Phase Transition, Dielectrics, Single-Ion Conductance, and

Article pubs.acs.org/IC

Phase Transition, Dielectrics, Single-Ion Conductance, and Thermochromic Luminescence of a Inorganic−Organic Hybrid of [Triethylpropylammonium][PbI3] Meng-Jin Wang,† Xuan-Rong Chen,*,‡ Yuan-Bo Tong,† Guo-Jun Yuan,† Xiao-Ming Ren,*,†,§,∥ and Jian-Lan Liu† †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry & Molecular Engineering and §College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ‡ School of Chemistry & Environmental Engineering and Instrumental Analysis Center, Yancheng Teachers University, Yancheng 224051, P. R. China ∥ State Key Lab & Coordination Chemistry Institute, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: In this study, we used the facile solvent evaporation method to achieve the inorganic−organic hybrid crystals of [triethylpropylammonium][PbI3], which have been characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, and differential scanning calorimetry as well as single-crystal X-ray structure analysis. The hybrid solid crystallizes in the monoclinic space group P21/c at room temperature and is composed of onedimensional [PbI3]∞ chains, where the neighboring PbI6 coordination octahedra connect together via the face-sharing mode and the organic cations fall in the spaces between [PbI3]∞ chains. The hybrid exhibits a dielectric phase transition with a critical temperature of ca. 432 K, dielectric relaxation at frequencies below 107 Hz, and single-ion conducting behavior, the conductivity of which increases rapidly from 9.43 × 10−10 S cm−1 at 383 K to 4.47 × 10−5 S cm−1 at 473 K. The variable-temperature single-crystal and powder X-ray diffraction analyses revealed that the dielectric phase transition is related to the disorder-to-order transformation of cations in the lattice. The electric modulus and impedance spectral analyses further disclosed that the dielectric relaxation arises from the ionic displacement polarization and molecular dipole orientation of cations. The single-ion conductance is due to the migration of cations that fall in the spaces of rigid inorganic [PbI3]∞ chains. The phase transition gives rise to this hybrid showing switchable ion-conducting nature around the critical temperature of the phase transition. Besides the fascinating functionalities mentioned above, the hybrid also exhibits a thermochromic luminescence feature originating from the electron transition between the valence and conduction bands of the inorganic [PbI3]∞ chain.

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INTRODUCTION Phase transitions in materials can often impart functionality to the system, sometimes leading to important technological innovations. The physical properties of a material may change massively across a solid-to-solid phase transition,1−6 and as a consequence, the material that undergoes fast and reversible phase transitions is very useful for applications as switching devices.1−7 In the context of designing and constructing phase-transition materials, in the past few years, significant efforts have been devoted to the development of new members of versatile families with the formula ABX3, which is the so-called perovskite-type inorganic−organic/organic perovskite hybrid materials,8 where the A- and/or X-site inorganic ions in a conventional perovskite crystal have been replaced by organic building blocks. Generally, the organic ammoniums occupy the A site and the small-size bridging ligands, such as N3−, CN−, or HCOO− anions, etc., reside in the X site.9−14 In most cases, the © 2017 American Chemical Society

phase transition has been observed in the organic perovskite materials mentioned above, which is triggered by the disorderto-order transformation of the organic ammoniums in the A site and the cooperative deformation of the framework of threedimensional perovskite as well. Such a phase transition commonly gives rise to the organic perovskite material showing switchable optical, dielectric, magnetic, etc., attractive functionalities.8−21 With respect to the organic perovskite materials containing N3−, CN−, or HCOO− anions, etc., in the X site favoring the formation of a three-dimensional framework, the family of ABX3, where the X− ions are halides, prefers to form onedimensional inorganic−organic hybrid crystals,22−27 and the three-dimensional framework forms, normally, when the A sites are occupied by the smallest-size organic ammoniums (i.e., Received: April 5, 2017 Published: August 7, 2017 9525

