Structural, Optical, Magnetic, and Dielectric Properties in Hybrid Solid

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Article Cite This: ACS Omega 2018, 3, 10725−10732

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Structural, Optical, Magnetic, and Dielectric Properties in Hybrid Solid Solutions of ZnαNi1−α(en)3Ag2I4 (0 < α < 1) by Varying the Relative Zn/Ni Content Chen Xue,† Yang Zou,*,† Jin Zhang,†,‡ Xiao-Ming Ren,*,†,‡,§ Katsuya Ichihashi,∥ Rio Maruyama,∥ and Sadafumi Nishihara*,∥ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering and College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China § State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210009, P. R. China ∥ Department of Chemistry and Center for Chiral Science, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

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S Supporting Information *

ABSTRACT: The hybrids of M(en)3Ag2I4 (M = Zn or Ni) are isostructural to each other and crystallize in space group P6322 with quite similar lattice parameters. The hybrid solid solutions ZnαNi1−α(en)3Ag2I4 (0 < α < 1) have been prepared via self-assembly in the mixed N,N-dimethylformamide solution of AgNO3, KI, and ethylenediamine, meanwhile, with certain relative amount of [Zn(en)3]2+ and [Ni(en)3]2+ ions at ambient condition. All hybrid solid solutions are isostructural to the parent hybrids M(en)3Ag2I4 (M = Zn or Ni). The UV−vis−near IR diffuse reflection spectra in solid state, thermogravimetric analysis, variable-temperature magnetic susceptibility, and dielectrics have been investigated for ZnαNi1−α(en)3Ag2I4 (0 < α < 1). The intensity of bands centered at 540 and 860 nm in UV−vis−near IR spectra, arising from the d−d electron transition in Ni2+ ion, as well as the Curie constant decreases linearly with the molar fraction of Zn (α)/Ni (1 − α), whereas the c-axis length, the C−N and C−C bond lengths in the ethylenediamine, the frequency-independent dielectric permittivity and the onset temperature of dielectric relaxation, and so forth show nonmonotonical alternation with the fraction of Zn (α)/Ni (1 − α) in the solid solution, and the origin for these differences is further discussed.

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below 315 °C.12,13 The band gap of orthorhombic phase CsPbI3 (2.25 eV) is larger than that of cubic phase CsPbI3 (1.73 eV); moreover, the orthorhombic phase CsPbI3 shows indirect band gap; these disadvantages make it less suitable for many optoelectronic applications. The cubic phase CsPbI3 is stabilizable in both NCs and thin films via forming solid solution, namely, by partially replacing I− ions in CsPbI3 with Br− or Cl− ions.12,14−17 In the study of organic solar cells, Zhang and co-workers found that a ternary solar cell, where two types of miscible donor molecules form solid solution to result in enhancement of hole mobility and abatement of charge recombination, shows higher cell efficiency than the corresponding binary solar cell with only one type of donor molecule.18 In hybrid ferroelectric material area, most recently, a series of A-site perovskite solid solutions, with a formula of [(NH 2NH3) x(CH3NH3) 1−x][Mn(HCOO)3 ] (x = 1.00− 0.67), were successfully synthesized and the ferroelectricity was investigated; the study revealed that with x value

INTRODUCTION Solid solutions are a class of unique materials. Generally, a solid solution not only preserves the important properties of its components but also in some cases, imparts synergistic properties to its components; for example, the well-known “bronze”, one of the first solid solutions made by humans and composed of the metals tin and copper, is much stronger and harder than either of its components and was an extremely useful material to the ancients. It is an efficient strategy to improve the performance and modulate the desirable nature of a functional material via making into a solid solution. For example, the nanocrystals (NCs) of cesium lead halide perovskites (CsPbX3 with X = Cl, Br, and I) have recently became a topic of extensive research,1−8 and among them, the CsPbBr3 NCs show photoluminescence quantum yields which are close to 100%9,10 and a wide range of applications.2−5,11 Regarding the CsPbBr3 NCs, the cubic phase of CsPbI3 NCs is more attractive for tunable lasers, light-emitting diodes, and solar cells owing to the lower band gap (1.73 eV) and larger exciton Bohr radius (6 nm for CsPbI3 with respect to 3.5 nm for CsPbBr3), but they are significantly less stable, and a phase transition occurs from cubic to orthorhombic phase of CsPbI3 © 2018 American Chemical Society

Received: June 18, 2018 Accepted: August 7, 2018 Published: September 6, 2018 10725

