A Three-Dimensional Molecular Perovskite Ferroelectric: (3

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A Three-dimensional Molecular Perovskite Ferroelectric: (3-Ammoniopyrrolidinium)RbBr3 Qiang Pan, Zhi-Bo Liu, Yuan-Yuan Tang, Peng-Fei Li, Rong-Wei Ma, RuYuan Wei, Yi Zhang, Yu-Meng You, Heng-Yun Ye, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00492 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Journal of the American Chemical Society

A Three-dimensional Molecular Perovskite Ferroelectric: (3-Ammoniopyrrolidinium)RbBr3 Qiang Pan, Zhi-Bo Liu, Yuan-Yuan Tang, Peng-Fei Li, Rong-Wei Ma, Ru-Yuan Wei, Yi Zhang, Yu-Meng You, Heng-Yun Ye*, Ren-Gen Xiong* Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. Supporting Information Placeholder ABSTRACT: Encouraged by CH3NH3PbI3 that is particularly promising for next-generation solar devices, molecular perovskite structures have recently received extraordinary attention from academic community because of their potential in producing unique physical properties. However, although great efforts have been made, molecular ferroelectrics with three-dimensional (3D) perovskite structures are still rare. So far, reported perovskite-like molecular ferroelectrics are basically one- or two-dimensional, significantly deviating from the inorganic perovskite ferroelectrics. Thus their ferroelectric properties have to be greatly improved to meet the requirements of practical applications. Here, we report a 3D molecular perovskite ferroelectric: (3-ammoniopyrrolidinium)RbBr3 [(AP)RbBr3], with a high Curie temperature (Tc = 440 K) beyond that of BaTiO3. To the best of our knowledge, such above-room-temperature ferroelectricity in the 3D molecular perovskite compound is unprecedented. Furthermore, (AP)RbBr3 has great application potential due to the high thermal stability, ultrafast polarization reversal (greater than 20 kHz) and fascinating multiaxial characteristic. This finding opens a new avenue to the design and controllable synthesis of molecular ferroelectric perovskites, where the metal ion, halogen ion, and organic cation can be easily tuned.

perovskite structure undergoes a distinct transition from the paraelectric point group m3̅ m to the ferroelectric point group m at around 440 K. As far as we are aware, it is the first example of hightemperature molecular 3D perovskite ferroelectric. Very importantly, (AP)RbBr3 possesses 12 equivalent ferroelectric axes, which provide possible ferroelectric related applications in polycrystalline forms.

For centuries, perovskites—of simplest generic formula ABX3 (A, B = two different cations, X = anion)—have intrigued chemists, physicists and material scientists. Thanks to the unique three-dimensional (3D) structure of corner-sharing BX6 octahedra enclosing nominal twelve-coordinate holes occupied by the A cations, they have rich and excellent physical properties, such as colossal magnetorestive effects, superconductivity, ionic conductivity, and a good dielectric and ferroelectric related properties, which are of great importance in microelectronics and telecommunication.1 Since recently, organic-inorganic hybrid perovskites are attracting tremendous interest because of their low production costs, simple processing and biocompatible characteristics. For example, CH3NH3PbX3 (X = I, Br or Cl) perovskites have been recognized as promising light harvesting materials for future high performance and ultralow-cost-per-watt photovoltaic devices.2

Figure 1. The crystal structure of (a) BaTiO3 and (b) (AP)RbBr3 in the ferroelectric phase. The structural unit of (AP)RbBr3 in (c) ferroelectric phase and (d) paraelectric phase. In (d), the atoms corresponding to the AP cation are shown in the space-filling mode.

As one of the important properties of perovskites, ferroelectricity has also been discovered in a few perovskite-type hybrids. Among them, the structures of metal formate frameworks resemble the cubic perovskite structure, except that the bridging formate ligand is a multi-atomic group.3 However, the low Tc below room temperature and the uniaxial characteristics severely limited their application potential4. Here, we found that (AP)RbBr3 adopting the 3D

(AP)RbBr3 has the familiar crystal structure of 3D framework of the corner-sharing RbBr6 octahedra, where the AP cations are confined in the cavities enclosed by the octahedra (Figure 1). Such arrangement of corner-sharing octahedra is common in the transition metal halides and halometallates of Sn(II), Pb(II), Sb(III) and

The crystals of (AP)RbBr3 can be easily synthesized by evaporation of the aqueous solution containing equal molar amounts of the organic ammonium and RbBr. The presence of HBr in the solution contributes to forming large crystals. The purity of the bulk phase was verified by the infrared spectrum and powder X-ray diffraction (Figures S1 and S2, Supporting Information).

