High-temperature biferroicity in a chiral 3D hybrid rare-earth-based double perovskite

Abstract

Recently, switchable physical properties have been discovered in functional hybrid rare-earth perovskites, which have gained greater attention for potential applications. By consciously introducing a dynamic and spherical organic cation, RM3HQ (the R-N-methyl-3-hydroxylquinuclidinium cation), we synthesized a three-dimensional (3D) nitrate-bridged hybrid rare-earth perovskite, (RM3HQ)₄K₂[Nd(NO₃)₆]₂ (1), possessing a 4-connected LON framework with a 6⁶ topological network (Schläfli symbol). Thermal measurements show that 1 undergoes two reversible phase transitions, at around 370 K and 410 K, respectively. Various-temperature single-crystal structural analyses reveal that 1 crystallizes in the polar space group P2₁ at 293 K and 390 K, respectively, and in the chiral space group P6₃22 at 428 K. The phase transitions support dielectric transitions, making 1 a type of switchable dielectric material. More importantly, the symmetry breaking at 410 K shows a ferroelastoelectric transition with the Aizu notation of 622F2. This study constructs a high-performance hybrid rare-earth perovskite and provides highly promising candidate materials for next-generation multifunctional electronic devices.

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Introduction

Responsive functional materials with switchable physical properties are currently a research hotspot, as their tunable characteristics hold great promise for next-generation smart devices.^1–5 Among these materials, hybrid perovskites—combining the advantages of organic and inorganic components—have attracted tremendous interest owing to their superior optical, electronic, and magnetic properties, with their structural phase transition behavior being one of their most attractive features.^6–11 Specifically, hybrid perovskites with structural phase transitions—i.e., those exhibiting a switchable dielectric constant, ferroelectricity, and/or ferroelasticity—have emerged as a key research focus, given their potential applications in the smart energy transition and piezoelectric devices.^12–19 However, a critical bottleneck persists in this field: achieving stable multiferroic coexistence at elevated temperatures. This challenge is particularly significant, as high-temperature multiferroics provide broader operational ranges and exhibit enhanced stability under harsh service conditions, which are both essential for advancing their practical implementation.^20–27

In recent years, organic–inorganic hybrid perovskites have emerged as a promising platform for designing high-temperature multiferroic materials, owing to their tailorable chemical compositions and dynamic structural characteristics.^21,28–35 In these systems, the synergistic interplay between the order–disorder transitions of flexible organic cations and the structural distortion of inorganic frameworks can drive crystallographic symmetry breaking and further induce multifunctional ferroic order, including ferroelectricity and ferroelasticity. Notably, in hybrid double perovskite systems—an important subclass of this family—the rational design of organic cations, combined with the tunable ionic radii and diverse coordination preferences of metal ions, has enabled the successful construction of a series of multifunctional materials.^36–41 For instance, Hu et al. integrated ferroelasticity, multiaxial ferroelectricity, and a magnetic response into a single material via a chirality-induction strategy;⁴² Jia et al. reported a rare-earth-based double perovskite with both pronounced ferroelasticity and a high piezoelectric response, which paves the way for its application in energy-conversion devices;⁴³ and Miao et al. developed a 3D hybrid metal halide perovskite multiferroic material with superior ferroelectric, ferroelastic, piezoelectric, and charge transport properties, endowing it with great potential for ferro-photoelectric device applications.⁴⁴ These advances collectively demonstrate that the ordered arrangement of two distinct metal centers, coupled with organic–inorganic synergy, can effectively regulate and optimize the multifunctional performances of such systems. Nevertheless, most studies to date have focused on hybrid double perovskites with only a single ferroic order, and reports on 3D hybrid perovskites that achieve the stable coexistence of multiple types of ferroic order remain scarce.^4,45–49

In this work, we report a new 3D hybrid rare-earth double perovskite crystal, (RM3HQ)₄K₂[Nd(NO₃)₆]₂ (1, RM₃HQ = R-N-methyl-3-hydroxylquinuclidinium), which features a 4-connected LON framework with a 6⁶ topological network (Schläfli symbol). It undergoes two thermally induced phase transitions with a high-temperature ferroelectric transition at 410 K, exhibiting prominent ferroelectricity (P_s ≈ 2.7 μC cm⁻²) and reversible ferroelasticity, realizing stable ferroelectric-ferroelastic coexistence. Hirshfeld surface analysis reveals that the extensive O–H⋯O/N–H⋯O hydrogen-bond network and K–O coordination interactions synergistically afford structural stability and account for the high phase-transition temperature. This work not only presents a new example of high-temperature multiferroics in rare-earth hybrid materials but also demonstrates, through molecular design and crystal engineering, a viable strategy for tuning the multifunctional properties.

