Tunable Dielectric Responses Triggered by Dimensionality

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Tunable Dielectric Responses Triggered by Dimensionality Modification in Organic−Inorganic Hybrid Phase Transition Compounds (C5H6N)CdnCl2n+1 (n = 1 and 2) Xiao-Fen Sun, Zhongxia Wang, Peng-Fei Li, Wei-Qiang Liao, Heng-Yun Ye, and Yi Zhang* Ordered Matter Science Research Center, College of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People’s Republic of China S Supporting Information *

ABSTRACT: Two hybrids (C5H6N)CdCl3 (1) and (C5H6N)Cd2Cl5 (2) were synthesized by stoichiometric regulation of reactants. 1 with a one-dimensional chain-like structure shows a step-like dielectric anomaly at around 158 K. 2 with a layered structure undergoes a prominent phase transition in the vicinity of 182 K, accompanying obvious dielectric relaxation behavior in a broad temperature range. Systematic characterization, such as differential scanning calorimetry (DSC), single-crystal X-ray diffraction, and dielectric measurements, has demonstrated that the phase transitions of 1 and 2 are both attributable to the dynamic motion of the organic cation. Significantly, dimensionality modulation triggers the tunable dielectric responses in these two compounds. Thus, regulation of the phase transition temperature and dielectric responses in the various dimensions of the structure is a potentially effective method to construct tunable dielectric phase transition materials.

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INTRODUCTION Phase transition materials, showing diverse physical properties that can be triggered by external stimuli such as temperature, light, pressure, and electric and magnetic fields, etc.,1 have attracted increasing attention for their potential applications in switchable dielectric devices, communication, sensors, and data storage, etc.2−5 Numerous efforts have been devoted to preparation of the phase transition materials. However, in most situations, it is hard to predict and design phase transition properties in crystal materials, and work for filtering phase transition compounds is accidental. Therefore, further exploring the relationship between the structure and the physical properties is very necessary. Fortunately, organic−inorganic hybrid compounds offer an important opportunity to promote the development of phase transition materials.6−12 It is due to the fact that in organic−inorganic hybrid phase transition materials, protonated organic amines usually serve as charge compensators and structure-directing templates13,14 and demonstrate dynamical disorder at relatively high temperatures and order below certain temperatures, leading to lattice symmetry breaking and structural phase transitions.15−18 Therefore, it is believed that hybrid organic−inorganic compounds are better candidates for phase transition materials.19 Recently, the strategy of substitution of the organic cations or inorganic anions to construct new systemic phase transition materials has been studied.20−22 For instance, replacement of the A site of the metal−formate system A[Mg(HCOO)3] (A stands for organic ammoniums) can obtain some new phase transition materials, such as [NH 4 ][Mg(HCOO) 3 ], © 2017 American Chemical Society

[CH 3 CH 2 NH 3 ][Mg(HCOO) 3 ], and [NH 3 (CH 2 ) 4 NH 3 ][Mg2(HCOO)6],23 which display phase transition behaviors at 255, 374, and 412 K, respectively. Although many efforts have been devoted into the development of phase transition materials,24,25 a study on the pathway to construct new multifunctional phase transition compounds still needs to be done. Because of the multivariate structure-directing template functionalities of organic cation, stoichiometric regulation of the same reactants can easily realize structural dimensionality modification, which may lead to different response characteristics in different frameworks of phase transition compounds. In this work, we successfully designed and synthesized compounds (C5H6N)CdCl3 (1) and (C5H6N)Cd2Cl5 (2) in a ratio of 1:1 and 1:2 of pyridinium (Py) and CdCl2, respectively. As expected, 1 and 2 demonstrate a quite different structural framework with an obvious difference in the phase transitions temperature, dielectric properties, and dynamic states of the organic cation, indicating that the tunable physical properties can be triggered by dimensionality modification. This strategy may provide an effective approach to further development of phase transition materials.

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EXPERIMENTAL SECTION

Synthesis. Compound 1. CdCl2·2.5H2O (2.28 g, 10 mmol) in a mixture of water (8.0 mL) and ethanol (95%, 8.0 mL) was added dropwise into Py hydrochloride (1.15 g, 10 mmol) aqueous solution

Received: December 19, 2016 Published: March 3, 2017 3506

DOI: 10.1021/acs.inorgchem.6b03074 Inorg. Chem. 2017, 56, 3506−3511

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1 and 2 at Different Temperatures (C5H6N)CdCl3 temp (K) Mw cryst syst space group a (Å) b (Å) c (Å) vol (Å3), Z F(000) no. of reflns collected/unique Rint GOF R1/wR2 [I > 2σ(I)]

298 298.87 orthorhombic Amma 6.7876(14) 7.3532(15) 17.718(3) 884.3(3), 4 568 2920/536 0.043.000 1.111 0.0519/0.1171