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Bi(III) ions, but rare in alkali metal halides.5 At room temperature, the crystal is polar with space group of Ia (the nonstandard set of the space group facilitates the comparison of the ferroelectric and paraelectric structures), and shows strong optical nonlinear behavior (Figure 2a). Organic molecules included in such organometal frameworks, such as the dimethylammonium, methylammonium and imidazonium, usually have large freedom of motion, thus they are normally disordered.6 In our case, the AP cation is well ordered and aligned in the lattice at room temperature. As shown in Figure S3 (Supporting information), the primary ammonium heads are oriented along the c direction, and the N atoms of the rings are located on the left of the molecules. Such an alignment should induce a polarization along the a- and c-directions. In order to estimate the spontaneous polarization, we performed calculations with point charge model. By assuming that the positive charges of the AP cation are located on the two N atoms, we obtained the polarization of 3.0 μC cm-2 with the components of 2.5 μC cm-2 along the a-axis and 1.7 μC cm-2 along the c-axis.

Figure 2. Ferroelectricity and related properties of (AP)RbBr3. (a) The temperature dependence of the SHG signal of the polycrystalline sample. (b) DSC data in heating and cooling runs. (c) The temperature dependence of the real part of the complex dielectric permitivity ( =  − i, where  and  are the real and the imaginary part, respectively), measured along [1 1 1] direction. (d) The applied-voltage dependence of the spontaneous polarization at various frequencies, measured from a Sawyer-Tower circuit. Thermogravimetric analysis reveals high thermal stability up to 570 K. A structural phase transition was observed at around 440 K (Figure S4, Supporting Information), and confirmed by the reversible thermal anomalies in the differential scanning calorimetry (DSC) measurement (Figure 2b). The vanishing of the SHG signal in high-temperature phase (HTP) also reveal a high-temperature phase transition from the polar room-temperature phase (RTP) into a centrosymmetric HTP. The high temperature structure determined at 453 K have a facecentered cubic (FCC) structure, space group Pm3̅m (Figure 1b). The relationship of the two temperature cells is aHTP  0.5aRTP, bHTP  0.5bRTP + 0.5 cRTP, cHTP  -0.5bRTP + 0.5cRTP. The ac plane of the RTP corresponds to the (0 1 1) plane of the HTP (Figure S5, Supporting Information). Both the Rb atom and the AP cation in the HTP are located on the special sites of m3̅m. The twist octahedron of the alkali metal halide framework become the regular octahedron. The high site symmetry of m3̅m requires a total disorder of the AP cation, and accordingly, the AP cation was modeled with a spherical structure regardless its molecular geometry. The large entropy

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changes(ΔS) of 36.66J/(mol∙K) supports such a transition. From the Boltzmann equation, ΔS = R ln N, where R is the gas constant and N is the ratio of the numbers of respective geometrically distinguishable orientations, we obtain N1 = 82.22. The AP cation can be considered to undergo a high degree of dynamic reorientation (tumbling), which complies the isotropic cubic phase.

Figure 3. Analyses of polarization directions. (a) 24 possible equivalent polarization directions of (AP)RbBr3 in the ferroelectric phase. (b) 6 possible equivalent polarization directions of BaTiO 3 within space group P4mm. Therefore, the ferroelectric origin is due to the freezing of the dynamics of the dipolar molecule and the subsequent alignment of the dipoles, which is different from the offcenter displacement of the central Ti atom in BaTiO3. We notice that this situation is very similar to that recently observed for the quinuclidinium, 3-hydroxlyquinuclidinium, and tetraethlyammonium salts.7 In those cases, the high temperature phase becomes cubic plastic phases, because the molecules have spherical geometries. The 3D spherical molecular structures such as tetramethylammonium, adamantane 1,4-diazabicyclo[2.2.2]octane tend to exhibit dynamical disorder in the close packed crystals because of weak van der Waals interactions. In our case, AP has a geometry significantly deviating from the spherical geometry. The transition to a freely rotating state should be due to the spacious cavity enclosed by the RbBr6 octahedral frame. According to the symmetry change, (AP)RbBr3 belongs to species of m3̅mFm(s) among the 88 species of ferroelectrics, where s indicates that the spontaneous polarization vector lies in the (0 1 1) face of the m3̅m phase.8 Such species has 24 crystallographically equivalent polarization vector directions (Figure 3), much greater than the six in BaTiO3, which allows more tunability in polarization direction. The large dielectric anomalies of the single-crystal and polycrystalline samples at around Tc = 440 K reveal the ferroelectric nature of the high-temperature phase transition (Figure 2c, Figure S6 & S7, Supporting Information). The dielectric permittivity jump at around Tc reveals the character of the first-order transition, consistent with that revealed by the sharp peaks in the thermal analysis. The excellent reversibility is consistent with the good stability of the alkali metal halide framework. Compared with BaTiO3, (AP)RbBr3 has the smaller dielectric permitivity (a few hundred for BaTiO3) and higher Tc. Tc is the temperature where ferroelectric materials lose their spontaneous polarization. Since such ferroelectric phase-transition is driven by thermal activation, height of the energy barrier between the order and disorder states is directly related to the transition temperature. In this case, considering the AP cation with relative big volume but restricted in a three dimensional {RbBr3}n2n- cage, a higher energy is expected to reach its disordered state (a more quantitative analysis based on DSC data can be found in Supporting Information, Figure S8). On the other hand, this phase transition also involves the distortion of the host metal-halide framework, resulting giant symmetry changes from monoclinic to cubic system. All these changes undoubtedly require large energy to activate the phase transition, leading the Curie temperature toward a high degree temperature.


