A Molecular Perovskite with Switchable Coordination Bonds for High

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A Molecular Perovskite with Switchable Coordination Bonds for High-Temperature Multi-Axial Ferroelectrics Wei-Jian Xu, Peng-Fei Li, Yuan-Yuan Tang, Wei-Xiong Zhang, Ren-Gen Xiong, and Xiao-Ming Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01334 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Journal of the American Chemical Society

A Molecular Perovskite with Switchable Coordination Bonds for High-Temperature Multi-Axial Ferroelectrics Wei-Jian Xu,† Peng-Fei Li,‡ Yuan-Yuan Tang,‡ Wei-Xiong Zhang,*,† Ren-Gen Xiong,‡ and Xiao-Ming Chen† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China

‡

Ordered Matter Science Research Center, Southeast University, Nanjing 211189, China.

ABSTRACT: The underlying phase transitions of ferroelectric mechanisms in molecular crystals are mainly limited in order-disorder and displacive types that are not involved in breaking of the chemical bonds. Here, we show that the bond-switching transition under ambient pressure is designable in molecular crystals, and demonstrate how to utilize the weaker and switchable coordination bonds in a novel molecular perovskite, [(CH3)3NOH]2[KFe(CN)6] (TMC-1), to afford a scarce multi-axial ferroelectrics with a high Curie temperature of 402 K and 24 equivalent ferroelectric directions (more than that of BaTiO3). The high-quality thin films of TMC-1 can be easily fabricated by a simple solution process, and to reveal perfect ferroelectric properties at both macroscopic and microscopic scales, suggesting TMC-1 as a promising candidate for applications in next-generation flexible electronics. The presented molecular assembly strategy, together with the achieved bond-switching ferroelectric mechanism, opens a new avenue for designing advanced ferroelectric materials.

INTRODUCTION Molecular ferroelectrics are of particular interest as their promising potential to alternate the conventional inorganic perovskite ferroelectrics (typically BaTiO3, BTO) in next-generation flexible devices, by taking their advantages of light weight, mechanical flexibility, environmentally benign synthesis, and easy process to thin films.1-12 The studies on molecular ferroelectrics were revived in the past decade by the emergence of numerous advances, such as the discovery of ferroelectricity in charge transfer complexes13,14 the molecular crystals with high spontaneous polarization catching up with inorganic perovskites15,16 and the ultrafast polarization switching in thin films based on biaxial molecular ferroelectrics.17,18 In view of the application as polycrystalline ferroelectric materials or thin films, the multi-axial molecular ferroelectrics are highly desirable, because more equivalent ferroelectric axes allow random spontaneous polarization to orient along the electric field, hence to reveal the ferroelectric properties effectively after poling. However, so far, the multi-axial molecular ferroelectrics are extremely limited in very few examples based on plastic crystals consisting of globular ions.19-21 Moreover, in teams of the underlying phase transition, the ferroelectric mechanisms in molecular crystals are mainly limited in order-disorder and displacive transitions that are not involved in breaking of the chemical bonds.22-25 The phase transition involving the cleavage and reformation of chemical bonds hence to cause a reorganization of the crystal structure, which generally requires applying high pressures in inorganic crystals (vide infra), to the best of our knowledge, has yet to achieve for ferroelectric mechanism in molecular crystals. Here, we show that the bond-switching transition under ambient pressure is designable in molecular crystals, and demonstrate how to utilize the weaker and switchable coordination bonds in a novel molecular perovskite, [(CH3)3NOH]2[KFe(CN)6]

(we named as TMC-1), to afford a new type of multi-axial ferroelectrics with a high Curie temperature of 402 K and 24 equivalent ferroelectric directions (more than that of BTO). In inorganic crystals, the phase transitions triggered via the cleavage and reformation of chemical bonds generally requires applying high pressures, as such transitions need to overcome a high activation barrier to make the sufficiently-large atomic displacements, such as the pressure-induced zircon-to-scheelitelike transition in ZrSiO4,26 and the perovskite-to-post-perovskite transition in MgSiO3.27 Recently, a switching of bridging-chelating modes of formate ion was observed in a metal formate with the ABX3 perovskite architecture,28 suggesting that the variable coordination bonds together with the structural flexibility in molecular perovskites may afford opportunities for achievement of bond-switching transition. Inspired by the recent investigations on cyanide-based molecular perovskites29-32 and the relevant analogues,33-37 as well as some flexible metal-organic frameworks,38-42 here, we consciously employed a polar organic cation, (CH3)3NOH+, to act as A-site cation, for constructing cyanide-based molecular perovskites. Distinguishing from the commonly-employed A-site cations in molecular perovskites, (CH3)3NOH+ has a unique hydroxy group that is capable of forming both coordination and hydrogen bonds. In the obtained compound TMC-1, as disclosed by the variable-temperature X-ray structural analyses, such A-site cation (CH3)3NOH+ does act as an unique moiety that has a thermal-sensitive coordination capability to B-site cations hence to allow a switching of coordination bonds to trigger a phase transition in the entire crystal. Furthermore, this bond-switching transition enable TMC-1 as a high-temperature multi-axial ferroelectric, which allows a facile fabrication for thin film that reveals perfect ferroelectric properties at both macroscopic and microscopic scales.

