Controlling Two-Step Phase Transitions and Dielectric Responses by

Article pubs.acs.org/crystal

Controlling Two-Step Phase Transitions and Dielectric Responses by A‑Site Cations in Two Perovskite-like Coordination Polymers Wei-Jian Xu, Kai-Ping Xie, Zhi-Feng Xiao, Wei-Xiong Zhang,* and Xiao-Ming Chen MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, P. R. China S Supporting Information *

ABSTRACT: Two new perovskite-like coordination polymers, A2[KFe(CN)6], were constructed by employing guanidinium and acetamidinium as A-site cations, respectively. Their cation-controlled two-step phase transitions as well as the relevant dielectric responses were uncovered by the combined techniques of the variable-temperature single-crystal X-ray structural analyses and dielectric measurements. With a similar size and shape, the A-site cations reveal similar two-step thermal-induced transitions on their motional dynamics, i.e., from a frozen order state to an in-plane rotational disorder state, and to a melt-like disorder state. However, the variation of the A-site cations on their symmetries and dipole moments makes noticeable impacts on the symmetry breaking, the critical temperatures, and the dielectric responses for the two-step structural phase transitions, i.e., the D3h nonpolar guanidinium results in an R3̅c ↔ R3̅m ↔ Fm3̅m transition, whereas the C2v polar acetamidinium results in a C2/m ↔ R3̅m ↔ Fm3̅m transition. Investigations of these two coordination polymers demonstrate a fine modulation on the phase transition behaviors and dielectric responses by changing the symmetries and dipole moments of A-site cations.

1. INTRODUCTION Solid−solid structural phase transition materials, with physical and/or chemical properties to enable responses to the application of external pressure, heat, or light, have attracted wide interest due to their potentially promising applications in modern optoelectronic materials, such as dielectric, ferroelectric, and nonlinear optical switches.1−19 The recent emergence of ABX3-type coordination polymers (CPs), which mimic the inorganic ABO3-peroviskite structure, takes advantage of the tunability of A and B components to develop phase transition materials with a switch of the dielectric, magnetic, and nonlinear optical properties.20−25 For example, a series of formate-bridged perovskite-like CPs, A[M(HCOO)3)] (where A+ = organic cations, and M = metal ion), and a family of azidobridged perovskite-like CPs, A[M(N3)3], exhibit a wide range of fascinating physical properties, such as ferroelectricity, ferroelasticity, magnetic bistability, switchable dielectric relaxation, and multiferroicity.26−28,29−35 Very recently, a family of dicyanamide-bridged perovskite-like CPs, [N(C3H8)4][M(N(CN)2)3] also revealed promising multiferroic behavior and an unusually large response under pressure and temperature.36,37 © XXXX American Chemical Society

The extensive studies on perovskite-like CPs indicated their thermal-induced phase transitions as well as the corresponding switchable dielectric properties could be controlled by A-site cations confined in the framework. For instance, Zhang et al. realized the switchable dielectric constants in several cyanobridged perovskite-like CPs owing to the thermal-induced order−disorder transitions of the A-site cations.38−41 Recently, by assembling a family of CPs with four different guest cations, (CH3)nNH4−n+ (n = 1−4), we have investigated the influence of the guest cations on the phase transitions and the dielectric switching behaviors.42 Nevertheless, as all the employed cations have a remarkably different size, it is hard to ascribe the distinct phase transition behaviors to the different symmetries (as well as the dipole moment) or the different sizes. In order to further investigate the template effect of ammonium cation in metalcyanide perovskite CPs for exploring advanced dielectric materials, in this manuscript, we selected two planar-shaped Received: September 23, 2016 Revised: October 24, 2016 Published: November 1, 2016 A

