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Fluorine Substitution Induced High T of Enantiomeric Perovskite Ferroelectrics: (R)- and (S)-3-(Fluoropyrrolidinium)MnCl 3

Yong Ai, Xiao-Gang Chen, Ping-Ping Shi, Yuan-Yuan Tang, Peng-Fei Li, Wei-Qiang Liao, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • 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

Fluorine Substitution Induced High Tc of Enantiomeric Perovskite Ferroelectrics: (R)- and (S)-3-(Fluoropyrrolidinium)MnCl3 Yong Ai,† Xiao-Gang Chen,§Ping-Ping Shi,§Yuan-Yuan Tang,† Peng-Fei Li,† Wei-Qiang Liao,† 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: The past decade has witnessed much progress in designing molecular ferroelectrics, whose intrinsic mechanical flexibility, structural tunability, and easy processability are desirable for the next-generation flexible and wearable electronic devices. However, an obstacle in expanding their promising applications in nonvolatile memory elements, capacitors, and sensors is effectively modulating the Curie temperature (Tc). Here, taking advantage of fluorine substitution on the reported molecular ferroelectric, (pyrrolidinium)MnCl3, we present enantiomeric perovskite ferroelectrics, namely, (R)- and (S)-3-(fluoropyrrolidinium)MnCl3. The close van der Waal’s radii and the similar steric parameters between H and F atoms ensure the minimum disruption of the crystal structure, while their different electronegativity and polarizability can trigger significant changes in the physical and chemical properties. As expected, the Tc gets successfully increased from 295 K in (pyrrolidinium)MnCl3 to 333 K in these two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step towards designing high-Tc molecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one could look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

INTRODUCTION High transition temperature (Tc), which is very preferred for ongoing design of electronic devices, has been sought amongst numerous functional materials including ferroelectric and superconductive systems.1 Strain engineering recently appeared to play a vital role in the much higher Tc of inorganic ferroelectric thin films, such as tetragonal PbTiO3 and 1-unit cell thick SnTe films, than that of the bulk.2-3 What is more well-known to have a large impact on the increase of Tc is the isotope effect. In oxide superconductors, it is the substitution of 16O with the heavier isotope 18O that can modify significantly the superconducting Tc if the motions of oxygen is involved in the formation of superconducting ground state.4 A similar case occurred in the quantum paraelectric, SrTiO3, where the oxygen isotope substitution induced ferroelectricity.5 Particularly, as a result of the change in proton tunneling frequency, the remarkable increase in Tc upon deuteration has been often observed in hydrogen-bonded ferroelectrics, while the Tc goes from 122 K in KH2PO4 and 310 K in PbHPO4 to 213 and 452 K in their deuterated forms KD2PO4 and PbDPO4, respectively.6-9 But nevertheless, given the fact that isotope effect works only in certain critical systems and may cause the issues of poor chemical stability or toxicity, there is still a long way to go in perfecting the simple, universal, and practical methodologies to enhance Tc. The past decade has witnessed the striking renaissance of molecular ferroelectrics, as their lightweight, flexible,

and biocompatible characteristics are especially promising for the future low cost, low energy consuming, wearable, portable, and environmental friendly electronic devices.10-12 The currently discovered species involve organic salts, metal-organic frameworks, and perovskite structures, with Tc ranging from as low as ~120 K to as high as ~450 K.1, 13-19 Given the subtlety, unpredictability, and complexity of those molecular ferroelectric systems, to disclose the general mechanism guiding the high Tc remains a great challenge. Fortunately, however, their unique capability of controlling the fundamental material properties by modulating the molecular structure provides a rich platform for designing High-Tc molecular ferroelectrics. Most recently, by introducing halogen atoms on the cation, we found that the Tc of 184 K in [(CH3)4N]PbI3 adopting the P63/m space group successfully increases to 189, 269, 291, and 312 K in [(CH3)3NCH2F]PbI3 (P63/m), [(CH3)3NCH2Cl]PbI3 (P63/mmc), [(CH3)3NCH2Br]PbI3 (P63/mmc), and the room-temperature ferroelectric [(CH3)3NCH2I]PbI3 (C2), respectively.20 Such precise molecular modifications that raise the potential energy barrier of the cationic tumbling motion point out an attractive approach to tune the Tc. And equally importantly, it was noted from this work that, replacing H with the F atom is more like an isostatic substitution in common with the isotope effect. The closest van der Waal’s radii and the most similar steric parameters between H and F atoms can ensure the minimum disruption of the crystal structure, whereas their different

