A series of bimetallic ammonium RbEu nitrates exhibiting switchable dielectric constant and photoluminescence properties†

Hua-Kai Li , Li-Ping Wang , Zong-Ze Cui , Qi Xu , Liang-Liang Zou , Na Wang , Le-Ping Miao , Heng-Yun Ye * and Chao Shi *
Chaotic Matter Science Research Centre, International Institute for Innovation, Jiangxi University of Science and Technology, Nanchang 330000, Jiangxi Province, P. R. China. E-mail: 15150517670@163.com; hyye@seu.edu.cn

First published on 31st July 2024

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Introduction

Multifunctional materials have become an extremely interesting part of physics & materials science, and thus their widespread exploration and investigation has been carried out. In recent decades, hybrid perovskite multifunctional materials (HPMMs) exhibiting combined excellent properties of both organic and inorganic sections have attracted great attention for their significance in both theory and application.^1–5 Among them, switchable HPMMs including ferroic materials,^6–11 tunable dielectrics,^12–15 and quadratic nonlinear-optical switches^16–19 are of growing interest because of their enormous application prospects in electro-optical intelligent devices, data processing and storage, and optical communication and so on. However, realization of HPMMs with predetermined properties is challenging, because most of them have complex structures, hindering switchable solid–solid transitions for such kinds of crystalline compounds under measurement conditions.²⁰ In this sense, it is urgent and necessary to search for new hybrid materials with multifunctional properties.

Rare-earth ions have unique photothermal, magnetism, and luminescence properties resulting from the localized nature of their 4f-electrons, which can construct a variety of coordination geometries.^21–23 As has been known, the hybrid perovskite based on rare-earth ions is the most suitable for the development of multifunctional materials.^24,25 Nevertheless, the reported rare-earth HPMMs exhibiting tunable and switchable properties are relatively scarce. This is mainly due to the complex coordination geometry of rare-earth ions and complicated interactions among the molecules, which makes it difficult to realize symmetry breaking, which is essential for dynamic responsive hybrid materials.⁶ Fortunately, we have demonstrated that hybrid rare-earth double perovskites are suitable for achieving multifunctionality.^26–31 For instance, a large family of hybrid double perovskites based on rare-earth nitrates and chiral cations have been successfully designed displaying photoluminescent properties,²⁶ superior piezoelectric responses,²⁷ circularly polarized luminescence activity,²⁸ and tunable ferroelectricity in a wide temperature range.²⁹ Of particular interest is the widespread and effective use of halogen substitution strategies to modify organic cations. This strategy not only reduces the symmetry of the molecule, but also increases the potential energy of the dynamic motion of the molecule.^32–35 As a result, different types of phase transitions occur in halogenated HPMMs, and the phase transition temperature increases accordingly.

Based on the previously excellent works, we have further investigated the hybrid rare-earth double perovskite system incorporating the rare-earth Eu³⁺ ion and alkali Rb⁺ ion. At the same time, we introduce halogen atoms Cl, Br and I into the cation. In this context, a series of bimetallic RbEu HPMMs, [(CH₃)₃NCH₂X]₂[RbEu(NO₃)₆] (X = Cl, 1; Br, 2; I, 3), were synthesized. They undergo order–disorder phase transitions between 200–430 K. Various-temperature single-crystal structural analysis and dielectric measurements reveal that the bistable thermodynamic behavior mainly originates from the reorientational changes of the cation in the anionic [Rb₄Eu₄(NO₃)₁₂] cage. By varying the cation in the A-site, the phase transition temperature is finely tuned. Furthermore, we have further examined the photoluminescence properties of 1–3. And the rare-earth Eu³⁺ ion acts as the activator of the luminescent properties. Here we report the preparation, various-temperature single-crystal structures, dielectric transitions, and photoluminescence properties of 1–3.

