English

• Ferroelectrics are a class of polar materials with spontaneous polarization and the polarization direction can be reversed under the stimulation of an external electric field [1]. The polarization of ferroelectrics can also change with the change of applied stress, electric field, or temperature, so it can show the physical effects of force, sound, light, electricity, heat, and other interactive responses, thus becoming one of the ideal materials for manufacturing multifunctional devices and sensitive components in the field of high technology [2-7].

  Since the discovery of the first ferroelectric Rochelle salt in 1920 [8], ferroelectric development has opened a new era, and more than a thousand ferroelectrics have been introduced so far [9-13]. At present, inorganic ceramic ferroelectrics (BTO, PZT, etc.) are still the most widely studied and used [14-21]. However, these materials require high-temperature sintering, which not only has high production cost and high energy consumption but also contains heavy metals posing a serious threat to environmental safety. Therefore, organic molecular ferroelectrics with excellent ferroelectric properties, simple production process, low cost, and environmental friendliness have become important supplements for the traditional inorganic ferroelectric materials [22-24].

  Ferroelectrics are often used in practical production and work in the form of ferroelectric thin films and ferroelectric crystals [25-32]. Ferroelectric thin films have great advantages in the expression of piezoelectric properties, ferroelectric domains, electric polarization switching, and so on [33,34].

  In the long-term molecular ferroelectric research [35-37], chemists and physicists found that organic ligands as stimulus responsive structural units play an important role in inducing ferroelectric properties [38,39]. Among them, the organic ligand with the spherical structure exhibits plastic characteristics, it is easy to form films and can be processed into various shapes, exhibiting outstanding performance on ferroelectric properties [40]. The typical compounds such as [(CH₃)₄N]FeCl₄ [41], [(CH₃)₄P]CdCl₃ [42], [1,4–2.2.2-dabco]ReO₄ [37], [1,4–2.2.2-dabco]ClO₄ [43], have been reported.

  Combined with our previous work [3.2.1-dabco]BF₄ [44], we designed and synthesized a caged molecule 1,5-diazabicyclo[3.2.2]nonane (1.5–3.2.2-dabcn) with a larger cage structure showing higher molecular rotational energy barrier when compared to 1,5-diazodicyclic[3.2.1]octane (3.2.1-dabco), which is more favorable for inducing high-temperature phase transitions.

  Reaction of equimolar amounts of 1.5–3.2.2-dabcn with HClO₄ or HReO₄ in a mixed solution of methanol and water produced two organic-inorganic hybrid crystals [1,5–3.2.2-Hdabcn]ClO₄ (1) and [1,5–3.2.2-Hdabcn]ReO₄ (2). The single crystal structure analysis showed that both compounds 1 and 2 crystallized in the ferroelectric space groups Pmn2₁ and Pca2₁, respectively. The obvious ferroelectric domains and polarization direction reversal of compounds 1 and 2 were observed by piezoresponse force microscopy (PFM) at room temperature.

  Differential scanning calorimetry (DSC) and dielectric measurement are effective methods to characterize the structural phase transition. As shown in Fig. 1a and Fig. S1 (Supporting information), compound 1 exhibits three reversible phase transitions with the first at the low-temperature region T₁ = 227/214 K, and the other two at the above-room temperatures T₂ = 381/357 K, T₃ = 406/372 K. While compound 2 shows two reversible and high-temperature phase transitions at T₁ = 378/353 K and T₂ = 396/379 K (Fig. 1b). The large thermal hysteresis (ΔT₂ = 33 K, ΔT₃ = 23 K of 1; ΔT₁ = 25 K, ΔT₂ = 16 K of 2) reveals the type of first-order phase transition. The N values in the phase transitions are about 16.71 (T₂) and 18.58 J mol⁻¹ K⁻¹ (T₃) for 1, and 42.45 (T₁) and 3.47 J mol⁻¹ K⁻¹ (T₂) for 2, indicating that the four pairs of phase transitions are ascribed to the order-disorder type phase transitions.

  Figure 1

  

  Figure 1.  (a, b) DSC and (c, d) the dielectric constants ε′ of compounds 1 and 2 in a heating and cooling cycle.