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decreasing from 1.00 to 0.67, the critical temperature TC is lowered; meanwhile, the transition from paraelectric to ferroelectric phase becomes more diffuse in the dielectric spectroscopy because of the A-site [NH2NH3]+ substituted by [CH3NH3]+ leading to decrease of the framework distortion and the polarization in the ferroelectric phase of solid solution, and this finding opens a new line of research into these MOFbased materials.19 In the study of molecule magnet, Ren et al. found that two spin-Peierls compounds, [Cl-BzPy][Ni(mnt)2]20 and [ClBzPy][Pt(mnt)2]21 (Cl-BzPy+ = 4′-chlorobenzyl-1-pyridinium and mnt2− = maleonitriledithiolate), show isomorphic and quite similar lattice parameters but distinct temperature of spin-Peierls transition, with TC ≈ 275 K in [Cl-BzPy][Pt(mnt)2] versus 106 K in [Cl-BzPy][Ni(mnt)2], and then they achieved the solid solutions of [Cl-BzPy][NixPt1−x(mnt)2] with x = 0.09−0.91 via mixing the acetonitrile solution at a certain molar ratio of [Cl-BzPy][Ni(mnt)2] to [Cl-BzPy][Pt(mnt)2] and crystallizing at ambient temperature. Interestingly, the spin-Peierls transition temperature, TC, is precisely tunable from 106 to 275 K in this solid solution family of [ClBzPy][NixPt1−x(mnt)2] (x = 0.09−0.91).22 Recently, Jiang et al. reported two new iodoargentate framework hybrids of M(en)3Ag2I4 (M = Zn and Ni) with the tridymite topology.23 The further studies disclosed that those two iodoargentate frameworks display distinct optical24 and electrochemical DNA recognition25 behaviors. Hybrid Zn(en)3Ag2I4 shows intense ultraviolet light absorbance, whereas Ni(en)3Ag2I4, besides the intense absorbance within the ultraviolet region being similar to that in Zn(en)3Ag2I4, displays two additional intense absorption bands, centered at 540 and 860 nm, in the visible and near infrared regions, respectively. Given that the hybrids M(en)3Ag2I4 (M = Zn and Ni) are isomorphic, moreover, possess rather similar cell parameters; it is possible to harvest the hybrid solid solutions of ZnαNi1−α(en)3Ag2I4. Herein, we present the study of the preparation strategy for ZnαNi1−α(en)3Ag2I4 via in situ selfassembly process in N,N-dimethylformamide (DMF) and the investigation for the Zn/Ni relative content dependence of structural, magnetic, optical, and dielectric properties of hybrid solid solutions ZnαNi1−α(en)3Ag2I4 (0 < α < 1).

solid solution sample is labeled as ZnNi-II, ZnNi-III, ZnNi-IV, ZnNi-V, and ZnNi-VI, respectively. Chemical and Physical Characterizations. Elemental analyses (for C, H, and N) were performed with an Elementar vario EL III analytical instrument. The molar fraction, α/1 − α, of Zn/Ni in each molecular solid solution sample was determined using a Sciex ELAN inductively coupled plasma (ICP) spectrometer. The magnetic susceptibility measurement was carried out using Quantum Design (SQUID) MPMS-5S magnetometers, with the magnetic field of 5 kOe within a temperature range from 2 to 300 K. Thermogravimetric analysis (TGA) was carried out with Rigaku TG-DTA 8120 in flowing N2 with a heating rate of 10 K/min. UV−vis−near IR diffuse reflectance spectra were recorded by Shimadzu UV 3101PC. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 diffractometer at ambient temperature, operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 1.5418 Å), and the measurement was performed in the 2θ range of 5°−50° with 0.01°/step. The variable-temperature PXRD measurements were performed using a Shimadzu XRD6100 diffractometer, operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 1.5418 Å). The diffraction angle 2θ ranges from 5 to 50° with a scan rate of 0.01°/step. During the measurements, the heating or cooling rate of temperature is ±10 K·min−1 and the sample was kept at the set temperature for 15 min to ensure that the sample and probe have the same temperature. The temperature-dependent ac dielectric permittivity was measured on a concept 80 system (Novocontrol, Germany); each powdered polycrystalline pellet, with 7 mm in diameter, was made under the static pressure of 6 MPa pressure. The pellet thicknesses are 0.49 mm (54 mg) for Ni(en)3Ag2I4 (ZnNi-I), 0.47 mm (52 mg) for ZnNi-II, 0.47 mm (53 mg) for ZnNi-III, 0.44 mm (50 mg) for ZnNi-I, 0.46 mm (52 mg) for ZnNi-V, 0.43 mm (47 mg) for ZnNi-VI, and 0.49 mm (54 mg) for Zn(en)3Ag2I4 (ZnNi-VII). The pellet was sandwiched by two copper electrodes with gold during the dielectric measurement. The ac frequency spans from 1 to 107 Hz, and the temperatures range from 173 to 453 K for ZnNi-I to V and from 173 to 438 K for ZnNi-VI and ZnNi-VII. X-ray Crystallography. Single-crystal X-ray diffraction data were collected for the selected single crystals using a Bruker diffractometer-SMART-APEX II with a charge-coupled device and D8-QUEST with a CMOS area detector. Both employed graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data reduction and absorption corrections were performed using the SAINT27 and SADABS28 software packages, respectively. The structures were solved by the direct methods using SHELXS-2014.29 The nonhydrogen atoms were anisotropically refined using a full-matrix leastsquares method on F2. All hydrogen atoms were placed at the calculated positions and refined as riding on the parent atoms. In the crystal structure of each hybrid solid solution ZnNi-x (x = II−VI), the Zn2+ and Ni2+ ions distribute randomly at the Wyckoff position 2b to form a solid solution and the occupied factor of site was determined based on the analysis of the relative content of Zn to Ni, which was achieved by ICP measurements for each sample of ZnNi-x (x = II−VI). The pretreatment of these hybrids for ICP measurement is dissolved in the samples with concentrated nitric acid. The details of data collection, structure refinement, and crystallography are summarized in Table S1. Further details can be obtained from the cifs deposited at the Cambridge Crystallo-