Experimental section

Synthesis

The organic amine, (CH₃)₃NCH₂X (X = Cl, Br or I), was custom made by WuXi AppTec (Shanghai, China). The corresponding nitrate [(CH₃)₃NCH₂X]NO₃ (X = Cl, Br or I) was prepared by reaction of [(CH₃)₃NCH₂X] (X = Cl, Br or I) and an equal molar amount of aqueous HNO₃, and used without further isolated. Other reagents were directly purchased from Aladdin Reagent Company (China, Shanghai). Single-crystals of 1–3 were obtained by the stoichiometric (2:1:1) combination of [(CH₃)₃NCH₂X] (10 mmol), RbNO₃ (5 mmol) and Eu(NO₃)₃·6H₂O (5 mmol) in water (10 mL). The clear solutions were allowed to evaporate slowly at room temperature for about a week. Colorless block-like crystals with size up to 6 × 6 × 4 mm³ were formed (Fig. S1, ESI†). The purity of the crystals was confirmed by the powder X-ray diffraction (PXRD) at room temperature (Fig. S2, ESI†). Thermogravimetric analysis was then performed with the sample, which shows that 1–3 are stable below 500 K (Fig. S3, ESI†). We also performed infrared spectroscopy of 1–3 in the frequency range of 500 to 4000 cm⁻¹ (Fig. S4, ESI†). As expected, 1–3 showed similar vibrational absorption peaks in the infrared spectrum. The frequencies of the infrared peaks have relatively strong peaks at 1446, 1333, 1050, 949, 819, and 743 cm⁻¹.

Physical property characterization

PXRD patterns were obtained on a Rigaku D/MAX 2000 PC X-ray diffractometer. Differential scanning calorimetry (DSC) measurements were collected on a netzsch differential scanning calorimeter (polyma) at atmospheric pressure, with a heating/cooling rate of 20 K min⁻¹. The infrared spectrum was measured in transmission mode by a Thermo Fisher Scientific Nicolet iS20 spectrometer with a scanning resolution of 4 cm−¹. Single-crystal diffraction data were carried out using a Rigaku synergy diffractometer with Mo-K_α radiation (λ = 0.71073 Å) from a graphite monochromator. The structures were resolved by direct methods and refined using the full-matrix least-square method based on F² using the SHELXLTL-2014 software package. In the crystal structures, the organic cation is totally disordered, and thus is not modelled as the chemical senses. This led to a few relatively unreasonable structural parameters and high R₁ values. For dielectric measurements, pressed-powder pellets were used. Carbon conducting paste deposited on the plate surfaces was used as the electrodes. A Tonghui TH2828A impedance analyzer was used to measure the complex dielectric constants. Photoluminescence data of the solid samples were recorded on an Edinburgh EI-980 fluorescence spectrometer. Emission/excitation spectra were measured using the same slit and iris on a FLS980 fluorescence spectrometer (Edinburgh Instrument) equipped with a 450 W continuous wavelength Xe lamp, using Hamamatsu R928P (visible) photomultipliers.

Results and discussion

Thermal properties

Generally, the DSC measurement is a concise thermo-dynamic method to confirm and identify the existence of the thermal anomalies triggered by temperature. The heat flow measurements during the cooling–heating cycles revealed that for 1–3, a reversible solid–solid phase transition is observed (Fig. 1). They undergo step-like transition processes with endo/exothermic peaks at 259/246 K for 1, 281/272 K for 2, and 327/316 K and 405/377 K for 3, upon heating/cooling. The corresponding thermal hystereses of 1–3 are 13 K, 9 K, 11 K and 28 K, respectively, at a scanning rate of 20 K min⁻¹. With the change of halogen atoms from Cl to I, the phase transition temperature shows a remarkable increasing trend. The averaged enthalpy changes, ΔH (by integration of DSC peaks), and corresponding entropy changes, ΔS (by ΔS = ΔH/T), in the cooling–heating cycles are shown in Table S1 (ESI†), indicating that the phase transition in the three compounds can be classified as an order–disorder type.³⁶ In Table S2 (ESI†), we present a comparative analysis of the structural phase transition temperatures for a range of rare-earth double perovskites.

