Fluoridation Achieved Antiperovskite Molecular Ferroelectric in [(CH3

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Fluoridation Achieved Antiperovskite Molecular Ferroelectric in [(CH)(F-CHCH)NH](CdCl)(CdCl) 3

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Zhong-Xia Wang, Yi Zhang, Yuan-Yuan Tang, Peng-Fei Li, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13109 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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Journal of the American Chemical Society

Fluoridation Achieved Antiperovskite Molecular Ferroelectric in [(CH3)2(F-CH2CH2)NH]3(CdCl3)(CdCl4) †

Zhong-Xia Wang,†,§ Yi Zhang,§ Yuan-Yuan Tang,† Peng-Fei Li,† Ren-Gen Xiong*, ,§ †

Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, People’s Republic of China

Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, People’s Republic of China §

ABSTRACT: Antiperovskites have developed to be one kind of important functional materials over the past few decades, showing abundant physical properties such as negative thermal expansion and superconductivity, etc. However, antiperovskite ferroelectric is scarcely discovered in inorganic ceramics. In this article, we report a new organic-inorganic hybrid antiperovskite ferroelectric [(CH3)2(F-CH2CH2)NH]3(CdCl3)(CdCl4) based on the strategy of molecular design. The replacement of one methyl in [(CH3)3NH]CdCl3 with ethyl produces the lower symmetric [(CH3)2(CH2CH3)NH]CdCl3 with non-polar perovskite structure, while the polar hexagonal antiperovskite structure with the formula of X 3BA (where X = [(CH3)2(F-CH2CH2)NH]+, B = [CdCl3]- and A = [CdCl4]2-) was received after further fluoridation of the ethyl group. Therefore, fluoridation successfully achieves the structural transformation from perovskite to antiperovskite, as well as the significant changes in physical properties from non-ferroelectric to ferroelectric. The antiperovskite [(CH3)2(FCH2CH2)NH]3(CdCl3)(CdCl4) exhibits typical ferroelectric phase transition above the room temperature (Tc = 333 K) including thermal anomalies, dielectric transitions and second harmonic generation (SHG) responses. Moreover, lower coercive fields and easy polarization switching are observed by the measurements of hysteresis loops and ferroelectric domains. The saturated polarization (Ps) of 4.0 μC/cm2 is almost 10 times as large as those recently discovered antiperovskite molecular ferroelectrics. This finding provides a novel strategy to design and explore more antiperovskite organic-inorganic hybrid ferroelectric materials.

INTRODUCTION Since 1930s, antiperovskites as a "reverse" analogue or a "twin" of perovskites with the formula of M3A’B’ (M is transition metals, B’ is from C, N, O and B and A’ is the metals in main group) have been discovered with rich physical properties, including superconductivity, large magnetocaloric effects, negative or zero thermal expansion, giant magnetoresistance and magnetostriction 1-11 that are comparable to perovskites. Although the early report of ternary tetrathiafulvalenium salts with antiperovskite structure has been found showing antiferromagnetic ordering,12 the design of ferroelectricity is still an important challenge in antiperovskites so far in contrast to the booming developments of perovskite ferroelectrics, because they usually crystallize in a centrosymmetric space group with highly symmetrical system.13 While for ferroelectrics, their crystallographic symmetry must meet one of the 10 polar point groups (C1, C2, C1h, C2v, C4, C4v, C3, C3v, C6, and C6v).14-16 Therefore, antiperovskite ferroelectrics are like new world waiting to be exploited to catch up with the rapid development of materials. Organic-inorganic hybrid perovskite as a young star shows impressive in multifunctional materials. It is precisely because of the flexibility and tunability of the structure, organic-inorganic hybrid perovskites with abundant dimensionality afford a platform to design various dimensional crystals with interesting properties, not only producing promise for future high performance in photovoltaic application,17-27 but also possessing

