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Discovery of an Antiperovskite Ferroelectric in [(CH3)3NH]3(MnBr3)(MnBr4) Zhenhong Wei, Wei-Qiang Liao, Yuan-Yuan Tang, Peng-Fei Li, Ping-Ping Shi, Hu Cai, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05037 • Publication Date (Web): 17 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018
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Journal of the American Chemical Society
Discovery of an Antiperovskite Ferroelectric in [(CH3)3NH]3(MnBr3)(MnBr4) Zhenhong Wei,†,‡ Wei-Qiang Liao,†,§,‡ Yuan-Yuan Tang,§ Peng-Fei Li,§ Ping-Ping Shi,§ Hu Cai,† and Ren-Gen Xiong*,†,§ †
Ordered Matter Science Research Center, Nanchang University, Nanchang, 330031, P. R. China Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, P.R. China §
Supporting Information Placeholder ABSTRACT: It is known that perovskites with the general chemical formula of ABX3 (A, B = cations, X = anion) have been intensively studied over the last half century because of their diverse functional properties, such as ferroelectricity in BaTiO3, piezoelectricity in PZT (lead zirconate titanate), and recently developed photovoltaic properties in CH3NH3PbI3. However, rather less attention has been paid to their ‘inverse’ analogs, antiperovskites, which have a chemical formula X3BA, where A and B are anions and X is a cation. Although most of important ferroelectrics are perovskites, no antiperovskite ferroelectrics have been found since the discovery of antiperovskites in 1930. Here, for the first time, we report a X3BA antiperovskite ferroelectric [(CH3)3NH]3(MnBr3)(MnBr4) (where (CH3)3NH is X, MnBr3 is B, and MnBr4 is A), which shows outstanding ferroelectricity with a significantly high phase transition temperature of 458 K as well as fascinating photoluminescence properties with two intense emissions. This finding opens a new avenue to explore the golden area of antiperovskites for high performance functional materials.
The intriguing functional properties such as ferroelectricity, superconductivity, colossal magnetoresistance, and ionic conductivity make ABX3-type perovskites (A, B = cations, X = anion) essential for many high-tech applications encompassing almost all aspects of modern life.1 Especially noteworthy is the rise of organic-inorganic perovskites over the past decade, while [CH3NH3]PbI3-based solar cells have been attracting worldwide interest due to the rapid growth of efficiency from 3.8% to 22.1%, rivaling that of high-purity crystalline silicon.2 More recently, an organic-inorganic perovskite ferroelectric, [Me3NCH2Cl]MnCl3 with extraordinary huge piezoelectric coefficient (d33) of 185 pC/N, appeared as a potential candidate for environmentalfriendly devices.3 For such hybrid system, the remarkable structural flexibility and tunability provide a fertile “playground” for achieving versatile crystal structures with diverse physical properties, and the simple, low-temperature, and energy-efficient processing is promising for reducing production cost.4 Actually, it is also possible to invert the ionic charges of perovskites, forming X3BA-type antiperovskite structures with anions on the A- and B-sites and cations on the X-site. As ‘inverse’ analogs or ‘twin’ of perovskites, antiperovskites have a long development history, too. It is since 1930s that metallic antiperovskites like Mn3CGa, Mn3NCu, Ni3CMg, Ni3NZn, and Li3OBr have been found to show a wide range of fascinating physical properties involving giant magetoresistance, large magnetocaloric effect,
