Article pubs.acs.org/cm
Unusual Sequential Reversible Phase Transitions Containing Switchable Dielectric Behaviors in Cyclopentyl Ammonium 18-Crown‑6 Perchlorate Yun-Zhi Tang,*,† Zhi-Feng Gu,† Jian-Bo Xiong,† Ji-Xing Gao,† Yi Liu,† Bin Wang,† Yu-Hui Tan,† and Qing Xu† †
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi Province, People’s Republic of China S Supporting Information *
ABSTRACT: Multisequential reversible phase transitions based on molecular materials have important applications in ferroelastic materials, ferroeletric materials, switchable dielectric materials, and temperature-controlling materials. Here, we report that a new compound, [Hcpa-(18-crown-6)]+[ClO4]− (1) (where Hcpa represents protonated cyclopentylamine cations) displays unusual multisequential reversible phase transitions accompanied by switchable dielectric behaviors. The stepwise synergistic disordering of Hcpa cations and ClO4− anions leads to the sequential reversible phase transitions and symmetry breaking. These unusual reversible phase transitions were further confirmed by the variable-temperature powder X-ray diffractometry (PXRD), thermal anomalies of differential scanning calorimetry (DSC) measurements, and abrupt dielectric anomalies in the heating and cooling processes.
1. INTRODUCTION Reversible structural phase transitions can be defined as the local or micro mutual transformations in crystal structures. Because they are often accompanied by many interesting physical response signals during the phase transitions processes, such as paraelastic−ferroelastic phase transition, paraelectric− ferroelectric transformations second harmonic generation (SHG) responses, dielectric and heat anomalies, etc.,1−6 they have been widely applied in ferroelastic materials, ferroelectric materials, energy storage, temperature controlling, data communication, infrared acquisition, signal processing and nonlinear optical (NLO) materials.7−10 Especially, the research on multisequential reversible phase transitions has attracted much attention, because of the combination of a variety of structural changes and physical response signals, which are promising to apply on modern multifunctional materials.11−29 A typical example for that, BaTiO3 (BT), is well-known for its multisequential reversible phase transition behaviors, which transforms from a cubic system to a tetragonal system (an orthogonal lattice), then to a trigonal system when the temperatures are varied from 130 °C to 0 °C, to −80 °C, respectively. However, most reports about multisequential reversible phase transitions are still mainly focused on inorganic materials such as compound metals, ceramics, BT and KH2PO4 (KDP), etc.2,3,29 Studies on multisequential reversible phase transitions based on molecule materials are still very rare.11−14,17−21 Recently, our research groups and others discovered that some order−disorder of host−guest molecules in crown-ether © XXXX American Chemical Society
clathrates can cause reversible structural phase transitions, depending on temperature.11−14,25 Using this methodology, we have discovered two new ferroelectrics: ([Hcha-(18-crown6)]+[BF4]− and [Hcha-(18-crown-6)]+[ClO4]− (Hcha = protonated cyclohexyl ammonium), with high Curie temperatures and large spontaneous polarization.25 Xiong et al. found that the slowing of the rotation of the 18-crown-6 molecule and the tumbling of the BF4 anion causes the symmetry breaking; the relative displacement between the cationic and anionic sublattices can induce spontaneous polarization.11−14 Actually, the crown-ether clathrates have displayed more and more advantages in constructing the multisequential reversible phase transitions materials: (i) they are easy to assemble into multiple ionic compounds containing both organic cations and inorganic anions, which are available for producing stepwise and synergistic disorder of host−guest molecules; (ii) there maybe exist diversified molecule motions including twist motion, displacement motion and order−disorder motion, etc.,11−14 which provide favorable conditions for the generation of multisequential reversible phase transition. Therefore, as ongoing research to explore the new type of molecular phase-transition materials especially for multiferroics materials, we committed to assemble several crown-ether Received: April 28, 2016 Revised: May 27, 2016
A
DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Scheme 1. Preparation of Compound 1
Figure 1. A change of symmetry elements of 1 from 8 (E, C2, 2 C2′, i, σh, 2 σv) in the 1-ITPto 4 (E, C2, i, σh) in the 1-LTP. orthorhombic, Pbcn, a = 12.4985(9) Å, b = 13.1847(10) Å, c = 13.6702(9) Å, V = 2252.7(3) Å3, Dc = 1.415 Mg m−3, Z = 4, μ = 0.226 mm−1, S = 1.114, R1[I > 2σ(I)] = 0.1241, wR2(all data) = 0.3636. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_ request/cif (CCDC No. 1444852 for 1-LTP, CCDC No. 1444853 for 1-ITP). 2.3. Measurement Methods. Infrared (IR) spectra were measured on a Shimadzu IR Prestige-21 system. Powder X-ray diffractometry (PXRD) analyses were recorded on a Rigaku D/MAX 2000 PC XRD instrument. Differential scanning calorimetry (DSC) measurements were conducted through a Perkin-Elmer Diamond DSC system under a nitrogen atmosphere with a heating or cooling rate of 5 K/min. Detailed heat analyses were treated on a Quantum Design PPMS system. Above room temperature, thermogravimetric analysis (TGA) measurements were performed, using a TA-Instruments STD 2960 system from 293 K to 900 K. Dielectric measurements were made with the single crystals. Silver conduction paste, which had been deposited on both surfaces, was used as the electrodes. The dielectric constants of the compounds were determined with an Agilent, Model TH2828A impedance analyzer over the frequency range from 1 kHz to 1 MHz.