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Preparation and phase transitions. A crystalline sample of TMC-1 was obtained by evaporation of stoichiometric amounts of K3Fe(CN)6 and trimethylamine N-oxide hydrochloride. Differential scanning calorimetry (DSC) unambiguously revealed that polycrystalline sample of TMC-1 undergoes a phase transition at ca. 402 K on heating, and the corresponding exothermic peak in the cooling mode was observed at ca. 390 K (Figure 1a). The large thermal hysteresis loop (ca. 12 K) indicated a first-order phase transition. It is notable that, a large latent heat of about 39.81 kJ/mol presented in TMC-1 is uncommon in molecular materials, and implied a drastic structural transition during phase transition. For convenience, we label the phase below and above the Tc(heating)/Tc(cooling) as lowtemperature (LT) and high-temperature (HT) phase, respectively. Variable-temperature powder X-ray diffraction (PXRD) studies were performed on polycrystalline samples of TMC-1 to further verify the phase transition. The PXRD patterns below Tc matched well with the simulated one from the single-crystal structure at 298 K, indicating the high crystallinity and purity of the phase. Upon heating above Tc, many diffraction peaks observed below Tc disappeared (Figure S4), and only a few observable peaks remained, suggesting a high-symmetry structure for HT phase (vide infra).

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Second harmonic generation effect. The structural phase transition of TMC-1 was also uncovered in its second harmonic generation (SHG) response. The temperature dependence of the

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RESULTS AND DISCUSSIONS

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Figure 1. DSC curve (a) and temperature dependence of SHG (b) data exhibiting a phase transition for TMC-1.

Figure 2. Thermal-induced bond-switching transition in TMC-1. (a) In the HT phase, the K–O bond breaks and the (CH3)3NOH+ cation becomes highly disorder on the 3m position. (b) In the LT phase, each cage consist of eight Fe–C–N–K and four Fe–C–N···O–K, the hydrogen bond N···H–O shown in red dash line. (c) The asymmetric unit of TMC-1 at LT phase, symmetric codes: A, (1/2+x, 3/2-y, -1/2+z); B, (1/2+x, 1/2+y, z); C, (x, 1-y, -1/2+z); D, (-1/2+x, -1/2+y, z); E, (x, 1-y, 1/2+z); F, (-1/2+x, 3/2-y, 1/2+z); G, (x, 2-y, 1/2+z); H, (-1/2+x, 1/2+y, z). (d) Relationship of the crystal lattice of TMC-1 between the LT (blue line) and HT (red line) phases: a’LT = 0.5aHT + 0.5bHT − cHT, b’LT = − 0.5aHT + 0.5bHT, c’LT = 0.5aHT + 0.5bHT + cHT. Only the Fe atoms are shown for clarity.