(C5H6N)Cd2Cl5 113 298.87 orthorhombic Pbca 6.724(6) 14.722(14) 17.494(13) 1736(3), 8 1136 11 361/1846 0.034.000 1.317 0.0273/0.0999

under strong stirring. Then the resulting solution was filtered and kept undisturbed at room temperature. Block colorless crystals were obtained after 2 weeks. Purity of 1 was verified by infrared (IR) spectroscopy and powder X-ray diffraction (PXRD) patterns. Powder X-ray diffraction (PXRD) patterns of 1 are shown in Figure S4 (Supporting Information), matching well with simulation. Compound 2. CdCl2·2.5H2O and Py with a molar ratio 2:1 were mixed in concentrated hydrochloric acid solvent and stirred for 30 min at room temperature. After 4 weeks, needle colorless crystals were obtained at 351 K (Figure S1, Supporting Information). Purity of 2 was verified by infrared (IR) spectroscopy and powder X-ray diffraction (PXRD) patterns. Powder X-ray diffraction (PXRD) patterns of 2 are shown in Figure S4 (Supporting Information), matching well with simulation. Crystallography. Variable-temperature X-ray single-crystal diffraction data of compounds 1 and 2 are collected with Mo Kα radiation (λ = 0.71073 Å) at 93, 113, and 298 K on a Rigaku Saturn 724 diffractometer. Structures are solved by direct methods and refined by the full-matrix method based on F2 by means of the SHELXLTL software package. All H atoms are generated geometrically and refined by using a “riding” model with Uiso = 1.2Ueq(C and N). Non-H atoms are refined anisotropically using all reflections with I > 2σ(I). All data processing including empirical absorption corrections are performed using the Crystalclear software package (Rigaku, 2005). Crystallographic data and structure refinement of compounds 1 and 2 at various temperatures are listed in Table 1. CCDC reference numbers are 1517085 (298 K) and 1517086 (113 K) for 1 and 1517087 (298 K) and 1517088 (93 K) for 2. DSC Measurements. DSC measurements were performed by heating and cooling the polycrystalline samples on a Perkin-Elmer Diamond DSC instrument. Measurements are carried out under nitrogen at atmospheric pressure in aluminum crucibles in the temperature range 120−270 K with a heating rate of 5 K/min. Dielectric Measurements. Complex dielectric permittivity ε (ε = ε′ − iε″) is measured on a Tonghui TH2828A over the frequency range from 2k Hz to 1 M Hz and in the temperature range from 110 to 300 K with the measuring ac voltage fixed at 1 V. The powder-pressed pellets and single-crystal samples with silver painted on both sides are used as the electrodes for dielectric studies, and the crystal faces selected in the measurements are based on the room-temperature structure.

298 482.18 orthorhombic Pmmn 10.262(2) 15.262(3) 3.8035(8) 595.7(2), 2 448 4010/778 0.024.000 1.166 0.0199/0.0470

93 482.18 orthorhombic Pnma 20.19(5) 15.24(4) 3.793(9) 1167(5), 4 896 5894/1165 0.043.000 1.16 0.0488/0.1503

Figure 1. DSC curves of compounds 1 (a) and 2 (b).

reveal a first-order type of phase transition in 1 (Figure 1a), while the phase transition in 2 is classified as a second-order type according to the broad peaks with narrow thermal hysteresis (Figure 1b). For convenience, we label the phase above 150 and 182 K as the high-temperature phase (HTP) and the phase below 158 and 184 K as the low-temperature phase (LTP) in 1 and 2, respectively. Variable-Temperature Structures of 1. Compound 1 consists of infinite linear anionic chains [CdCl3]−1n and Py cations. At 298 K, 1 crystallizes in the centrosymmetric orthorhombic space group Amma. Each Cd ion is coordinated by six bridged Cl ions to give a weakly distorted octahedron geometry with Cd−Cl distances ranging from 2.644(2) to 2.645(2) Å (Table S1, Supporting Information), which is in good agreement with those found in other structurally similar compounds.23−25 These face-sharing Cd octahedrons form one-dimension chains along the crystallographic a axis. Py cation weaves in the space between two adjacent chains and locates on the special position with 2mm symmetry (Figure S7, Supporting Information). To satisfy the crystal symmetry requirements, Py cation is orientationally disordered over two equivalent positions related by the mirror plane, which is perpendicular to the b axis. Thus, the C and N atoms of the Py cation occupy the same crystallographic sites with an occupancy of 0.5 (Figure 2c). The weak N−H···Cl hydrogen bonds (3.619 Å) linked inorganic chains give freedom for the motion of the Py cation. At 113 K in the LTP, 1 crystallizes in the orthorhombic space group Pnma. During the phase transition processes, the configuration of the inorganic anionic chains remains nearly the same as that in the HTP (Table S1, Supporting