DOI: 10.1021/acs.cgd.6b01404 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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1, and 1.89 g of acetamidine hydrochloride for 2) in the ratio of 1:2 at room temperature. Yield: 86% for 1 and 82% for 2 based on K3Fe(CN)6. Elemental analysis, calcd (%) for 1: C, 25.88; N, 45.28; H, 3.26. Found, C, 25.18; N, 44.04; H, 3.15. calcd (%) for 2: C, 32.53; N, 37.94; H, 3.82. Found, C, 32.13; N, 37.48; H, 3.67. 2.2. Materials and Physical Measurements. Elemental (C, H, and N) analyses were performed on a PerkinElmer Vario EL elemental analyzer with as-synthesized samples. Variable-temperature powder Xray diffraction (PXRD) patterns (Cu Kα, λ = 1.54056 Å) were collected on Bruker Advance D8 DA VANCI θ−2θ diffractometer. Thermogravimetric analyses (TGA) were carried out on a TA Q50 system with a heating rate of 5 K min−1 under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was carried out on a TA DSC Q2000 instrument under a nitrogen atmosphere in aluminum crucibles with heating and cooling rates of 10 K/min from 113 to 460 K. The dielectric measurements were carried on a TH2828A impedance analyzer at 5 frequencies from 100 kHz to 1 MHz, with an applied voltage of 1.0 V, and a temperature sweeping rate of approximately 2 K/min in a Mercury iTC cryogenic environment controller of Oxford Instrument. The pressed-powder pellets of 1−2 deposited with silver conducting glue and using gold wires as electrodes. 2.3. X-ray Crystallographic Analyses. The in situ variabletemperature single-crystal diffraction intensities were collected on a Rigaku XtaLAB P300DS single-crystal diffractometer for 1 and on a Rigaku R-AXIS SPIDER IP diffractometer for 2, with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The CrystalClear software package was used for data collection, cell refinement and data reduction. Absorption corrections were applied by using multiscan program REQAB47 for 1 and ABSCOR48 for 2, respectively. The structures were solved by direct methods and refined using full-matrix least-squares methods with the SHELX program package and Olex2 program.49,50 Anisotropic ADPs were refined for all non-hydrogen atoms, and the positions of the hydrogen atoms were generated geometrically. HKL5 refinement were applied for 2γ, as the twinning occurs during the transition from the hexagonal phase (2β) to monoclinic phase (2γ). CCDC numbers: 1482240−1482245. Crystallographic data and structural refinements for 1−2 are summarized in Table 1.

cations with almost the same size to construct metal-cyanide perovskite CPs, and thereby to genuinely investigate how the symmetry, dipole, and H-bonding character of A-site cation affect the phase transition behavior (also the dielectric responses) by excluding the size effect. As shown in Scheme 1, one of the cations is nonpolar guanidinium (Gua, C(NH2)3+) Scheme 1. Symmetries of A-Site Cations

with D3h symmetry, and the another one is polar acetamidinium (Ace, CH3C(NH2)2+) possessing a dipole moment of 1.2 D with C2v symmetry.43−46 Accordingly, we obtained two new perovskite-like CPs, A2[KFe(CN)6] (A = Gua for 1, and Ace for 2), and herein we report their cation-controlled two-step phase transitions as well as the switchable dielectric responses uncovered by the combined techniques of the variabletemperature single-crystal X-ray structural analyses and dielectric measurements.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals were commercially available and were used without further purification. The red block crystals of 1−2 were obtained by slow evaporation of the aqueous solution containing 3.29 g of K3Fe(CN)6 and amine salt (1.93 g of guanidine hydrochloride for

Table 1. Summary of Crystal Data and Structural Refinements for 1−2 at Different Temperatures

a

compound

1

2

formula T (K) phases crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z Dc /g cm−3 reflns coll unique reflns Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data)a wR2 (all data)a GOF CCDC number

[C(NH2)3]2[KFe(CN)6] 425(2) 1β trigonal R3̅m 8.521 (2) 8.521 (2) 19.915(7) 90 1252.4(5) 3 1.536 2662 303 0.1074 0.0832 0.3027 0.0928 0.3143 1.009 1482242