Experimental section

Synthesis

All chemicals are commercially available from Aladdin and used directly without further purification.

Synthesis of (RM₃HQ)I. (R)-3-Hydroxylquinuclidinium (0.508 g, 4 mmol) and CH₃I (0.710 g, 5 mmol) were added to 50 mL of ethyl acetate and stirred at room temperature. After reacting for 24 hours, a large amount of white powder was formed and filtered. Powdered (RM3HQ)I was obtained by drying at 333 K for 30 minutes; yield: 1.04 g, 85.1%, based on (R)-3-hydroxylquinuclidinium.

Synthesis of 1. To eliminate the interference of iodide ions in the reaction system, the synthesized (RM3HQ)I (1.012 g, 5 mmol) was first reacted with 1.84 mL of HNO₃ (68%, aq., 10 mmol) at room temperature, and iodide ions were oxidized to elemental iodine. Elemental iodine was removed by heating to 363 K until the solution was clear and transparent. A solution of (RM3HQ)NO₃ was obtained, which is named the A solution. Then, Nd(NO₃)₃·6H₂O (0.877 g, 2 mmol), KCl (0.149 g, 2 mmol), and 1.10 mL of HNO₃ (68%, aq., 6 mmol) were added to 25 mL of deionized water, which was stirred and dissolved to form the B solution. A mixture of A solution and B solution was stirred, and finally, 10 mL of ionized water was added to reduce the crystal precipitation rate and improve the crystal quality. The mixture was filtered and placed on a heating stage at 313 K to remove volatile solvent, and light-purple single crystals were formed over about 3 days and then collected and dried to obtain (RM3HQ)₄K₂[Nd(NO₃)₆]₂ with a yield of 71.8% based on Nd(NO₃)₃·6H₂O.

Element analysis for 1. Anal. (%) calc.: C, 22.88; H, 3.84; N, 13.34. Found (%): C, 22.84; H, 3.86; N, 13.38 (Table S1).

Physical property characterization

A single crystal was crushed into powder for DSC measurements. The P–E loop measurements were performed on a single crystal with an area of ∼30 mm² and a thickness of ∼1 mm along the polar axis in the direction of the monoclinic phase [010]. For more information on performance measurements and single-crystal structure determination, see the SI.

Results and discussion

Phase transition properties

The phase purity of 1 was confirmed based on the good consistency between its experimental and simulated PXRD patterns (Fig. S1). TGA analysis indicated that 1 exhibits thermal stability up to 536 K (Fig. S2). Differential scanning calorimetry (DSC) measurements revealed two pairs of reversible endothermic/exothermic peaks during heating–cooling cycles (Fig. 1 and Fig. S3), with phase-transition temperatures of 370/360 K and 410/400 K, respectively, corresponding to thermal hysteresis of 10 K. Entropy changes (ΔS) were calculated from the average enthalpy changes (ΔH) obtained over the whole phase-transition process. Upon heating, ΔS₁ ≈ 30.6 J mol⁻¹ K⁻¹ at 370 K (N₁ ≈ 6.3, ΔS = R·ln N) and ΔS₂ ≈ 9.25 J mol⁻¹ K⁻¹ at 410 K (N₂ ≈ 1.7, see Note S1). Both N values are much greater than 1, indicating the structural transition of 1 from an ordered to a disordered state with a higher degree of freedom during phase transitions. This DSC-derived phase-transition behavior is consistent with that of [TMAEA]Pb₂Cl₆ reported by Zhang et al.,⁵⁰ suggesting that 1 undergoes an isostructural phase transition followed by a ferroelectric phase transition.