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RESULTS AND DISCUSSION Phase Transitions. DSC measurement is one of the best methods to detect the phase transition triggered by temperature. As shown in Figure 1, DSC curves of 1 and 2 measured in a heating−cooling run display a pair of exothermic and endothermic peaks at 158/150 and 184/182 K, respectively, indicating the reversible characteristic of the two phase transitions. The sharp peaks with large thermal hysteresis 3507

DOI: 10.1021/acs.inorgchem.6b03074 Inorg. Chem. 2017, 56, 3506−3511

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changes that deviate from the original plane with a distinct deviation angle θ (θ = 29.38°) (Figure 3c and 3d). Besides, the Py cation is more ordered than that in the HTP. It is noted that the crystallographic a axis is double that at 298 K (Figure 3a and 3b). With temperature decreasing, the distances of the N− H···Cl hydrogen bonding interactions, related to the plane (101), are unequal, being responsible for the swing of the Py cations along the c axis (Figure S10b, Supporting Information). Dielectric Properties. Dielectric Properties of 1. The temperature dependence of the real part (ε′) of the singlecrystal sample along various axes of 1 over selected frequencies is plotted in Figure 4. The striking reversible dielectric anomalies along different axes were recorded. On cooling, ε′ along the a axis falls from 32 to 21 at about 160 K, and further it sharply increases from 21 to 32 at around 150 K. ε′ values of the b and c axes have a similar trend with that of the a axis. ε′ values at 1 M Hz along the a and c axes are obviously larger than that along the b axis. Such a strong dielectric anisotropy can be responsible for the intrinsic structural changes in the phase transition processes.27−31 As structure analysis shows in Figure 2, the Py cation lies in the ac plane perpendicular to the b axis. Thus, the orientational motions of the Py cations arouse motions along the a and c axes, which yield more dipolemoment components, leading to visible dielectric anomalies in these two axes. Additionally, in the frequency range from 10k to 1 M Hz (Figure S6, Supporting Information), the dielectric anomalies appear at the same temperature, indicating that the phase transition temperature is independent of the frequency. Dielectric Properties of 2. As demonstrated in Figure 5, 2 displays clear dielectric anomaly with remarkable dielectric relaxation behavior in a large temperature range as well as notable dielectric anisotropy along the three crystallographic axes. At 1 kHz, ε′ along the c axis displays an abrupt change from 40 to 12.5, which means the ε′ at 158 K is about 3 times that at 128 K. In addition, ε′ along the b axis shows a 2.67-fold change on cooling, while only a very tiny anomaly is recorded in the direction of the a axis. This phenomenon can be explained by the dynamic motions of the Py cation during the cooling process. As illustrated in Figure 3, the Py cation sways along the c axis, perpendicular to the bc plane, indicating that the dynamical motions of the Py cation along the c axis yield more dipole-moment components in this direction. This result gives rise to the positive dielectric response of the c axis. To comprehensively investigate its relaxation characterization, the real part (ε′) of the polycrystalline samples of 2 was measured over the frequency range from 1k Hz to 1 M Hz. As shown in Figure 6a, ε′ displays significant frequency dispersion below room temperature, indicating that the dielectric anomalies move progressively toward lower temperatures with a decrease of frequency. Moreover, the maximum peak of ε″ also shifts from 195 K at 1 M Hz to 135 K at 1 kHz (Figure 6b).33 Meanwhile, the dielectric relaxation behavior is also specially studied at select temperatures. As presented in Figure 6c, the Cole−Cole diagrams of 160, 170, and 180 K deviate from semicircles, which confirm the obvious dielectric dispersion in 2. This dispersive character obeys the Cole−Cole function ε0 − ε∞ ε = ε∞ + 1 + (iωτ )1 − α

Figure 2. Molecular structures of 1 shown at different temperatures. (a) HTP (298 K): pyridine cations are strongly disordered, and N atoms orientate over two positions with an occupancy of 0.5. (b) LTP (113 K): Py cations are totally ordered. (c) Asymmetric unit of 1 shown at 298 K. (d) Asymmetric unit of 1 shown at 113 K. All hydrogen atoms are omitted for clarity. Thermal ellipsoids for all atoms are shown at the 30% probability level.