clathrates by selecting diversified organic amines, varied inorganic ions, and even different crown ethers.11−14,25,30 Fortunately, we successfully captured a new class of pervoskite structure clathrates: [Hcpa-(18-crown-6)]+[ClO4]−, 1, which not only shows unusual two sequential reversible phase transitions, but also exhibits switchable dielectric behaviors. Here, we demonstrate its interesting reversible structural phase transitions accompanied by a large heat anomaly and sensitive switchable dielectric changes, as well as variable-temperature powder X-ray diffractometry (PXRD).
2. EXPERIMENTAL SECTION 2.1. Syntheses of [Hcpa-(18-crown-6)]+[ClO4]− (1). All the reagents and solvents used were of commercially available quality. As shown in Scheme 1, 1 mmol (0.085 g) cyclopentylamine of methanol solution (5 mL) was added slowly in a 5 mL of methanol solution of 18-crown-6 (1 mmol, 0.264 g), then followed by the dropwise addition of 0.14 g of perchloric acid (72%) into a trichloromethane solution (1 mL). The large colorless block single crystals of 1 (Figure S1 in the Supporting Information) were obtained via slow evaporation from the mixed solution at room temperature over 2 weeks. For 1: yield, 0.297 g (66%), based on CPA. Infrared (IR) (Figure S2 in the Supporting Information) (KBr, cm−1): 3419 (w), 3106 (s), 2916 (s), 2466 (w), 2111 (w), 1611 (m), 1471 (m),1350 (m), 1286 (m), 1250 (m), 1090 (s), 958 (s), 832 (m). 2.2. Single-Crystal X-ray Crystallography. X-ray diffraction (XRD) determinations were carried out above and below the phase transitions temperatures for 1 through a Bruker Smart Apex II singlecrystal diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 150 and 301 K, respectively. We also tried to determined its structure over 400 K; however, we are not able to get their completed structures, because the diffraction data are very poor. All the treatments including data collection, cell refinements, and data reduction were conducted by using the Crystal Clear software package. The structures were resolved by direct methods and refined by the full-matrix method based on F2, using the SHELXLTL software package.31−33 All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were generated geometrically. Cell parameters and refinements for 1 at 150 K: C17H36ClNO10, Mr = 449.92, monoclinic, C2/c; a = 12.661(2) Å, b = 12.485(2) Å, c = 13.677(2) Å, β = 92.124(4)°, V = 2160.5(6) Å3, Dc = 1.476 Mg m−3, Z = 4, μ = 0.236 mm−1, S = 1.027, R1[I > 2σ(I)] = 0.0761, wR2(all data) = 0.2406. At 301 K: C17H36ClNO10, Mr = 449.92,
3. RESULTS AND DISCUSSION 3.1. Crystal Structure Determinations. For convenience, we label the first phase transition for compound 1 as Tcl, and the second phase transition as Tch; the structure below Tcl is called 1-LTP, that between Tcl and Tch is called 1-ITP, and that above Tch is labeled as 1-HTP. By comparison of their cell parameters and refinements data, we can discover obvious changes between 1-LTP and 1-ITP. First, the unit-cell length b extends from 12.485(2) Å to 13.185(10) Å for 1, increasing ∼5.6%; the unit-cell volume is enlarged from 2160.5 Å3 to 2252.7 Å3 for 1, increasing by ∼4.3%. Second, a significant change for the first phase transition (Tcl) is that the β angle has changed from 92.124° to 90°. Lastly, below Tcl, 1-LTP crystallized in the monoclinic system, in the C2/c (No. 15) space group, while between Tcl and Tch, 1-ITP, belonging to the orthorhombic crystal system with Pbcn space group (No. 60), crystallized. According to the viewpoint of symmetry breaking, a phase transition exists between the paraelastic highB
DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 2. (a) Molecular structure of compound 1-LTP at 150 K. (b) The partly disordered Hcpa cations and ordered ClO4− anions at 150 K. (c) Molecular structure of compound 1-ITP at 300 K. (d) The partly disordered Hcpa cations and disordered ClO4− anions at 300 K. (e) Molecular structure of compound 1-ITP at 300 K, highlighting the positional and orientational variation.