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EXPERIMENTAL SECTION Chemicals and Materials. The solvents and chemicals, such as DMF, ethylenediamine (en), KI, AgNO3, Zn(NO3)2· 6H2O, and Ni(NO3)2·6H2O, were supplied by Sinopharm Chemical Reagent Co. Ltd. of China, which are of analysis pure grade, and used as received without further purification. Sample Preparation. Polycrystalline samples of Ni(en)3Ag2I4 and Zn(en)3Ag2I4 were achieved using the published method23,24,26 and labeled as ZnNi-I and ZnNiVII, respectively. Each polycrystalline sample of solid solution was prepared using the same process, which is also similar to the preparation for Ni(en)3Ag2I4 and Zn(en)3Ag2I4. The difference in the process of preparing each solid solution sample only concerns the relative content of Zn2+ to Ni2+ ions in the starting DMF solution containing Zn(NO3)2 and Ni(NO3)2. In the starting DMF solution, the total concentration of Zn2+ and Ni2+ ions was maintained at 0.15 mol L−1 and the molar ratio of Zn(NO3)2 and Ni(NO3)2 is x/(1 − x) for each solid solution sample (x = 0.1, 0.3, 0.5, 0.7, and 0.9), and the corresponding 10726

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analogous lattice parameters. Thus, only the crystal structure of ZnNi-VII is described in detail. There are one silver ions, two distinct I ions, one zinc ions, and half of an ethylenediamine molecule contained within an asymmetric unit of ZnNi-VII, as shown in Figure 1a. The Zn1

graphic Data Centre and can be obtained free of charge on request via http://www.ccdc.cam.ac.uk/data_request/cif. The CCDC reference numbers range from 1580489 to 1580494 and 1584776.

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RESULTS AND DISCUSSION Characterization for the Composition. The molar fraction of Zn/Ni was determined for each polycrystalline sample of ZnNi-x (x = II−VI) using the ICP technique, and the results are summarized in Table 1. It is found that the Table 1. Comparison of Molar Ratio of Zn to Ni in ZnNi-x (x = II−VI) between the Starting Materials and Determination Using Different Techniques starting material

ICP

sample

x/(1 − x)

α/(1 − α)

ZnNi-II ZnNi-III ZnNi-IV ZnNi−V ZnNi-VI

0.1:0.9 0.3:0.7 0.5:0.5 0.7:0.3 0.9:0.1

0.10:0.90 0.31:0.69 0.49:0.51 0.70:0.30 0.89:0.11

Figure 1. (a) Asymmetric unit with nonhydrogen atom labeling with 50% probability thermal ellipsoids, (b) Zn(en)32+ coordination octahedron, and (c) [AgI4]3− coordination tetrahedron viewed along threefold axis in ZnNi-VII.