Fig. 1 DSC curves from a heating–cooling cycle of 1.

Crystal structure analysis

Single-crystal X-ray diffraction experiments were performed at different temperatures to elucidate the phase-transition mechanism of 1. In the low-temperature phase (LTP), 1 crystallizes in the chiral space group P2₁, with detailed unit-cell parameters provided in Table S2. The asymmetric unit of the LTP comprises an anionic {K₂[Nd(NO₃)₆]₂}⁴⁻ framework accompanied by ordered RM3HQ cations (Fig. 2a). This anionic assembly is constructed via two distinct coordination modes: (i) each Nd³⁺ center is chelated by six nitrate ligands to form a distorted Nd(NO₃)₆ octahedron; and (ii) each K⁺ acts as a dual-coordination linker, bridging two oxygen atoms of the RM3HQ cation and donor oxygen atoms from nitrate groups of adjacent [Nd(NO₃)₆] units. Notably, K⁺ displays two bridging modes toward nitrate oxygen atoms: μ₂-κ³(O, O′, O″) and μ₂-κ²(O, O′). These intricate linkage patterns collectively construct a 3D framework (Fig. 2b). The packing diagram further reveals that the metal nodes are interconnected through a combination of boat-shaped and chair-shaped hexahedral units, forming a highly interconnected topological network that underpins the thermal stability and phase-transition behavior exhibited by 1 (Fig. 2c).

Fig. 2 (a) The minimum asymmetric unit of 1. (b) The three-dimensional metal framework diagram of 1. For clarity, ligands were omitted while metal atoms were retained. (c) The topological diagram of 1. For clarity, ligands were omitted while metal atoms were retained. (d) The stacking diagram of 1 and the structural changes of RM3HQ and Nd(NO₃)₆ between the LTP, ITP, and HTP.

Unlike conventional 3D bimetallic halide perovskites, 1 features a hybrid of face- and corner-sharing linkages between K⁺/Nd³⁺-centered polyhedra. Specifically, each [Nd(NO₃)₆] octahedron connects with an adjacent KO₈ dodecahedron via face sharing to form a bimetallic dimer, and these dimers are further linked to adjacent dimers through corner sharing (Fig. 2d). Topological analysis via the Topos program reveals that the K⁺/Nd³⁺ 3D framework possesses a 4-connected LON topology with the vertex symbol 6⁶. At 390 K, 1 retains the monoclinic space group P2₁, confirming the occurrence of an isostructural phase transition. The primary structural change is manifested in the progressive disordering of nitrate ligands: two of the six NO₃⁻ groups in each [Nd(NO₃)₆] unit adopt a two-fold disordered configuration, while the thermal vibrations of guest cations are intensified. Despite these dynamic perturbations, the local coordination geometry remains well-defined: the Nd–O bond lengths range from 2.54 to 2.61 Å, K–O bond distances range from 2.84 to 3.16 Å, and O–K–O bond angles range from 27.3° to 159.8°. These structural parameters confirm that the inorganic {K₂[Nd(NO₃)₆]₂}⁴⁻ framework preserves its structural integrity upon heating, thus providing a stable scaffold for the order–disorder transitions of nitrate and organic moieties (Fig. 2d).

Upon heating to 428 K, 1 transforms into a high-temperature phase (HTP), crystallizing in the hexagonal space group P6₃22. This transition corresponds to a ferroelectric phase change, which is primarily characterized by a pronounced increase in dynamic disorder: all nitrate ligands in the [Nd(NO₃)₆] units adopt a twofold disordered configuration, while the guest cations exhibit threefold disorder. Despite the substantially enhanced motional freedom of these moieties at elevated temperatures, the inorganic {K₂[Nd(NO₃)₆]₂}⁴⁻ framework retains structural integrity (Fig. 2d). Variable-temperature PXRD measurements were further performed to elucidate the phase transition behavior of 1 (Fig. S4). A comparison of the diffraction patterns between the LTP and ITP shows no diffraction peak disappearance, indicating the absence of symmetry breaking and thus confirming an isostructural phase transition at this stage. Upon further heating to the HTP, a notable reduction in the number of resolvable diffraction peaks is observed—this phenomenon is consistent with the structural characteristics of a higher-symmetry space group and aligns with the aforementioned single-crystal structural analysis and physical property measurements.