disorder with N1 and C9 atoms located on the crystallographic 2-fold axis, and both N1 and C9 atoms distribute equally over the mirror plane in the [0 0 1] direction due to orientational disorder24 (Figures 2a and 2b), therefore, the disordered Hcpa cations can be viewed as the rotation motions of the N1 and C9 atoms. The disorder leads to positive charges carried by the N atoms centered in the mirror planes. The ClO4− anion is completely ordered and has a tetrahedral geometry, the Cl−O distances are 1.427(3) and 1.439(3) Å, respectively. At room temperature, 1-ITP also has one 18-C-6 host molecule, one Hcpa cation, and one ClO4− anion, as previously observed; however, there are distinguishable differences between them: (i) The degree of disorder of the Hcpa cations has greatly increased, since all the C atoms in cyclopentyl rings show
temperature high-symmetry phase and ferroelastic low-temperature low-symmetry phase. This ferroelastic phase transition creates a lattice distortion. The C2/c space group possess a lower centrosymmetric point group C2h with four symmetric elements (E, C2, i, σh), and Pbcn has a higher centrosymmetric point group D2h with eight symmetric elements (E, C2, 2C2′, i, σh, 2σv). Therefore, the space group of 1-LTP (C2/c) is the subgroup of 1-ITP (Pbcn), which agrees well with the principle of Aizu notation of mmmF2/C (Figure 1). Figure 2 afford the crystal structures of 1 before and after the first phase transition (Tcl). Below 184 K, the asymmetric unit of 1-LTP contains one 18-C-6 host molecule, one Hcpa cation, and one ClO4− anion. Although the Hcpa cations seems like a “cis-1,4-cyclohexane diamine”, it is actually a 2-fold orientational C
DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. Packing view along the b-axis, showing the changes in the β angle at (a) 150 K and (b) 300 K.
Figure 4. Packing view along the c-axis, showing the changes of cell length b and c for 1 at (a) 150 K and (b) 300 K.
1-ITP, all the ClO4− anions and Hcpa cations in the vertical direction are completely parallel to the c-axis, so there obviously exist two new C2′-axes: one parallel to the a-axis, and the other parallel to the c-axis. In addition, we can clearly observe that the cell length (b) has been extended from 12.485 Å to 13.185 Å when viewed from the c-axis in 1-LTP and 1-ITP (Figures 4a and 4b). The reason can be attributed to the elongated displacement ellipsoids of highly disordered of ClO4− anions and Hcpa cations almost pointing toward the same b-axis direction. This can be further confirmed by their distinct changes from donor−acceptor distances of intermolecular hydrogen bonds between 1-LTP and 1-ITP, since C9− H9A···O5 distances varied from 2.648 Å to 3.302 Å. It should be emphasized that, recently, Xiong et al. have reported another crown ether clathrate [(DIPA)(18-crown6)]ClO4 (where DIPA = 2,6-diisopropylanilinium) (referenced hereafer as 2), which also undergoes sequential reversible phase transitions.16 Different from the title compound, the order− disorder change of the 18-crown-6 molecule and ClO4− anion in 2 is the major reason for the reversible phase transition; moreover, the most important observation is that 1 has unusual high-temperature phase transitions above 400 K, far more than that of 2 (278 K), indicating that 1 is promising with regard to practical applications in modern molecular dielectric materials. In addition, we have measured its solid-state 13C chemical shift of 1 at room temperature. As shown in Figure S3 in the Supporting Information, three distinct characteristic peaks occur, at 24.5°, 33.0°, and 53.8°, respectively. Considering the different environment of hydrogen atoms, we can credit them
elongated displacement ellipsoids and almost point to the b-axis directions (see Figures 2c and 2d), indicating strong dynamics of the in-plane swinglike motion about the N atom fixed by N−H···O hydrogen-bonding interactions. (ii) The ClO4− anions vary from an ordered state to a disordered state, since the O4 atom displays a highly dynamic displacement ellipsoid. As further explained in Figure 2e, the Hcpa cation can be completely distributed over two equivalent positions, which exhibit an almost-inverted structure, while the O4 atoms of ClO4− anions is also split into a 2-fold symmetry (O4, O4B, O4′, and O4B′). One of the most important things is to