ion locates at the special crystallographic site, the Wyckoff position 2b, with a threefold rotational axis passing through. In addition, there are three ethylenediamine molecules coordinated to that zinc ion forming a bivalence cation. The coordinated ethylenediamine molecule possesses a C2 point with the twofold rotational axis passing through the midpoint and Zn1. All of these symmetry operations make the bivalence cation, [Zn(en)3]2+, possessing the D3 point group symmetry (ref. Figure 1b which was viewed along threefold rotational axis). The Ag1 is coordinated with four iodine ions I forming a tetrahedral coordination sphere, [AgI4]3−, which has a symmetry with C3 point group (ref. Figure 1c and viewed along threefold rotational axis). The threefold rotational axis passes through I2 and Ag1 ions in the coordination tetrahedron. Both I1 and I2 ions act as the μ2-bridged ligands and the I2 ion connecting two neighboring Ag+ ions arrange in a linear fashion, whereas the I1 ion linking two neighboring Ag+ ions adopt in a nonlinear manner with bond angle ∠Ag1− I1−Ag1 of 143.62(3)° in ZnNi-VII. In addition, the bond length is 2.194(7) Å for Zn−N, 2.8556(4) for Ag1−I1, and 2.8829(15) Å for Ag1−I2 in ZnNi-VII, and these values are comparable to those in the literature.23 As for the hexagonal screw axis along c-axis and twofold axis along a-/b-axis, the [AgI4]3− coordinated tetrahedrons connected with each other by sharing vertex forming a helical inorganic anionic open framework [Ag2I4]n2n− and forming two types of hexagonal channels along the c-axis and a-/b-axis, shown as in Figure 2a,b. The charge-balanced cations [Zn(en)3]2+ are fulfilled within the channels. The nearest Zn···Zn distance is 7.2194(4) and 9.0542(3) Å along the c-axis

molar fraction of Zn/Ni in the final polycrystalline product is rather close to that in the starting material for the preparation of each hybrid solid solution, and the final polycrystalline sample corresponds to a formula of ZnαNi1−α(en)3Ag2I4 with α = 0 for ZnNi-I, 0.1 for ZnNi-II, 0.31 for ZnNi-III, 0.49 for ZnNi-IV, 0.7 for ZnNi-V, 0.89 for ZnNi-VI, and 1 for ZnNiVII. These results demonstrate that Zn can be substituted by Ni at an arbitrary molar ratio in the hybrid Zn(en)3Ag2I4 to form a solid solution and vice versa. This conclusion is further supported by the single-crystal structure analysis. The microanalysis for C, H, and N elements was also performed for this series of hybrids, and the theoretically calculated values for C, H, and N elements were gained on the formula ZnαNi1−α(en)3Ag2I4, where the α value was obtained from ICP measurement for each sample of ZnNi-x (x = I− VII). The contents of C, H, and N elements found together with calculated on the formula ZnαNi1−α(en)3Ag2I4 are summarized in Table 2 for each sample of ZnNi-x (x = I− VII), and they are coincided with each other. Table 2. Microanalysis for C, H, and N Elements Found and Calculated (Parentheses) in ZnNi-x (x = I−VII) sample ZnNi-I ZnNi-II ZnNi-III ZnNi-IV ZnNi-V ZnNi-VI ZnNi-VII

C% 7.47 7.49 7.46 7.45 7.48 7.37 7.48

(7.49) (7.48) (7.47) (7.46) (7.45) (7.44) (7.43)

H% 2.52 2.53 2.50 2.54 2.55 2.51 2.49

(2.51) (2.51) (2.51) (2.50) (2.50) (2.50) (2.50)

N% 8.66 8.65 8.67 8.63 8.60 8.54 8.58

(8.73) (8.83) (8.71) (8.70) (8.69) (8.68) (8.67)

Crystal Structure. The crystal structures of ZnNi-I and ZnNi-VII were previously reported to be isomorphic with space group P63.23 However, to check the published cifs of both ZnNi-I and ZnNi-VII using the PLATON program30 indicates that their space group should be P6322. In this study, the crystal structures of ZnNi-x (x = I−VII) were determined at ambient temperature, revealing that they are isomorphic to each other and crystallize in space group P6322 with rather

Figure 2. Packing diagrams viewed along (a) c-axis and (b) a-axis directions in ZnNi-VII. These show the open framework with hexagonal channels formed by [AgI4]3− tetrahedra and [Zn(en)3]2+ fulfilled within the channels. 10727

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Figure 3. Plots of the normalized (a) cell parameters and (b) band lengths of M−N, C−N, C−C, Ag−I1, Ag−I2, and bond angles of ∠I2−Ag−I1 and ∠I1−Ag−I1 against the molar fraction of Zn (α) in ZnαNi1−α(en)3Ag2I4.