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electronegativity and polarizability may trigger significant changes in the physical and chemical properties.21-23 Compared with the H/D isotope effect necessitating proton ordering, apparently, there will be a larger place for H/F substitution to enrich the molecular ferroelectric family.

relationship of the chirality of crystal packing in (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 crystals is consistent that of the chirality of (R)- and (S)-3-fluoropyrrolidinium cations.

Figure 1. Experimentally measured VCD and IR spectra for (R)- and (S)-3-(fluoropyrrolidinium)MnCl3.

Scheme 1. Design of molecular ferroelectrics through H/F substitution. As shown in scheme 1, based on the reported hybrid ferroelectric, (pyrrolidinium)MnCl3, undergoing a paraelectric-to-ferroelectric phase transition from Cmcm to Cmc21 at 295 K, herein we modified the cation with a F atom to obtain (R)-3-(fluoropyrrolidinium)MnCl3 and (S)-3-(fluoropyrrolidinium)MnCl3.24 The H/F substitution, leading to minor structural changes, enables them to maintain ferroelectric polarization. Furthermore, since homochiral compounds have a high chance to crystallize in the five chiral-polar point groups: 1 (C1), 2 (C2), 4 (C4), 3 (C3), and 6 (C6), the introduced chiral center on the modified cations is favorable for the generation of the ferroelectric phase adopting the chiral-polar space group P21.25 Inspiringly, just as expected, the Tc gets successfully increased up to 333 K in the two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step towards designing High-Tc molecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one can look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

RESULTS AND DISCUSSION The vibrational circular dichroism (VCD) measurement is a powerful technique for identifying the homochiral structures since the circular dichroism signal depends on the chiral chromophores. As shown in Figure 1, the VCD spectra of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 exhibit five pairs of strong signals (Δε) at 1628, 1610, 1429, 1387, 945, and 879 cm-1, and several relative weak dichroic signals centered at 1301, 1284, 1237, 1069, 1000, and 850 cm-1, corresponding exactly to the specific IR vibration peaks. The fact that the two VCD spectra are nearly mirror images of each other confirms the enantiomorphic nature of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 crystals. It is note that the enantiomorphic

The differential scanning calorimetry (DSC) results depicted in Figure 2a indicate the occurrence of a reversible phase transition in both (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 at around 333 K, which was determined by the peak temperature of the endothermic peak in the heating run. Notably, the marked temperature hysteresis or tailing effect in the heating and cooling processes is typical for the second-order phase transitions, with corresponding entropy changes (ΔS) of about 1.91 and 2.07 Jmol-1K-1 for (R)- and (S)-3-(fluoropyrrolidinium)MnCl3, respectively. The phase above Tc was defined as high temperature phase (HTP) and the phase below Tc as the low temperature phase (LTP). To further detect the symmetry change accompanying the phase transition, the temperature-dependent second harmonic generation (SHG) signal, that is sensitive to the inversion symmetry breaking and the generation of ferroelectric polarization, has also been recorded on the title compounds. As shown in Figure 2b, below 333 K, the detectable SHG activity indicates a non-centrosymmetric crystal structure in the LTP, being consistent with the 2 (C2) polar point group (see below). At around 333 K, the gradually decreased SHG signal corresponds to a transition to the nonpolar but chiral 222 (D2) point group in the HTP (see below). Landau relationship gives the formula: χ(2) = 6ε0βPs, where Ps is the spontaneous polarization and χ(2) is the second-order nonlinear optic susceptibility.10 In the temperature dependent SHG response, Ps contributes to χ(2) because β is almost independent of the temperature. The calculated Ps at 298 K in LTP for (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 is 4.83 μC/cm2 and 4.84 μC/cm2, respectively (see below). While at 363 K in HTP, the Ps for (R)and (S)-3-(fluoropyrrolidinium)MnCl3 is zero because of the non-polar nature of the 222 (D2) point group. Due to the contribution of spontaneous polarization to secondorder nonlinear optic susceptibility in LTP, the SHG intensity shows obvious enhancement from HTP to LTP. Such phenomenon is similar to that found in the typical molecular ferroelectric Rochelle salt, where the SHG intensity in the ferroelectric phase with the polar 2 (C2) point group is obviously stronger than that in the paraelectric phase with the nonpolar chiral 222 (D2) point group (Figure S1).10