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second-order nonlinear optical susceptibility χ(2) of polycrystalline sample of TMC-1 is illustrated in Figure 1b. The value of χ(2) is nonzero (0.11) at room temperature, indicating TMC-1 is SHG active. Upon heating, the value of χ(2) decreases slightly but still remains positive, suggesting the TMC-1 belongs to a noncentrosymmetric space group at the LT phase. However, the value of χ(2) quickly decreases to zero upon heating to 395 K and maintains SHG inactive with further increasing temperature, indicating a transition from the non-centrosymmetric LT phase to a centrosymmetric HT phase. Crystal structures and transition mechanism. Single crystal Xray diffraction analysis at 298 K (LT phase) showed that TMC-1 crystallizes in a polar space group of Cc, with cell parameters: a = 15.3166(9) Å, b = 8.8618(4) Å, c = 14.4531(7) Å and β = 98.440(2)o. As shown in Figure 2c, each Fe3+ ion is surrounded by six cyano groups, four of which act as bridging ligands between the Fe3+ and K+ ions. Each K+ ion locates in an elongated N4O2 octahedron with two axially-occupied hydroxy groups from two (CH3)3NOH+ cations, which meanwhile interact with two uncoordinated cyano groups via two O–H···N hydrogen bonds [O···N distances of 2.651(5) and 2.664(5) Å, respectively]. The linkage of the entire structure can be simplified as a three-dimensional (3D) NaCl type framework (Figure 2b), by regarding Fe3+ and K+ metal ions as two kinds of nodes meanwhile both the hydrogen-bonding interactions and coordination interactions as linkers. To probe the structure for HT phase, the variable-temperature single-crystal X-ray diffractions were attempted but all failed, as the structural transition was so drastic that the single-crystalline could not be maintained for HT phase. Alternatively, the PXRD data was collected at 408 K for HT phase, which could be well indexed by a cubic F-center lattice with cell length of a = 12.578(2) Å, a same lattice type and a similar cell length for HT phase of a known analogue of TMC-1, [(CH3)4N]2[KFe(CN)6] (space group Fm3̅ m),29 in which non-polar tetrahedral tetrame-thyl-ammonium acts as highly-disordered A-site cation. Accordingly, a double perovskite structure model in cubic space group Fm3̅ m was constructed for HT phase of TMC-1, and led to an easily converged Rietveld refinement at Rwp = 2.81% and Rp = 2.16% (Figure S1). In the structure of HT phase, each Fe3+ and K+ are connected by six cyano groups to a 3D cage-like framework (Figure 2a). Each anionic [K4Fe4(CN)12] cage enclosed by 12 Fe–C–N–K fragments encapsulates a (CH3)3NOH+ guest cation, and the guest cations are not involved in coordination interactions but highly disordered over 24 orientations as required by the crystallographic symmetry. With the structural information, the structural differences between the LT and HT phases become clear. During the transition from HT to LT phase, the hydroxy group of (CH3)3NOH+ revealed a “bond switching” behavior from a coordination-free state to a coordinatively-bonded state. Meanwhile, each K+ ion are involved in the cleavage of two K–N bonds (3.074 Å in HT phase) and formation of two K–O bonds (K–O1 2.901 Å and K–O2 2.903 Å in LT phase), thus, in each cage of perovskite structure, four Fe–C– N–K edges were replaced by Fe–C–N···H–O–K linkages, meanwhile the other eight edges remain unchanged. In short, benefiting from the switchable coordination bonds of an unique A-site cation, TMC-1 reveals an unprecedented bond-switching transition that greatly distinguishes from the conventional order-disorder one in

the other molecular perovskites and displacive one in inorganic perovskites.43-51 The transition enables TMC-1 to be a new type of hightemperature multi-axial ferroelectric. Inorganic perovskite BTO is the most famous example of the multi-axial ferroelectrics, because it undergoes a displacive transition from a cubic (Pm3̅ m) to a tetragonal (P4mm) phase at 393 K with an Aizu notation of m3mF4mm,52 hence leads to 6 crystallographically equivalent polarization directions that are of importance for its valuable applications in forms of polycrystalline ceramics and thin films. For TMC-1, thanks to the drastically phase transition, the symmetry breaking occurs during the transition from a cubic HT phase, with 48 (E, 8C3, 3C2, 6C2, 6C4, i, 8S6, 3σh, 6σd, 6S4) symmetry elements, to a monoclinic LT phase, with only 2 (E, σh) symmetry elements. Such symmetry breaking with an Aizu notation of m3mFm52 results in up to 24 crystallographically equivalent polarization directions, more than that of BTO. In this sense, TMC-1 presented new type of molecular ferroelectrics, i.e., induced by bond-switching transition. Moreover, such unique ferroelectric mechanism differs greatly from the existing ones induced, such as, by order-disorder transition in molecular perovskites53-56 or plastic crystal,19-21 by guest water molecular in some MOFs,57-59 and by charge-transfer in organic crystals.60 Ferroelectric properties. We measured the complex dielectric constant as function of the temperature in frequency range of 100 kHz to 1 MHz on polycrystalline sample of TMC-1. The real part (ε’) of the dielectric constant of TMC-1 exhibited a clear increase with an abrupt slope around Tc with heating (Figure S5), and the maximum value was about 72 at 100 kHz corresponding to a high dielectric state in the HT phase. Different from the conventional second-order proper ferroelectrics, whose dielectric constants typically show a large λ-shape peak at vicinity of Tc, the step-like dielectric switching behavior indicates a first-order improper ferroelectric transition for TMC-1, consistent with the observation in the DSC measurement. The macroscopic ferroelectric polarization reversal was initially investigated on a single crystal of TMC-1 along the c-axis at 298 K (Figure S6-S8). A flat ferroelectric hysteresis loop was observed at room temperature at 1 Hz, with a relatively small coercive field (Ec) of 5.7 kV/cm and an electric polarization of 0.58 μC/cm2 at 12 kV/cm. The polarization value is in good accordance with that calculated one (0.54 μC/cm2) from the point charge model (Figure S9). To comprehensively investigate the ferroelectric properties for TMC-1, we prepared a thin film by a simple and operative chemical solution routine. A drop of precursor aqueous solution contains TMC-1 was carefully spread on a freshly cleaned ITO (Indium Tin Oxide)-coated glass (conductive ITO was used as the bottom electrode). With controlled substrate temperature and edge-pinned crystallization,17 a uniform film consists of continuous microcrystals with high coverage was obtained. With this high-quality thin film, ferroelectric polarization reversal was successfully achieved at room temperature through macroscopic polarization vs. external voltage hysteresis loop measurements. To be specific, a layer of sputtered Au was used as the top electrode, so the whole ferroelectric hysteresis measurements were conducted with capacitor architecture (Au/TMC-1/ITO) under the Sawyer-Tower circuit.