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Figure 4. Domain structure recorded for an as-grown crystal. (a, b) The VPFM phase and amplitude mages. (c, d) Phase and amplitude signals as functions of the tip voltage for a selected point, showing local PFM hysteresis loops. Polarization reversal was investigated on a thin film capacitor with the configuration of ITO (indium-tin oxide glass)/thin crystallite/GaIn because of high coercive filed (see Supporting Information for experimental details). The polarization–filed dependence at various temperatures and frequencies was recorded. The remnant polarization (Pr) and the shape of the loops change little as the temperature increase from 303 K to 333 K (Figure S9) and similar frequency dependence was observed. As shown in Figure 2d, the high rectangularity retains up to 20 kHz without any significant change of Pr. The measured Pr is around 2.3 μC cm-2, a little smaller than calculated value. Compared with those of recently developed molecular ferroelectrics, Pr of (AP)RbBr3 is among the moderate level. For a thin crystallite of 700 nm in thickness, the coercive voltage for the polarization reversal is about 38 V, corresponding to a coercive field (Ec) of 542 kV/cm. Such a high coercive field is due to the surface effect as well as that the polarization reversal involves a great amplitude of reorientation of the cations. Considering that domain structure and domain dynamics is important for understanding ferroelectric mechanism and practical applications, we studied the domain properties with help of the piezoresponse force microscopy (PFM). PFM is a general ferroelectric characterization method, which provides information of the magnitude [by recording the vertical and lateral (VPFM and LPFM) amplitude images] and direction (by recording the VPFM and LPFM phase image) of the polarization. By applying DC voltage to the conductive tip, one can easily manipulate the local polarization direction and study the polarization reversal process. Figure 4a,b demonstrates the stripe-like domain structure in an area of 1 × 1 μm2 of an as-grown thin crystallite. The neighbored domains have about 180° phase contrast and nearly the same vertical amplitudes, indicating 180° domains. More measurements revealed the existence of non-180° domains, as shown in the red rectangle area in Figure S10 (Supporting Information). This result is consistent with the symmetry change in the ferroelectric m3̅mFm(s) species.

Figure 5. Domain manipulating. The panels in each row are arranged as the sequence: topographic image (left), VPFM amplitude image (middle) and VPFM phase image (right). (a) Images for the initial state of the as-grown crystallite. (b) Images for the state after the first switching operation in the region of the blue rectangle, produced by scanning with the tip bias of 11 V. (c) Images for the state after the succeeding back-switching operation in the region of the smaller red rectangle, produced by scanning with the tip bias of -8 V. (d) Images for the state after the succeeding back-switching operation in the region of the smaller blue rectangle, produced by scanning with the tip bias of 9 V. To test the polarization reversal properties, we measured the phase and amplitude by scanning the applied tip voltage from -15 to 15 V (Figure 4c,d). The typical rectangular phase hysteresis loop and butterfly amplitude loop were developed, due to ferroelectric polarization reversal. We then carried out local polarization manipulation experiments by writing in the selected rectangle areas. As shown in Figure 5, clear and successive reversals of phase contrasts (~180°) were demonstrated by different tip-bias. The generated domain shapes deviated from the rectangle due to domain diffusion, which may be caused by the roughness of sample surface. The domain manipulation indicates that the polarization direction of the crystallite is switchable in a controllable manner. The written domains are stable, and remain unchanged after nine hours (Figure S11, Supporting Information). The frequency (speed) of polarization reversal is closely related to the ferroelectric domain dynamics during the ferroelectric switching. Typically, the polarization reversal involves following steps: 1) domain nucleation, 2) forward extension and 3) lateral expansion. Among those steps, the reversal time are normally limited by the



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lateral expansion speed of domains. In our case, the lateral movement of domain walls can be easily activated and rapidly diffused as shown by the PFM switching results (Figure 5), which indicates the fast lateral expansion of domains in this material can be achieved. On the other hand, our ferroelectric hysteresis loop measurement is based on a micro crystal with thickness of ~700 nm. Such small thickness also reduces the forward extension time leading to a high switching frequency. In summary, we have rationally designed a 3D molecular perovskite ferroelectric. The revealed properties suggest great application potential of this kind of material. For example, good thermal stability and high Tc of molecular ferroelectrics have been intensively pursued. The exceptional thermal stability and unprecedentedly high Tc seems to be inborn for the alkali metal perovskites owing to the stability of the alkali metal halide framework. The realization of ultrafast polarization would be an important factor improving the realistic device speed. Finally, multiaxial characteristic attributes it more polarization states and makes the polycrystalline applications become possible. Considering both the framework and the cation have much room to be designed and tailored, one can expect more alkali metal halide perovskite ferroelectric to be discovered with better properties and greater application potential.

