An abnormal decrease in Curie temperature in pseudohalide rare-earth double perovskite ferroelastics driven by lanthanide contraction

Abstract

Hybrid rare-earth double perovskites (HREDPs) have received increasing attention due to their structural tunability and multifunctionality. However, the impact of lanthanide contraction on the properties of HREDPs remains to be investigated. Herein, we synthesized a series of isostructural HREDP ferroelastics, (DMSOX)₂LnCs(NO₃)₆ (DMSOX = dimethylsulfoximine and Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺). The results demonstrate that decreasing the rare-earth ionic radius modulates the Curie temperature (T_C), exhibiting a monotonic decrease from 411.1 to 395.8 K across the lanthanide series. Structural analysis reveals that the shrinkage of the ionic radius weakens hydrogen bonding interactions, which is the primary origin of the lowered T_C. Among them, Eu-based (DMSOX)₂EuCs(NO₃)₆ possesses an orange-red luminescence behavior with a lifetime of 4.87 ms and a quantum yield of 44%. This work provides a distinct strategy for fine-tuning multifunctional HREDP materials.

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Introduction

Perovskite materials have garnered significant research interest due to their exceptional physical properties, including ferroelectricity, piezoelectricity, pyroelectricity, second harmonic generation (SHG), and superconductivity.^1–12 Consequently, perovskite materials have recently experienced rapid development, giving rise to diverse families, such as lead-based halide perovskites, lead-free halide double perovskites, metal-free perovskites, and formate perovskites.^13–26 Among them, as an emerging perovskite family, hybrid rare-earth double perovskites (HREDPs) have demonstrated extraordinary potential, owing to the unique 4f electron configurations and variable coordination geometries of rare-earth cations.^27–33 In 2020, Shi et al. reported the first two-dimensional (2D) HREDP, (R/S-3HQ)₄KCe(NO₃)₈ (R/S-3HQ = R/S-hydroxylquinuclidinium), which exhibits room-temperature ferroelectricity and photoluminescence.³⁰ Subsequently, 3D HREDPs were demonstrated, which possess relaxor ferroelasticity and large piezoelectric responses.²⁹ Using molecular engineering, Wang et al. designed an HREDP ferroelectric with out-of-plane polarization and circularly polarized luminescence (CPL).³⁴ Our group designed 3D rare-earth double perovskite ferroelectrics with the highest Curie temperature through cation regulation and perovskite framework substitution and explored their application in the field of X-ray detection.³¹

Lanthanide contraction, featuring the progressive decrease in ionic radii across the lanthanide series, has served as a powerful tool for precisely tailoring the physicochemical properties of functional materials.^9,35–40 For instance, Escudero-Escribano et al. utilized lanthanide contraction in Pt–lanthanide alloys to tune the surface compressive strain, thereby optimizing the oxygen reduction activity.⁴¹ Li et al. demonstrated that substituting smaller lanthanide ions can effectively suppress the disorder of organic cations in polyiodides, optimizing the SHG responses.⁴² Despite these advances, the specific effects of lanthanide contraction on the properties of HREDPs have not been fully explored.

In this work, we investigate the influence of lanthanide contraction on the Curie temperature (T_C) of a newly synthesized series of HREDP ferroelastics, (DMSOX)₂LnCs(NO₃)₆ (DMSOX = dimethylsulfoximine and Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺). Our results demonstrate that the T_C can be modulated by the ionic size of rare-earth elements, exhibiting a decreasing trend as the ionic radius decreases (Scheme 1). Structural analysis reveals that as the ionic radius of the rare-earth ion decreases, the N–H⋯O hydrogen bond angles between organic cations and inorganic frameworks are progressively reduced, indicating a weakening of hydrogen bonding interactions that leads to a lowering of the phase transition temperature. Furthermore, the Eu-based HREDP (DMSOX)₂EuCs(NO₃)₆ exhibits typical orange-red luminescence behavior, with a lifetime of 4.87 ms and a quantum yield of 44%.

Scheme 1 Lanthanide contraction induces a decrease in T_C in (DMSOX)₂LnCs(NO₃)₆.

