Lone-Pair-Electron-Driven Ionic Displacements in a Ferroelectric

Sep 27, 2016 - A displacive-type mechanism, which accounts for the occurrence of .... an exceptionally thermally stable, guest-free low dielectric mat...
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Lone-Pair-Electron-Driven Ionic Displacements in a Ferroelectric Metal−Organic Hybrid Wen-Ping Zhao,† Chao Shi,† Alessandro Stroppa,*,‡,§ Domenico Di Sante,‡,# Fanica Cimpoesu,⊥ and Wen Zhang*,† †

Ordered Matter Science Research Center, Southeast University, Nanjing 211189, Jiangsu, China Consiglio Nazionale delle Ricerche (CNR-SPIN), Via Vetoio, I-67010 L’Aquila, Italy § International Centre for Quantum and Molecular Structures and Physics Department, Shanghai University, 99 Shangda Road, Shanghai 200444 China ⊥ Institute of Physical Chemistry of Roumanian Academy, Splaiul Independentei 202, Bucharest 060021, Romania # Institut fuer Theoretische Physik und Astrophysik, Universitaet Wuerzburg, Am Hubland Campus Sued, Wuerzburg 97074, Germany ‡

S Supporting Information *

ABSTRACT: A displacive-type mechanism, which accounts for the occurrence of ferroelectricity in most inorganic ferroelectrics, is rarely found in molecule-based ferroelectrics. Its role is often covered by the predominant order−disorder one. Herein, we report a lone-pairelectron-driven displacive-type ferroelectric organic−inorganic hybrid compound, [H2dmdap][SbCl5] (1; dmdap = N,N-dimethyl-1,3diaminopropane). The structure of 1 features a typical zigzag chain of [SbCl5]∞ containing cis-connected anionic octahedra. The compound undergoes a second-order paraelectric−ferroelectric phase transition at 143 K (P21/c ↔ Pc) with a saturation polarization of 1.36 μC·cm−2 and a coercive field of 3.5 kV·cm−1 at 119 K. Theoretical study discloses the ferroelectricity mainly originating from the relative displacements of the Sb and Cl ions in the crystal lattice, which are driven by the 5s2 lone-pair electrons of the SbIII center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric polarization in this class of compounds.

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INTRODUCTION Ferroelectric materials distinguish themselves from other advanced materials by switchable macroscopic polarizations, which endow the materials with excellent electrical, optical, mechanical, thermal, and even magnetoelectric properties.1−4 Molecule-based ferroelectrics, including organic−inorganic hybrids and molecular salts, have recently drawn much attention because of their advantages in structural tunability, multifunctionality, and processability.5−10 Some of them can show high working temperatures and large spontaneous polarizations that are comparable to conventional highperformance inorganic oxide ferroelectrics.6 Up to now, however, practically usable molecular ferroelectrics are still very scarce. This is primarily because prediction and screening of new ferroelectrics are hampered by the lack of a deep understanding of the structural origins of paraelectric−ferroelectric phase transitions. In this sense, it is fundamentally important to search for new examples with unique ferroelectric origins. Halogenometallate hybrids constitute a large family of organic−inorganic hybrid ferroelectrics.5b Attractive classes are the halogenoantimonates(III) and halogenobismuthates© XXXX American Chemical Society

(III), namely, Ay[MmXn], where A is the protonated amine or its analogues, M is the SbIII or BiIII ion, and X is the Cl, Br, or I anion.11−15 Their structures contain basic deformed MX6 octahedral units that connect with neighboring ones by corners, edges, or faces to form anionic discrete bioctahedra, chains, or layers. The occurrence of ferroelectricity is associated with order−disorder transitions of the organic cations and/or displacements of the anions. It is thought that distortion of the MX6 octahedron is triggered by the ns2 lone-pair electrons of the MIII center,14 like that in complex oxides such as multiferroic BiFeO3 and BiMnO3 and ferroelectric PbTiO3, where the lone-pair-electron-driven structural distortions are responsible for the appearance of ferroelectricity.16,17 However, this mechanism has not been finely understood because of the interference of the order−disorder transitions of the organic cations13 or incompleteness of the structural data.14b Herein, we report a new SbIII-based ferroelectric compound, [H2dmdap][SbCl5] (1; dmdap = N,N-dimethyl-1,3-diaminopropane), with a structure featuring a 1D chain of cis-connected Received: July 5, 2016