make clear how the symmetry breaking happened in a reversible structural phase transition. As analyzed in above text, the structure of 1-ITP located in a higher symmetrical space group Pbcn with point group D2h. Compared by C2h, D2h contains two more 2-fold axes (2 C2′) and the two σv symmetric elements; in fact, σv can be also obtained by the symmetrical rotating operation of 2-fold axes (2 C2′), according to the symmetric operation principle, so the key problem is to reveal the formation of two new C2′ axes in 1-ITP. As depicted in Figure 3, when viewed from the b-axis, we can clearly observe that, in 1-LTP (Figure 3a), all the ClO4− anions along the vertical direction have deviated away (2.124°) from the c-axis little by little. Besides, all Hcpa cations have tilted toward the −a-axis direction. Namely, all Hcpa cations and ClO4− anions can not superimpose; thus, there is not any symmetrical axis or mirror plane in this direction. However, for D
DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials for the β-hydrogen, the γ-hydrogen of −CH2−, and the αhydrogen of R3CH− groups, respectively. In addition, the ratio among them is estimated to be 2:2:1, according to the rough calculation on their peak areas. A slight deviation in peak area can be tentatively attributed to the high disorder of Hcpa cations. 3.2. Powder X-ray Diffractometry (PXRD). Since it is very difficult to perform single-crystal X-ray diffraction (XRD) determinations above 400 K, we carried out PXRD of temperature evolution from 173.15 K to 433.15 K for 1. As is well-known, a structure has more Bragg diffraction peaks below the phase-transition temperature than it has above the phase-transition temperature.18 Variable-temperature PXRD patterns of 1 are shown in Figure 5. For the first phase
restoration of diffraction patterns discloses that the degree of crystalline of 1 is well preserved (back to 293 K), which favors their switchable molecular dielectric materials. 3.3. Thermal Properties. For a reversible structural phase transition triggered by temperature, the compound will display heat anomaly behaviors in the vicinity of the phase transition temperature. Therefore, it is significant for one to perform its DSC measurement during the heating and cooling process. As illustrated in Figure 6a, the crystalline sample of 1 undergoes its first phase transition at a crystallization temperature (Tc) of ∼188 K in a cooling and heating cycle, uncovering an exothermic peak at 184.6 K and an endothermic peak at 191.9 K. Remarkably, both the exothermic and endothermic peaks are very “steep” and have a large thermal hysteresis (7.3 K) between them, clearly revealing that it is characterized by first-order phase transition features, which are further proven by their large entropy changes. By using precise integration and calculus methods, we calculated its entropy changes (ΔS), with a value of 7.93 J mol−1 K−1 on the heating process (see the Supporting Information). Through the formula ΔH = T ΔS
(T = 191.9 K)
we deduced that the enthalpy change (ΔH) is ∼1520 J mol−1. Furthermore, from the Boltzmann equation,
ΔS = R ln N where R is the gas constant and N represents the ratio of possible orientations, N can be calculated to be 2.595 at T ≈ 191.9 K. With the same procedure, the calculated entropy change (ΔS = 7.91 J mol−1 K−1) and N values (∼2.588) for the cooling process (at T ≈ 184.6 K) were calculated, which are very close to those of the heating process.29 The most striking feature is that compound 1 exists at another unusual hightemperature phase transition (Tch). As shown in Figure 6b, during the heating and cooling process from 298 K to 430 K, there clearly exists another two peaks, which occur at ∼403.9 K and ∼402.8 K, respectively. Totally different from Tcl, the thermal hysteresis in Tch is only 1.1 K, which is far less than that of Tcl. Moreover, both the exothermic endothermic peaks are smoother and wider than those of Tcl. Besides, there also exist large entropy (ΔH) and enthalpy changes (ΔS) in Tch; their calculated values are 3.817 and 4.244 J mol−1 K−1 (ΔS) and 1541.7 and 1709.5 J mol−1 (ΔH) for the heating and cooling processes, respectively (see the Supporting Information), we tentatively attribute that they also belong to first-order phase
Figure 5. Variable-temperature powder X-ray diffractometry (PXRD) patterns of 1.