Results and discussion

Thermal and dielectric properties

(DMSOX)₂LnCs(NO₃)₆ single crystals (DMSOX = dimethylsulfoximine and Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺) were grown via slow evaporation of aqueous solutions containing (DMSOX)NO₃, CsNO₃, and Ln(NO₃)₃·6H₂O in the molar ratio of 2 : 1 : 1 (Fig. S1). Powder X-ray diffraction (PXRD) measurements confirmed their phase purity (Fig. S2). Thermogravimetric analysis (TGA) measurements indicate that the decomposition temperatures of (DMSOX)₂LnCs(NO₃)₆ are 480 K (La³⁺), 478 K (Ce³⁺), 476 K (Pr³⁺), 471 K (Nd³⁺), 462 K (Sm³⁺), and 453 K (Eu³⁺) (Fig. 1a), exhibiting a continuous downward trend.⁴³ To investigate the phase transition behavior of (DMSOX)₂LnCs(NO₃)₆, we carried out differential scanning calorimetry (DSC) tests. As shown in Fig. 1b, the endothermic peaks of (DMSOX)₂LnCs(NO₃)₆ in the heating run were observed at 411.1 K (La³⁺), 408.6 K (Ce³⁺), 405.6 K (Pr³⁺), 405.5 K (Nd³⁺), 397.7 K (Sm³⁺), and 395.8 K (Eu³⁺), while the corresponding exothermic peaks appeared in the cooling run, confirming the reversible phase transition (Fig. S3). The thermal hysteresis ranges from 6.7 to 11.2 K, and the enthalpy change (ΔH) ranges from 2.640 to 3.062 kJ mol⁻¹ (Table S1), which are characteristics indicative of a first-order phase transition. Notably, the phase transition temperature of the (DMSOX)₂LnCs(NO₃)₆ decreases as the rare-earth ion radius decreases, exhibiting a trend governed by lanthanide contraction (Fig. 1b). Furthermore, the dielectric constant measurement as a function of temperature corroborated this phenomenon. As shown in Fig. 1c, the real part of the dielectric constant (ε′) increases continuously with temperature and presents an anomaly at the phase transition temperature; the corresponding cooling curves also display similar dielectric anomaly behavior (Fig. S4), in agreement with the results of DSC measurements.^44,45

Fig. 1 (a) TGA curves of (DMSOX)₂LnCs(NO₃)₆. (b) DSC curve of (DMSOX)₂LnCs(NO₃)₆ in the heating run. (c) Temperature-dependent ε′ of (DMSOX)₂LnCs(NO₃)₆ at 1 MHz.

Crystal structure analysis

To elucidate the phase transition mechanism, crystal structures of (DMSOX)₂LnCs(NO₃)₆ (Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺) were determined via single-crystal X-ray diffraction. For convenience, the phases before and after the transition are referred to as the ferroelastic phase (FP) and paraelastic phase (PP), respectively. Due to their similar crystal structures and phase transition mechanisms, we selected (DMSOX)₂EuCs(NO₃)₆ as a representative example for detailed discussion. The structural data for the remaining compounds are shown in the SI (Fig. S5–S9 and Tables S2–S6). As illustrated in Fig. 2a, in the FP, (DMSOX)₂EuCs(NO₃)₆ crystallizes in the triclinic space group P1̄ with the unit cell parameters: a = 9.120(6) Å, b = 9.139(6) Å, c = 9.147(6) Å, γ = 63.85(2)°, and V = 614.2(7) ų (Table S7). (DMSOX)₂EuCs(NO₃)₆ features a typical three-dimensional (3D) double perovskite framework characterized by a corner-sharing network of Eu(NO₃)₆ and Cs(NO₃)₆ octahedra. Ordered DMSOX organic cations occupy the interstitial cavities, and their oxygen atoms form strong coordination bonds with Cs⁺ cations (Fig. 2b).⁴⁶ In the PP, (DMSOX)₂EuCs(NO₃)₆ transforms into the higher-symmetry space group R1̄, with the unit cell parameters: a = b = 10.127 Å, c = 21.379 Å, γ = 120°, and V = 1898.8 ų (Fig. 2d, e and Table S7). Notably, the DMSOX organic cations exhibit a three-fold substitution disorder around the C₃ rotation axis (Fig. 2c). Based on the above analysis, it is suggested that the phase transition is driven by an order–disorder transition of the organic cations.

Fig. 2 Packing diagrams of the single crystal structure of (DMSOX)₂EuCs(NO₃)₆ in the FP (a) and PP (d). For clarity, the NO₃⁻ ions were regarded as a ball. The 3D perovskite cage of (DMSOX)₂EuCs(NO₃)₆ in the FP (b) and PP (e). (c) Triple substitution disordered DMSOX cation. (f) The relationship of the crystal lattice of (DMSOX)₂EuCs(NO₃)₆ between the FP and PP.