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Figure 2. Phase transitions of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3. (a) DSC curves in a heating-cooling mode. (b) Temperature dependent SHG response. Real part (ε′) of the complex dielectric constant as a function of temperature at several frequencies for (c) (R)- and (d) (S)-3-(fluoropyrrolidinium)MnCl3. Inset: Plot of 1/ε′ vs temperature in the vicinity of Tc.

Generally, in the vicinity of Tc, the structural phase transitions including the ferroelectric ones can also be evidenced by the dielectric anomalies. Figure 2c gives the dielectric constant response versus temperature measured at various frequencies for (R)-3-(fluoropyrrolidinium)MnCl3. The real part (ε′) of the dielectric constant shows a notably anomalous peak at 333 K, whose maximum peak value is more than 15 times larger than that of the peak valley. And more importantly, the dielectric response obeys the Curie−Weiss behavior, revealing a proper ferroelectric phase transition. Based on the formulas ε′ = Cpara/(T− T0) (T > Tc) or Cferro/(T0′ − T) (T < Tc), where Cpara and Cferro are the Curie constants, and T0 and T0′ are the Curie−Weiss temperatures for the paraelectric and ferroelectric phases, respectively, for (R)-3-(fluoropyrrolidinium)MnCl3, at 100 Hz one can obtain the fitted Cpara = 3333 K and Cferro = 1111 K. The Cpara/Cferro ratio of 3 and the λ shape of the dielectric anomaly are both direct evidence for the second-order ferroelectric phase transition, in good agreement with the above DSC and SHG results.26 Similar case was also observed in (S)-3-(fluoropyrrolidinium)MnCl3 (Figure 2d), with the Cpara/Cferro ratio fitted as 3.3 at 100 KHz. The Curie constants fitted at different frequencies are given in the Tables S1 and S2.

Figure 3. Crystal structures of the ferroelectric phases of (a) (R)- and (b) (S)-3-(fluoropyrrolidinium)MnCl3 and of the respective paraelectric phases (c) and (d).

To better understand the mechanism of the phase transition, for both (R)- and (S)-3-(fluoropyrrolidinium)MnCl3, the single crystal structures were determined in the LTP and HTP. The two enantiomeric compounds adopt the same polar monoclinic space group P21 (point group 2 (C2)) at 298K, corresponding to the ferroelectric phase. The detailed crystal data and parameters are listed in the Table S3. Their typical hybrid metal halide perovskite structures with the general formula of ABX3 (A, B = different cations, X = anion) resemble that of the parent (pyrrolidinium)MnCl3 as reported elsewhere.24 The crystal structure of (S)-3-(fluoropyrrolidinium)MnCl3 is an enantiomorphism to that of (R)-3(fluoropyrrolidinium)MnCl3, having a mirror-image relationship. The asymmetric unit includes one 3-fluoropyrrolidinium cation and one (MnCl3)- moiety. The enantiomeric cations are all in an ordered state with a single site for N atom, and the chiral C atom connecting the F atom has “R” and “S” conformation in the (R)- and (S)-3(fluoropyrrolidinium)MnCl3, respectively (Figure 3). With Mn-Cl bond length in the range of 2.520-2.560 Å, the adjacent one-dimensional (MnCl3)n- anionic chains are parallel to each other. The ferroelectric polarization is mainly originated from the ordered arrangement of the homochiral 3-fluoropyrrolidinium cations. At 363 K, corresponding to the HTP, there is no significant change in the anionic chain. The average Mn−Cl bond length of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 shows almost no change from HTP (2.5510Å and 2.5518Å) to LTP (2.5545Å and 2.5545Å). The average shift of Mn atomic coordinate between HTP and LTP is only 0.0065 Åalong the b axis in one unit cell (Figures S2 and S3 and Tables S4 and S5). However, the 3-fluoropyrrolidinium cation occupies a special position of two-fold axis in HTP, which makes the cation 2fold orientationally disordered with the N atomic coordinate distributing equally over the two-fold axis. In the HTP, the disordered state indicates that the N atomic coordinate is centered on the two-fold axis. In the LTP, the vanishing of two-fold axis and the ordered state show that the N atomic coordinate moves. From 363 K in HTP to 298 K in LTP, the average shift of the N atomic coordinate is about 0.662 Åand 0.664 Åalong the b axis in one unit cell for each (R)- and (S)-3-fluoropyrrolidinium cation, respectively, which is significantly larger than that of Mn atomic coordinate. Therefore, the ferroelectric mechanism of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 is mainly attributable to the order-disorder transition of (R)- and (S)-3-fluoropyrrolidinium cation, respectively. The positive charges of the (R)- and (S)-3-fluoropyrrolidinium cations are assumed to locate on the N atoms. From the calculations of a point charge model (Figures S4 and S5 and Tables S6 and S7), the shift of N atomic coordinate may generate a polarization of about 4.83 μC/cm2 and 4.84 μC/cm2, respectively. As a result of the order−disorder transition of the (R)and (S)-3-(fluoropyrrolidinium) cations, the enantiomers undergo a phase transition from chiral-polar P21 to the nonpolar chiral space group C2221 (point group 222 (D2)). C2221 is one of the non-isomorphic super groups of P21. Such a group-to-subgroup relationship between the paraelectric and ferroelectric space groups meets the requirement of the Curie symmetry principle. Symmetry



