Unique Freezing Dynamics of Flexible Guest Cations in the First Molecular Postperovskite Ferroelectric: (C ₅ H ₁₃ NBr)[Mn(N(CN) ₂ ) ₃ ]

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2019Unique Freezing Dynamics of Flexible Guest Cations in the First Molecular Postperovskite Ferroelectric: (C5H13NBr)[Mn(N(CN)2)3] Sha-Sha Wang, Xiao-Xian Chen, Bo Huang, Rui-Kang Huang, Wei-Xiong Zhang and Xiao-Ming Chen Sha-Sha Wang , Xiao-Xian Chen , Bo Huang , Rui-Kang Huang , Wei-Xiong Zhang *Corresponding author: E-mail Address: [email protected] and Xiao-Ming Chen https://doi.org/10.31635/ccschem.019.20190012 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Ferroelectricity is usually found in cubic and hexagonal perovskites consisting of corner- and faced-shared BX6 octahedra, respectively. In contrast, another important branch of perovskites, postperovskites, which contain layers constructed by both edge- and corner-shared BX6 octahedra, have been much more scarcely studied and virtually absent in the fields of ferroelectricity since the discovery of postperovskite structure in 1965. In this study, we present for the first time a molecular postperovskite ferroelectric, (bromocholine)[Mn(N(CN)2)3], and demonstrate how spontaneous polarization arises from the freezing dynamics of bromocholine cations in a unique space confined by postperovskite layers. These findings suggest that postperovskites could serve as a new type of host–guest model in controlling the molecular dynamics of abundant polar cations in the development of novel ferroelectric materials. Download figure Download PowerPoint Introduction In the past century, inorganic perovskites have displayed great success in serving as fundamental materials in manifold fields, such as ferroelectricity, superconductors, and colossal magnetoresistance.1–4 By topologically mimicking perovskite structure via diverse molecular components, molecular perovskites (also known as organic–inorganic hybrid perovskites) with a general formula of ABX3 (where A and B are cations and X is an anion), have exhibited exotic properties that are generally hard to achieve with inorganic materials, such as flexible photovoltaics,5,6 flexible ferroelectrics,7,8 and unique energetic properties.9,10 Moreover, the advantages of mechanical flexibility, environmentally benign synthesis, and easy processing enable molecular perovskites to be promising alternatives for use in next-generation flexible devices, and thus making these materials an important research topic in modern material science.7,11–13 In view of structural topology (Figure 1), ABX3-type perovskites have three typical subclasses that are assigned based on the linkage of the BX6 octahedra: cubic perovskite, consisting of corner-shaped octahedra;14–16 postperovskite, consisting of both edge- and corner-shared octahedral, and hexagonal perovskite, consisting of face-shared octahedra. These three classes, respectively, feature a three-dimensional host framework (cubic perovskite), two-dimensional layers (hexagonal perovskite), and one-dimensional chains (postperovskite) filled with and/or spaced by A-site guest cations. In the past decade, efforts have been made to use such simple host–guest models to achieve and tune ferroelectric polarization and multifunctional switchable properties for which the molecular dynamics of the differing polar A-site cations in the confined space was an essential consideration. These efforts have yielded numerous cubic and hexagonal molecular perovskites that have made important contributions to the rapid development of molecular ferroelectrics and piezoelectrics.7,17,18 Perhaps even more important has been their contribution to the creation of practical and desirable high-temperature multiaxial ferroelectrics equipped with up to 12 polarization axes via a conventional order–disorder mechanism19,20 or an unconventional bond-switching mechanism,21 as well as piezoelectrics with stronger piezoelectricity than lead zirconate titanate.22 Figure 1 | Structural topologies for (a) cubic perovskite, (b) hexagonal perovskite, and (c) postperovskite. Download figure Download PowerPoint However, compared with the cubic and hexagonal perovskites, postperovskites have received much less attention and are still essentially absent in the field of ferroelectricity, despite the fact that the first postperovskite structure, CaIrO3, has been known since 1965.23 The main reason for this lack of interest may be the fact that the double-oxygen bridge usually leads to a structural tension in the layer for postperovskite oxides. Because of this, CaIrO3 is the only oxide with a stable structure at ambient pressure.24 In contrast, molecular postperovskites have multiatomic anions that serve as double-ligand bridges, including formate (HCOO−),25 thiocyanide (SCN−),26 and dicyanamide (N(CN)2−, dca),27–30 which enable the postperovskite layers to be stable under