transition (Tcl, 184.6 K), a new Bragg diffraction peak clearly appears at 29.1° at a temperature of 173.15 K, in comparison with those of 298.15 K, and then they disappear again when the temperature reaches 303.15 K. For the second phase transition (Tch), in comparison with those at 298.15 K, we can easily observe that a strong Bragg diffraction peak at ∼21.7° and another one at 19.8° disappeared when the temperature is higher than that at the Tch transition (402 K). Then, both Bragg diffraction peaks appear again when the temperature returns to room temperature. These results suggest that 1-ITP has a higher symmetrical structure than 1-LTP, as does 1-HTP, relative to 1-ITP. It is worth mentioning that the complete
Figure 6. Thermal measurements of the DSC curves for 1 in the heating−cooling cycle, showing a heat anomaly at (a) ∼188 K and (b) ∼403 K. E
DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 7. Dielectric properties for 1, measured as a function of temperature: (a) from 165 K to 225 K under frequencies of 100 kHz and 1 MHz, respectively; (b) from 385 K to 420 K under a frequency of 1 MHz.
transition.25 However, we cannot determine its dynamic mechanism, because of the lack of accurate crystallographic data at such a high temperature. To further verify that the Tch phase transitions do not originate from the decomposition of 1, we carried out their thermogravimetric experiments. According to the TG-DTA curves from Figure S4 in the Supporting Information, decomposition of 1 occurred at a temperature of 469.5, which is far greater than 403 K. 3.4. Dielectric Properties. Significant changes of dielectric anomaly in the vicinity of the phase transition is a key factor for a switchable dielectric materials. To further understand its dielectric behavior during the phase transition process, we performed measurements of the temperature dependence of the dielectric constants with single-crystal samples in a heating−cooling cycle mode. As illustrated in Figure 7a, for compound 1 in the first phase transition (Tcl), an abrupt decrease in the cooling cycle exists. The dielectric constant displays a remarkable steplike change from 2.3 to 1.8 when the temperature decreased in the vicinity of 194.5 K. As temperature decreased below 187 K, the dielectric constants remained almost constant, at ∼1.8, corresponding to a low dielectric state. In the heating cycle, when the temperature is closed to phase transition (stating from 187.4 K), it exhibited a strong regression with maximum values of 2.0 and 2.05 for frequencies of 1 MHz and 100 kHz, respectively, corresponding to a high dielectric state. Different from that in Tcl, the graph of the dependence of the dielectric constant of the second phase transition temperature (Tch) appears to show a sharp “peak” pattern, instead of a steplike pattern. As shown in Figure 7b, first the dielectric constant display a remarkable change, at ∼406.7 K in the cooling cycle: the maximum peak value reaches 5.9, which is much greater than the value reached at temperatures above 410 K (∼4.5). Until the temperature is below 400 K, the dielectric constant remains low (∼4.0). While in the heating cycle, the dielectric constant exhibited a rapid increase, with another sharp peak at ∼199.5 K, the maximum value is beyond 6.0. After that, the dielectric constant quickly decreases with increasing temperature.
anions affords new insights into investigating the sequential reversible phase transitions and symmetry breaking. Especially, their remarkable dielectric changes, large heat anomalies, and high-temperature reversible structural phase transitions make them a practical application in switchable dielectric materials and temperature-controlling 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.chemmater.6b01726. Supplementary Figures S1−S4 and the calculation of ΔS and N (PDF)
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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 Y.-Z.T. is grateful for the support from the National Natural Science Foundation of China (Grant Nos. 21261009, 21461010, and 21471070), Young Scientist Foundation of Jiangxi Province and Jiangxi Province Science and Technology Support Program (No. 20133BBE50020), the Patent Sustentation Fund from Jiangxi Province (No. 3203304622).
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4. CONCLUSION We have successfully explored a new crown ether clathrates, which display unusual sequential reversible phase transitions. The stepwise synergistic disordering of Hcpa cations and ClO4− F
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Chemistry of Materials
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DOI: 10.1021/acs.chemmater.6b01726 Chem. Mater. XXXX, XXX, XXX−XXX