giant magnetostriction, superconductivity, superionic conductivity, etc.5 However, in contrast to the fact that most technologically important ferroelectrics are inorganic perovskites, up to now antiperovskites contribute nothing to the field of ferroelectrics. Ferroelectricity occurs only in crystals belonging to the 10 polar point groups, i.e. 1, 2, m, mm2, 4, 4mm, 3, 3m, 6, and 6mm,6 but the majority of metallic antiperovskites usually crystallize in highsymmetric and centrosymmetric space groups.7 Moreover, the conductivity presented in these metallic materials is also not beneficial to induce and measure ferroelectric nature. As a consequence, organic-inorganic antiperovskites come into view of researchers. The flexible antiperovskite structure can allow for rotation or reorientation of the organic cations, and the asymmetry of them helps to template polar structure and include electric dipole, both enabling the generation of ferroelectricity. Inspired by [CH3NH3]PbI3, a class of organic-inorganic antiperovskites containing the same cation has been theoretically proposed and investigated.8 With the pronounced off-center displacement of ions, they are predicted to be stable, polar, possess large polarization, and demonstrate the potential for ferroelectrics. This points out the interesting future direction of organic-inorganic antiperovskite ferroelectrics, and to bring them into practice is urgently required for adding luster to the vast perovskite universe. The real ferroelectric property of antiperovskites is a remaining puzzle that has resisted solution for more than seventy years. In terms of the exploration of ferroelectric candidates, molecular tailoring or modification used for tuning the structural properties, ferroelectric properties as well as other functionalities is naturally a powerful tool. Based on years of experience on seeking ABX 3type functional perovskites,3,9 we found that the structural feature of [(CH3)3NH]3(MnBr3)(MnBr4) (1) is consistent with that of X3BA antiperovskites, with X = (CH3)3NH, B = MnBr3, and A = MnBr4. It is really exciting this antiperovskite crystallizes in the polar space group P63mc, and a ferroelectric phase transition occurs at as high as 458 K. It is for the first time that the existence of ferroelectricity in antiperovskites can be confirmed experimentally. Moreover, 1 shows intriguing photoluminescence properties with two strong emissions. Compositional and chemical versatility of this family has many possibilities for systematically determining structure-property relationships to help targeted design antiperovskite ferroelectrics. This work will definitely trigger more research efforts to further explore functional antiperovskite materials for many applications. Compound 1 was easily obtained by slow evaporation of stoichiometric amounts of [(CH3)3NH]Br and MnBr2 in an ethanol solution. Phase purity of the as-grown crystals was confirmed by
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lectric phases: 6/mmmF6mm. Based on the group-subgroup relationship, there is just one mother group of the P63mc belonging to the point group 6/mmm: P63/mmc, which further suggests that the paraelectric phase should have the space group P63/mmc. The mirror symmetry in the paraelectric phase will be satisfied by the disorder of the [(CH3)3NH]+ cations and the [MnBr4]2- anions, which induces the disappearing of spontaneous polarization in the paraelectric phase.
powder X-ray diffraction (PXRD) (Figure S1) and elemental analysis (Table S1). The single-crystal structure of 1 was determined at different temperatures from 93 K to 423 K (Table S2). At 293 K, 1 crystallizes in the space group P63mc. The structure is comprised of [MnBr3]- chains, [(CH3)3NH]+ cations around the chains and the [MnBr4]2- anions siting between the chains (Figure 1a and Figure S2a). Such a structure can be further understood as the packing of the chains [MnBr3][(CH3)3NH]3 separated by the [MnBr4]2- anions. The chain [MnBr3][(CH3)3NH]3 is formed by tilting face-sharing [MnBr3][(CH3)3NH]6 octahedra with the MnBr6 octahedra as the centers and the [(CH3)3NH]+ cations as the vertexes. Such a packing is similar to that for the hexagonal perovskite BaNiO3,10 except that the signs of the charges of the cations and anions in 1 are opposite to those in BaNiO3, with [MnBr4]2- anion corresponding to the Ba2+ ion, the [(CH3)3NH]+ cation to the O2- ion, and the (Mn(Br(0.5))6)- octahedron to the Ni4+ ion. Therefore, 1 is a X3BA hexagonal antiperovskite. As shown in Figure 1a, the [MnBr4]2- anions have the same orientation, and the N-H bonds of the organic cations point upper right or left, which should lead to a spontaneous polarization.