prominent ferroelectric and piezoelectric properties that can rival with the traditional inorganic perovskites.28-41 Especially, the scarce antiperovskite ferroelectrics can be reached by organic-inorganic hybrid structural strategy.42 Very recently, first two new hybrid molecular antiperovskites of [(CH3)3NH]3(MnX3)(MnX4) (X = Cl, Br) were discovered by us with extraordinary ferroelectric and photoluminescent properties.43,44 While holding on to the bonus from organic-inorganic hybrid strategy, an inevitability is that a slight change in organic or inorganic component possibly triggers structural change with a drastic variation of physical performances. For example, when the Mn2+ of [(CH3)3NH]3(MnCl3)(MnCl4) is substituted by isomorphic Cd2+, a centrosymmetric perovskite-type structure of [(CH3)3NH]CdCl3 is received, exhibiting non-ferroelectric/paraelectric phase transition behavior.45 Simultaneously, the molecular structure undergoes the transformation from antiperovskite to perovskite. Imagining that how to produce a polar structure with a slight modulation in the part of organic cation? We previously reported that the introduction of methyl into dabco (1,4-diazabicyclo[2.2.2]octane) molecule breaks its mirror plane and generates a molecular dipole moment, resulting in a reduced crystallographic symmetry and excellent ferroelectricity in [MeHdabco]RbI3.46 Moreover, incorporating highly symmetrical organic cation with electrophilic halogen may also promote polar molecular structure and acquire the expected physical properties. Remarkably, a noncentrosymmetric crystal of [(CH3)3NCH2Cl]CdCl3 is produced by mean of the replacement of one methyl in

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centrosymmetric [(CH3)4N]CdCl3 using methyl chloride, and thus it exhibits excellent ferroelectricity and super large piezoelectricity.28 In addition, halogen regulation achieved ferroelectric [(CH3)3NCH2I]PbI3 suggests that the symmetry of crystal can be tuned by the halogen in the organic amine, subsequently adjusting the regarding physical properties.47 Such the strategies encourage us that the chemical tailoring or modification in a small portion of the organic component is a shortcut that can bring in ferroelectricity. Based on the molecular design apporach (Figure 1), we considered to replace one of methyl groups in [(CH3)3NH]CdCl3 with ethyl at the beginning to break the existed mirror plane, possibly producing a polar crystal to pick up a ferroelectric compound. However, the single crystal X-ray diffraction confirmed a centrosymmetric structure of [(CH3)2(CH2CH3)NH]CdCl3 with reduced crystal symmetry. Then further fluorination of the ethyl group was carried out. In surprise, we found a new organic-inorganic hybrid X3BA antiperovskite ferroelectric: [(CH3)2(F-CH2CH2)NH]3(CdCl3)(CdCl4) (1). We introduced fluorine into the ethyl group of (CH3)2N(CH2CH3), succeeding in the structural transformation from perovskite to antiperovskite, accompanied by the realization of ferroelectric. 1 displays prominent ferroelectric phase transition above the room temperature (Tc = 333 K) with larger spontaneous polarization, lower coercive electric field and easy domain reversal, compared to the previously reported hybrid molecular antiperovskite ferroelectrics. This work will open a new approach for discovering more molecular ferroelectrics, especially for pending development of antiperovskite ferroelectric compounds.

and [(CH3)3NCH2I]PbI3 suggest us that the further halogen decoration on the ethyl of (CH3)2N(CH2CH3) would address the issue and eventually reach the goal of ferroelectricity. Subsequently, we tried to synthesize the fluorinated organic amine, that is (CH3)2N(F-CH2CH2) (see Experimental section). A polar crystal of [(CH3)2(FCH2CH2)NH]3(CdCl3)(CdCl4) was then confirmed by single crystal X-ray diffraction. The phase purity of bulk phase was performed by infrared (IR) analysis and powder X-ray diffraction (PXRD) (Figure S2 and Figure S3). Infrared characteristic peak of the C−F bond from organic cation is located at around 1050 cm-1. At 293 K, 1 crystalizes in a polar space group of P63 (No. 173) with a = 15.2403(5) Å, b = 15.2403(5) Å, c = 6.7047(3) Å, α = β = 90° , γ = 120°and V = 1348.64(11) Å3, a typically hexagonal cell parameter (Table S1). Compared to the molecular structure of [(CH3)2(CH2CH3)NH]CdCl3, another tetracoordinated CdCl4 breaks into the perovskite, forming a kind of composite metal framework structure in 1 (Figure S4). The packing structure of 1 contains isolated zerodimensional [CdCl4]2- anions and infinite onedimensional chain [CdCl3]- anions arranged along the caxis, which are surrounded by [(CH3)2(F-CH2CH2)NH]+ cations (Figure S5). Therefore, 1 can be viewed

Figure 1. The approach of molecular design for ferroelectric. The chain-like framework of [CdCl3]- (left) and the decoration of organic cation achieving the polar structure with ferroelectricity (right).

