdBr₄ measured at a heating rate 10 K min⁻¹. T_d is the decomposition temperature.

3.3. X-ray diffraction experiment on single-crystal

Single-crystal X-ray diffraction (SCXRD) analysis was carried out on [N(C₂H₅)₄]₂CdBr₄ at 300 K. The compound crystallizes in the tetragonal system with space group P4̄2₁m, and the lattice parameters were determined to be a = b = 13.4605(3) Å, c = 14.4058(4) Å, with α = β = γ = 90°. The number of formula units per unit cell (Z) is 4, as summarized in SI S2. Fig. 3(a) and (b) depict the tetragonal crystal structure and thermal ellipsoids of each atom in [N(C₂H₅)₄]₂CdBr₄ at 300 K. Within the unit cell, two crystallographically distinct tetraethylammonium cations, denoted as [N(1)(C₂H₅)₄] and [N(2)(C₂H₅)₄], are present. The organic–inorganic framework consists of these cations and CdBr₄²⁻ anions. A summary of selected bond-lengths and angles is provided in SI S3. The Cd–Br bond-lengths range from 2.582 Å to 2.587 Å, and the Br–Cd–Br bond angles vary between 107.69° and 112.58°, indicating a distorted tetrahedral coordination geometry around the Cd²⁺ ion. As also shown in SI S3, the distances from N(1) to its four surrounding carbon atoms range from 1.46 Å to 1.49 Å, while all four N(2)–C distances are uniformly 1.50 Å. This suggests that the local symmetry around N(2) is higher than that around N(1). The crystallographic information file (CIF) corresponding to the SCXRD data collected at 300 K is available in SI S4. Additionally, the complete crystallographic dataset has been deposited with the Cambridge Crystallographic Data Centre (CCDC 2477720). And, the morphology of the colorless and transparent single crystal grown in this study is shown in Fig. 3.

Fig. 3 (a) Tetragonal structure and (b) thermal ellipsoids for two type of [N(C₂H₅)₄] cations and CdBr₄ anion of [N(C₂H₅)₄]₂CdBr₄ crystal at 300 K (inset: morphology of a colorless and transparent single crystal at 300 K) (CCDC no. 2477720).

A simulated powder X-ray diffraction (PXRD) pattern generated from the CIF file closely agrees with the experimental PXRD pattern obtained from powdered [N(C₂H₅)₄]₂CdBr₄ samples, as shown in Fig. 4. Notably, two prominent diffraction peaks were observed at 11.09° and 13.15°, corresponding to the (111) and (200) plane, respectively. Peak indexing was carried out using the Mercury software. Powder X-ray diffraction (PXRD) measurements were conducted on crushed single crystals of [N(C₂H₅)₄]₂CdBr₄ over a 2θ range of 8–50° at temperatures above 300 K, and the results are presented in Fig. 5. The diffraction patterns recorded below 450 K remained nearly identical, indicating that the crystal structure is stable within this temperature range. In contrast, a clear change in the PXRD pattern was observed at 500 K, which corresponds well with the endothermic peak near 476 K identified in the DSC analysis. Furthermore, the presence of well-defined diffraction peaks at 500 K suggests the formation of a new crystalline phase rather than melting. The PXRD pattern obtained at 300 K after cooling the sample from 500 K (denoted as 300 K (A)) showed a significant difference compared to the original 300 K pattern. This indicates that the structural change is irreversible, in agreement with the DSC results. Based on the combined evidence from DSC and PXRD analyses, the phase transition temperatures of [N(C₂H₅)₄]₂CdBr₄ are estimated to be approximately 232 K (T_C1) and 476 K (T_C2).

Fig. 4 Powder X-ray diffraction (PXRD) patterns with temperature variation and the simulated XRD pattern of [N(C₂H₅)₄]₂CdBr₄. The 300 K (A) pattern represents the PXRD pattern measured after cooling the single crystal from 500 K to 300 K.

Fig. 5 ¹H NMR spectra for CH₂ and CH₃ in [N(C₂H₅)₄]₂CdBr₄ with increasing temperature (inset: recovery traces measured with the delay time of 10 ms–15 s).

