English

• Hybrid organic-inorganic perovskites (HOIPs) are renowned for their superior material performance and functional versatility [1-6]. Rich chemical diversity and structural tunability of HOIPs endow them with various properties such as luminescence, magnetism, ferroelectricity, and photoelectric properties [7-13]. Structurally, the directional alignment of internal components would cause polar stacking of HOIP structures to generate spontaneous polarization, whose directions can be further possibly switched by applying an electric field [14,15]. The resulting ferroelectricity has been utilized for wide applications in memory, sensing, optoelectronics and micro transducers [16-19]. It should be emphasized that, ferroelectricity exists only in certain polar phases below Curie temperature (T_C), which directly determines the working temperature range and performance stability of material [20]. Over the past decade, committed to effectively improving T_C of HOIP ferroelectrics has long been a continuous pursuit, as the relatively low T_C poses great challenges for their practical uses [21,22]. With the collaborative development of intelligent devices, another academic hot topic is the versatility of integrating properties in multi-physical channels of HOIP ferroelectrics for expanding the utilizing freedom and novel exotic effects [23-27].

  The structural characteristics of HOIPs that combines the advantages of organic and inorganic components implies promising solutions to address the aforementioned challenges [28]. Based on the order-disorder phase transition mechanism, great efforts have been made to develop chemical modification strategies (e.g., H/F substitution) for A-site organic cations of HOIP structures to enhance T_C, as demonstrated in previous several works [22,29,30]. Besides A-site organic cations, the inorganic anions composed of BX₆ octahedrons also play a crucial role in optimizing physical and chemical performance [31,32]. For example, the halogen substitution or doping strategy at the X-position has been proven to be an effective means to regulate material properties [21,33,34]. Notably, one of the greatest charms of regulating B-site metal ions lies in the ability to conveniently customize the additional physical functions [35-37]. However, the importance of B-site metal ions in HOIP ferroelectrics has been usually overlooked, partly because the changes of metal ions may cause structural collapse to loss crystal polarity. The massive interactions between organic and inorganic cations not only provide enormous opportunities for performance regulation, but also bring complexity and unpredictability [1,28]. In this stage, systematically investigating the B-site regulation strategy of HOIP ferroelectrics has become a very interesting and challenging task.

  In this work, we have successfully achieved significant T_C enhancement and versatility of HOIP ferroelectrics through the B-site regulation strategy (Scheme 1). By replacing Cd²⁺ of parent (3-pyrrolinium)CdBr₃ with Mn²⁺ with smaller ionic radii, we obtained the HOIP ferroelectric (3-pyrrolinium)MnBr₃ with a considerable saturation polarization (P_s) of 6.06 µC/cm² and a T_C of 303 K [38]. The significant improvement in ferroelectricity demonstrates the feasibility of B-site regulation of HOIP structure, which is closely related to the stronger intermolecular interactions and the reduced void occupancy in lattice caused by the reduction of ion radius. Further, by selecting Ni²⁺ with a smaller ion radius as the B-site ion, the resulting (3-pyrrolinium)NiBr₃ possesses a higher T_C up to 346 K, realizing a jump enhance of 99 K compared to parent (3-pyrrolinium)CdBr₃. More strikingly, the introduction of Mn²⁺ and Ni²⁺ ions brings fascinating photoluminescence and magnetism respectively, which will promote the multifunctional integration research of HOIP ferroelectrics towards novel application possibilities.

  Scheme 1

  

  Scheme 1.  The diagram of design strategy of (3-pyrrolinium)BBr₃ (B = Mn and Ni).

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  The prism-liked crystals of (3-pyrrolinium)BBr₃ (B = Mn and Ni) were easily obtained by slow evaporation of clear aqueous solution of hydrobromic acid containing equal molar amounts of (3-pyrrolinium)Cl and BBr₂ (B = Mn and Ni) at 343 K respectively. X-ray single crystal diffraction technology determine that both of (3-pyrrolinium)BBr₃ (B = Mn and Ni) crystallized in orthorhombic Cmc2₁ space group that belongs to the polar mm2 point group, with similar cell parameters listed in Tables S1 and S2 (Supporting information). They adopt a typical ABX₃ perovskite structure where each B-site Mn²⁺ or Ni²⁺ cation coordinates with six Br⁻ anions to form [BBr₆] (B = Mn and Ni) octahedrons, and A-site organic 3-pyrrolinium cations locate in the gaps surrounded by octahedrons-formed one-dimensional inorganic chains (Fig. S1 in Supporting information). From packing view of structure, a notable feature is that all 3-pyrrolinium cations are aligned in an orientationally ordered manner towards the c-axis direction, which leads to spontaneous polarization along [001] direction (Fig. 1a).