age from 1 kHz to 5 kHz, indicating all the dipoles in the closelypacked crystal lattice can be synchronically switched under external electric field of such high frequencies. Compared with the recentlyreported molecular ferroelectrics,53-60 Pr of TMC-1 is among the moderate level.

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Figure 3. Room-temperature ferroelectric hysteresis loops of thin film of TMC-1 at different frequencies.

As shown in Figure 3, comparing with the single crystal, the thin film well presented a rectangular P-E hysteresis loops at different frequencies, affording indispensable proof for the ferroelectricity in TMC-1. From Figure 3, the coercive voltage is about 35 V for a film with 1.5 μm thickness at 1 kHz, corresponding to a coercive field (Ec) of 233 kV/cm, which is significantly larger than that of common bulk molecular ferroelectrics (typically 5 ~ 20 kV/cm). The process of ferroelectric reversal in TMC-1 involves relatively great amplitude of the motion of the cations, which needs a relative large coercive field essentially. With increasing the driving AC frequency (1 kHz to 5 kHz), Ec increases rapidly from 233 kV/cm to 450 kV/cm, revealing that high frequency ferroelectric switch needs higher coercive field to energize the ferroelectric domain nucleation and domain wall motion. The considerable remnant polarization (Pr) of TMC-1 film is 1.25 μC/cm2, which is comparable with calculated value of 1.47 μC/cm2 (Figure S9). This value of remnant polarization can be maintained through different driving AC volt-

Domain structures and polarization reversal. Piezoresponse force microscopy (PFM) has emerged as an extremely useful method for the characterization of ferroelectric materials at the nanometer scale, which can provide nondestructive visualization of the statics and dynamics of ferroelectric domains with unprecedented spatial resolution.61-66 Each PFM image can be characterized by the amplitude and phase parameters to provide information about the value of the piezoelectric coefficient (proportional to the local polarization) and the orientation of the domain polarization, respectively. To verify the presence of non-180° domains, the vertical and lateral PFM (VPFM and LPFM) mappings of the same area of thin film were acquired simultaneously (Figure 4). First, the LPFM piezoresponse was overlaid on a 3D topography, and the fact that no obvious correlation was found between them provides a direct proof for the existence of ferroelectric domains (Figure S11). Meanwhile, it is notable that the domain distributions in the VPFM and LPFM modes are very different. As shown in Figure 4c, the VPFM phase image shows the single-domain state, whereas an obvious bipolar domain pattern emerges in the LPFM phase image, suggesting that there exist two different polarization directions in this region and the angle between them should be non-180° (Figure 4f). Moreover, both the VPFM and LPFM amplitude images have various contrasts and display similar patterns with that of the LPFM phase image, corresponding to the complex polarization directions (Figure 4b, 4e). Based on these results, the presence of abundant non-180° domains, or the presence of multiple polar axes, can be established.

Figure 4. Topographic image (a), vertical (b, c) and lateral (e, f) PFM (VPFM and LPFM) images of the film surface for TMC-1. (d) Phase-voltage hysteresis loop (up) and amplitude-voltage butterfly loop (down) for a selected point on the film surface.