DOI: 10.1021/acs.inorgchem.7b00881 Inorg. Chem. 2017, 56, 9525−9534

Article

Inorganic Chemistry Table 1. Crystallographic Data and Refinement Parameter for 1 at 373, 298, and 100 K 373 K chemical formula fw wavelength (Å) CCDC numbers cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)/Z ρ (g cm−3) F(000) abs coeff (mm−1) θ ranges of data collection (deg) index range Rint indep reflns/restraints/param refinement method GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)a residual (e Å−3) a

298 K

100 K

C9H22I3Pb 732.16 0.71073 1535131 monoclinic P21/c 8.141(5) 19.664(12) 12.540(6) 90 116.00(3) 90 1804.29/4 2.695 1296 14.460 2.749−27.524



−10 ≤ h ≤ 10, −20 ≤ k ≤ 25, −13 ≤ l ≤ 16 0.0715 4123/0/134

−10 ≤ h ≤ 10, −21≤ k ≤ 25, −12 ≤ l ≤ 15 0.0580 4040/0/134 full-matrix least squares on F2 1.032 0.0365, 0.0638 0.0699, 0.0721 0.905/−1.485

−10 ≤ h ≤ 10, −24 ≤ k ≤ 25, −15 ≤ l ≤ 12 0.0603 3909/0/134

1.037 0.0454, 0.0770 0.1254, 0.0925 0.818/−1.153

1.163 0.0303, 0.0665 0.0361, 0.0683 1.292/−2.141

R1 = ∑(||Fo| − |Fc||)/∑|Fo|; wR2 = ∑w(|Fo|2 − |Fc|2)2/∑w(|Fo|2)2]1/2. Preparation of [triethylpropylammonium][PbI3] (1). PbI2 (0.003 mol) and triethylpropylammonium iodide (0.003 mol) were dissolved in N,N-dimethylformamide (15 mL) under ultrasonic conditions at ambient temperature. The yellow rod single crystals of 1 were obtained by slow evaporation of the mixed solution after 2 weeks. Yield: ∼60% estimated from the amount of reactant PbI2. Elem anal. Calcd for C9H22NI3Pb: C, 14.70; H, 2.99; N, 1.91. Found: C, 14.75; H, 2.99; N, 1.19. Physical Measurement. Differential scanning calorimetry (DSC) measurements were carried out on a NETZSCH DSC 204F1 Phoenix calorimeter for powdered samples between 223 and 423 K, with a temperature scanning rate of 10 K min−1. Fourier transform infrared (FT-IR) spectroscopy in the wavenumber range of 400−4000 cm−1 was performed on a Bruker VERTEX80 V Fourier transform infrared spectrometer (KBr disk) at room temperature. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance powder diffractometer operating at 40 kV and 40 mA using Cu Kα radiation with λ = 1.5418 Å. The 2θ angles span from 5 to 50° with 0.01° step−1. During measurements of the temperature-dependent PXRD data, the temperature changing rate is 10 K min−1. It is worth mentioning that PXRD measurement at a certain temperature starts after the sample is kept at the set temperature for 15 min to ensure that the sample and probe have the same temperature. Other parameter settings are the same as those in the PXRD measurement at room temperature. The dielectric permittivity and impedance spectra were measured for the powered sample using a Concept 80 system (Novocontrol, Germany) under a N2 atmosphere in the temperature range of 123− 473 K and the alternating-current (ac) frequency span from 1 to 107 Hz. The powdered sample was prepared in a disk form with a diameter of 10 mm and a thickness of ca. 0.56 mm under ca. 8 MPa static pressure; the disk sample was sandwiched between two parallel copper electrodes. The variable-temperature fluorescence spectra of the sample in the solid state were recorded on a Fluorolog Tau-3 fluorometer in the temperature range of 10−300 K. The solid-state