Figure 2. Variable-temperature PXRD patterns of 1. The phase transition of 1 was evidenced by a sharp heat anomaly at around 458 K in the differential scanning calorimetry (DSC) curve (Figure S4). The temperature-dependent ε′ (the real part of the dielectric permittivity) at 500 Hz of 1 also shows a notable dielectric anomaly near this temperature (Figure 3a), further confirming the phase transition at around Tc = 458 K. The maximum ε′ value of the dielectric anomaly is few hundred times larger than those in the stable states, revealing a ferroelectric-to-paraelectric transition. It is note that Tc of 1 is among the highest one in molecular ferroelectrics, which makes 1 adaptable to hightemperature working environment. The Tc is higher than those for organic salts such as diisopropylammonium bromide (426 K),12 molecular perovskites including (3Ammoniopyrrolidinium)RbBr3 (440 K),9d (benzylammonium)2PbCl4 (438 K),13 [Me3NCH2ClMnCl3 (406 K),3 and some inorganic perovskites such as BaTiO3 (393 K). We also performed the measurement of temperature dependent SHG (second harmonic generation) signal to detect the symmetry change during the transition process. With temperature increasing, the SHG intensity remained stable below Tc, and suddenly vanished at around Tc (Figure 3b), which manifests a transition from a noncentrosymmetric phase to a centrosymmetric one, consistent with the space group change from P63mc to P63/mmc. The ferroelectricity of 1 were then directly verified by the measurements of polarization−electric field (P−E) hysteresis loops and pyroelectric effect. Figure 3c shows typical ferroelectric J−E (current density–electric filed) curve with two opposite peaks at 343 K. From the current accumulating, we get a standard P−E hysteresis loop with the saturate polarization (Ps) of 0.45 μC/cm2. This value is slightly larger than those found in some typical molecular ferroelectrics such as Rochelle salt (0.2 μC/cm2),14 [C7H10NO(18-crown-6)][BF4](0.35 μC/cm2),15 and ammonium sulfate (0.25 μC/cm2);14 but smaller than those of recently developed molecular perovskite ferroelectrics (3.5-13 μC/cm2).3,9,13 By integrating the pyroelectric current of 1, temperature dependent spontaneous polarization was obtained. Polarization appears be-
Figure 1. Comparison of the crystal structures of 1 and BaNiO3. (a) The packing view of 1 viewed along the [110]-direction. H atoms bonded to the C atoms were omitted for clarity. (b) The packing view of BaNiO3. Compound 1 was found to exhibit a ferroelectric phase transition at around Tc = 458 K by thermal and dielectric analysis below. Unfortunately, the single-crystal structures above Tc are not able to be determined because of weak diffraction. We thus performed the variable-temperature PXRD measurements. When the temperature increases, the PXRD patterns below Tc are consistent with those at 293 K (Figure 2). However, at 463 K above Tc, the PXRD patterns show an obvious decrease in the number of the diffraction peaks, indicating that the paraelectric phase has a higher symmetry than the ferroelectric phase. The indexing of the PXRD patterns at 463 K reveals a hexagonal lattice with a = b = 15.6620(6) Å, c = 6.6607(3) Å (Figure S3). From Pawley refinements, the space group P63/mmc is the most possible space group. According to Aizu rule on ferroelectric phase transitions,11 for the ferroelectrics with point group 6mm, there is only one kind of permitted symmetry change between the paraelectric and ferroe-
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Journal of the American Chemical Society low Tc and abruptly drops to zero at around Tc (Figure 3d), in accordance with the transition from a ferroelectric phase to a paraelectric one.
Figure 3. Ferroelectricity and related properties of 1. (a) The ε′ as a function of temperature in the heating run. (b) The temperature dependence of the SHG intensity. (c) P–E hysteresis loops measured at 343 K. (d) The temperature dependence of the spontaneous polarization.
Figure 5. Photoluminescence properties of 1. (a) Ultraviolet-vis (UV-vis) absorption spectra and photoluminescence excitation (PLE) spectrum. (b) Emission spectra at various excitation wavelengths. Insets of (b) represent the [MnBr3]- chain and the [MnBr4]2- anion.