  Figure 1

  

  Figure 1.  Packing view of the crystal structures of (3-pyrrolinium)BBr₃ (B = Mn and Ni) in (a) ferroelectric phase and (b) paraelectric phase. The DSC curves of (c) (3-pyrrolinium)MnBr₃ and (d) (3-pyrrolinium)NiBr₃. (e) Equatorial plane projection of point group of mmm in the paraelectric phase and mm2 in the ferroelectric phase. The blue plane represents mirror symmetry elements in the paraelectric phase.

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  Differential scanning calorimetry (DSC) analysis reveal that (3-pyrrolinium)MnBr₃ experiences a reversible phase transition at 303 K (T_C), by observing a pair of endothermic/exothermic peaks during the heating/cooling process (Fig. 1c). After replacing the B-site with Ni²⁺ cations with a smaller ion radius, the T_C was successfully elevated to 346 K for (3-pyrrolinium)NiBr₃ (Fig. 1d). Thermogravimetric (TG) measurements shows the good thermal stabilities of (3-pyrrolinium)BBr₃ (B = Mn and Ni), and the decomposition temperatures of (3-pyrrolinium)BBr₃ are much higher than the phase transition temperatures (Fig. S2 in Supporting information). In terms of crystal structure, when the temperature rises above T_C, the crystal symmetry of (3-pyrrolinium)BBr₃ (B = Mn and Ni) transforms into the space group Cmcm in centrosymmetric mmm point group, entering into the paraelectric phase (PP) (Fig. 1e). 3-pyrrolinium cations in both of (3-pyrrolinium)BBr₃ (B = Mn and Ni) are located on a symmetrical site 2mm in their respective lattice, resulting in a 2-fold orientational disordered state and cancelling the macroscopic dipole moment (Fig. 1b). During the phase transition process, the distorted [BBr₆] octahedrons have not undergone significant variations (Fig. S3 and Tables S3-S6 in Supporting information). From paraelectric phase (PP) to ferroelectric phase (FP), symmetry breaking occurs with the disappearance of the mirror plane that perpendicular to the c-axis direction, inducing crystal polarity. Accordingly, the phase transition mechanism of (3-pyrrolinium)BBr₃ (B = Mn and Ni) can be mainly attributed to the order-disorder transition of organic cations, whose orientational ordering is responsible for the generation of spontaneous polarization.

  According to Aizu rules, the phase transition from PP to FP of these (3-pyrrolinium)BBr₃ (B = Mn and Ni) can be defined as the mmmFmm2-type one. By utilizing non-centrosymmetric crystals with high sensitivity to second-harmonic generation (SHG) signals, we carried out the SHG measurements to verify the crystal symmetry. As shown in Fig. 2a and Fig. S4a (Supporting information), the SHG response of (3-pyrrolinium)BBr₃ (B = Mn and Ni) are active and their intensity weaken gradually as the temperature increases until the T_C, corresponding to the polar Cmc2₁ space group. Then, the SHG signals disappear when the temperature above T_C, entering into the PP with centrosymmetric Cmcm space group. Near T_c, the dielectric testing of (3-pyrrolinium)BBr₃ (B = Mn and Ni) at frequency of 500 Hz to 1 MHz exhibit significant dielectric anomalies (Fig. S5 in Supporting information). From temperature-dependent real part (ε') of dielectric constant, both of (3-pyrrolinium)BBr₃ (B = Mn and Ni) experience λ-shaped significant dielectric anomalies peaked up to 5348 and 2042 near their respective T_C, which suggest typical paraelectric-ferroelectric phase transitions accompanying drastic changes in macroscopic dipole moments (Fig. 2a and Fig. S4a).