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  The phase transition of 1 and 2 was further verified by the temperature-dependent of the real part (ε′) of the complex permittivity ε (ε = ε′ - iε′′, where ε′′ is the imaginary part of the permittivity). The ε′ of compounds 1 and 2 show two distinct dielectric anomalies near their respective high-temperature phase transition points (Figs. 1c and d), which are consistent with their DSC results. As the temperature increases, the ε′ values of 1 increase from 10 to 90 to 106 and then decrease to 56, the high dielectric state is about 10.6 times that of the low dielectric state. The ε′ value of 2 increases from 3.4 to 11 and then continues to rise to 21, the maximum value is 6.2 times that of the minimum value.

  The T-ε′ curve of 1 shows a sharp peak shape, indicating that it may be an intrinsic ferroelectric, while the stepped dielectric anomaly curve of 2 suggests that it is an extrinsic ferroelectric. In addition, compounds 1 and 2 exhibit significant dielectric losses (the imaginary part) at their respective phase transition points at 1 MHz (Figs. S2a and b in Supporting information).

  Because both compounds 1 and 2 were found to undergo multiple reversible phase transitions, the variable-temperature X-ray single-crystal diffractions at the different phases were recorded to gain insight into the phase transition mechanism. As shown in Table S1 (Supporting information), the crystal structures of 1 in phase Ⅰ (173 K, below T₁) and phase Ⅱ (293 K, between T₁ and T₂) are crystallized in the orthorhombic system, with the polar space groups Pna2₁ and Pmn2₁ (point group mm2), meeting the symmetry requirement for ferroelectrics,

  In phase Ⅰ, the stacking diagram of 1 (Fig. 2a) shows that the monoprotonated [1,5–3.2.2-Hdabcn]⁺ cations are connected through the hydrogen bonds N—H···N to form a wavy one-dimensional chain, this connection is also found in similar molecular ferroelectrics [1,4–2.2.2-dabco]ReO₄, [1,4–2.2.2-dabco]ClO₄ and [1,4–2.2.2-dabco]BF₄ [37,43,45], but unlike the ideal linear N—H···N hydrogen bonds in the perfectly spherical molecule 1,4–2.2.2-dabco. As shown in Fig. 2d, one organic amine cation is surrounded by eight anions, just like a twisted cage structure enclosing an organic cation.

  Figure 2

  

  Figure 2.  The packing views of 1 in ferroelectric phases (a, d) 173 K, (b, e) 293 K and (c, f) paraelectric phase 393 K.

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  To be noted, in phase Ⅱ, the asymmetric unit consists of half deprotonated [1,5–3.2.2-H₂dabcn]²⁺ cation, half unprotonated [1,5–3.2.2-dabcn] molecule, and one ClO₄⁻ anions. Different from the head-to-tail hydrogen-bonding interactions of monoprotonated [1,5–3.2.2-Hdabcn]⁺ in phase Ⅰ, the deprotonated [1,5–3.2.2-H₂dabcn]²⁺ is connected with unprotonated [1,5–3.2.2-dabcn] molecule to for a dimer [1,5–3.2.2-H₂dabcn]₂²⁺ by intramolecular hydrogen bond N—H···N. Then the dimers are further connected to form a wavy chain [1,5–3.2.2-H₂dabcn]_nⁿ⁺ through hydrogen bonds, Fig. 2b. The cage structure surrounding the cations becomes a relatively regular wedge-shaped hexahedron (Fig. 2e).

  In phase Ⅲ, 1 still crystallizes in the orthorhombic crystal system but adopts mmm point group with Cmma space group (a = 11.466(3) Å, b = 13.468(4) Å and c = 6.7503(19) Å, α = β = γ = 90°). As shown in Fig. 2c, the cation and anionic are severely disordered, which makes the molecular configuration change from the original low-symmetry cage structure to a high-symmetry cage structure. At this stage, the hexahedron wrapping the cations also becomes more regular, presenting as a regular rectangle (Fig. 2f).

  Compound 2 crystallizes in the polar space group Pca2₁ of mm2 at 293 K (a = 17.9283(7) Å, b = 6.9767(2) Å, c = 8.5325(3) Å, α = β = γ = 90°, Table S2 in Supporting information). As shown in Fig. 3a, the structure of 2 is constructed with the monoprotonated [1,5–3.2.2-Hdabcn]⁺ cations and ReO₄⁻ anions, which are connected by the N—H···O hydrogen bond to form a discrete molecule. In the ReO₄⁻, the Re—O bond lengths range from 1.701(4) Å to 1.717(4) Å, and the O—Re—O bond angles range from 108.2(3)° to 110.7(2)°, indicating that the anion group is an irregular tetrahedral structure. Compound 2 has a zero-dimensional structure in phase Ⅰ, which is different from the other spherical molecules, such as [1,4–2.2.2-dabco]ReO₄ and [1,4–2.2.2-dabco]ClO₄, in which the cations are 1D chains connected by hydrogen bonds [37,43].