A

DOI: 10.1021/acs.inorgchem.6b01545 Inorg. Chem. XXXX, XXX, XXX−XXX

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7.795(2) Å, b = 14.623(3) Å, c = 14.101(4) Å, β = 118.91(2)°, and Z = 4. The asymmetric unit contains the pair of H2dmdap cation and an anionic [SbCl5] unit (Figure 1a). The dmdap

anionic octahedra. It shows a paraelectric−ferroelectric phase transition at 143 K. On the basis of detailed structural analysis and first-principles calculations, the origin of the ferroelectricity is clearly assigned to the relative displacements of the Sb and Cl ions in the crystal lattice that are triggered by the 5s2 lone-pair electrons of the SbIII center.

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EXPERIMENTAL SECTION

Synthesis. An aqueous solution (30 mL) containing SbCl3 (2.28 g, 10 mmol), dmdap (1.02 g, 10 mmol), and concentrated HCl (5 mL) was slowly evaporated at room temperature to yield colorless block crystals of 1 (yield 75%, based on SbCl3). X-ray Diffraction Experiments. Powder X-ray diffraction was measured on a Rigaku SmartLab X-ray diffraction instrument. Singlecrystal X-ray diffraction was performed on a Rigaku Saturn 724+ CCD diffractometer equipped with a Rigaku low-temperature gas spray cooler device using Mo Kα (λ = 0.71075 Å) radiation from a graphite monochromator. Data collection, cell refinement, and data reduction were performed using the Rigaku CrystalClear1.3.5 software package.18 The structures were solved by direct methods and successive Fourier synthesis and then refined by full-matrix least-squares refinements on F2 using the SHELXTL program.19 All non-H atoms were refined anisotropically, and the positions of all H atoms were generated geometrically. CCDC 1478804 and 1478805 contain the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. [fax (+44) 1223-336-033 or e-mail [email protected]]. Thermal Measurements. Differential scanning calorimetry (DSC) curves were obtained on a PerkinElmer Diamond differential scanning calorimeter under nitrogen at atmospheric pressure with a 10 K·min−1 heating/cooling rate. Specific heat analysis was carried out on a Quantum Design Physical Property Measurement System. Second Harmonic Generation (SHG) Measurement. A SHG experiment was carried out on an INSTEC Ins 1210058 instrument with the laser Vibrant 355 II, OPOTEK (pulsed Nd:YAG at a wavelength of 1064 nm, a pulse duration of 5 ns, a peak power of 1.6 MW, and a repetition rate of 10 Hz). The numerical value of the nonlinear optical coefficients for SHG was determined by a comparison with the KDP reference. Dielectric and Ferroelectric Measurements. For dielectric measurements, both the single-crystal samples with well-defined faces and the crystalline-powdered samples were used in the form of disks. Silver conduction paste was deposited on the surfaces as electrodes. Dielectric constant curves were measured on a Tonghui TH2828A impedance analyzer over the frequency range of 500 Hz and 1 MHz at a heating/cooling rate of 5 K·min−1. Hysteresis loops were recorded on a Radiant Precision Premier II instrument. The dielectric contribution was subtracted from the curves. Theoretical Calculation. Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package.20 The projector-augmented-wave method21 was employed with the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation functional.22 The energy cutoff was set to 400 eV, and a 2 × 4 × 4 Monkhorst−Pack grid of k points was used. The Berry phase approach was employed to calculate the ferroelectric polarization P.23 We also used the Heyd−Scuseria−Ernzerhof (HSE) hybrid functional,24 which mixes a fraction of Fock exchange with the exchange density functional. Symmetry mode analysis has been done using the tools of the Bilbao Crystallographic Server, such as Pseudosymmetry and Amplimodes.25 For more details, see the Supporting Information.