Hirshfeld surface analysis and two-dimensional fingerprint plots were employed to elucidate the intermolecular interaction origins of the structural stability of 1 (Fig. 3). The results reveal that O–H⋯O/N–H⋯O hydrogen bonds dominate the crystal interactions (≈65.8%), followed by H⋯H van der Waals interactions (≈25.5%). Sharp peaks and high-density regions concentrated at low d_e and d_i values in the fingerprint plots (Fig. 3b–d) confirm the strong directionality and high density of the hydrogen-bond network. This network is primarily formed from multidirectional linkages between hydroxyl/amino hydrogen atoms of the organic RM3HQ cations and nitrate oxygen atoms in the inorganic framework; it interpenetrates with the K⁺/Nd³⁺ coordination network to construct a robust 3D interaction network. Upon heating, the hydrogen-bond network exhibits dynamic behavior without complete dissociation, introducing an additional cooperative energy barrier that suppresses inorganic framework distortion and organic cation disordering, thus elevating the phase-transition temperature above 410 K.

Fig. 3 Analysis of the Hirshfeld surface of 1. (a) A schematic plot of the surface of 1. (b–d) The fingerprint plots for 1, displayed against the background of all intermolecular contacts, highlighting the H⋯H, H–O and H–N hydrogen-bonding interactions, respectively.

Ferroelectricity and ferroelasticity

Variable-temperature second-harmonic generation (VT-SHG) measurements were conducted to probe symmetry evolution during phase transitions. In its low-temperature phase, 1 exhibits a SHG response with an intensity approximately 0.25 times that of the KDP reference (Fig. S5). The SHG intensity slightly attenuates near T₁ (≈370 K), drops significantly near T₂ (≈410 K), and eventually falls to a negligible level (Fig. 4a). This trend is consistent with the crystal structure phase transition pathway (Fig. 1), further confirming the reversibility of the phase transition process. SHG switching cycling test results were further obtained for 1 (Fig. S6), which demonstrate that the SHG signal exhibits good reversibility and stability over multiple cycles. To clarify the dielectric response characteristics associated with the phase transitions, we measured the dielectric constant of 1 at multiple frequencies over the temperature range of 310–440 K (Fig. 4b and Fig. S7). Consistent with the symmetry evolution and phase transition behavior revealed by VT-SHG measurements, the real part of the dielectric constant (ε′) exhibits reversible changes during both heating and cooling at 1 MHz, with two distinct dielectric anomalies observed near the corresponding phase transition temperatures (T₁ ≈ 370 K and T₂ ≈ 410 K, Fig. 4b). Thermal hysteresis of ∼10 K is observed between the cooling and heating curves, which is consistent with the DSC results and further confirms the structural reversibility of the phase transitions (Fig. 1).

Fig. 4 (a) The temperature dependence of the SHG intensity of 1 during a heating–cooling cycle. (b) The temperature dependence of the real part of the dielectric constant of 1 at 1 MHz during a heating–cooling cycle. (c) P–E hysteresis loops of 1 measured along the b-axis at 293 K and 392 K. (d) A comparison of the phase transition temperature of 1 with those of representative organic, inorganic, hybrid, and rare-earth hybrid double perovskite ferroelectric materials.

Polarization–electric field (P–E) hysteresis loop measurements were performed on the (010) crystallographic face of a single crystal of 1 to characterize its ferroelectric switching behavior (Fig. 4c). At 293 K, the material displays a typical ferroelectric hysteresis loop with saturation polarization (P_s) ≈ 2.5 μC cm⁻² and a coercive field (E_c) of 95 kV cm⁻¹. Upon heating to 392 K, P_s increases to ∼2.7 μC cm⁻², while E_c decreases significantly to 75 kV cm⁻¹. The well-defined open P–E loop confirms the intrinsic ferroelectric nature of 1. The temperature dependence of P_s and E_c originates from thermally activated domain dynamics and a reduced energy barrier for polarization switching, and such temperature-sensitive ferroelectric responses endow 1 with great potential for application in thermally tunable ferroelectric devices. Among newly developed hybrid rare-earth double perovskites, 1 exhibits a notably high Curie temperature (T_c). This represents a high T_c value within this specific system and falls in the moderate T_c range based on reports of ferroelectrics across diverse structural systems (Fig. 4d and Table S3 ).^{6,10,15,40,42–44,50–62}