  Figure 2

  

  Figure 2.  (a) The temperature-dependent real part of the dielectric constant and SHG intensity of (3-pyrrolinium)MnBr₃. (b) Ferroelectric hysteresis loops of (3-pyrrolinium)MnBr₃ measured at different temperatures. (c) Variation of polarization as a function of the dynamic path connecting the ferroelectric phase (λ = 1) to the reference phase configuration (λ = 0). (d) The calculation model of (3-pyrrolinium)MnBr₃.

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  One of the most direct evidence for becoming a ferroelectric material is the switchability of polarization and its nonlinear hysteresis relationship with the electric field. We performed the polarization (P)-electric field (E) hysteresis loop measurements on bulk crystal samples of (3-pyrrolinium)BBr₃ (B = Mn and Ni). By using Sawyer-Tower method, we obtained shaped-well P-E hysteresis loops, indicating its good ferroelectricity. For (3-pyrrolinium)MnBr₃, the almost linear relationship between polarization and electric field at 320 K indicates the characteristics of ordinary dielectrics in the PP (Fig. 2b). When temperature decreases below T_C, the ferroelectric loops can be observed in FP. Upon cooling, the P-E hysteresis loops expand and the saturation polarization (P_s) increases gradually to reach about 6.06 µC/cm² at 291 K, in accordance with the estimated value of 5.67 µC/cm² from the point-charge model of crystal structure (Fig. 2b, Figs. S6 and S7 in Supporting information). The polarization value of 6.06 µC/cm² is higher than most of the reported ferroelectrics with photoluminescence (Table S7 in Supporting information). The (3-pyrrolinium)NiBr₃ also shows typical P-E hysteresis loops in the atmospheric environment, offering solid evidences of ferroelectricity (Fig. S4b in Supporting information). To gain a deeper insight of the ferroelectricity, the density functional theory (DFT) calculations were carried out. Two ferroelectric states (λ = ±1) are based on the single crystal structure of (3-pyrrolinium)BBr₃ (B = Mn and Ni) in FP, and the reference phase (λ = 0) is related to the PP of (3-pyrrolinium)BBr₃ (B = Mn and Ni) (Fig. 2d and Fig. S4d in Supporting information). The dynamic evaluation path from λ = 1 to 0 and −1 to 0 are necessary for the estimation of theoretical polarization value. For isomorphic structures and same phase transition mechanism of (3-pyrrolinium)BBr₃ (B = Mn and Ni), we list computational model of (3-pyrrolinium)MnBr₃ as a represent (Fig. S8 in Supporting information). We considered the rotation and displacement of the 3-pyrrolinium cations as well as slight displacements of the inorganic skeleton for the reasonable estimation. The variation of polarization as a function of the dynamic path show the smooth polarization value changes during the ferroelectric switching process (−1 < λ < 1) and the zero polarization at λ = 0, which indicates the PP states of (3-pyrrolinium)BBr₃ (B = Mn and Ni) (Fig. 2c and Fig. S4c in Supporting information). Taking (3-pyrrolinium)MnBr₃ as an example, its theoretical polarization value of 6.08 µC/cm² along the c-axis agrees well with the experimental value (6.06 µC/cm²) at 291 K.

  Besides ferroelectricity, B-site ion substitution strategy also provides opportunities to introduce additional physical functions. As expected, the (3-pyrrolinium)MnBr₃ shows intriguing photoluminescent properties, as observed in (pyrrolidinium)MnBr₃ and(3-pyrrolinium)MnCl₃ [35,39]. Under ultraviolet lamp irradiation, it emits bright orange-red light with the excitation wavelength of 456 nm (Fig. 3a). The corresponding PL emission peaked at about 654 nm can be attributed to the (t_2g)³(e_g)²–(t_2g)⁴(e_g)¹ electronic transition of Mn²⁺ ion with an octahedrally coordinated crystal field [40,41]. Based on the PL data, the emitted light color coordinate is determined as (0.63, 0.36) for (3-pyrrolinium)MnBr₃, and the PL decay lifetime (τ) was estimated to be 104.6 µs with the quantum yield at room temperature of 23.93% (Figs. 3b and c, Fig. S9 in Supporting information).