[CH3C(NH2)2]2[KFe(CN)6] 298(2) 2β trigonal R3̅m 8.7278(4) 8.7278(4) 19.424(1) 90 1281.4(1) 3 1.375 4205 395 0.0366 0.0329 0.0861 0.0335 0.0865 1.164 1482245

298(2) 1γ trigonal R3̅c 8.4290(2) 8.4290(2) 39.662(2) 90 2440.4(2) 6 1.516 8400 542 0.0581 0.0240 0.0648 0.0248 0.0650 1.197 1482240

455(2) 1α cubic Fm3̅m 11.911(6) 11.911(6) 11.911(6) 90 1690(3) 4 1.459 1717 130 0.0934 0.0375 0.1020 0.0401 0.1047 1.140 1482243

150(2) 2γ monoclinic C2/m 13.737(1) 8.874(1) 7.984(1) 120.82(1) 835.8(2) 2 1.467 877 802 0.0813 0.2216 0.0834 0.2268 1.097 1482241

395(2) 2α cubic Fm3̅m 12.051(1) 12.051(1) 12.051(1) 90 1750.1(3) 4 1.401 3552 136 0.0346 0.0217 0.0455 0.0252 0.0471 1.018 1482244

R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2. B



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3. RESULTS AND DISCUSSION 3.1. Detection of Thermal-Induced Phase Transitions. Compounds 1−2 are stable when exposed to air, and the TGA analyses showed that both of them are stable up to 480 K (Figure S1). Their thermally induced phase transitions were detected by differential scanning calorimetry measurements with a rate of 10 K/min. Upon heating/cooling (Figure 1), two

i.e., three-dimensional framework consisting of [Fe4K4(CN)12] cages formed by Fe−CN−K fragments and fully filled by the guest cations. It should be noted that such perovskite-type structure is completely different from the structure of Prussian blue analogues, AxMAII[MBIII(CN)6]y·zH2O, in which the alkali ion A and a portion of water molecular are accommodated in the cavities and the other portion of coordinated water molecules occupy the vacancy of the [MB(CN)6] site.51−53 The two-step phase transitions in both of 1 and 2 could be mainly ascribed to the dynamic changes of the encapsulated cations, i.e., from a nearly arbitrarily orientational disorder state at α phases to an in-plane rotational disorder state at β phases, and then further frozen to an orientational order state at γ phases (vide infra). Take a close look at Figure 2: the guest cation in α phases rotates dynamically nearly over arbitrary orientation and the CN− bridges disordered over four sites, as required by the imposed symmetry of cubic space group Fm3̅m. When cooling to the β phases that crystallize in trigonal space group R3̅m, the guest cations rotate in-plane to be disordered over six sites; meanwhile, the CN− bridges in the host frameworks are slightly disorder over two sites. Consequently, during the α → β phase transitions for both 1 and 2, symmetry breaking occurs with a change of the total symmetry elements of the crystallographic point group decreasing from 48 (E, 8C3, 3C2, 6C2, 6C4, i, 8S6, 3σh, 6σd, 6S4) to 12 (E, 2C3 3C2 i, 2S6, 3σv), which is a ferroelastic transition with an Aizu notation of m3mF3̅m.54 With further cooling to the γ phases, the guest cations for both 1 and 2 are frozen to be orientational ordered. Because of the different symmetries of guest cations, their γ phases crystallize in different space groups; i.e., 1γ with D3h-symmetric Gua cations crystallizes in a high-symmetry trigonal space group R3̅c, whereas 2γ with C2v-symmetric Ace cations crystallizes in a low-symmetry monoclinic space group C2/m. Accordingly, during the 1β → 1γ transition, the total symmetric elements of the crystallographic point group remain unchanged, i.e., 12 (E, 2C3, 3C2, i, 2S6, 3σv), while a unit cell multiplication was observed as the c axis is doubled from 19.915(7) Å at 425 K (1β) to 39.662(2) Å at 298 K (1γ). In contrast, for the 2β → 2γ transition, the total symmetric elements of the crystallo-