Figure 1. Single-crystal structure of 1 at 293 K: (a) asymmetric unit with thermal ellipsoids at the 50% probability level; (b) top and side views of the [SbCl5]∞ zigzag chain; (c) packing diagram along the a axis. Hydrogen bonds are depicted as cyan dashed lines. H atoms are omitted for clarity.

cation is totally ordered and shows an all-trans conformation of the N1−C4 chain, being almost the most favored conformation trapped in the lattice. In the [SbCl5] unit, the Sb ion adopts a distorted octahedral coordination geometry with six Cl ions. The Sb−Cl lengths vary from 2.476 to 2.906 Å, among which the four terminal Cl3−Cl6 ions form the relatively shorter bond lengths with the Sb ion (2.476−2.828 Å) and the two bridging Cl1 and Cl2 ions form the relatively longer bond lengths (2.902 and 2.859 Å, respectively). The Cl1 and Cl2 ions locate at the middle point of the line between two neighboring Sb ions with the bond angle ∠Cl1Sb1Cl2 = 85.07°. Thus, typical zigzag chains of [SbCl5]∞-containing cis-connected anionic octahedra run along the a axis (Figure 1b). The cations locate in the cavities formed by the anionic chains in the crystal lattice. Several hydrogen bonds develop between the NH groups and Cl ions, with the N···Cl distances in the range 3.218−3.448 Å (Figures 1c and S2). Paraelectric−Ferroelectric Phase Transition. The paraelectric−ferroelectric phase transition of 1 was confirmed by the specific heat capacity (Cp), second-harmonic generation (SHG), dielectric constant, and electric hysteresis loop measurements (Figures 2 and S3). The Cp curve shows a peak at 143 K upon cooling, corresponding to the phase transition point (Tc; Figure 2a). The cusplike peak indicates that the phase transition has a second-order characteristic, together with the fact that there are no observable thermal peaks in the DSC measurement between 123 and 293 K. The enthalpy change of the phase transition is extracted from the Cp curve with a value of 32.3 J·mol−1, and the corresponding entropy change is 0.226 J·mol−1·K−1. The SHG measurement was performed on the crystalline powder of 1 between 123 and 293 K (Figure S4). With a decrease of the temperature, a rise of the signal appears at about

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RESULTS AND DISCUSSION Crystal Structure. Compound 1 is a blocklike crystal with well-developed {011} crystal faces (Figure S1). At 293 K, it crystallizes in the monoclinic space group P21/c with a = B

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the frequency, suggesting that no relaxation process occurs during the phase transition. From the curve of the reciprocal of the dielectric constant 1/ε′ as a function of the temperature, the Curie constant C in both the paraelectric and ferroelectric phases (Cpara and Cferro) can be drawn by fitting to the Curie− Weiss law, ε′ = C/(T − T0), where T0 is the Curie−Weiss temperature. At 10 kHz, the ratio of |Cpara/Cferro| is 3.3 and T0 is 143 K, equal to Tc. These results indicate that the phase transition in 1 is of second order, which is consistent with the thermal measurements. Hysteresis loops of 1 were measured in the temperature range covering Tc (Figure 2c). Above Tc, only linear responses are observed, corresponding to the paraelectric phase. Nonlinear responses and typical electric hysteresis loops develop below Tc. With a decrease in the temperature, the values of the remanent polarization (Pr) and coercive field (Ec) increase gradually, which are extracted from the intercepts when E and P are zero, respectively. At 119 K, Pr reaches a value of 1.36 μC· cm−2 and Ec a value of 3.5 kV·cm−1. A temperature-dependent Pr curve shows continuous changes during the occurrence of the ferroelectric phase, consistent with the characteristics of a second-order phase transition (Figure 2c, inset). Origin of the Ferroelectricity. In order to elucidate the structural origin of the ferroelectricity in 1, a structural comparison between the paraelectric and ferroelectric phases is made. At 113 K, 1 crystallizes in the polar monoclinic space group Pc with a = 7.6816(7) Å, b = 14.545(2) Å, c = 13.974(2) Å, β = 118.32(1)°, and Z = 4 (Figure S6 and Table S1). The data show little difference from the ones at 293 K. The H2dmdap (A) cation still shows the all-trans conformation without any observable changes. When one focuses on the anionic part, however, striking ionic displacements occur (Figures 3a and S7). Large variations of the Sb−Cl distances in the ferroelectric phase at 113 K are found in the bridging Cl1, Cl2, and terminal Cl3/Cl7 ions with values (by reference to the paraelectric phase at 293 K) of +4.7%/−6.1%, + 2.1%/− 4.5%, and +3.0%/−1.9%, respectively (Table S2). In contrast, the remaining terminal Cl ions, i.e., Cl4/Cl8, Cl5/Cl10, and Cl6/Cl9, show only slight displacements with values of less than 1.4%, which is comparable with the crystal contraction ratio of the two phases (ca. 1.0%). Overall, ionic net displacement is supposed to occur roughly along the a axis, consistent with the dielectric constant and hysteresis loop measurements. To gain a deeper understanding of the origin of the ferroelectricity, we perform DFT simulations and symmetrymode analysis (see the Supporting Information for details). In order to estimate the ferroelectric polarization, we introduce a reference nonpolar structure in terms of which the atomic displacements leading to the polar phase are described. The polarization is evaluated along the path connecting the nonpolar and polar structures. The parameter λ represents the amplitude of the distortion mode connecting the two phases, and it is normalized to one, i.e., λ = 0 and 1 for the nonpolar and polar structures, respectively. The total polarization is estimated to be 0.30 (0.45) μC·cm−2 in PBE (HSE), which is roughly comparable with the experimental result (1.36 μC·cm−2 at 119 K). The discrepancy between the theoretical estimate and experimental value may have different causes, such as the use of different exchange-correlation functional or relaxation effects on the unit-cell volume. All of the details have been carefully discussed and analyzed in a study of the organic compound croconic acid.26 It is interesting to decompose the