breaking occurs with an Aizu notation of 222F2, belonging to the 88 species of potential paraelectric-to-ferroelectric phase transitions. Furthermore, from the variable temperature powder Xray diffraction (PXRD) measurements (Figure S6). The experimental PXRD below Tc matches well with the one simulated from the single crystal structure at 298K, indicating the high purity of the corresponding phase. When heated to above Tc, a few of the diffraction peaks disappear, suggesting a higher symmetric structure of the HTP. The number of polarization directions in the ferroelectric phase is expressed as n = Np/Nf, where Np and Nf represent the sum of symmetry elements of paraelectric and ferroelectric point groups, respectively.27-29 In this case, where n = 2, (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 are defined as uniaxial with two opposite polarization directions. No doubt that the molecular design strategy of fluorine substitution on the pyrrolidinium cation did maintain the vital role of cation in the formation of ferroelectric crystals.

of ferroelectric materials at nanometer scale without destroying sample.27-29 The amplitude and phase parameters acquired from PFM image provide the information about the value of the piezoelectric coefficient and the direction of the polarization, respectively. The static and dynamic issues of ferroelectric domains are closely related to the basic physics of ferroelectrics and the optimization of device performance. To further discover the ferroelectric properties of the title compounds, we performed the PFM measurements on the thin films. Figures 5a and 5b depict the out-of-plane PFM phase and amplitude images of (R)-3-(fluoropyrrolidinium)MnCl3 thin film coated on the ITO surface. As depicted in Figures 5a and 5b, the phase and amplitude images correspond well to each other, demonstrating a ferroelectric domain pattern with alternating bands of upward and downward polarizations separated by domain walls. Such stripe-like domains are usually found in classically uniaxial molecular ferroelectrics, such as diisopropylaminium bromide13 and triglycine sulfate (TGS)30. The electrically reversible spontaneous polarization is the intrinsic property of molecular ferroelectrics. Hence, local PFM-based hysteresis loop measurement was conducted by applying a DC voltage between the conductive tip and ITO substrate. As shown in Figures 5c and 5d, the hysteresis loop and butterfly-shape curve suggest the dominant polarization switching for (R)-3(fluoropyrrolidinium)MnCl3. The PFM signals of (S)-3(fluoropyrrolidinium)MnCl3 are quite similar as its enantiomer form (Figure S7). These results further confirm the ferroelectricity of the title compounds.

Figure 4. Ferroelectric hysteresis loops of (R)- and (S)-3(fluoropyrrolidinium)MnCl3 measured on the single-crystal sample along the polar b axis at 313 K.