ASSOCIATED CONTENT Supporting Information Supplementary method, Figures S1-S11. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by 973 project (2014CB932103) and the National Natural Science Foundation of China (21290172, 91422301, 21427801 and 21573041).

REFERENCES

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(1) Tejuca, L. G.; Fierro, J. L. G. Properties and Applications of Perovskite-Type Oxides; Marcel Dekker: New York, 1993. (2) (a) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050; (b) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Nature 2013, 499, 316; (c) Liao, W.; Zhao, D.; Yu, Y.; Shrestha, N.; Ghimire, K.; Grice, C. R.; Wang, C.; Xiao, Y.; Cimaroli, A. J.; Ellingson, R. J.; Podraza, N. J.; Zhu, K.; Xiong, R. G.; Yan, Y. J. Am. Chem. Soc. 2016, 138, 12360; (d) Saparov, B.; Mitzi, D. B. Chem. Rev. 2016, 116, 4558; (e) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y. Science 2014, 345, 295; (f) Liao, W.; Zhao, D.; Yu, Y.; Grice, C. R.; Wang, C.; Cimaroli, A. J.; Schulz, P.; Meng, W.; Zhu, K.; Xiong, R. G.; Yan, Y. Adv. Mater. 2016, 28, 9333. (3) (a) Xu, G. C.; Ma, X. M.; Zhang, L.; Wang, Z. M.; Gao, S. J. Am. Chem. Soc. 2010, 132, 9588; (b) Xu, G. C.; Zhang, W.; Ma, X. M.; Chen, Y. H.; Zhang, L.; Cai, H. L.; Wang, Z. M.; Xiong, R. G.; Gao, S. J. Am. Chem. Soc. 2011, 133, 14948; (c) Zhang, W.; Xiong, R. G. Chem. Rev. 2012, 112, 1163. (4) (a) Ye, H. Y.; Zhou, Q. H.; Niu, X. H.; Liao, W. Q.; Fu, D. W.; Zhang, Y.; You, Y. M.; Wang, J. L.; Chen, Z. N.; Xiong, R.-G. J. Am. Chem. Soc. 2015, 137, 13148; (b) 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; (c) Zhang, Y.; Liao, W. Q.; Fu, D. W.; Ye, H. Y.; Chen, Z. N.; Xiong, R. G. J. Am. Chem. Soc. 2015, 137, 4928; (d) Ye, H. Y.; Liao, W. Q.; Hu, C. L.; Zhang, Y.; You, Y. M.; Mao, J. G.; Li, P. F.; Xiong, R. G. Adv. Mater. 2016, 28, 2579. (5) Paton, L. A.; Harrison, W. T. Angew. Chem. Int. Ed. 2010, 49, 7684. (6) (a) Zhang, W.; Ye, H.-Y.; Graf, R.; Spiess, H. W.; Yao, Y. F.; Zhu, R. Q.; Xiong, R.-G. J. Am. Chem. Soc. 2013, 135, 5230; (b) Jain, P.; Ramachandran, V.; Clark, R. J.; Zhou, H. D.; Toby, B. H.; Dalal, N. S.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc. 2009, 131, 13625; (c) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc. 2008, 130, 10450; (d) Besara, T.; Jain, P.; Dalal, N. S.; Kuhns, P. L.; Reyes, A. P.; Kroto, H. W.; Cheetham, A. K. Proc. Natl. Acad. Sci. 2011, 108, 6828. (7) (a) Harada, J.; Shimojo, T.; Oyamaguchi, H.; Hasegawa, H.; Takahashi, Y.; Satomi, K.; Suzuki, Y.; Kawamata, J.; Inabe, T. Nat. Chem. 2016, 8, 946; (b) Ye, H. Y.; Ge, J. Z.; Tang, Y. Y.; Li, P. F.; Zhang, Y.; You, Y. M.; Xiong, R.-G. J. Am. Chem. Soc. 2016, 138, 13175; (c) Li, P. F.; Tang, Y. Y.; Wang, Z. X.; Ye, H. Y.; You, Y. M.; Xiong, R.-G. Nat. Commun. 2016, 7, 13635. (8) Aizu, K. J. Phys. Soc. Jpn. 1969, 27, 387.


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