Figure 2. (a) Specific heat capacity Cp, (b) real part of the dielectric constant ε′ (inset: curve fitting of the 1/ε′ as a function of the temperature at 10 kHz, shown as brown dashed lines), and (c) selected hysteresis loops (inset: temperature dependence of the remanent polarization Pr) of 1. The electrical measurements were performed on the single-crystal sample along the [100] direction.

145 K, indicating the occurrence of a noncentrosymmetric structure of the sample. At the lower temperature (130 K), the SHG intensity reaches a value of 5.4 times that of the reference KH2PO4. This measurement affords strong proof to support the conclusion that 1 undergoes a transition from a centrosymmetric to a noncentrosymmetric structure, more exactly, a polar structure in the ferroelectric phase. The dielectric constant (ε = ε′ − iε″) was measured on the single-crystal sample along the [100] direction (Figures 2b and S5). A huge peak of ε′ appears at 143 K upon cooling. The peak values vary from 6300 at 0.5 kHz to 1130 at 1 MHz, a striking decrease with an increase of the electric-field frequency. This phenomenon is a common feature for normal ferroelectrics and can be explained by a domain wall model; that is, the process of nucleation and growth of domain walls that contribute to the dielectric constant value is time-consuming or frequency-dependent.1 There is no shift of Tc with variation of C

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polarization according to the functional units in the unit cells, i.e., the A organic cations and the SbCl5 unit. As shown in Figure 3c, the total polarization increases linearly as a function of the normalized amplitude of the distortion mode λ. The functional mode decomposition shows that the A organic cations contribute little to the total polarization, while the SbCl5 units account for more than 90% of the total polarization. This confirms that the anionic unit is the main structural origin for the ferroelectric polarization in 1. Another interesting feature is that the two contributions of A and SbCl5 add linearly to give the total polarization. This suggests that the two subsystems are essentially decoupled or weakly interacting. A more detailed microscopic analysis is provided by focusing on the effects of the lone pairs of the SbCl5 units (Figures 3b, 4, and S8). It is found that the lone pairs induce distortions in the linear chains of the octahedra, which, in turn, account for the polarization. The main role of the lone-pair distortion in the ferroelectric polarization is further confirmed by the main contribution to polarization, which comes from the SbCl5 units. The mechanism of local distortions of the vertex-shared SbCl6 octahedral units, as well as the dynamics determining the ferroelectric behavior, can be assigned to the stereochemical activity of the lone pair of the SbIII site.27 In 1, the location of the lone pair can be easily identified by considering the triad of the longer Sb−Cl bonds in each unit. Intuitively, the vector aligned with the lone pair, corresponding also to the dipole moment of the distorted octahedral moiety, can be considered as a vector resulting from the longer three Sb−Cl bond axes. In Figure 4, the sequence of one [SbCl5]∞ chain is shown. The spatial arrangements of the lone pairs give rise to alternately up and down orientations with respect to the mean plane containing the Sb−Cl−Sb bridges. One of the apical Sb−Cl bonds (perpendicular to the slab plane) is elongated. The direction of the elongated and perpendicular Sb−Cl bond also alternates from unit to unit. The structural distortions are detailed in the Supporting Information.