Figure 1. DSC curves of 1 (blue) and 2 (red).

pairs of reversible sharp endothermic/exothermic peaks were observed at critical temperatures (Tc(heating)/Tc(cooling)) of 411/ 406 K (T1) and 439/435 K (T1′) for 1, 201/199 K (T2) and 385/381 K (T2′) for 2, respectively. These phase transitions were further confirmed by the in situ variable-temperature powder X-ray diffraction analysis (Figure S2). For convenience, we label 1γ (2γ), 1β (2β), and 1α (2α) for the low-temperature phases below T1 (T2), the intermediate-temperature phases between T1 (T2) and T1′ (T2′), and the high-temperature phases above T1′ (T2′), respectively. 3.2. Crystal Structures, Structural Phase Transitions, and the Relevant Mechanism. The single-crystal X-ray structural analyses for all phases of 1 and 2 reveal that, as shown in Figure 2, they have a common double-perovskite structure,

Figure 2. Crystal structures of 1 (a) and 2 (b) at different phases. C



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graphic point group decrease from 12 (E, 2C3, 3C2 i, 2S6, 3σv) to 4 (E, C2, i, σh), which is a ferroelastic transition with an Aizu notation of 3̅mF2/m.54 The different hydrogen-bonding character of −NH2 and −CH3 groups significantly affects the host−guest interactions in these two CPs. As shown in Figure 2, each −NH2 group could donate two hydrogen to the N atoms of CN− bridges, forming two N−H···N classical hydrogen bonding interactions. Therefore, each Gua cation in the 1γ phase forms six N−H···H hydrogen bonding interactions (N···N distances: 3.11−3.35 Å, Table S1), whereas each Ace cation in the 2γ phase forms only four N−H···H hydrogen bonds with longer N···N distances of 3.27−3.39 Å. In other words, 1 has stronger hydrogen-bonding interactions than 2, very consistent with the fact that 1 has significantly higher phase transition temperatures (439 and 411 K) that those (385 and 201 K) observed at corresponding twostep phase transitions in 2. In addition, such distinct hydrogenbonding interactions are also responsible for the different enthalpy change during the β → α transitions; i.e., 6.95 kJ/mol for 1 is larger than 4.34 kJ/mol for 2. However, it should be noted that, for the γ → β transitions, the enthalpy change in 1 (1.47 kJ/mol) is smaller than that in 2 (2.13 kJ/mol). As indicated by the varied Fe···K distances and the ∠FeKFe angles (Figure S3), in contrast to the 1γ → 1β transition (R3̅c → R3̅m), the 2γ → 2β transition (C2/m → R3̅m) is associated with a very remarkable deformation of the host framework, which significantly contributes to the entire enthalpy change. For an order−disorder transition, ΔS = R ln(N), where R is the gas constant and N is the ratio of the number of configurations in the disordered and ordered system. Taking the ΔS estimated from DSC analyses, the N values were estimated to be 1.54, 6.68 and 3.58, 3.87 for 1 and 2, respectively. These estimated N values were not very close to the N values (6, 4 and 6, 4, respectively) expected by single-crystal X-ray analyses, indicating these phase transitions are more complicated than a typical order−disorder transition mechanism undergoing reorientations of guest cations. By and large, although guest cations with a similar size and shape, their different symmetries and dipoles lead to distinct phase transition behaviors, which also make noticeable impacts on the thermal properties, e.g., the critical temperatures as well as the enthalpy change. 3.3. Dielectric Properties. The temperature-dependent dielectric permittivities were measured on the powder-pressed pellets for 1−2. As shown in Figure 3a, upon cooling from 450 K, a reflective point of the dielectric constant (ε′) was found in the vicinity of the 1α → 1β transition (435 K), but no noticeable change was observed in the vicinity of 1γ → 1β transition (406 K). For 2, ε′ at below 199 K (2γ) remains nearly constant of ca. 4 (i.e., a low dielectric state), and rapidly changes to ca. 8 (i.e., a high dielectric state) at 2β phase. In addition, a small peak was observed at 381 K (2α → 2β transition), owing to the competition between thermal disordering and electric ordering, similarly observed in polar liquids.41 For such a host−guest system, ε′ is mainly determined by the possessing dipole moment of guest cation and the separate positive−negative point charge of each cage in different phases. Taking 2 as an example, two possible rotation models could be proposed for the polar Ace cation at 2β and 2α phases, respectively (Figure 4). In 2β phase (model A), the Ace cations reveal an axial rotation about the pseudo-C3 axis perpendicular to the Ace plane, which changes the orientation of the possessing dipole moment and thus is dielectrically active.