Information). Nevertheless, the dynamic behavior of the Py cation shows evident changes that there is an ordered cation with a large deviation from the mirror plane (Figure 3b). In addition, N−H···Cl hydrogen bonds (3.366 Å) are apparently shorter than those in the HTP. The obvious order−disorder transition of the Py cation contributes to the phase transition of 1, which is in accordance with the DSC results. Variable-Temperature Structures of 2. 2 demonstrates a layered packing structure in which the Py cation occupies the space enclosed by inorganic anionic layers. The basic unit of the crystal structure at 298 K contains 2 Py cations, 4 Cd atoms, and 10 Cl atoms. Each Cd atom is surrounded by 6 Cl atoms, forming a distorted octahedron with Cd−Cl distances ranging from 2.582(4) to 2.673(5) Å and adjacent Cl-atom angles Cl− Cd−Cl varying from 87.07(2)° to 172.67(5)° (Table S3, Supporting Information). The inorganic frameworks are analogous to (C5H5NH)(MnCl3)H2O (Mn−Cl distances, 2.583(2)−2.674(2) Å; Cl−Mn−Cl angles, 87.15(5)− 173.49(5)°).27 Corner-sharing Cd octahedrons are linked together by Cl atoms to build up a two-dimensional layered network (Figure S9, Supporting Information). The Py cation locates on a special position, where the symmetry requires the equivalent thermal vibration of the Py cation around the mirror (101) (Figure 3b). Also, the N−H···Cl hydrogen bonds linking the Py cation with Cl atoms are the same (Figure S10a, Supporting Information), which makes the Py cation on the plane perpendicular to the c axis. At 93 K, the structure of the inorganic layers is nearly unchanged (Figure 3b), while the Py cation has obvious

where ε0 and ε∞ are the low- and high-frequency limits of the electric permittivity, respectively, ω is the angular frequency, τ is the macroscopic relaxation time, and α is the distribution of 3508

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Figure 3. Structures of 2 shown at different temperatures. (a) HTP (298 K): N atom and adjacent C atoms of Py cations are disordered. (b) LTP (93 K): N and adjacent C atoms of Py cations are slightly ordered. (c) Compound 2 along the c axis shown at 298 K. (d) Compound 2 along the c axis shown at 93 K. All hydrogen atoms are omitted for clarity. Thermal ellipsoids for all atoms are shown at the 30% probability level.

Figure 4. Anisotropic dielectric permittivity (ε′) of 1 along the a, c, and b axes measured in a cooling−heating run at 1 M Hz.

the relaxation times parameter. Taking into account the shifts of the maximum peak of ε″, the relaxation behavior of the dielectric response in 2 may be interpreted by the Arrhenius relation using the macroscopic relaxation time τ = τ0 exp(Ea/ kBT), where τ0 is the inverse of the frequency factor, Ea denotes the activation energy, kB denotes the Boltzmann constant, and T is the temperature. For a Debye peak, the equation can be rewritten as ln τ = ln(2πf)−1 = ln(τ0) + Ea/kBTp, in which f is the frequency and Tp is the temperature of the peak. Ea and τ0 can be approximately calculated to be 0.275 eV and 1.19 × 10−14 s, respectively, using a ln τ versus 1/T plot of the polycrystalline sample (Figure S11, Supporting Information). Such dielectric relaxation behavior can also be observed in some other phase transition compounds.25,26,32−34 Accordingly, the dielectric relaxation behavior of 2 may be explained by the swing and the reorientational motions of the Py cations. During the cooling process, the frozen orientational motions of Py

Figure 5. Anisotropic dielectric permittivity (ε′) of 2 along the c, b, and a axes at selected frequencies.

cations are unable to make enough responses to the change in the external electric field with deceasing temperature.

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CONCLUSION In conclusion, two structure-adjustable organic−inorganic hybrid compounds, (C5H6N)CdCl3 (1) with a chain-like perovskite-type structure and (C5H6N)Cd2Cl5 (2) with layered networks, have been synthesized by stoichiometric regulation of reactants. Systematic characterization was performed to reveal their phase transitions and dielectric behavior. The dynamic motions of the Py cation contribute to the phase transitions and prominent dielectric responses in 1 and 2. Moreover, the phase 3509

DOI: 10.1021/acs.inorgchem.6b03074 Inorg. Chem. 2017, 56, 3506−3511

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21522101, 21371032) and the Outstanding Young Teachers of Southeast University Research Fund (2242015R30025).

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Figure 6. (a) Real part (ε′) of the polycrystalline samples of 2 at selected frequencies on heating. (b) Imaginary part of dielectric constants (ε″) of the dielectric permittivity of 2 at selected frequencies on heating. (c) Cole−Cole diagrams for the polycrystalline sample 2 at selected temperatures.

transition temperature and dielectric responses show an obvious difference in these two compounds. This is due to the dimensionality modification, which makes the cation have different motion states in the two compounds. This strategy of dimensionality modulation promotes the development of dielectric phase transition materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03074. Crystal pictures, IR spectra, variable-temperature PXRD patterns, and other dielectric characterization (PDF) (CIF) (CIF) (CIF) (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yi Zhang: 0000-0002-6375-1712 Notes

The authors declare no competing financial interest. 3510

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