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Figure 5. Polarization reversal of TMC-1 measured from a film. Topographic images (top) and VPFM phase (bottom) images of the film surface. (a) Initial state. (b) After the first switching with positive bias at +70 V. (c) After the succeeding back-switching with negative bias at −70 V. (d) After the third switching with positive bias at +70 V. The yellow and purple regions in phase images indicate the polarizations oriented upward and downward, respectively.

Domain switching—the phenomenon wherein a ferroelectric changes between two spontaneously polarized states under electrical or mechanical loads—is a crucial feature of ferroelectric materials. In view of the local PFM spectroscopic measurements for the thin film (Figure 4d), the characteristic hysteresis and butterfly loops triggered by the applied voltage are typical for the 180° polarization switching of ferroelectric domains. To further manifest the switching behavior, the switching process of ferroelectric domains for the thin film of TMC-1 is clearly illustrated in Figure 5, with a selected area of 25 μm × 25 μm. For the initial state, the uniform contrast in the VPFM phase image indicates the single-domain state in this area (Figure 5a). When a tip bias of +70 V was applied, the polarization direction of the central region was switched, as the color contrast in the phase image changed from yellow to purple (Figure 5b). Significantly, then by an opposite −70 V tip bias, the polarization direction of this region can be switched back, appearing as a box-in-box pattern (Figure 5c). It demonstrated the ability of polarization to be switched forth and back perfectly. Finally, once poling with a tip bias of +70 V, the polarization direction of the domain was switched down again, meaning a reproducible polarization switching process (Figure 5d). The poling has no effect on the surface topography, and the switched domains were able to remain stable for more than 16 hours (Figure S12). Consequently, charging effects can be ruled out as a possible origin,67 and the good ferroelectric retention for the thin film of TMC-1 is confirmed. Such stable and switchable polarization revealed by the above PFM results is an intrinsic characteristic of ferroelectric materials, which makes TMC-1 distinguishable from the pyroelectric materials.

switching phase transition under ambient pressure in molecular crystals. During this thermal-induced phase transition at a muchabove room temperature (402 K), the intrinsic “bond switching” behavior affords a symmetry breaking, from a cubic paraelectric phase (Fm3̅ m) to a monoclinic ferroelectric phase (Cc), that enable TMC-1 to be a new type of multi-axial ferroelectric with up to 24 equivalent polarization directions. Furthermore, the high-quality ferroelectric thin films of TMC-1 can be easily fabricated by a simple aqueous solution process at room temperature. These thin films can exhibit prefect ferroelectric properties at both macroscopic and microscopic scales, such as a well-defined rectangular P−E hysteresis loops at a relatively high frequency of 5 kHz, and a reversibly controllable polarization reversal of the ferroelectric domains by poling with DC biases. Such high thin-film performances enable TMC-1 to be a promising candidate for applications in nextgeneration flexible electronics.

CONCLUSION

ASSOCIATED CONTENT

By introducing an unique cation with switchable coordination capability, i.e., (CH3)3NOH+, as A-site cation into the molecular perovskite, i.e. TMC-1, we achieved an unprecedented bond-

Experimental procedures, crystallographic files in CIF format, thermogravimetry curves, PXRD patterns, additional analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

Ultimately, regarding the diversities of the ionic components with switchable coordination capabilities, the metal sites with variable coordination geometries, as well as the structures with deformable flexibilities, molecular crystals provide promising systems to design and achieve diverse bond-switching transitions, especially those hard to be accessible in inorganic crystals under ambient pressure. In this context, the molecular assembly strategy presented by TMC-1, together with the achieved ferroelectric mechanism, not only opens a new avenue to construct advanced molecular ferroelectrics for a wide of applications, but also has significant implications for designing and tuning the physical properties of molecular functional materials based on bond-switching transitions.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the NSFC (21290173, 21427801, 91422302, and 21671202). W.-X.Z. is thankful to the Pearl River S&T Nova Program of Guangzhou. We thank Prof. J.-H. Luo and Mr. C.-M. Ji for their helpful discussions on SHG measurement.

Notes The authors declare no competing financial interests.

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Table of Contents

An unprecedented bond-switching ferroelectric phase transition under ambient pressure was achieved in a molecular perovskite, [(CH3)3NOH]2[KFe(CN)6] (TMC-1), to afford a scarce high-temperature multi-axial ferroelectrics. High-quality thin films of TMC-1 reveal perfect ferroelectric properties at both macroscopic and microscopic scales.

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A Molecular Perovskite with Switchable Coordination Bonds for High

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