RESULTS AND DISCUSSION The crystal structure of [(CH3)2(CH2CH3)NH]CdCl3 adopts a centrosymmetric P21/c (No. 14) with a = 6.7388(3) Å, b = 15.1099(7) Å, c = 10.4269(7) Å, β = 118.749(4)° , and V = 930.83(9) Å3 (Table S1). The packing structure is made of organic [(CH3)2(CH2CH3)NH]+ cations and face-sharing CdCl6 octahedrons formed onedimensional chain-like perovskite structure along the aaxis (Figure S1). The [(CH3)2(CH2CH3)NH]+ cation is in an ordinary position, compared to the [(CH3)3NH]+ cation sitting on the special mirror place in [(CH3)3NH]CdCl3.45 Therefore, the replacement of methyl with ethyl in the [(CH3)3NH]+ organic cation leads to symmetry breaking, reducing the crystal symmetry (Figure 1), but it is not enough to create a polar crystal in this structural model. Afterwards, the compelling cases of [(CH3)3NCH2Cl]CdCl3

Figure 2. (a) The packing of 1 viewed along the chains of (CdCl3)[(CH3)2(F-CH2CH2)NH]6 octahedra. Packing views of 1 in (b) ferroelectric phase (293 K) and (c) paraelectric phase (373 K) in the direction of [110]. The red planes are mirror only existed in paraelectric phase and the H atoms of the cations were omitted for clarity.


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Journal of the American Chemical Society as the chains of {[(CH3)2(F-CH2CH2)NH]3(CdCl3)}2+ separated by the [CdCl4]2- anions. The organic cation with hexa-coordinated CdCl6 forms tilted face-sharing [(CH3)2(F-CH2CH2)NH]6(CdCl3) octahedra (Figure 2a). Such a packing model is familiar with BaNiO3, which has a typically hexagonal perovskite structure.48,49 When the part of organic cation is corresponded to oxygen atom, the [CdCl4]2- component sits on the position of Ba atom, and the [Cd(Cl0.5)6]- replaces the site of Ni one, 1 reaches a hexagonal antiperovskite structure, being scarcely discovered in the organic-inorganic hybrid system. The length of Cd−Cl bond (2.4337(15) − 2.6566(14) Å) and the angles of adjacent Cl atoms (Cl−Cd−Cl) ranging from 84.30(5) to 115.21(5)°are comparable to those recently reported cadmium-chloride compounds.28,41,50,51 The organic cation in the crystal occupies a general position where there is no any symmetry element. The [CdCl4]2anions with the unique orientation upward, and the vector direction of N−H bonds of organic cations is slightly downward (Figure 2b). Therefore, the spontaneous polarization along the c-axis should be due to the nonoverlapping of charges in this structural arrangement. When the crystal was heated to be at 373 K, 1 crystallized in a centrosymmetric space group of P63/m (No. 176) with a = 15.2801(3) Å, b = 15.2801(3) Å, c = 6.7272(2) Å, α = β = 90° , γ = 120°and V = 1360.25(7) Å3 (Table S1). The molecular component and basic packing structure of [CdCl3]- chain and [CdCl4]2- tetrahedron remain the same. The Cd−Cl bond length and Cl−Cd−Cl bond angle are also similar to those at 293 K. Unlikely, the organic cation sits on a special position possessing the 6/m symmetry site, where a mirror appears in the crystal. In order to attain the required crystallographic symmetry, all carbon atoms of the organic cation were refined with distributing equally over the mirror plane (001), which is perpendicular to the c-axis (Figure 2c and Figure S6). Interestingly, tetra-coordinated [CdCl4]2- anions also

Figure 3. (a) Differential scanning calorimetry (DSC) curve measured on the powder sample of 1. (b) The temperature dependence of dielectric real part ε’ (ε = ε′ − iε″, where ε″ is imaginary part of ε) performed along the c-axis on a single crystal sample of 1 in a heating / cooling run. (c) The ε’ measurements carried out in different frequencies upon heating. (d) The temperature dependent second harmonic generation (SHG) responses.