  Figure 3

  

  Figure 3.  (a) Excitation and emission spectra of (3-pyrrolinium)MnBr₃. (b) The CIE chromaticity coordinates as well as (c) the decay curves and PL lifetimes of (3-pyrrolinium)MnBr₃.

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  Crystal structure of (3-pyrrolinium)NiBr₃ consists of [NiBr₃]⁻ anionic chains and 3-pyrrolinium cations. The [NiBr₃]⁻ chain is made of 4 + 2 elongated face-sharing [NiBr₆] octahedra, which is expected that the intra-chain Ni…Ni magnetic couplings will be strong. The direct current magnetism data of (3-pyrrolinium)NiBr₃ was collected in the temperature range of 1.8–300 K in 1.0 kOe. At room temperature, the magnetic susceptibility χ_M T values are 1.40 cm³ K/mol for (3-pyrrolinium)NiBr₃, which is large than expect value 1.0 cm³ K/mol for Ni(Ⅱ) (g = 2.0; S = 1; Fig. 4a). The χ_M T value was decreased with the temperature decrease while the value of χ_M was increased and then reach max at 6.5 K, which indicates that the antiferromagnetic coupling between nearly Ni(Ⅱ) ions. Continue cooling temperature, the χ_M value sharply decreases and reach mini value 0.013 cm³ K/mol. The field-dependent isothermal magnetization (M vs. H) curve also proves that strong antiferromagnet coupling in (3-pyrrolinium)NiBr₃ (Fig. 4b).

  Figure 4

  

  Figure 4.  (a) Temperature dependence of the magnetic susceptibility (χ_M T). (b) Field-dependent isothermal magnetization (M) at 2.0 K.

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  Obviously, the B-site regulation strategy works well in (3-pyrrolinium)BBr₃, whose T_C increases sequentially as the ion radius decreases from Cd²⁺ to Mn²⁺ to Ni²⁺. In organic-inorganic hybrid systems, the structural phase transition of crystals is closely related to the molecular orientation dynamics, involving ionic size, shape and molecular interactions, etc. We thus calculated the crystal void of (3-pyrrolinium)BBr₃ (B = Mn and Ni) by Multiwfn software respectively [42]. As shown in Figs. 5a–c and Table S8 (Supporting information), the void occupancy in lattice changes from 31.58 of (3-pyrrolinium)CdBr₃ to 30.12 of (3-pyrrolinium)MnBr₃ and then to 28.54% of (3-pyrrolinium)NiBr₃. The decrease in void occupancy of lattice results in confined orientation movement of 3-pyrrolinium cations to make the increase of phase transition energy barrier. Molecular interactions between cations and anions that act like a rope for pulling each other play an important role in determining and affecting the physic properties of materials. As an effective means, Hirshfeld d_norm surfaces and related 2D fingerprint plots can iconically visualize the environment and surrounding interactions of molecules/ions in the lattice. The intensity of molecular interaction can be mapped onto the Hirshfeld d_norm surface by using a red–blue–white color scheme, where the van der Waals contact, closer contacts, and longer contacts are expressed by white regions, red regions and blue regions respectively (Figs. 5d–f). 3-pyrrolinium cations in these three crystals have similar surrounding environments but different interaction intensity, with the d_norm value change from 0.4490 in (3-pyrrolinium)CdBr₃ to 0.4481 in (3-pyrrolinium)MnBr₃ and then to 0.4047 in (3-pyrrolinium)NiBr₃. By adjusting the ionic radius from Cd²⁺ to Mn²⁺ to Ni²⁺, the reduced crystal void occupancy and stronger molecular interactions result in limited orientational dynamics of cations and more distorted [BX₆] octahedron, which contribute to the notable enhancement of T_C. Additionally, the introduction of Mn²⁺ and Ni²⁺ ions with octahedral coordination also bring intriguing red emission and magnetism respectively, which demonstrate the good feasibility of B-site ion regulation strategy for optimizing ferroelectricity and multifunctional integration.