Piezoresponse force microscope (PFM) has become a standard method for both probing and switching the local ferroelectric polarization.16 To further study its ferroelectricity, we performed the PFM measurements in the thin film of 1. Figure 4 shows the PFM phase image overlaid on three-dimensional (3D) topography, where the domains are mainly irregular shape. It is clear that the signal has no obvious correlation with the local topology of sample surface, providing a solid experimental proof for the existence of ferroelectric domains. The most intrinsic property of ferroelectrics is the electrically switchable spontaneous polarization. Therefore, local PFM-based hysteresis loop measurement was carried out for 1, where the obvious 180°reversal of PFM phase signal and the characteristic butterfly loop of amplitude signal are typical for the polarization switching of ferroelectric domains.
In addition to the ferroelectric properties, 1 shows extraordinary luminescence properties in the solid state. As is known, many organic-inorganic metal halides especially hybrid perovskites have been reported to exhibit a single emission,17 while it is interesting to find that 1 can show two emissions. The absorption spectrum of 1 presents several absorption peaks ranging from visible to UV region (Figure 5a). By exciting the sample at different wavelength from absorption band, the photoluminescence (PL) spectra of 1 were recorded in Figure 5b. Two strong emissions are clearly observed at 609 nm and 507 nm, respectively. The 609 nm and 507 nm emission is ascribed to the 4T1g→6A1 transition of the octahedrally coordinated Mn2+ ion from the [MnBr3]- chain and the 4T1→6A1 transition of the tetrahedrally coordinated Mn2+ ion from the [MnBr4]2- anion, respectively.18 The quantum yield is 41.96% with the lifetime of 281.7 μs for the combined two emissions (Figures S5 and S6), which makes 1 have potential applications in optoelectronic devices such as displays and light-emitting diodes. In summary, we have successfully demonstrated an antiperovskite ferroelectric, [(CH3)3NH]3(MnBr3)(MnBr4), which exhibits excellent ferroelectricity with extremely high Curie temperature of 458 K. It also possesses unprecedented photoluminescence properties, showing two emissions. This finding throws light on searching for new antiperovskite ferroelectric. Considering the flexible compositional and structural tunability of organicinorganic hybrids, as represented by the development of lead halide perovsikites, one can expect more antiperovskite ferroelectrics to be discovered with outstanding performance and great application prospects.
Figure 4. PFM phase image (a) and Amplitude image (b) overlaid on the 3D topography. Phase (c) and amplitude (d) signals as functions of the tip voltage for a selected point, showing the local PFM hysteresis loops.
ASSOCIATED CONTENT Supporting Information
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Supplementary methods, figures S1−S6, Tables S1 and S2, and Xray crystallographic data (CIF) for this article. This material is available free of charge via the Internet at http://pubs.acs.org.
(13) Liao, W. Q.; Zhang, Y.; Hu, C. L.; Mao, J. G.; Ye, H.-Y.; Li, P. F.; Huang, S. D.; Xiong, R.-G. Nat. Commun. 2015, 6, 7338. (14) Jona, F.; Shirane, G. Ferroelectric Crystals; Pergamon Press: Oxford, U.K., 1962. (15) Fu, D.-W.; Zhang, W.; Cai, H.-L.; Zhang, Y.; Ge, J.-Z.; Xiong, R.G.; Huang, S. P. D. J. Am. Chem. Soc. 2011, 133, 12780. (16) Tang, Y.-Y.; Li, P.-F.; Zhang, W.-Y.; Ye, H.-Y.; You, Y.-M.; Xiong, R.-G. J. Am. Chem. Soc. 2017, 139, 13903. (17) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Inorg. Chem. 2013, 52, 9019. (18) Rodriguez-Lazcano, Y.; Rodriguez, F.; Nataf, L. Phys. Rev. B 2009, 80, 085115.
Corresponding Author *
[email protected] Author Contributions ‡These
authors contributed equally to this work.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by 973 project (2014CB932103), the National Natural Science Foundation of China (21427801 and 21703033) and Natural Science Foundation of Jiangsu Province (BK20170658).
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