demonstrates disordered over the mirror plane, leading to the disappearance of inner polarization in crystal, which further gives experimental evidences for the explanation of unachieved high temperature crystal structures in ferroelectric antiperovskites of [(CH3)3NH]3(MnX3)(MnX4) (X= Cl, Br). The variabletemperature PXRD measurements show the similar powder diffraction spectra upon heating and cooling, indicating the small change in crystallographic symmetry (Figure S7). As a matter of fact, the mirror symmetry has no obvious effect on powder diffraction peak. The obvious phase transition occurred with the change of symmetry from P63 to P63/m points to a ferroelectric-paraelectric phase transition with an Aizu principle of 6/mF6.52 The symmetry change and structural phase transition are generally supported well by relevant physical properties. Firstly, we confirmed the phase transition of 1 using DSC measurements. As plotted in Figure 3a, a pair of anomaly peaks occurs at 333 K (Tc) and 318 K in the heating and cooling run, respectively, indicating that 1 experiences a reversible phase transition. The thermal hysteresis of 15 K reveals the first-order phase transition behavior in 1.15 The entropy change (ΔS) was calculated to be approximately 0.794 J (mol K)−1 for the phase transition at Tc, according to the records of DSC curve. Secondly, the phase transition of 1 was further verified by the temperature-dependent real part (ε’) of complex dielectric permittivity on a single crystal along the c-axis (Figure 3b). The sharp dielectric peaks at Tc in the heating process and 318 K upon cooling measured at 1 kHz are consistent with DSC results, referring to a ferroelectricparaelectric phase transition in 1.14 In addition, the ε’ shows strong frequency dependence that is shown in Figure 3c. The value of ε’ gradually increases with the decreasing of applied electric filed frequencies and the type of dielectric anomaly changes from step-like at high frequencies to peak at low ones. In general, ferroelectricparaelectric phase transition is accompanied by symmetry change that is symmetry breaking.14 Therefore, the structural phase transition triggered symmetry change in 1 was investigated by temperature-dependent SHG response. SHG signals, only active in noncentrosymmetric phase, should give a variation as the space group of 1 changes from polar to centrosymmetric. In Figure 3d, the SHG intensity is obvious below Tc, indicating a polar phase and being consistent with the ferroelectric C6 point group. As the temperature increases above Tc, the intensity of SHG signal sharply drops to be zero due to the appearance of mirror symmetry in the centrosymmetric phase P63/m. The step-like change of SHG intensity in the vicinity of Tc suggests a first-order phase transition feature, agreeing with the DSC results. The measurements of the hysteresis loop are the most straightforward way to prove ferroelectricity. We systematically characterize the ferroelectricity of 1 using Sawyer-Tower electric circuit and double-wave method. Based on the results of crystal structure analyses and dielectric measurements, c-axis is polar axis. Therefore, we measured the ferroelectric hysteresis loop of 1 along this direction. As shown in Figure 4a, a well opened hysteresis loop with saturated polarization (Ps) of 4.0 μC/cm2 and coercive electric field (Ec) of 4.6 kV/cm is observed. The higher Ps and lower Ec of 1 make it more practical than those in recently reported molecule-based



antiperovskite ferroelectrics.43,44 Moreover, we performed double wave method to further confirm the ferroelectricity of 1.53,54 A typical

of c-axis observed from the starting state of λ = 1 experiences gradually decrease to be zero λ = 0, and then reversely reaches the end state of λ = -1 with a polarization of -3.99 μC/cm2, pointing to the ferroelectricparaelectric-ferroelectric phase transition, which agrees well with the experimental ferroelectric and pyroelectric results. Moreover, the calculated polarization is multivalued due to the quantum polarization (eR/Ω, where e > 0 is the charge quantum, Ω is the primitive cell volume and R is a lattice vector) according to the modern theory of polarization.55-57 From Figure 4d, when adding or subtracting a quantum polarization (7.95 μC/cm 2), same paths are received and the branches are separated by the polarization quantum. Notice that the branches of the lattice run exactly parallel to each other, so that the differences in polarization along each branch for the same structural distortion are identical.