Generally, due to the confinement effect, reducing the void volume increases the phase transition energy barrier, resulting in a higher phase transition temperature.^47–52 However, in the (DMSOX)₂LnCs(NO₃)₆ series, an anomalous decrease in the T_C is observed with the lanthanide contraction (Tables S1 and S8).⁵³ To uncover the microscopic mechanism underlying this anomalous phenomenon, we systematically investigated the interactions between the organic cations and the inorganic framework. As shown in Fig. 3a, as the rare-earth ion varies from La³⁺ to Eu³⁺, the volume of Ln(NO₃)₆ octahedra gradually decreases, which in turn reduces the volume of the lattice cavities enclosing the organic cations. This compression of the cavity volume further strains and distorts the hydrogen bonds formed between the organic cations and NO₃⁻ (Fig. 3b). Specifically, in (DMSOX)₂LaCs(NO₃)₆, the DMSOX organic cations can freely adjust to form nearly straight and strong N–H⋯O hydrogen bonds with the surrounding NO₃⁻, with a bond angle of approximately 161.3°. In sharp contrast, in (DMSOX)₂EuCs(NO₃)₆, the reduction of cavity volume forced the N–H⋯O hydrogen bonds to become distorted, with the bond angle decreasing to 158.8° (Fig. 3c). Accordingly, with the decrease in the rare-earth ionic radius, the N–H⋯O hydrogen bond angle gradually decreases, indicating that the hydrogen bonding interactions between the organic cations and the inorganic framework gradually weaken.^54,55 Such weakened interactions reduce the phase transition energy barrier, which ultimately gives rise to the anomalous decrease in the phase transition temperature. Furthermore, we analyzed the origin of the gradual decrease in T_C from a thermodynamic perspective. As the rare-earth ionic radius decreases, ΔH and ΔS exhibit a systematic decreasing trend, consistent with the weakening of hydrogen bonding interactions induced by lanthanide contraction. The systematic evolution of these thermodynamic parameters provides a thermodynamic basis for the gradual decrease in T_C.

Fig. 3 (a) The relationship between the volume of Ln(NO₃)₆ octahedra and the radius of the Ln³⁺ ion. (b) The T_C and N–H⋯O hydrogen bond angle as a function of the radius of the Ln³⁺ ion. (c) N–H⋯O hydrogen bond angle in (DMSOX)₂LaCs(NO₃)₆ and (DMSOX)₂EuCs(NO₃)₆.

Evolution of ferroelastic domains

The symmetry change of (DMSOX)₂LnCs(NO₃)₆ corresponds to a ferroelastic phase transition following the Aizu notation 3̄F1̄. The temperature-dependent evolution of ferroelastic domains constitutes one of the most direct pieces of evidence for ferroelastic materials. Ferroelastic domains with different orientations exhibit distinct birefringence properties, resulting in alternating bright and dark striped patterns.⁵⁶ Accordingly, we observed the evolution of ferroelastic domains via polarized light microscopy and selected (DMSOX)₂EuCs(NO₃)₆ as a representative example for detailed illustration. As shown in Fig. 4b and c, when (DMSOX)₂EuCs(NO₃)₆ crystals transform from FP to PP, the striped ferroelastic domains disappear. As (DMSOX)₂EuCs(NO₃)₆ crystals revert to FP, the ferroelastic domains reappear (Fig. 4d). Subsequently, during multiple phase transition cycles, the domain structures exhibited consistent changes (Fig. 4e and f), verifying the reversibility of the ferroelastic phase transition (Video S6). No domain structure was detected under natural light (Fig. 4a), proving that the striped patterns are the result of ferroelastic domains rather than the crystal morphology.⁵⁷ In addition, the evolution behavior of the ferroelastic domains in (DMSOX)₂EuCs(NO₃)₆ under applied stress was investigated. As shown in Fig. S10, after applying stress to the thin film, clear domain wall motion and reorientation were observed. These experimental results confirm the ferroelasticity of (DMSOX)₂EuCs(NO₃)₆, demonstrating that the evolution of its ferroelastic domains can be controlled by stress. The evolution of ferroelastic domains of other HREDP ferroelastics is included in the SI (Fig. S11–S15 and Videos S1–S5).

Fig. 4 (a) Topography of the (DMSOX)₂EuCs(NO₃)₆ crystal in the FP. (b–f) Evolution of the ferroelastic domain of (DMSOX)₂EuCs(NO₃)₆ between the FP and PP.