Figure 3. (a) Relative displacements of the Cl ions in the anionic chain in the ferroelectric phase of 1 (113 K). Blue digits denote the Sb−Cl bond lengths (Å). Ionic-displacement-induced dipole moments (p1 + p2) are drawn schematically as arrows (the length denotes the strength of the moment). The vector sum p (p = p1 + p2) of the major polarization components p1 and p2 gives the direction of the overall ionic polarization roughly along the a axis. (b) Two views of the [SbCl5]∞ chains showing the lone-pair (brown) orientations toward the most open octahedral faces. (c) Total polarization decomposed according to the functional groups H2dmdap (A) and SbCl5. See the text for details.

Figure 4. Different views of the [SbCl5]∞ chains, showing the lone-pair orientations toward the most open octahedral faces. The dipole carried by a distorted octahedron is represented by an arrow along the lone pair. The longer three bonds of a [SbCl6]3− moiety are represented by a lighter gray tone than the shorter ones (that point outward). In the right-side pictures, the dipole vectors of the surrounding organic cations are also shown. The arrows are directed toward the barycenter of negative charge in each unit, namely, the most substituted N atom (see the text and Figure S8). D

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Shanghai University and the Special Program for Applied Research on Super Computation of the NSFC−Guangdong Joint Fund (the second phase). D.D.S. acknowledges financial support from the German Research Foundation (DFG-SFB Project No. 1170).

The possibility of tuning the ferroelectric polarization by the appropriate choice of the organic cation should stimulate further study in hybrid framework materials.28 In 1, it shows that, even though the cation carries a large dipole moment, this is not a sufficient condition to have a large contribution to the polarization of the crystal. Indeed, the spatial arrangement of the cation dipoles may give rise to a negligible contribution. Therefore, a possibility of enhancing the polarization would be to use the appropriate framework geometry in connection with the hydrogen-bond topology, which could favor cation tilting, i.e., cation-tilting engineering. On the other hand, acting on the halogen atoms, i.e., changing the electronegativity difference with respect to the metal atoms, would also change the resulting dipole moment of the octahedral units and thus change the resulting ferroelectric polarization of the crystal. We suppose that the interplay between the dipole moments of the octahedral unit, which indirectly depends on the lone pair, and the tilting of the cation dipoles is one of the further directions in which high-performance molecule-based ferroelectrics can be explored.