Crystal structures

To have deep insight into the preferentially oriented crystallization of 1–3, we determined the various-temperature single-crystal structures below and above the phase transition temperatures (T_tr), labelled as low-, and high-temperature phases (LTP and HTP). The structures of 1–3 adopt the 3D double-perovskite structures, which are composed of corner-shared Rb(NO₃)₆ and Eu(NO₃)₆ octahedra, and the organic cations, (CH₃)₃NCH₂Cl, (CH₃)₃NCH₂Br or (CH₃)₃NCH₂I are confined in the A-site enclosed by the octahedra (Fig. 2(a)).

In the HTP at 293 K, compound 1 crystallizes in the cubic space group Fmm, showing a perfect NaCl-type double metal–nitrate framework (Fig. 2(b)). With the changes of the A-site cations, the cell parameters change a little (Table S3, ESI†). In addition, the space required for the movement of organic cations is related to the crystal lattice void volume. The void occupancy of compounds 1, 2 and 3 was calculated by Multiwfn software.³⁷ The results show that the void occupancy of compounds 1, 2 and 3 is 7.57%, 22.01% and 28.06%, respectively. This fact is ascribed to the relatively flexible characteristic of the anionic cage [RbEu(NO₃)₆], which leaves much room to adapt the organic cation via tilting and/or distortions of the anion octahedra (Table S4, ESI†).

In the host cage, [Rb₄Eu₄(NO₃)₁₂], the Eu³⁺ ion, located at Wyckoff site 4a, is dodecahedrally coordinated by six NO₃⁻ ligands centered on the Wyckoff 24e site, in a bidentate mode. The Rb⁺ ion in 1–3, located at Wyckoff site 4b, is octahedrally coordinated by six NO₃⁻ ligands in a monodentate mode. Noteworthily, the oxygen atoms of the nitrate ions were found to be disordered and modelled over four-sites with equal occupancies. This is related by a four-fold rotation axis across the nitrogen atom, indicating a roughly axial rotation of the NO₃⁻ ligand. Meanwhile, the orientation of the cation in 1–3 was determined based on the shape of the electron density. The cation resides in the cage and locates centered on Wyckoff site 8c, showing a highly orientational disorder, analogous to the K⁺/Rb⁺-LN³⁺ ones.^23,38

Although the structures of 1–3 at HTP are clarified well, their diffraction data at LTP become worse due to twinning and/or crystal cracking after reversible phase transitions. Fortunately, the structure of 1 at LTP was successfully solved. Single-crystal structural analysis of 1 at HTP and LTP can help understand the driving force for the phase transitions in this series of bimetallic ammonium RbEu nitrate hybrid perovskite compounds. The thermal-triggered reversible phase transitions were also evidenced by changes of the cell parameters, bond strengths and corresponding angles during the phase transitions in variable-temperature single-crystal structures of 1 (Tables S3 and S4, ESI†). In the LTP, the degree of high disorder of cations in the crystal lattice gradually decreases. Besides the orientationally disordered cations, thermal vibrations of the flexible metal-nitrate framework are also evidenced by the large anisotropic thermal vibrations of O atoms of the NO₃ groups (Table S5, ESI†). The occurrence of the reversible solid-state phase transition is mainly ascribed to the changed dynamic motions of cations, as well as relative displacement and tilting of anion framework units.

The single-crystal structures of 2 and 3 at LTP have not been obtained due to their poor diffraction data. We then measured the variable-temperature PXRD spectra of 2 and 3 to further verify the phase transitions. In the case of 2, the PXRD before the phase transition does not change, but after reaching the phase transition temperature, the diffraction peak of 17–19° appears (Fig. 3(a)). In order to visually observe the changes, we made a false-color map, we can clearly see that in the red dotted box, when the temperature drops to 270 K, the peak intensity suddenly appears, indicating that there is a new phase (Fig. 3(b)). For compound 3, the PXRD pattern at different temperatures proves that it does have two phase transitions at high temperature. The change in the reflection peak of the first structural phase transition is marked with a purple circle, and the change in the reflection peak of the second structural phase transition is marked with a black circle. We also made a false-color map of 3, however because the difference of diffraction peak is not obvious, we fail to see the disappearance and appearance of peak intensity (Fig. S5, ESI†). These results are attributed to structural changes during the phase transition process, which are consistent with the DSC results. The results indicate that the local environments around the main groups are relatively anisotropic at LTP and isotropic at HTP because of the rotation-induced averaged field, corresponding to the mainly order–disorder transition of the reported crystal.^23,39