Variable-temperature single-crystal X-ray diffraction confirms the symmetry evolution from P2₁ to P6₃22 during the ITP-to-HTP transition, a structural characteristic closely associated with ferroelasticity. Complementary to this structural evidence, in situ polarized light microscopy was employed to directly visualize the ferroelastic domain evolution. Characteristic striped ferroelastic domains emerge progressively on the crystal surface upon cooling from 420 K to 380 K, marking the onset of the ferroelastic phase (Fig. 5). These domains vanish completely upon reheating to 420 K, with the crystal recovering its higher-symmetry state. Reversible domain formation and disappearance during repeated thermal cycling provide direct microscopic evidence for the ferroelastic phase transition, and these observations further confirm the coexistence of ferroelectricity and ferroelasticity in 1, establishing it as a promising candidate for advanced multiferroic device applications.

Fig. 5 The reversible evolution of ferroelastic domains under polarized light microscopy during thermal cycling. (a–c) Domain formation during initial cooling. (c and d) Domain disappearance upon heating. (d–f) Domain reappearance during subsequent cooling.

Semiconductor properties

Compound 1 was further characterized by UV-vis absorption spectroscopy, and first-principles calculations based on density functional theory (DFT) were performed to determine its band structure characteristics. The UV-vis diffuse reflectance spectrum shows that the highest absorption peak of the compound is located at 576 nm. The optical band gap was determined using the modified Tauc equation, which is expressed as follows:

[hvF(R_∞)]^1/2 = A(hv − E_g)

where h is Planck's constant, ν is the photon frequency, F(R_∞) is the Kubelka–Munk function, and A is the proportionality constant. Using this method, the optical band gap (E_g) of 1 was determined to be 3.73 eV (Fig. 6a). Meanwhile, theoretical calculations were carried out to further verify the band gap characteristics. The results indicate that the compound exhibits indirect band gap semiconductor behavior, with a theoretical band gap value of 3.48 eV (Fig. 6b). The experimental and theoretical band gap values are in good agreement, confirming the accuracy of the structural model derived from single-crystal analysis.

Fig. 6 (a) The solid-state UV-vis absorption spectrum. The inset shows the fitting of the absorption spectrum for band gap determination. (b) The calculated band structure of 1.

Conclusions

In this work, we successfully synthesized a rare-earth/alkali–metal hybrid double perovskite featuring an unusual three-dimensional topological structure, which undergoes two consecutive phase transitions and exhibits the coexistence of ferroelectricity and ferroelasticity. This study not only verifies that rational molecular design and structural modulation can achieve high-temperature multiferroicity in rare-earth hybrid systems but also reveals an order–disorder synergistic transition mechanism that provides an important design strategy and material platform for the future development of novel smart materials with multiple responsive functionalities.

Author contributions

Li-Ping Wang and Chang-Chun Fan: measurements, formal analysis, investigation, writing-original draft; Zheng-Hui Hu and Hong-Fei Zhao: single-crystal analysis, structural depiction, ferroelastic domain measurements and hysteresis loop measurements; Chang-Chun Fan and Chao Shi: funding acquisition; Jian-Rong Li and Chao Shi: conceived the idea of the work, formal analysis, investigation, visualization and writing-review & editing draft.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

All data included in this study are available upon request by contacting the corresponding author. Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6qi00400h.

CCDC 2445289–2445291 contain the supplementary crystallographic data for this paper.^63a–c

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22175079), the Natural Science Foundation of Jiangxi Province (No. 20225BCJ23006 and 20224ACB204002), the Talent Introduction Project of Jinling Institute of Technology (JIT-B-202414) and the Open Project Program of Jiangxi Province Key Laboratory of Functional Crystalline Materials Chemistry, Jiangxi University of Science and Technology (2024SSY05161).

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Footnote

† These authors contributed equally to this work.

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