Figure 4. Plots of variable-temperature magnetic susceptibility of ZnNi-x (x = I−VII) in different temperature/magnetic susceptibility scales.

and a-/b-axis, respectively. Only weak van der Waals interactions are found between the inorganic framework and [Zn(en)3]2+ coordinated octahedral guests. Unit Cell Parameters and Bond Lengths. The plots of normalized lattice parameters, a, c, and V, versus the molar fraction (α) of Zn in ZnαNi1−α(en)3Ag2I4 are shown in Figure 3a for ZnNi-x (x = I−VII), indicating that the a-/b-axis length and the cell volume increase monotonically with α increasing, whereas the plot of the c-axis length versus the molar fraction (α) of Zn in ZnαNi1−α(en)3Ag2I4 displays a double-peak shape and two peaks locate at ca. α = 0.31 and 0.7. As the molar fraction α of Zn in ZnαNi1−α(en)3Ag2I4 changes from 0 to 1, the c-axis length and the cell volume increase by 0.4 and 1.2%, respectively. The bond lengths of M−N, C−N in the [M(en)3]2+, as well as Ag−I1 and Ag−I2 are plotted against the molar fraction (α) of Zn in ZnαNi1−α(en)3Ag2I4, which is depicted in Figure 3b for ZnNi-x (x = I−VII). The bond length of M−N almost linearly increases with the molar fraction (α) of Zn in ZnαNi1−α(en)3Ag2I4, and this observation is in good agreement with the fact that the Zn2+ ion radius (0.74 Å) is larger than the Ni2+ ion radius (0.69 Å).31 The bond lengths of both Ag−I1 and Ag−I2 in the inorganic framework are almost independent on the change of the molar fraction (α) of Zn in ZnαNi1−α(en)3Ag2I4 for ZnNi-x (x = I−VII). However, the bond lengths of N−C and C−C in the ethylenediamine molecule change irregularly. Magnetic Susceptibility. Because the Ni2+ ion shows paramagnetism, the variable-temperature magnetic susceptibility was measured in the temperature range of 1.8−300 K for the polycrystalline samples of ZnNi-x (x = I−VI) and the corresponding plots of χm against T are depicted in Figures 4 and S3, where the symbol χm denotes the molar magnetic susceptibility, corresponding to per formula unit of ZnαNi1−α(en)3Ag2I4. All samples of ZnNi-x (x = I−VI) display

the Curie−Weiss-type magnetic behavior in 1.8−300 K and that is understandable. In the series of ZnαNi1−α(en)3Ag2I4, the paramagnetism of each sample contributes from the [Ni(en)3]2+ ions occupied in the channels of framework because the distances between the neighboring [Ni(en)3]2+ are large enough; for example, the nearest [Ni(en)3]2+ ions show the Ni···Ni distances of 9.002(2) Å along a-/b-axis direction and 7.218(7) Å along caxis direction even if in the crystal structure of ZnNi-I (Ni(en)3Ag2I4), the magnetic couplings between the neighboring [Ni(en)3]2+ is rather weak and each solid solution can be considered as an isolated spin system. Accordingly, the magnetic susceptibility data are fitted using eq 1 for each solid solution χm =

C + χ0 T−θ

(1)

In eq 1, the symbols C and θ denote the Curie constant and Weiss constant, respectively, and the χm and χ0 represent the molar magnetic susceptibility and the temperature-independent paramagnetic susceptibility, respectively. The temperatureindependent paramagnetic susceptibility generally includes two parts, in which the diamagnetism contributes from the atom core of molecules as well as the possible temperatureindependent van Vleck-type paramagnetism originates from the coupling between the magnetic ground and excited states through a magnetic field.32 The best fits were performed for the magnetic susceptibility data of ZnNi-x (x = I−VI) in the range of 1.8−300 K, to give the parameters of C, θ, and χ0, which are summarized in Table S2. The Weiss constant (θ) obtained from the best fits of magnetic susceptibility is so small for all samples of ZnNi-x (x = I−VI), indicating the existence of almost absence of magnetic exchange interaction between the neighboring paramagnetic [Ni(en)3]2+ ions. The Curie constant decreases with the relative content of Ni2+ ions reducing in ZnNi-x (x = I−VI). The Curie constant C = 1.311 10728

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Figure 5. (a,b) UV−vis−near IR diffuse reflectance spectra of ZnNi-x (x = I−VII) in different wavelength ranges and plots of absorption band intensity against the molar fraction, α, of Zn in ZnαNi1−α(en)3Ag2I4 for (c) 540 (d) 860 nm.