Photoluminescence properties

The incorporation of rare-earth lanthanide ions into hybrid double perovskite frameworks facilitates the achievement of a broad range of fluorescence emission colors.⁵⁸ As illustrated in Fig. 5b, under 365 nm ultraviolet (UV) irradiation, (DMSOX)₂EuCs(NO₃)₆ crystals (Fig. 5a) exhibit the characteristic orange-red luminescence behavior of Eu³⁺. Fig. 5d presents the excitation (blue curve) and emission (red curve) spectra of (DMSOX)₂EuCs(NO₃)₆. The excitation spectrum features a dominant peak at 396 nm, which matches the absorption spectrum (Fig. 5c) and corresponds to the characteristic ⁵L₆ ← ⁷F₀ excitation transition of Eu³⁺. The emission spectrum shows two intense emission peaks: the 592 nm peak arises from the ⁵D₀ → ⁷F₁ magnetic dipole transition and the 617 nm peak is attributed to the ⁵D₀ → ⁷F₂ electric dipole transition.^31,59 In addition, as shown in Fig. S16, the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (DMSOX)₂EuCs(NO₃)₆ were (x = 0.6267, y = 0.3732), which is located in the deep orange-red region, consistent with the characteristic red emission originating from the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ ions.⁶⁰ Furthermore, the fluorescence lifetime of (DMSOX)₂EuCs(NO₃)₆ is 4.87 ms (Fig. S17a), and the photoluminescence quantum yield (QY) reaches 44% (Fig. S17b).

Fig. 5 Optical properties of (DMSOX)₂EuCs(NO₃)₆. Photographs of (DMSOX)₂EuCs(NO₃)₆ crystals under natural light (a) and 365 nm ultraviolet (UV) irradiation (b). (c) UV-vis absorption spectra of (DMSOX)₂EuCs(NO₃)₆. (d) Emission and excitation spectra of (DMSOX)₂EuCs(NO₃)₆.

Conclusions

In summary, we synthesized a series of isostructural hybrid rare-earth double perovskite (HREDP) ferroelastics (DMSOX)₂LnCs(NO₃)₆ (DMSOX = dimethylsulfoximine and Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Eu³⁺) and systematically investigated the impact of lanthanide contraction on their Curie temperature (T_C). Our work demonstrates that continuous shrinkage of the rare-earth ionic radius can modulate the T_C, leading to a monotonic decrease from 411.1 to 395.8 K. The reduction of T_C is attributed to the weakening of hydrogen bonding interactions induced by lanthanide contraction. Our findings offer a precisely controllable strategy for the design and fine-tuning of advanced multifunctional HREDPs.

Author contributions

M.-J. Lin was responsible for the experimental design, execution, and manuscript writing, with a primary focus on the collection and analysis of dielectric and DSC data and the investigation of mechanisms. J.-Y. Li assisted in the observation of ferroelastic domains. P.-G. Liu was responsible for the determination of crystal structures. Y.-T. Zou was responsible for fluorescence data testing. H.-F. Ni and G. Teri assisted in data analysis. L. Pan was responsible for thermogravimetric data measurement. C.-F. Wang, D.-W. Fu and Y. Zhang supervised and guided this work. All authors collectively discussed the results and participated in the revision of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: synthesis methods, materials and methods, Fig. S1–S17, Tables S1–S14, and Videos S1–S6. See DOI: https://doi.org/10.1039/d6qi00521g.

CCDC 2537242 ((DMSOX)₂EuCs(NO₃)₆_423 K), 2537243 ((DMSOX)₂LaCs(NO₃)₆_423 K), 2537244 ((DMSOX)₂NdCs(NO₃)₆_423 K), 2537245 ((DMSOX)₂SmCs(NO₃)₆_423 K), 2537246 ((DMSOX)₂PrCs(NO₃)₆_423 K), 2537247 ((DMSOX)₂CeCs(NO₃)₆_423 K), 2537248 ((DMSOX)₂SmCs(NO₃)₆_300 K), 2537249 ((DMSOX)₂NdCs(NO₃)₆_300 K), 2537250 ((DMSOX)₂PrCs(NO₃)₆_300 K), 2537251 ((DMSOX)₂PrCs(NO₃)₆_300 K), 2537252 ((DMSOX)₂CeCs(NO₃)₆_300 K) and 2537253 ((DMSOX)₂EuCs(NO₃)₆_300 K) contain the supplementary crystallographic data for this paper.^61a–l

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (LQN25B010004 and LZ24B010001), the National Natural Science Foundation of China (22505227, 22371258, and 22375182), and the Science and Technology Plan Project of Jinhua (2026-1-010).

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61. (a) CCDC 2537242: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56jp; (b) CCDC 2537243: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56kq; (c) CCDC 2537244: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56lr; (d) CCDC 2537245: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56ms; (e) CCDC 2537246: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56nt; (f) CCDC 2537247: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56pv; (g) CCDC 2537248: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56qw; (h) CCDC 2537249: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56rx; (i) CCDC 2537250: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56sy; (j) CCDC 2537251: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56tz; (k) CCDC 2537252: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56v0; (l) CCDC 2537253: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2r56w1.

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