In addition, to better understand the influence of halogen substitution strategies on the molecular dynamics of this series of compounds, we performed Hirshfeld surface analysis on them, and we obtained visual and 2D fingerprint plots (Fig. S6, ESI†). The red part indicates that there is a strong hydrogen bond interaction between nitric acid and cationic halogen. The 2D fingerprint plots show that with the substitution atoms from Cl to Br, the interaction force between halogen and nitrate also increases from 13.3% to 16.3%. Interestingly, unlike 1 and 2, the organic cation in 3 has hydrogen bonds, resulting in an A–H⋯O interaction of 28.9%. The reason is that as Cl to I ions gradually increase, organic cations also increase, which leads to the expansion of the inorganic framework. At the same time, halogen ions form alkali metal–halogen coordination bonds with Rb⁺ ions. The increase of halogen ions enhances the coordination bond interaction force, and the molecular volume increases, which leads to the increase of the energy barrier for free rotation and phase transition temperature. This result further confirms the previous observations.^40,41

Dielectric measurements

The thermal-triggered phase transitions were also evidenced by the anomaly of the dielectric constant response during the phase transitions in temperature vs. the real part of the dielectric constant spectra of 1–3 (Fig. 4). The switchable properties of the dielectric constants of 1–3 were measured on powdered samples. Compound 1 has a large dielectric change of about 1.76 times at 259/238 K (heating/cooling). This result can be ascribed to the disordering (CH₃)₃ClN⁺ and anionic cage [Rb₄Eu₄(NO₃)₁₂]. They are dielectrically inactive at LTP, however, dielectrically active at HTP. This type of dynamic change has been extensively investigated in the hybrid double perovskite analogue [(CH₃)₄]₂[KFe(CN)₆].⁴² They display step-like dielectric changes at around 281/272 K for 2 and 330/320 K for 3 in the heating–cooling cycles. The corresponding ε′ changes are about 1.78 time for 2 and 1.72 time for 3, respectively. The results show that the phase transition temperatures of the compounds increase gradually with the change of halogen ions, which is consistent with the DSC results. The apparent dielectric switching behaviors mainly stem from the order–disorder transition of polar cations in the crystal lattices, resulting in the switchable property of the dielectric constant. The dipolar reorientation contributes greatly to the dielectric constant in molecular-based materials. These results can be well explained based on the theory of dielectrics.⁴³ In addition, through the previous analysis, we can determine that the increase of halogen ion radius increases the kinetic energy barrier of cation overturning and the heat energy required for coordination bond breaking, resulting in an enhanced interaction force between the organic cation and anionic skeleton, which is the reason for the gradual increase in phase transition temperature, indicating that the phase transition temperature of compounds can indeed be regulated by regulating halogen ions.

In addition, compounds 1–3 demonstrate promising potential for application in the realm of dielectric switching technologies. When characterized by a low dielectric constant, these materials correspond to an “off” state within a switching context, conversely, a high dielectric constant signifies an “on” state. Two potential switching mediators in this material enable a rapid transition from switching “off” to switching “on” as the temperature rises near its phase transition point. This thermally activated dielectric switching behavior renders the material particularly suitable for integration into contemporary intelligent sensor apparatuses and rapid-response feedback mechanisms. As shown in Fig. 4(d), when the dielectric switching material reaches its phase transition point, the circuit can be controlled to make the bulb light up, showing its practical application potential in electrical appliances. Therefore, the innovation of novel dielectric switching materials holds immense importance in advancing the field of technology.