Figure 6. Plots of dielectric permittivity (ε′) vs temperature of ZnNi-VII and the data obtained from (a) two consecutive heating−cooling cycles at f = 104 Hz and (b) first cooling process at the selected frequencies.

emu K mol−1 is obtained from magnetic susceptibility fit for ZnNi-I (Ni(en)3Ag2I4), which is slightly larger than the spinonly value 1.0 emu K mol−1 for an S = 1 spin system owing to the existence of unquenched orbital magnetic moments in Ni2+ ion with electronic configuration d8. For other hybrids ZnNi-x (x = II−VI), the corresponding Curie constant Cx (x = II−VI) is theoretically related to the C value of ZnNi-I (Ni(en)3Ag2I4) and the molar fraction of Ni2+ ion in ZnNi-x (x = II−VI) according to eq 2 Cx = (1 − α)C

being rather similar to ZnNi-VII in the ultraviolet light region, but possesses two additionally intense absorption bands with a maximum at ca. 540 and 860 nm. The absorption bands in the ultraviolet light region in ZnNi-I and ZnNi-VII attribute to the electron transitions from the valence bands to conducting bands within the {Ag2I42−}∞ inorganic framework. It is no doubt that the absorption band centered at 860 nm is assigned to the d−d electron transition within the Ni2+ ion, and the absorption band with a maximum at 540 nm arises probably from the d−d electron transition within the Ni2+ ion or the charge-transfer transition from the donor of anionic framework to the acceptor of Ni2+ ions in the UV−vis−near IR diffuse reflectance spectrum of ZnNi-I.24,33 Other hybrid solid solutions, ZnNi-x (x = II−VI), show analogous UV−vis− near IR diffuse reflectance spectra to ZnNi-I owing to the existence of Ni2+ in the crystals, but the relative intensity of both bands centered at 540 and 860 nm reduces with the molar fraction of Ni reducing in the crystal (Figure 5b). The relative intensities of two bands in the visible and near IR spectroscopy regions are plotted against the molar fraction of Zn (α)/Ni (1 − α) for each hybrid solid solution of ZnNi-x (x = II−VI), which are displayed in Figure 5c,d, respectively, indicating good linear relationship, and this is further confirmed by the linear fits with more than 0.99 adjusted coefficient of determination for two plots.

(2)

In eq 2, the symbol α is the molar fraction of Zn ions in the hybrid solid solution ZnNi-x (x = II−VI) with a formula of ZnαNi1−α(en)3Ag2I4. Therefore, the relative content of Ni2+ in the hybrid solid solution ZnNi-x (x = II−VI) can be calculated using the Curie constants and eq 2 and the corresponding α values are listed in Table S2, which are basically in agreement with the corresponding results determined using the ICP technique. UV−Vis−Near IR Diffuse Reflectance Spectroscopy. The UV−vis−near IR diffuse reflectance spectra of ZnNi-x (x = I−VII) are displayed in Figure 5a,b. Hybrid ZnNi-VII (Zn(en)3Ag2I4) shows two absorption bands in the wavelength ranges of 200−400 nm and absence of any absorbance in the visible and near IR spectroscopy regions (400−1200 nm); hybrid ZnNi-I (Ni(en)3Ag2I4) shows the absorption bands 2+