Figure 4. (a) Ferroelectric hysteresis loops of 1 measured along the c-axis using a Sawyer-Tower electric circuit. (b) P–E hysteresis loop of 1 performed using double-wave method. (c) The temperature dependent spontaneous polarization integrated from pyroelectric current of 1. (d) Calculated polarization as a function of structural distortion (λ) from the high symmetry non-polar structure (λ=0, P63/m) to two ferroelectric states (λ=±1, P63) structure. J−E (current density−electric filed) curve for 1 is shown in Figure 4b. It shows two peaks due to charge displacement, which corresponds to two stable states with opposite polarity. According to the J−E curve, the P−E hysteresis loop can be obtained by integrating the polarization switching current, in which the Ps (3.9 μC/cm2) is consistent with that using Sawyer-Tower electric circuit. In addition, ferroelectrics as a special kind of pyroelectrics, showing a sensitive spontaneous polarization response to the temperature. As depicted in Figure 4c, the temperature dependent polarization was carried out by integration of pyroelectric current. A sharp pyroelectric current response was captured around the Tc, indicating the appearance of spontaneous polarization that points to a ferroelectric-paraelectric phase transition. The obtained polarization below Tc is about 3.9 μC/cm2, matching well with that measured from P−E loop. The sudden change of polarization curve in the vicinity of Tc in Figure 4c follows the same pathway as that of SHG signal (Figure 3d). For the polarization variation of ferroelectricparaelectric phase transition, density functional theory (DFT) calculation can provide further theoretical supports for the polarization switching dynamics. Accordingly, the crystal structure of 1 at room temperature was selected to be as ferroelectric configuration (λ = 1), then the “paraelectric” (high symmetry non-polar structure, λ = 0) configuration was from the structural distortion of the ferroelectric phase taking the organic cations and inorganic anions into account based on the crystal structure symmetry in paraelectric phase. The polarization change as the function of dynamic path is shown in Figure 4d. The ferroelectric polarization of 3.99 μC/cm2 in the direction

Figure 5. The vertical PFM phase images (a), the amplitude images (b) and the topography (c) of the asgrown crystal surface for 1. (d) The dependence of phase and amplitude for a selected point, displaying a hysteresis loop and a butterfly curve. The vertical PFM phase (e) and amplitude (f) images for 1 after successive switchings with reverse DC bias ±40 V. Piezoresponse force microscopy (PFM) as a characteristic feature of ferroelectric materials is powerful to investigate the statics and dynamics of ferroelectric polarization with the visualization of microscale domain structures.36,58,59 PFM tests on bulk crystal sample of 1 along the polar c-axis were carried out, where the signal of vertical PFM is strong, and the lateral PFM response is negligible. As shown in Figure 5a, the phase image definitely reveals two antiparallel domains with a clear contrast, which is inconsistent to the topography (Figure


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Journal of the American Chemical Society 5c). And from Figure 5b, we can clearly see the domain wall in the amplitude image, separating these two different domains, between which the piezoelectric signals are very close. These findings demonstrate the existence of 180°antiparallel domains, which provides the evidence for crystal analyses (6/mF6). Moreover, the phase and amplitude as functions of the tip voltage exhibits a hysteresis loop and a butterfly curve, respectively, suggesting the polarization switching of ferroelectric domains (Figure 5d). The polarization reversal is reproducible and stable even the crystal was repeated heating and cooling. Then, we selected a small region on the surface of the bulk crystal sample, which is a single phase with single domain in this area. Subsequently, the box pattern in the center with reversed DC biases was applied to check the domain polarization switching. A tip bias of +40 V was applied to pole the 4 μm × 4 μm square region, followed by another poling with a tip bias of -40 V in the central area of 1 μm × 1 μm (Figures 5e and 5f). Interestingly, a hexagon domain appears in the center of a diffused one, instead of box-in-box shape domains. Such the results indicate the little larger voltage was applied, leading to the diffusion of domain based on the symmetry of hexagonal point group.

CONCLUSIONS In summary, we have synthetized and discovered a new hybrid organic-inorganic antiperovskite ferroelectric [(CH3)2(F-CH2CH2)NH]3(CdCl3)(CdCl4) based on the chemical design. The fluoridation successfully achieves the structural transformation from perovskite to antiperovskite, accompanied by the drastic changes of physical properties from non-ferroelectric to ferroelectric. The title compound exhibits prominent ferroelectric phase transition behavior above the room temperature. Particularly, the larger saturated spontaneous polarization and lower coercive electric field than recently discovered antiperovskite ferroelectrics are captured. Moreover, the clear polarization reversal detected by PFM is very easy to implement under low switching voltage. Such the outstanding performances are attributed to the unique structural arrangement with the orientational disordered organic cation and inorganic framework. Since the strategy of organic-inorganic hybrid demonstrates great potential to regulate crystal structure and its physical properties, this report would start a new beginning for exploring more hybrid antiperovskite structures and promote the further development of antiperovskite ferroelectrics.