ambient pressure. As a result of this stability, some studies on these postperovsite layers have appeared in the literature over the past few decades. Nevertheless, only three molecular postperovskites have been documented exhibiting structural phase transitions.31,32 To date, little is known about the molecular dynamics of polar guest cations, let alone how to achieve ferroelectric polarization, in molecular postperovskites. To rationally construct molecular postperovskites so that they exhibit ferroelectric polarization is an important piece in the understanding of perovskite ferroelectrics and is important in the exploration novel molecular ferroelectrics. In our ongoing efforts to construct dicyanamide (N(CN)2−, dca)-bridged molecular postperovskites using different A-site polar cations, we obtained a new molecular postperovskite, (bc)[Mn(dca)3] ( 1, bc+ = bromocholine), which, to the best of our knowledge, is the first known postperovskite ferroelectric. Herein, we present the temperature-dependent two-step phase transitions of (bc)[Mn(dca)3], including a typical ferroelectric and an unusual phase transition, which mainly originates from a unique freezing dynamic of flexible polar bc+ confined in the space between postperovskite layers. The transitions were studied in depth using several techniques, including specific heat capacity (Cp), differential scanning calorimetry, variable-temperature X-ray diffraction analyses, dielectric constant, second harmonic generation (SHG), pyroelectric measurements, and Hirshfeld surface analyses. Experimental Methods Materials All reagents and solvents were commercially available and were used without additional purification. Synthesis (C5H13NBr)[Mn(dca)3] ( 1) was synthesized using the following procedure: bromocholine bromide (1 mmol, 247 mg), Na(dca) (3 mmol, 267 mg), and 50 wt% Mn(NO3)2 aqueous solution (0.24 mL) were mixed into water (∼5 mL), and then ultrasound was applied for ten minutes. The filtered solution was placed in a desiccator, and the water was allowed to evaporate slowly at room temperature. After one week, colorless, rhombic crystals of 1 were obtained with 92% yield. Elemental analysis: calculated C: 31.45, H: 3.12, N: 33.34%; found C: 31.77, H: 3.05, N: 33.25%. Results and Discussion Rhombic crystals of 1 were obtained by slow evaporation of an aqueous solution containing bromocholine bromide, Na(dca), and Mn(NO3)2 in the ratio of 1∶3∶1 at room temperature (see details in the "Experimental Methods" section). The single-crystal X-ray diffraction analysis for 1 revealed that each Mn|| ion is octahedrally coordinated with six terminal N atoms from six dca anions, where each dca anion acts as a bridging ligand linking two adjacent Mn|| ions. With the Mn(dca)6 coordination units as BX6 octahedra and the bc+ cations as A-site cations, the full structure of 1 can be topologically described as postperovskite type (Figure 3a) wherein the BX6 octahedra are edge-sharing via double-dca bridges along the c-axis and corner-sharing via single-dca bridges along the b-axis, respectively, forming two-dimensional layers interleaved by A-site cations. A low-temperature phase transition was detected by the variable-temperature specific heat capacity, Cp, and second harmonic generation, SHG, measurements. As shown in Figure 2a, a small broad Cp anomaly was observed at around 174 K (T1), implying a second-order phase transition.33 The estimated ΔH and ΔS are 78.0 J·mol−1 and 0.45 J·K−1·mol−1, respectively. As the temperature was decreased from 200 K, the SHG intensity rose around 165 K from a value near 0 to about 0.2 times that of KH2PO4, indicating a structural transition from centric to acentric upon cooling. This transition was further confirmed by single-crystal X-ray diffraction analyses (see below). For convenience, the structural phases below and above 174 K were labeled as α and β phases, respectively. Figure 2 | (a) The temperature-dependent Cp (left) and SHG (right) responses (gray line stands for the baseline), (b) dielectric constant at different frequencies (the inset shows a linear fitting to the Curie–Weiss law at a frequency of 19 kHz), and (c) spontaneous polarization integrated by the pyroelectric currents under a reversal electric field in the vicinity of phase transition between α and β phases for 1. Download figure Download PowerPoint The temperature-dependent dielectric permittivity (ɛ = ɛ′ + iɛ″) was measured on a single crystal along the b-axis (i.e., the polar axis in ferroelectric phase; Figure 2b). A pronounced, λ-shaped dielectric anomaly, typical for normal ferroelectrics, was observed around 174 K. By fitting the reciprocal dielectric constant (1/ɛ′) versus T plot around 174 K based on the Curie–Weiss law, ɛ′ = Cferro/(T0 − T) (when T < Tc) or ɛ′ = Cpara/(T − T0) (when T > Tc), where C is the Curie–Weiss constant and T0 is the Curie temperature, a Cpara/Cferro ratio of 0.6 (at 