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(1) Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Oxford University Press: Oxford, U.K., 1991. (2) Scott, J. F. Ferroelectric Memories; Springer: Berlin, 2000. (3) Ye, Z. G., Ed. Handbook of advanced dielectric, piezoelectric and ferroelectric materials: Synthesis, properties and applications; Woodhead Publishing: Cambridge, U.K., 2008. (4) Scott, J. F. Applications of Modern Ferroelectrics. Science 2007, 315, 954. (5) (a) Horiuchi, S.; Tokura, Y. Organic ferroelectrics. Nat. Mater. 2008, 7, 357. (b) Zhang, W.; Xiong, R.-G. Ferroelectric Metal-Organic Frameworks. Chem. Rev. 2012, 112, 1163. (c) Tayi, A. S.; Kaeser, A.; Matsumoto, M.; Aida, T.; Stupp, S. I. Supramolecular ferroelectrics. Nat. Chem. 2015, 7, 281. (6) (a) Horiuchi, S.; Tokunaga, Y.; Giovannetti, G.; Picozzi, S.; Itoh, H.; Shimano, R.; Kumai, R.; Tokura, Y. Above-room-temperature ferroelectricity in a single-component molecular crystal. Nature 2010, 463, 789. (b) Horiuchi, S.; Kagawa, F.; Hatahara, K.; Kobayashi, K.; Kumai, R.; Murakami, Y.; Tokura, Y. Above-room-temperature ferroelectricity and antiferroelectricity in benzimidazoles. Nat. Commun. 2012, 3, 1308. (c) Fu, D.-W.; Zhang, W.; Cai, H.-L.; Ge, J.-Z.; Zhang, Y.; Xiong, R.-G. Diisopropylammonium Chloride: A Ferroelectric Organic Salt with a High Phase Transition Temperature and Practical Utilization Level of Spontaneous Polarization. Adv. Mater. 2011, 23, 5658. (d) Fu, D.-W.; Cai, H.-L.; Liu, Y.; Ye, Q.; Zhang, W.; Zhang, Y.; Chen, X.-Y.; Giovannetti, G.; Capone, M.; Li, J.; Xiong, R.-G. Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal. Science 2013, 339, 425. (7) Akutagawa, T.; Koshinaka, H.; Sato, D.; Takeda, S.; Noro, S.; Takahashi, H.; Kumai, R.; Tokura, Y.; Nakamura, T. Ferroelectricity and polarity control in solid-state flip-flop supramolecular rotators. Nat. Mater. 2009, 8, 342. (8) (a) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. Order−Disorder Antiferroelectric Phase Transition in a Hybrid Inorganic−Organic Framework with the Perovskite Architecture. J. Am. Chem. Soc. 2008, 130, 10450. (b) Jain, P.; Ramachandran, V.; Clark, R. J.; Zhou, H. D.; Toby, B. H.; Dalal, N. S.; Kroto, H. W.; Cheetham, A. K. Multiferroic Behavior Associated with an Order− Disorder Hydrogen Bonding Transition in Metal−Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture. J. Am. Chem. Soc. 2009, 131, 13625. (c) Xu, G.-C.; Ma, X.-M.; Zhang, L.; Wang, Z.M.; Gao, S. Disorder-Order Ferroelectric Transition in the Metal Formate Framework of [NH4][Zn(HCOO)3]. J. Am. Chem. Soc. 2010, 132, 9588. (d) Xu, G.-C.; Zhang, W.; Ma, X.-M.; Chen, Y.-H.; Zhang, L.; Cai, H.-L.; Wang, Z.-M.; Xiong, R. G.; Gao, S. Coexistence of Magnetic and Electric Orderings in the Metal−Formate Frameworks of [NH4][M(HCOO)3]. J. Am. Chem. Soc. 2011, 133, 14948. (e) Nagabhushana, G. P.; Shivaramaiah, R.; Navrotsky, A. Thermochemistry of Multiferroic Organic−Inorganic Hybrid Perovskites [(CH3)2NH2][M(HCOO)3] (M = Mn, Co, Ni, and Zn). J. Am. Chem. Soc. 2015, 137, 10351. (f) Ghosh, S.; Di Sante, D.; Stroppa, A. Strain Tuning of Ferroelectric Polarization in Hybrid Organic Inorganic Perovskite Compounds. J. Phys. Chem. Lett. 2015, 6, 4553. (g) Tian, Y.; Shen, S.; Cong, J.; Yan, L.; Wang, S.; Sun, Y. Observation of Resonant Quantum Magnetoelectric Effect in a Multiferroic MetalOrganic Framework. J. Am. Chem. Soc. 2016, 138, 782. (9) (a) Zhao, H.-X.; Kong, X.-J.; Li, H.; Jin, Y.-C.; Long, L.-S.; Zeng, X. C.; Huang, R.-B.; Zheng, L.-S. Transition from one-dimensional water to ferroelectric ice within a supramolecular architecture. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 3481. (b) Zhou, B.; Kobayashi, A.; Cui, H. B.; Long, L.-S.; Fujimori, H.; Kobayashi, H. Anomalous

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CONCLUSIONS In conclusion, a clear displacive-type mechanism is found in a new ferroelectric organic−inorganic hybrid compound. By analysis of the structures and exploitation of symmetry-mode analysis and first-principles calculations, the origin of the ferroelectricity is ascribed to the relative displacements of the Sb and Cl ions in the 1D [SbCl5]∞ zigzag anionic chains in the crystal lattice, which are driven by the 5s2 lone-pair electrons of the SbIII center. This study clarifies a lone-pair-driven displacive-type mechanism that is rare in ferroelectric organic−inorganic hybrids and affords a new strategy to design and screen new molecular ferroelectrics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01545. Details on the experimental data and structural analysis (PDF) X-ray crystallographic data in CIF format (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (Grant 21225102). A.S. and F.C. are thankful for the Bilateral Agreement CNR/RA (Joint Project 2014-2016) No. 0006436 (1/27/2014). A.S. and D.D.S. acknowledge the CARIPLO Foundation through the MAGISTER Project Rif2013-0726. A.S. thanks the High-End Foreign Expert program hosted by E

DOI: 10.1021/acs.inorgchem.6b01545 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01545 Inorg. Chem. XXXX, XXX, XXX−XXX