EXPERIMENTAL SECTIONS Materials. Dimethylamine (Aladdin, 2M in THF), 2fluoroethyl bromide (Adamas-beta, 98 %), N,Ndimethylethylamine (Aladdin, 98 %), CdCl2· 2.5H2O (Macklin, 98 %), potassium hydroxide (Aladdin, AR 90 %), toluene, hydrochloric acid (36 ~ 38 wt % in H2O) were used as received. Synthesis. A mixture of 80 mL dimethylamine in THF, 20 g 2-fluoroethyl bromide, 22 g KOH and 150 mL toluene was stirred at room temperature for 6 hours. Then, the mixture was heated and kept at 57 ° C for 20 hours. The reaction solution was experienced with atmospheric distillation to collect the fraction of 78-80 ° C, which was passed into an ethanol solution of hydrochloric acid to

give colorless 2-fluoroethyldimethylamine hydrochloric acid. Yield: 12.5 %. M.p 165 ° C. The molecular structure and crystal data of (CH3)2N(F-CH2CH2)· HCl are shown in Figure S8 and Table S1. The colorless crystals of [(CH3)2(CH2CH3)NH]CdCl3 and [(CH3)2(F-CH2CH2)NH]3(CdCl3)(CdCl4) were obtained by slow evaporation of a clear aqueous containing CdCl2· 2.5H2O and the hydrochloride of the corresponding amine with the ratio of 2:3 at room temperature. X-ray, DSC, dielectric and SHG measurements. Variable-temperature X-ray single-crystal diffraction data were collected on a Rigaku Saturn 724+ diffractometer with Mo–Kα radiation (λ = 0.71073 Å) at 173, 293 and 373 K for 1. Data processing including empirical absorption corrections was performed using the Crystalclear software package (Rigaku, 2018). The structures were solved by direct methods and refined by the full-matrix method based on F2 by means of the SHELXLTL software package. Non-H atoms were refined anisotropically using all reflections with I > 2σ (I). All H atoms were generated geometrically and refined using a "riding" model with Uiso = 1.2Ueq (C and N). The asymmetric units and the packing views were drawn with DIAMOND (Brandenburg and Putz, 2005). Angles and distances between some atoms were calculated using DIAMOND, and other calculations were carried out using SHELXLTL. DSC was performed by heating and cooling the polycrystalline samples on a Perkin–Elmer Diamond DSC instrument in the temperature range 280–360 K with a heating rate of 20 K/min under nitrogen atmospheric pressure in aluminum crucibles. For dielectric measurements, the samples were made with single-crystals cut into thin plate perpendicular to the crystal c-axis. Silver conduction paste deposited on the plate surfaces was used as the electrodes. Complex dielectric permittivity was measured with a TH2828A impedance analyzer over the frequency range from 1 kHz to 1 MHz with an applied electric ﬁeld of 0.5 V. Polycrystalline samples were ground and sieved into particle sizes of 100−150 μm for the powder SHG measurements. An unexpanded laser beam with low divergence (pulsed Nd: YAG at a wavelength of 1064 nm, 5 ns pulse duration, 1.6 MW peak power, 10 Hz repetition rate) was used. The instrument model was FLS 920 from Edinburgh Instruments, and the temperature was 280−360 K with the cooling and heating rate of 10 K/min with the DE 202 system, while the laser was Vibrant 355 II instrument from OPOTEK. Ferroelectric, pyroelectric and PFM measurements. For ferroelectric and pyroelectric measurements, the single crystal sample is the same with the dielectric one. The hysteresis loop was recorded on a Radiant Precision Premier II using a Sawyer-Tower electric circuit at a measurement frequency of 20 Hz. In the double-wave method measurement, the J–V curve was measured according to Ref (53), and one polarization inversion period is 10 seconds. Pyroelectric property was measured with an electrometer/high resistance meter (keithley 6517B) with a heating or cooling rate of 10 K/min. For PFM measurements, polarization imaging in nanoscale and local switching were carried out using a resonant-enhanced piezoresponse force microscopy



(MFP-3D, Asylum Research) on crystal sample along the c-axis.

ASSOCIATED CONTENT Supporting Information. IR spectrum, PXRD, crystal data and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21831004, 21427801 and 91422301).

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Fluoridation Achieved Antiperovskite Molecular Ferroelectric in [(CH3

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