3.4. ¹H MAS NMR chemical shifts and spin–lattice relaxation times

The ¹H MAS NMR spectra of [N(C₂H₅)₄]₂CdBr₄ single crystals were recorded at a Larmor frequency of 400.13 MHz under a 9.4 T magnetic field to investigate structural changes near T_C1. Tetramethylsilane (TMS) was used as an external reference for accurate calibration of chemical shifts. The spectra, acquired at a spinning rate of 5 kHz, are presented in Fig. 5. Peaks marked with asterisks (*) correspond to spinning sidebands, located approximately ±12.5 ppm from the central peak, consistent with the 5 kHz MAS rate. Below T_C1, the spectrum exhibits a single resonance, whereas above T_C1, two partially overlapping signals become apparent. At 300 K, signals at 1.04 ppm and 2.91 ppm are assigned to the protons of the CH₃ and CH₂ groups, respectively. The signal at 1.04 ppm shows significantly higher intensity than the one at 2.91 ppm, consistent with the relative number of hydrogen atoms in the CH₃ and CH₂ groups. Overall, the ¹H chemical shifts exhibit minimal variation with temperature, indicating weak temperature dependence.

At 300 K, the spin–lattice relaxation time T_1ρ value for ¹H in [N(C₂H₅)₄]₂CdBr₄ was determined, and the recovery trace of nuclear magnetization can be well described by a single exponential function, as expressed by the following equation:^66,67

P(t) = P(0) exp(−t/T_1ρ) (1)

In this context, P(t) denotes the magnetization measured after a spin-lock period of duration t, whereas P(0) represents the initial magnetization prior to spin-locking. The ¹H MAS NMR spectra were acquired at 300 K by varying the spin-lock delay time. The resulting signals, obtained over a delay range of 10 ms to 15 s, are shown in the inset of Fig. 5, and the decay of magnetization follows the exponential behavior described by eqn (1). From the slope of the magnetization decay curves, the spin–lattice relaxation time in the rotating frame (T_1ρ) was determined. The T_1ρ values for the CH₃ and CH₂ groups were found to be identical, each measuring 2.68 s.

3.5. ¹³C MAS NMR chemical shifts and spin–lattice relaxation times

The ¹³C MAS NMR spectra of [N(C₂H₅)₄]₂CdBr₄ were recorded at resonance frequencies of 100.61 MHz and 125.77 MHz under magnetic fields of 9.4 T and 11.7 T, respectively. Tetramethylsilane (TMS) was used as the external reference to ensure accurate chemical shift calibration. At 300 K and a spinning speed of 10 kHz, two distinct signals were observed at 54.24 ppm and 10.30 ppm, corresponding to the CH₂ and CH₃ carbon environments, respectively. The temperature dependence of the ¹³C chemical shifts was examined at a reduced spinning rate of 5 kHz for both Larmor frequencies, as presented in Fig. 6. The square and triangle symbols in the plot represent data obtained at 100.61 MHz and 125.77 MHz, respectively, and show good agreement. Due to instrumental limitations, low- and high-temperature spectra were not acquired at 10 kHz, as such conditions could impose excessive mechanical stress on the MAS NMR probe. Overall, the ¹³C chemical shifts exhibited minimal variation with increasing temperature. However, below the T_C1 transition temperature, the spectra for both CH₃ and CH₂ groups displayed splitting into multiple peaks. As the temperature increased and approached T_C1 (e.g., at 238 K), the split signals began to merge; above T_C1, only one or two signals were observed, as illustrated in the inset of Fig. 6. The peak labeled “x” near 54 ppm is a spinning sideband associated with the CH₂ group. At 218 K, well below T_C1, the CH₃ signal is split into several distinct peaks, indicating reduced symmetry. This spectral behavior suggests that the local structural environment around the ¹³C nuclei is less symmetric at lower temperatures and becomes more symmetric above T_C1.

Fig. 6 ¹³C NMR chemical shifts for CH₂ and CH₃ in [N(C₂H₅)₄]₂CdBr₄ measured at Larmor frequency of 100.61 and 125.77 MHz with increasing temperature (inset: ¹³C NMR spectra at 218, 238, 253, and 333 K).

The spin–lattice relaxation time in the rotating frame (T_1ρ) for ¹³C nuclei in [N(C₂H₅)₄]₂CdBr₄ at 300 K was determined using eqn (1), following a method analogous to that used for the ¹H T_1ρ measurements. The ¹³C MAS NMR spectra were analyzed by tracking changes in signal intensity as a function of delay time. Spectral data collected over a delay range of 10 ms to 15 s are shown in Fig. 7. The T_1ρ values were extracted from the slopes of the magnetization recovery curves. As a result, the T_1ρ values for the CH₂ and CH₃ carbon atoms were found to be very similar, measured at 2.13 s and 2.14 s, respectively.