  Figure 3

  

  Figure 3.  (a) Crystal structure of 2 in the ferroelectric phase at 293 K. (b) PXRD tests of the powder sample of 2 at different temperatures.

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  In high-temperature phase Ⅱ, due to the highly disordered molecules making crystal structure solution difficult, the variable temperature PXRD tests at 390 K and 427 K were performed for 2. As shown in Fig. S2b, the XRD pattern at 293 K is in good agreement with the single crystal simulation data from the room temperature. Compared with 293 K, the diffraction peaks at 390 K are significantly reduced, indicating that compound 2 has undergone a structural phase transformation and entered a new phase (Fig. 3b). Based on the PXRD data at 390 K, it was deduced that compound 2 crystallizes in the orthorhombic crystal system (Fig. S3a in Supporting information), and the space group with the maximum probability speculated by the Pawley refinements is Pmm2. This is also consistent with the subsequent SHG activity response.

  Compared with the two groups of peaks at phase Ⅰ and Ⅱ, the diffraction peaks at 427 K (phase Ⅲ) are significantly reduced, and the diffraction characteristics are very similar to those of the paraelectric phases found in the quasi-spherical structures [quinuclidinium][X] (X = ReO₄, ClO₄, and PF₆), and [2.2.2-dabco]ReO₄ [37,46]. The PXRD data at 427 K also revealed that 2 may be crystallized in the tetragonal system (Fig. S3b in Supporting information), and the space group with the maximum probability speculated by the Pawley refinements is P4/mmm.

  The polar space groups are usually accompanied by SHG activity, and the centrosymmetric space group shows SHG inactivity. In general, the ferroelectric-paraelectric phase transition is usually accompanied by a transition of crystal structure from the polar space group to the centrosymmetric space group, therefore, the temperature-dependent SHG response measurement of compounds 1 and 2 was performed. As shown in Fig. 4a, compound 1 shows an obvious SHG activity below 390 K, suggesting a polar phase, and consistent with the ferroelectric space groups Pna2₁, Pmn2₁ in phases Ⅰ, Ⅱ. With the increase in temperature, the SHG signal intensity rapidly drops to zero at around 400 K and remains stable thereafter, which demonstrates a centrosymmetric crystal structure, corresponding to the centrosymmetric space group Cmma in phase Ⅲ.

  Figure 4

  

  Figure 4.  Temperature dependence of SHG signals for polycrystalline samples (a) 1 and (b) 2.

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  As shown in Fig. 4b, it can be seen that the SHG signal of compound 2 is active before T₂, this is consistent with its crystallization in the non-centrosymmetric space groups Pca2₁ (phase Ⅰ) and Pmm2 (phase Ⅱ). When the temperature reaches the vicinity of T₂, the SHG intensity value suddenly dropped to zero, which implies that compound 2 enters a centrosymmetric space group. Combined with the variable temperature PXRD, it should be the centrosymmetric space group of P4/mmm. Therefore, combined with SHG and crystal structure analysis, compounds 1 and 2 undergo the mmmFmm2 and P4/mmmFmm2 type ferroelectric phase transitions, respectively [47].

  Piezoresponse force microscopy has extremely high spatial resolution and can realize the visualization of ferroelectric domain structures at the microscopic nanoscale, which is a powerful tool to demonstrate the existence of nanoscale ferroelectricity. Figs. 5a and b show the PFM phase, topography, and amplitude images for the thin films of 1. The distinct tonal domain structures in the phase image suggest the existence of antiparallel domains. A clear domain wall can be observed in the amplitude map, transforming the original single-domain structure into a double-domain structure, where the intensity of the piezoelectric signals is very close, proving the existence of 180° antiparallel domain structure. Figs. 5c and d show the vertical and lateral components of 180° domain structure for the thin films of 2, the phase diagram clearly shows two antiparallel domains, and the piezoresponse of adjacent domains are similar in the amplitude images. This provides sufficient evidence for the existence of 180° antiparallel domains.