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independent dielectric permittivity (ε′) and the corresponding temperature region for ZnNi-x (x = I−VII). It is worth noting that there is no simple linear relationship between the frequency-independent dielectric permittivity (ε′) and the molar fraction of Zn/Ni in the ZnNi-x as well as between the Tonset and the molar fraction of Zn/Ni in the ZnNi-x. Relationship between Physical Properties and Relative Content of Zn/Ni. In the solid solutions of ZnαNi1−α(en)3Ag2I4 (0 < α < 1), there exists good linear relationship between the intensity of bands centered at 540 and 860 nm as well as the Curie constant and the molar fraction of Zn/Ni. This is because that the bands centered at 540 and 860 nm arise from the d−d electron transition in the Ni2+ ion and the magnetic susceptibility also contributes from the Ni2+ ion. In the crystal structure of ZnαNi1−α(en)3Ag2I4 (0 < α < 1), the distance between the nearest neighboring Ni2+ ions is large enough so that there is neglectable interaction between them, as a result, the relationship between the intensity of d−d electron transition band and the concentration of chromophore follows the Beer−Lambert law, as well as the temperature-dependent magnetic susceptibility obeys the Curie−Weiss law and the Curie constant is proportional to the molar fraction of paramagnetic centers in solid solutions of ZnαNi1−α(en)3Ag2I4 (0 < α < 1). On the other hand, the lattice parameters, thermal stability (the characteristic onset temperature of starting release of the coordinated ethylenediamine molecules), the frequencyindependent dielectric permittivity, and the onset temperature of thermally activated dielectric relaxation change nonmonotonically with the molar fraction of Zn/Ni in ZnαNi1−α(en)3Ag2I4 (0 < α < 1). As shown in Figure 3a,b, the nonmonotonic alternation of cell parameters are related to the nonhomogeneous change of C−N and C−C bond lengths in the guest [M(en)3]2+ ion. In the guest [M(en)3]2+ ion, the bonds of metal ion (Zn2+/ Ni2+) with ethylenediamine molecules are mainly governed by the electrostatic interaction; as displayed in Figure 3b, the M− N bond length increases linearly with the molar fraction of Zn (α) in ZnαNi1−α(en)3Ag2I4 (0 < α < 1), indicating that the bind energy of M−N in [M(en)3]2+ ion decreases with increasing the molar fraction of Zn (α); accordingly, the thermal stability of [M(en)3]2+ ion should decrease with increasing the molar fraction of Zn (α) in ZnαNi1−α(en)3Ag2I4 (0 < α < 1), and this conclusion is basically consistent with the alternation of the onset temperature of losing ethylenediamine molecules of ZnαNi1−α(en)3Ag2I4 (0 < α < 1). The electric polarizability is the tendency to allow an externally applied electric field to induce electric dipoles (separated positive and negative charges) in a material. Polarization P is related to the applied electric field E by

In a dilute solution, the light absorbance of a solute follows the Beer−Lambert law, namely, the light absorbance of a solute is proportional to its concentration. The good linear relationship between the relative intensities of two bands at 540 and 860 nm and the molar fraction of Ni in the solid solution demonstrate the Ni2+ ions homogeneously distribute in the lattice of each solid solution; this conclusion is further supported by the single-crystal structure analysis. Although the UV spectra of ZnNi-x (x = I−VII) in the ultraviolet light region of 200−400 nm show high similarity, the band edges exhibit obvious distinction from each other. As illustrated in Figure 5a, the band edge shifts toward lower energy side as the molar fraction of Zn (α) reduces in the hybrid solid solution family of ZnαNi1−α(en)3Ag2I4, namely, the energy band gap of {Ag2I42−}∞ inorganic framework decreases with α reducing. This fact demonstrates that the slight change of guest molecule geometry nature, such as the molecular volume of guest, directly affects the bonding nature of the {Ag2I42−}∞ inorganic framework. Dielectrics. To remove the moisture adsorbed on the surface of sample pellet, prior to the measurement of dielectrics, the pellet sandwiched by two copper electrodes covered gold was heated to 383 K (110 °C) and then the measurements of dielectrics were performed for two consecutive heating−cooling cycles. As shown in Figures 6a and S5, the plots of dielectric permittivity (ε′) versus temperature measured in two consecutive heating−cooling cycles are almost the same for ZnNi-x (x = I−VII), indicating that the trace amount of moisture adsorbed on the surface of sample pellet was completely eliminated. The temperature dependences of dielectric permittivity (ε′) obtained from the first cooling process are displayed in Figures 6b and S6 for ZnNi-x (x = I−VII) at the selected frequencies. The dielectric spectra of ZnNi-x (x = I−VII) show common characteristics, namely, the ε′ value is independent on the ac frequency in the lower temperature region, while increases quickly with the temperature, which depends strongly on the ac frequency, in the higher temperature region, indicating the existence of thermally activated dielectric relaxation behavior in the higher temperature region.34,35 For instance, as depicted in Figure 6b, ZnNi-VII shows a temperature-independent dielectric permittivity at the temperature below ca. 373 K, which value varies slowly from 11 to 11.7 as the temperature is elevated from 173 to 373 K. The dielectric permittivity of ZnNi-VII depends strongly on the ac frequency at the temperature above 373 K. The temperature, where the thermally activated dielectric relaxation is clearly visible in ε′−T plot, is defined as the onset temperature (Tonset), which is summarized in Table 3 together with the ac frequencyTable 3. Tonset, Frequency-Independent Dielectric Permittivity (ε′), and the Corresponding Temperature Region for ZnNi-x (x = I−VII) sample