Fluorescence properties

Finally, we would like to discuss the photoluminescence property in the title crystals. As shown in Fig. 5(a), when irradiated with ultraviolet light, the colorless bulk crystals give off a strong orange-red color. The emission and excitation spectra are illustrated. Meanwhile, the assignment of the transitions of those typical peaks is also included. In the emission spectrum, two remarkable emissions can be observed at 592 nm and 617 nm, revealing the magnetic dipole transition (⁵D₀ → ⁷F₁) and the electric dipole transition (⁵D₀ → ⁷F₂), respectively (Fig. 5(b) and Fig. S7, ESI†). In the excitation spectrum, the excitation bands are the same as those for the analogous 3-quinuclidinone (3HQ)-templated layered perovskites, such as (3HQ)₄[RbEu(NO₃)₈].²¹ The typical Eu³⁺ excitation peaks are observed, indicating that the Eu³⁺ ion still acts as the activator of the photoluminescence property. These results indicate that the compound 1–3 exhibits room temperature phosphorescence properties. In addition, Fig. 5(c) displays that the CIE color coordinates of 1 are [0.63, 0.37], which agrees with the characteristic of orange-red display under an ultraviolet lamp (Fig. S8, ESI†). Fluorescence lifetime (FLT) and fluorescence quantum yield (FQY) are also important parameters for evaluating fluorescence properties. Fig. 5(d) shows the fluorescence decay curve of 1 measured at the peak wavelengths of the emission and excitation spectra. The fluorescence lifetime of compounds 1–3 was fitted with a double exponential function:⁴⁴

where I, A, t and τ are photoluminescence intensity, contrast, decay time and decay contrast, respectively. Average life is calculated using the following formula:

The FLTs of 1, 2, and 3 were 4.344 ms, 4.598 ms, and 4.715 ms, respectively, indicating phosphorescent characteristics (Fig. S9, ESI†). The decay of 1 is divided into fast decay and slow decay processes, indicating that the photoluminescence comes from Eu³⁺ ions at the same lattice position, respectively. This is consistent with the fluorescence characteristics of rare earth materials. The FQYs of 1, 2 and 3 were 57.85%, 49.7% and 37.55%, respectively (Fig. S10, ESI†). In the inorganic 3D framework of compounds 1–3, excitation and emission come entirely from Eu³⁺. Increasing the radiative transition and reducing the non-radiative transition may be an effective way to increase the FQY. According to the previous Hirshfeld surface calculation, due to halogen substitution, the increase in the cation radius from Cl to I leads to the distortion of the inorganic framework and damages the symmetry of the structure. Therefore, the freedom of motion of the inorganic three-dimensional frame is also larger, the anionic frame is more disordered, the rigid structure is reduced, and the energy consumed by the non-radiative transition is inhibited, thus reducing the FQY.^45,46

Conclusions

In conclusion, we have designed and synthesized a series of RbEu bimetallic ammonium metal nitrate hybrid analogues, [(CH₃)₃NCH₂X]₂[RbEu(NO₃)₆] (X = Cl, 1; Br, 2; I, 3). They undergo reversible structural phase transitions between 200–430 K. The switchable dielectric constant benefits from the caged-like perovskite structure and the use of the spherical organic template cation. Meanwhile, the realization of photoluminescence properties is owing to the inclusion of the Eu³⁺ ion. Considering the rich photoluminescence luminescent properties of rare-earth ions, the presented molecular assembly strategy should lead to the preparation of a new class of smart materials for novel applications.

Author contributions

C. Shi conceived the project. H.-K. Li prepared the samples. H.-K. Li, W.-L. Ping and Z.-Z. Cui performed the dielectric, DSC and PXRD measurements. Q. Xu, L.-L. Zou and N. Wang measured the emission spectrum, FLT and FQY. H.-Y. Ye and L.-P. Miao contributed to single crystal measurement and analysis. H.-Y. Ye and C. Shi wrote the manuscript, with inputs from all other authors.

Data availability

All data included in this study are available upon request by contact with the corresponding author.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22175079 and 22275075) and the Natural Science Foundation of Jiangxi Province (no. 20225BCJ23006, 20224ACB204002 and 20204BCJ22015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2310567–2310569 and 2314668. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc01910e

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