  Figure 5

  

  Figure 5.  The PFM phase, topography, and amplitude images for the thin film of (a, b) 1 and (c, d) 2 at one region with 180° domain in the (a, c) vertical and (b, d) lateral components, respectively.

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  The spontaneous polarization of electrical switching is the most essential feature of ferroelectrics. It reveals the repeated switching of ferroelectrics between two or more polarization states under the excitation of the external electric field. Therefore, the local PFM-based hysteresis loop measurement was performed to verify the ferroelectric characteristic of 1 and 2. In Figs. 6a and e, the typical hysteresis loops and butterfly curves of phase and amplitude as a function of the bias tip voltage indicate the successful polarization switching of domains. The local coercive voltages of 1 and 2 were estimated to be about 13 V and 25 V by averaging the minima of the amplitude loops.

  Figure 6

  

  Figure 6.  Phase and amplitude signals as functions of the tip voltage for a selected point, showing a hysteresis loop and a butterfly curve of (a) 1 and (e) 2. Vertical PFM phase (top) and amplitude (down) images of the films for 1 and 2: (b, f) Initial state; (c, g) After the first polarization reversal by applying +110 V and +120 V tip bias. (d, h) After the succeeding back-switching produced by applying −110 V and −120 V tip bias.

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  For a more intuitive observation of the switching process of domains, the localized polarization operations were performed on thin films of 1 and 2 in the as-grown state. First, a single domain region was selected as the initial state for polarization switching tests, as shown in the red boxes of Figs. 6b and f, and then, a DC tip bias of +110 V and +120 V were applied to the selected areas on the thin films. It can be clearly observed that a new domain appears in the phase diagram, and an obvious domain wall also appears in the amplitude diagram, confirming that the ferroelectric domain has completed a polarization reversal (Figs. 6c-g). Finally, when an opposite bias of −110 and −120 V was applied, the polarization direction of the selected region (Figs. 6d and h) can be reversed back, indicating that the polarization of these domains can be switched reversibly and repeatedly.

  In summary, we have successfully designed and prepared two molecular ferroelectrics [1,5–3.2.2-Hdabcn]ClO₄ (1) and [1,5–3.2.2-Hdabcn]ReO₄ (2), which show two high-temperature phase transitions. The variable-temperature single crystal structures revealed compound 1 undergoes a mmmFmm2 type ferroelectric phase transition near 380 K. While compound 2 crystallizes in the polar point group mm2 in both phase Ⅰ and phase Ⅱ, and the centrosymmetric point group 4/mmm in phase Ⅲ, with a 4/mmmFmm2 type ferroelectric phase transition near 396 K. At room temperature, the switching of ferroelectric domains and rectangular P-E hysteresis loops were successfully achieved on the polycrystalline films of 1 and 2. In addition, the low coercive voltages of 13 V (1) and 25 V (2) and easy domain reversal make them considerable potential for practical applications.

  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.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 21865015, 22071094 and 22075123) and the Department of Science and Technology in Jiangxi Province (No. 20213BCJ22055).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108809.

  
  
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• 

  Figure 1  (a, b) DSC and (c, d) the dielectric constants ε′ of compounds 1 and 2 in a heating and cooling cycle.

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  Figure 2  The packing views of 1 in ferroelectric phases (a, d) 173 K, (b, e) 293 K and (c, f) paraelectric phase 393 K.

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  Figure 3  (a) Crystal structure of 2 in the ferroelectric phase at 293 K. (b) PXRD tests of the powder sample of 2 at different temperatures.

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  Figure 4  Temperature dependence of SHG signals for polycrystalline samples (a) 1 and (b) 2.

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  Figure 5  The PFM phase, topography, and amplitude images for the thin film of (a, b) 1 and (c, d) 2 at one region with 180° domain in the (a, c) vertical and (b, d) lateral components, respectively.

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  Figure 6  Phase and amplitude signals as functions of the tip voltage for a selected point, showing a hysteresis loop and a butterfly curve of (a) 1 and (e) 2. Vertical PFM phase (top) and amplitude (down) images of the films for 1 and 2: (b, f) Initial state; (c, g) After the first polarization reversal by applying +110 V and +120 V tip bias. (d, h) After the succeeding back-switching produced by applying −110 V and −120 V tip bias.

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