Tonset/K

ε′

temperature region/K

ZnNi-I ZnNi-II ZnNi-III ZnNi-IV ZnNi-V ZnNi-VI ZnNi-VII

430 393 392 373 360 380 373

10.8−11.9 9.8−10.3 9.7−10.1 12.4−13.0 11.2−11.9 10.7−11.9 11−11.7

173−430 173−393 173−392 173−373 173−360 173−380 173−373

P = ε0(εr − 1)E

(3)

where the symbols ε0 and εr represent the permittivity of the free space and the relative dielectric constant of a material. On the other hand, the P is also the density of atomic-induce electric dipole per unit volume P=

∑ p/V = Np

(4)

Therefore, Np = ε0(εr − 1)E 10730

(5) DOI: 10.1021/acsomega.8b01372 ACS Omega 2018, 3, 10725−10732

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where p is the electric-induce dipole moment and N is the density of dipoles. As mentioned above, the εr of a material is relevant to the electric-induce dipole moment, which depends on the crystal structure. Consequently, both the nonmonotonic change of unit cell volume and the electric-induce dipole moment in a unit cell give rise to the nonmonotonic change of frequency-independent dielectric permittivity and the onset temperature of thermally activated dielectric relaxation in ZnαNi1−α(en)3Ag2I4 (0 < α < 1).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (X.-M.R.). *E-mail: [email protected] (S.N.). ORCID

Chen Xue: 0000-0002-0462-9637 Yang Zou: 0000-0002-1320-004X Xiao-Ming Ren: 0000-0003-0848-6503

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CONCLUSIONS The hybrid solid solutions ZnαNi1−α(en)3Ag2I4 (0 < α < 1) have been prepared and characterized, and the crystal structures, TGAs, the UV−vis−near IR diffuse reflection spectra in solid state, variable-temperature magnetic susceptibility, and dielectric properties have been investigated. The molar fraction of Zn (α)/Ni (1 − α) in each hybrid solid solution is rather close to the value calculated from the molar ratio of Zn/Ni in the starting mixed solution for preparation of each solid solution. The Zn2+ and Ni2+ form solid solution in ZnαNi1−α(en)3Ag2I4 (0 < α < 1) owing to two isostructural parent hybrids having rather similar lattice parameters. The distance is large enough between the neighboring Ni2+ ions in the lattice of ZnαNi1−α(en)3Ag2I4 (0 < α < 1), leading to the existence of neglectable electric and magnetic dipole interaction as well as spin coupling in the lattice of ZnαNi1−α(en)3Ag2I4 (0 < α < 1); thus, the relationship between the intensity of bands centered at 540 and 860 nm in UV−vis−near IR spectra and the relative content of Zn/Ni shows dilute solution behavior, as well as the solid solutions of ZnαNi1−α(en)3Ag2I4 (0 < α < 1) show magnetic feature of an isolated magnetic system. However, the lattice parameters, the frequency-independent dielectric permittivity, and the onset temperature of thermally activated dielectric relaxation exhibit nonmonotonic change with the fraction of Zn/Ni in ZnαNi1−α(en)3Ag2I4 (0 < α < 1), attributing to the nonhomogeneous alternation of N−C and C−C bond lengths in the [M(en)3]2+ guests and synergistic interaction. This study provided an insight into understanding the correlation between the given functionality and the relative content of components in hybrid solid solution.

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Article

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the National Nature Science Foundation of China (grant nos. 21671100 and 21771104).

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REFERENCES

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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/acsomega.8b01372. Optical images of ZnNi-x (x = I−VII); PXRD patterns of ZnNi-x (x = I−VII) obtained at room temperature and the corresponding calculated profiles obtained from the single-crystal structure data using Mercury 3.1 program; magnetism and Curie−Weiss fit results for ZnNi-x (x = I−VII); variable-temperature PXRD of ZnNi-IV and ZnNi-V; plots of dielectric permittivity (ε′) versus temperature; plots of dielectric permittivity (ε′) versus temperature; summary of crystallographic data for ZnNi-x (x = I−VII); summary of magnetic properties of ZnNi-x (x = I−VII); and band length M− N, C−N, and Ag−I in ZnNi-x (x = I−VII) (PDF) 10731

DOI: 10.1021/acsomega.8b01372 ACS Omega 2018, 3, 10725−10732

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DOI: 10.1021/acsomega.8b01372 ACS Omega 2018, 3, 10725−10732