The ferroelectricity was then confirmed by the measurements of polarization-electric field (P–E) hysteresis loops by the double-wave method. Concerning the ferroelectric space group P21, the crystals of both (R)- and (S)3-fluoropyrrolidinium)MnCl3 exhibit spontaneous polarization (Ps) along the b axis in the ferroelectric phase. As shown in Figure 4, the typical P–E hysteresis loops obtained at room temperature show the measured Ps of around 5.0 and 5.4 μC/cm2 for (R)- and (S)-3-(fluoropyrrolidinium)MnCl3, respectively, comparable to that of (pyrrolidinium)MnCl3 and the calculated results (Tables S6 and S7). Besides, the corresponding coercive field (Ec) values of (R)- and (S)-3-(fluoropyrrolidinium)MnCl3 are 6.8 and 8.2 kV/cm, respectively. Remarkably, the fluorine substitution persists the fine ferroelectric properties of the (pyrrolidinium)MnCl3 system as well as greatly enhances the Tc, endowing the molecular ferroelectrics the adaptability to above-room-temperature working environment. Piezoelectric force microscopy (PFM) has been widely performed as a very powerful tool for the characterization

Figure 5. Phase (a) and amplitude images (b) of the piezoresponse force microscope for the thin film of (R)-3-(fluoropyrrolidinium)MnCl3. (c) Phase and (d) amplitude signals as functions of the tip voltage for a selected point, showing local PFM hysteresis loops.

CONCLUSIONS In conclusion, the ability to modulate the Tc is addressing the challenge of effectively designing molecular ferroelectrics that play a unique role in the future electronic devices with high working temperatures. We have adopted the fluorine substitution strategy to construct the high-Tc enantiomeric perovskite ferroelectrics, namely, (R)- and (S)-3-(fluoropyrrolidinium)MnCl3. The introduced chiral


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Journal of the American Chemical Society center resulting from F substitution on the modified cations ensures the formation of the ferroelectric phase adopting the chiral-polar space group P21. And, this work demonstrates that, replacing H with the F atom in the molecular ferroelectric systems is an available Tc-enhancing method that can be on par with the isotope effect, while the title compounds not only persist the moderate ferroelectric performance of the prototypical one but also displays a noteworthy enhancement of 38 K in the resulting Tc. By providing a new way of modulating the Tc, we anticipate that there will be a large place for H/F substitution to enrich the molecular ferroelectric family.

METHODS Synthesis: The title compounds were obtained through slowly evaporation of a solution of acetonitrile/ethanol (3:1) containing (R)- or (S)-3-fluoropyrrolidinium hydrochloride (1.25 g, 10 mmol), and MnCl2· 4H2O (1.98 g, 10 mmol) at room temperature. The red rod-like crystals were formed after about one week. Differential scanning calorimetry (DSC), second harmonic generation (SHG) and Dielectric measurements: Perkin-Elmer Diamond instrument (model: Ins 1210058 INSTEC Instruments, laser: Vibrant 355 II, OPOTEK) was used in DSC measurement through heating the samples under inert atmosphere. The heating rate is 20 K min-1 with a temperature of 260–360 K. The sample pellets was prepared with 15 mm2 in area and 0.3 mm in thickness for Dielectric constant measurements. X-Ray crystallographic and Powder X-ray diffraction (PXRD) measurements: Single crystal structure was measured with Rigaku Saturn 924 CCD diffractometer and Mo-Kα radiation (λ = 0.71073 Å) at 298 K and 363 K, respectively. CrystalClear software package was used for data collection, Cell refinement, and data reduction. The SHELXL 2014 software package was used for structures refine. Rigaku D/MAX 2000 was performed for PXRD measurement under the various temperature between 293–373 K. Polarization−electric field (P−E) hysteresis loops: P−E hysteresis loops measurements were recorded using the double-wave method at 313 K. The double-wave method was carried out with a homemade system, including high voltage amplifier (Trek 623B), waveform generator (Agilent 33521A) and low-current electrometer (Keithley 6514). The measuring frequency was 0.05 Hz.

ASSOCIATED CONTENT Supporting Information Temperature dependent SHG intensity of Rochelle salt, fitted Curie constants, summary of crystal data, analysis of the Mn atomic displacement, calculation of polarization according to a point charge model, PXRD characterization and PFM for (S)-3-(fluoropyrrolidinium)MnCl3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*[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 91856114).

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Fluorine Substitution Induced High - American Chemical Society

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