  Figure 5

  

  Figure 5.  The calculated void occupancy of (a) (3-pyrrolinium)CdBr₃, (b) (3-pyrrolinium)MnBr₃ and (c) (3-pyrrolinium)NiBr₃. The Hirshfeld d_norm surfaces and two-dimensional (2D) fingerprint plots (bottom) of (d) (3-pyrrolinium)CdBr₃, (e) (3-pyrrolinium)MnBr₃ and (f) (3-pyrrolinium)NiBr₃. The d_i of the x-axis coordinate and the d_e of the y-axis represent the distance (Å) from the atoms inside the cation to the Hirshfeld surface and the external atoms to the Hirshfeld surface respectively.

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  In summary, we investigated the roles of regulating B-site ions in performance optimizing for HOIP ferroelectrics. By substituting Mn²⁺ and Ni²⁺ with smaller ionic radii for the Cd²⁺ of the parent (3-pyrrolinium)CdBr₃ respectively, we successfully obtain two new HOIP ferroelectrics (3-pyrrolinium)BBr₃ (B = Mn and Ni). The T_C has been significantly improved, realizing a jump enhance of 99 K to offer a wide ferroelectric working temperature window. Systematic experimental characterizations and theoretical calculations strongly confirmed their ferroelectricity. Structural analysis reveal that the phase transition mechanism is attributed to the order-disorder transition of 3-pyrrolinium cations. The reduction of ion radius of B-site components caused stronger molecular interactions and reduced void occupancy in lattice, which result in limited orientational dynamics of cations and more distorted [BX₆] octahedron. The former promotes T_C from 247 K of parent (3-pyrrolinium)CdBr₃ to 303 K of (3-pyrrolinium) MnBr₃ and then up to 346 K of (3-pyrrolinium)NiBr₃. Also striking is the introduction of additional physical properties of fascinating photoluminescence and magnetism. The electronic transition of Mn²⁺ ion with an octahedrally coordinated crystal field brings red-light PL emission, and the magnetism is originated from the [NiBr₃]⁻ framework. Our work would offer instructive clues for performance optimizing of HOIP ferroelectrics, and should inspire further exploration of the multifunctional integration between ferroelectricity and other intriguing physical properties.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgment

  This work was financially supported by the National Natural Science Foundation of China (Nos. 22375182, 92056112 and 21991141).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109690.

  
  
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  Scheme 1  The diagram of design strategy of (3-pyrrolinium)BBr₃ (B = Mn and Ni).

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  Figure 1  Packing view of the crystal structures of (3-pyrrolinium)BBr₃ (B = Mn and Ni) in (a) ferroelectric phase and (b) paraelectric phase. The DSC curves of (c) (3-pyrrolinium)MnBr₃ and (d) (3-pyrrolinium)NiBr₃. (e) Equatorial plane projection of point group of mmm in the paraelectric phase and mm2 in the ferroelectric phase. The blue plane represents mirror symmetry elements in the paraelectric phase.

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  Figure 2  (a) The temperature-dependent real part of the dielectric constant and SHG intensity of (3-pyrrolinium)MnBr₃. (b) Ferroelectric hysteresis loops of (3-pyrrolinium)MnBr₃ measured at different temperatures. (c) Variation of polarization as a function of the dynamic path connecting the ferroelectric phase (λ = 1) to the reference phase configuration (λ = 0). (d) The calculation model of (3-pyrrolinium)MnBr₃.

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  Figure 3  (a) Excitation and emission spectra of (3-pyrrolinium)MnBr₃. (b) The CIE chromaticity coordinates as well as (c) the decay curves and PL lifetimes of (3-pyrrolinium)MnBr₃.

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  Figure 4  (a) Temperature dependence of the magnetic susceptibility (χ_M T). (b) Field-dependent isothermal magnetization (M) at 2.0 K.

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  Figure 5  The calculated void occupancy of (a) (3-pyrrolinium)CdBr₃, (b) (3-pyrrolinium)MnBr₃ and (c) (3-pyrrolinium)NiBr₃. The Hirshfeld d_norm surfaces and two-dimensional (2D) fingerprint plots (bottom) of (d) (3-pyrrolinium)CdBr₃, (e) (3-pyrrolinium)MnBr₃ and (f) (3-pyrrolinium)NiBr₃. The d_i of the x-axis coordinate and the d_e of the y-axis represent the distance (Å) from the atoms inside the cation to the Hirshfeld surface and the external atoms to the Hirshfeld surface respectively.

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