19 kHz) was obtained, agreeing well with typical features of a second-order ferroelectric transition.33 The ferroelectric transition was further confirmed by a reversible, or "switchable," electric polarization revealed by measuring the temperature dependence of pyroelectric current on a polished single crystal under reverse electric field conditions applied along the polar axis (i.e., the b-axis; see the ).34–36 As shown in Figure 2c, the spontaneous polarization (Ps) integrated by the pyroelectric currents changes from almost 0 at 200 K to ca. 0.053 μC cm−2 at 120 K after a poling procedure of applying an electric field of 3.77 kV cm−1. Moreover, a reversed polarization in α phase can be obtained after a poling procedure of applying a reverse electric field, confirming a reversible spontaneous polarization by electric field, that is, ferroelectricity. In addition, the spontaneous polarization does not occur abruptly but gradually increases below T1, consistent with the characteristics of a second-order phase transition.33 To interrogate the nature of ferroelectric transition, variable-temperature, single-crystal X-ray diffraction analyses were performed for 1 at 139 K ( α phase) and 189 K ( β phase), respectively. The paraelectric β phase crystallizes in a centrosymmetric monoclinic space group P21/m, in which each bc+ cation is located over two crystallographically equivalent positions in the same mirror plane perpendicular to the b-axis, and the double-dca bridges sway about two positions (Figure 3b). For the ferroelectric α phase, crystallizatioin occurs in the polar monoclinic space group P21, within which both the guest cations and the double-dca bridges are collectively ordered and slightly tilt along the b-axis in the same direction (Figure 3a) with position offsets of 0.019 Å for Br atoms and 0.13/0.34 Å for the middle N atoms of double-dca bridges. Such a collective freeze of disordered moieties in a polar manner results in a spontaneous electric polarization of 0.024 μC cm−2, estimated by a point-charge model, which is consistent with the experimental pyroelectric measurement value of 0.053 μC cm−2. Such a transition from β to α phase is accompanied by a breaking of symmetry having an Aizu notation of 2/mF2; that is, four symmetry elements (E, C2, i, and m) in β phase are halved into two (E and C2) in α phase, thus meeting the symmetry requirements for a typical second-order ferroelectric transition.33,37,38 In short, the underlying mechanism for ferroelectricity of 1 is mainly ascribed to the order–disorder transition of guest cations and double-dca bridges, which is different from the off-center displacement of the central Ti4+ cation in inorganic BaTiO3.1 Figure 3 | Ferroelectric transition of 1 from (a) α phase (139 K) to (b) β phase (189 K). The red plane stands for the crystallographic mirror. Each organic cation is monochromatic (blue for the α phase and purple for the β phase), and hydrogen atoms are omitted in packing views for clarity. Download figure Download PowerPoint Moreover, the unique confined space provided by the postperovskite layers in 1 gives rise to an interesting dynamic behavior of guest cations. With further heating from the β phase, a new phase (labeled the γ phase) was detected above 327 K (T2), by differential scanning calorimetry, dielectric, and variable-temperature measurements, as well as X-ray crystal diffraction (see the ). In comparison with the β phase, the γ phase belongs to the same space group P21/m, but has a double c-axis (c γ = 15.0423 Å, c β = 7.5009 Å) and a larger β angle (113.68° vs. 108.76°) associated with an interlayer glide along the c-axis (Figure 4). This phenomenon leads to an unusual phase transition because the higher temperature γ phase has double the asymmetric units of the lower temperature β phase. The underlying mechanism of the transition is that half of the guest cations (shown in orange in Figure 4b) change their conformation from trans form in the β phase to cis form in the γ phase, and they also undergo a significant change in orientation. In other words, the guest cations rotate ∼110° in the anticlinal plane. Accordingly, the –CH2CH2– moieties in the cis-form cations strongly sway about the mirror plane perpendicular to the b-axis in the γ phase. Figure 4 | Structural transition of 1 from (a) β phase (189 K) to (b) γ phase (338 K) viewed along the b-axis. Each organic cation is monochromatic (purple for the β phase and orange for the γ phase), and hydrogen atoms are omitted for clarity. Download figure Download PowerPoint To further understand the essential role of the intermolecular interactions on the phase transitions, we performed a Hirshfeld surface analysis for the guest cations in 1 and compared the results with the observations in the chloride analog that we recently reported, (C5H13NCl)[Mn(dca)3] ( 1_Cl).32 Unilke 1, 1_Cl undergoes two-step Cmcm (Z = 4) → Pbcm (Z = 4) → Pbca (Z = 8) nonferroelectric phase transitions upon cooling. Such phase transition behaviors in 1 and 1_Cl are mainly attributed to the distinct conformations of their guest cations, guest–guest interactions, and host–guest interactions. For 1_Cl, the guest cation (denoted as cc+) contains a relatively smaller Cl atom (compared with the Br atom in bc+) that barely interacts with adjacent cations making the guest–guest interactions are negligible, as indicated by the Hirshfeld surfaces mapped with normalized contact distance (dnorm; Figure 5a). Meanwhile, cc+ maintains the cis form in all phases of 1_Cl to interact with the single-dca bridges from the host framework and displays a trivial two-step freezing behavior from a fourfold disorder, to a twofold disorder, and then to a frozen state. Figure 5 | The Hirshfeld surfaces mapped with dnorm for (a) the ordered cc+ cations in 1_Cl and (b) the bc+ cations in 1β, in which the intermolecular contacts shorter and longer than van der Waals separations show up as red and blue spots, respectively. For clarity, the hydrogen atoms, metal ions, and double-dca bridges are omitted. As indicated by the red spots, the cc+ cation mainly interacts with two single-dca bridges (pink dashes) but not adjacent cations. In contrast, each bc+ cation strongly interacts with two adjacent bc+ cations (red dash) and the double-dca bridges (black arrows). Download figure Download PowerPoint In contrast, the cis-form bc+ cations could only be observed in the high-temperature, entropically favored γ phase for 1. When cooling γ into the β phase, all bc+ cations adopt the trans form, and for each bc+ cation, the relatively larger Br atom comes into contact with a methyl group from adjacent bc+ cations (C–H⋯Br, 3.22 Å) to give two strong guest–guest interactions, as indicated by two large red spots in the Hirshfeld surface for a bc+ cation (Figure 5b). Moreover, the trans-form conformation allows each bc+ cation to have relatively strong interactions with the double-dca bridges (see the ) via the short H⋯C and H⋯N contacts indicated by the red spots (and marked by black arrows in Figure 5b). These types of host–guest interactions, together with the significant guest–guest interactions in 1, permit a mirror symmetry along the b-axis in the β phase when both the bc+ cation and the double-dca bridges are dynamically disordered, but induce polar behavior along the b-axis in the α phase when both cation and dca ligands are frozen, that is, a spontaneous polarization in a ferroelectric phase. Conclusions In summary, we present a new molecular postperovskite, 1, which is to the best of our knowledge, the first ferroelectric postperovskite, and we disclose the essential role of the molecular interactions of the cations on the changeable molecular dynamics triggering unique phase transitions for 1. Upon cooling, the guest cations change from a highly disordered state to a slightly disordered state, accompanied by unusual conformational and orientational changes, and then reach a frozen state that results in a typical order–disorder ferroelectric transition to generate switchable spontaneous electric polarization. These unique dynamic behaviors of the flexible guest cations, together with the resulting ferroelectric and exceptional phase transitions, show that such postperovskites could serve as a promising new host–guest model to control molecular dynamics of abundant polar cations in the development of advanced functional materials. Supporting Information Supporting information is available. CCDC numbers are 1900811, 1900812, and 1900813 for α, β, and γ phases, respectively. Conflict of Interest The authors declare no conflict of interest. Funding Information This work was supported by the National Natural Science Foundation of China (21722107, 21671202, and 21821003) and the local innovative and research team's project of Guangdong Pearl River talents program (2017BT01C161). Acknowledgment W.-X.Z. is thankful to the Pearl River S&T Nova Program of Guangzhou (201610010027). References 1. Cohen R. E.Origin of Ferroelectricity in Perovskite Oxides.Nature1992, 358, 136–138. Google Scholar 2. Prellier W.; Singh M. P.; Murugavel P.The Single-Phase Multiferroic Oxides: From Bulk to Thin Film.J. Phys.: Condens. Matter2005, 17, R803–R832. Google Scholar 3. Franchini C.Hybrid Functionals Applied to Perovskites.J. Phys.: Condens. Matter2014, 26, 253202. Google Scholar 4. Tejuca L.; Fierro J.Properties and Applications of Perovskite-Type Oxides; CRC Press: New York, 1993. Google Scholar 5. Kazim S.; Nazeeruddin M. K.; Gratzel M.; Ahmad S.Perovskite as Light Harvester: A Game Changer in Photovoltaics.Angew. Chem., Int. 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Google Scholar Chen Zhou Zhang Chen Huang and Zhang Hybrid Perovskite a , Information is thankful to the Pearl River S&T Nova Program of Guangzhou (201610010027). times