Figure 3. Temperature-dependent dielectric constants for 1 (a) and 2 (b).

Figure 4. Proposed rotation models for the Ace cation in 2β (model A) and 2α phases (model B), respectively.

Therefore, the transition between 2β and 2γ phases brings about a step-like dielectric switching at around 199 K. For 2α phase (model B), the Ace cations reveal an out-plane oscillatory fluctuation in addition to the C3 rotational motion, namely, being in nearly arbitrary rotation. The transition from a C3rotational motion in 2β to a melt-like disorder state in 2α for a free polar Ace cation should not significantly change the dielectric constants for a powder sample, as both of them are dielectrically active motions. However, as indicated by singlecrystal X-ray analyses, the bridging ligands are also remarkably swaying in 2α and 2β, such dynamically disorders of the ionic bridging ligands together with the cationic guests could dynamically separate the negative- and positive-point charge for each cage. As a result, temperature-dependent instantaneous dipole moments were induced to cause the frequencydependent increase of ε′ at above 300 K as well as the reflective point of ε′ in the vicinity of the 2β ↔ 2α transition. For 1, both rotational models A and B of a free nonpolar Gua guest cation are dielectrically inactive, so no dielectric switching was observed during the 1γ ↔ 1β transition, except a frequency-dependent dielectric constant was observed at above 400 K together with a reflective point in the vicinity of the 1β ↔ 1α transition, owing to the instantaneous dipole D



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moments of cages. On the whole, although the encapsulated guest cations in 1−2 undergo almost the same dynamic behaviors, their dielectric responses are significantly different, reflecting that the dipole moments of the guest cations play a crucial role on modulating dielectric response.

4. CONCLUSION In summary, two new cyano-bridged perovskite-like CPs were constructed by employing similar-sized guanidinium and acetamidinium as A-site cation, respectively. The similar-shaped cations reveal similar thermal-induced two-step transitions on their motional dynamics, i.e., from a frozen order state to an inplane rotational disorder state, and to a melt-like disorder state. However, the variation on their symmetries and dipole moments makes noticeable impacts on the symmetry breaking, the critical temperatures, and the dielectric responses for the two-step structural phase transitions, i.e., R3̅c ↔ R3̅m ↔ Fm3̅m transition for the nonpolar D3h-symmetric Gua cation, whereas C2/m ↔ R3̅m ↔ Fm3̅m transition for the polar C2v-symmetric Ace cation. Investigations on these two CPs illustrated well a fine modulation on the two-step phase transitions as well as the dielectric responses by the symmetries and dipole moment of A-site cations. The study on such system provides insights into the crucial role of A-site cation in perovskite systems and may be a key to understanding the dynamic motion of guest cations in the confined spaces, affording a useful strategy to design novel thermoresponsive dielectric materials.

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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.cgd.6b01404. Thermogravimetric analysis, in situ variable-temperature PXRD patterns for the title compounds (PDF) Accession Codes

CCDC 1482240−1482245 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21290173, 21301198, and 91422302), NSF of Guangdong (S2012030006240). W.X.Z. is thankful to the Pearl River S&T Nova Program of Guangzhou and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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