Fig. 7 ¹³C NMR recovery traces in [N(C₂H₅)₄]₂CdBr₄ measured with the delay time of 10 ms–15 s at 300 K.

3.6. ¹¹³Cd MAS NMR chemical shifts

The ¹¹³Cd MAS NMR experiments were performed across a temperature range encompassing the phase transition temperature T_C1, in order to gain insight into the coordination environment of the Cd²⁺ ion within the CdBr₄²⁻ anion. The temperature-dependent ¹¹³Cd chemical shifts, measured at a Larmor frequency of 88.75 MHz, are presented in Fig. 8. The chemical shift and line width in the ¹¹³Cd NMR spectrum as a function of temperature are shown in detail in Fig. 9. The ¹¹³Cd resonance shifts toward lower (more negative) values with increasing temperature, exhibiting an abrupt discontinuity near T_C1. Specifically, the chemical shift changes from approximately 392 ppm at 180 K below T_C1 to around 362 ppm at 380 K above this transition, suggesting a structural modification in the local environment surrounding the Cd²⁺ ion. Similarly, the line width follows a temperature-dependent trend analogous to that of the chemical shift. Notably, a pronounced narrowing of the line width occurs near T_C1, decreasing sharply from roughly 15 ppm below T_C1 to approximately 1.3 ppm at 380 K. The narrowing of the ¹¹³Cd line widths with increasing temperature suggests an enhanced mobility of the ¹¹³Cd nuclei. This significant reduction implies enhanced dynamic behavior of the Cd²⁺ ion, which is tetrahedrally coordinated by four Br⁻ ions, at elevated temperatures.

Fig. 8 ¹¹³Cd NMR spectra in [N(C₂H₅)₄]₂CdBr₄ with increasing temperature.

Fig. 9 ¹¹³Cd NMR chemical shifts and line widths in [N(C₂H₅)₄]₂CdBr₄ near the phase transition temperature T_C1.

4. Conclusions

Colorless and transparent single crystals of [N(C₂H₅)₄]₂CdBr₄ were grown via an aqueous solution method. Differential scanning calorimetry and related analyses revealed two phase transition temperatures at approximately 232 K (T_C1) and 476 K (T_C2). The compound crystallizes in a tetragonal system with space group P4̄2₁m, and demonstrates thermal stability up to approximately 560 K. On the other hand, although the ¹H MAS NMR chemical shifts showed minimal variation near T_C1, the ¹³C MAS NMR spectra exhibited notable changes in peak multiplicity across this temperature, indicating a lowering of symmetry below T_C1. This suggests that while the crystal maintains tetragonal symmetry above T_C1, it transitions to a lower-symmetry phase at lower temperatures. Furthermore, ¹¹³Cd MAS NMR spectra revealed more pronounced changes near T_C1 compared to those of ¹H and ¹³C. The change in chemical shifts with temperature indicates a change in local electronic configuration at the ¹¹³Cd sites, which in turn means that the neighboring atoms change their configurations. The line width behavior is consistent with increased rotational freedom of the CdBr₄²⁻ tetrahedra upon phase transition; the changing line widths are attributed to various internal molecular motions. Additionally, spin–lattice relaxation times in the rotating frame (T_1ρ) for both CH₃ and CH₂ groups were found to be nearly identical: 2.68 s for ¹H and 2.14 s for ¹³C. This similarity implies comparable molecular mobility for both functional groups across the transition. The phase transition in the [N(C₂H₅)₄]₂CdBr₄ crystal occurs at T_C1 = 232 K and is accompanied by distinct structural changes, primarily due to variations in atomic coordinates and the rotation of the CdBr₄²⁻ tetrahedra. A detailed investigation of the structural and physical properties of this organic–inorganic hybrid compound will demonstrate its potential for various technological applications in the future.

Author contributions

A. R. Lim performed crystal growth and wrote the manuscript. S. H. Kim and Y.-J. Ko measured NMR experiments, and D. Shin measured X-ray experiments.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2477720 contains the supplementary crystallographic data for this paper.⁶⁸

All relevant data are within the manuscript and the SI. See DOI: https://doi.org/10.1039/d5ra05828g.

Acknowledgements

This research was supported by the Regional Innovation System & Education (RISE) program through the Jeonbuk RISE Center, funded by the Ministry of Education (MOE) and the Jeonbuk State, Republic of Korea (2025-RISE-13-JJU).

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