methylammoniums such as methylammonium lead−trihalide perovskites) or the B sites are occupies by the bigger-size divalence metal ions.28 It is worth mentioning that, most recently, a three-dimensional molecular perovskite ferroelectric, (3-ammoniopyrrolidinium)RbBr3, was reported in which the organic cation is a heterocycle, aminium (3-ammoniopyrrolidinium).29 Besides the difference in the crystal structure, another distinction between the organic perovskite materials with N3−, CN−, or HCOO− anions, etc., and with halides in the X site is that the phase transition has rarely been found in the family of one-dimensional ABX3, where the X− ions are halides. It is noted that a haloplumbate-based hybrid crystal with anisotropic molecular order and controllable dynamics30,31 is achievable if building blocks with volume-conserving motion (dynamic disordering) are introduced into the haloplumbate hybrid lattice because the disorder-to-order structural transformation frequently occurs in an amphidynamic crystal. In this study, we have successfully achieved the [triethylpropylammonium][PbI3] hybrid crystals by introducing the anisotropic molecular order and potentially dynamic disordering triethylpropylammonium cations into the onedimensional [PbI3]∞ lattice and found that the transformation of disorder−order of cations results in the dielectric phase transition and switchable single-ion conducting behavior. In addition, the multifunctional hybrid solid also exhibits luminescence thermochromism.

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EXPERIMENTAL SECTION

Chemicals and Materials. All reagents and chemicals were purchased from commercial sources and directly used without further purification. Triethylpropylammonium iodide was synthesized according to a similar procedure described in the literature.26,27 9526


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Figure 1. (a) Asymmetric unit of 1 with non-hydrogen atom labeling and thermal ellipsoids drawn at the 30% probability level. (b) Face-sharing octahedral chain of [PbI3]∞ (the Pb1- and Pb2-type octahedra are represented using different colors and the symmetric codes # = 1 − x, 2 − y, 1 − z). (c) Packing diagram viewed along a axis. (d) Shorter contacts between the alkyl chains in the cations and the [PbI3]∞ chains. tional.37 The long-range van der Waals interaction corrections utilized Grimme’s semiempirical approach (DFT-D).38 The plane-wave basis set energy cutoff was set at 240 eV for 1. The convergence parameters were set as follows: SCF tolerance, 1 × 10−6 eV atom−1; total energy tolerance, 2 × 10−5 eV atom−1; maximum force tolerance, 0.05 eV Å−1; maximum stress component, 0.1 GPa; displacement of convergence tolerance, 0.002 Å. Other calculation parameters were set at the default values in the CASTEP code.

UV−vis absorption spectra were recorded on a PerkinElmer Lambda 950 UV−vis−near-IR spectrophotometer at ambient temperature. X-ray Crystallography. Single-crystal X-ray diffraction data were collected for 1 at 100, 298, 373, and 413 K using the graphitemonochromated Mo Ka (λ = 0.71073 Å) radiation on a CCD area detector (Bruker SMART). Data reduction and absorption corrections were performed with the SAINT and SADABS software packages,32 respectively. Structures were solved by direct methods using the SHELXL-97 software package.33 The non-hydrogen atoms were anisotropically refined using a full-matrix least-squares method on F2. All hydrogen atoms were placed at the calculated positions and refined as riding on the parent atoms. The details of data collection, structure refinement, and crystallography are summarized in Table 1.34 Details of Density Functional Theory (DFT) Calculation. Geometric optimization was carried out for the triethylpropylammonium cation in the DFT framework using the Gaussian 98 program.35 The cation was not constrained to any symmetry in the process of geometric optimization. The calculations are performed at the B3LYP/ 6-31(d,p) level, and the convergence criterion of the self-consistent field (SCF) is set as 10−8. Geometric optimization of the crystal structure and electron-bandstructure calculation were performed for 1 in the DFT framework. In the process of geometric optimization, the cell unit parameters of 1 were constrained to the values obtained from the single-crystal X-ray diffraction at 298 K, and the positions of all atoms were fully optimized. The bond lengths and angles in the triethylpropylammonium cation and the PbI6 coordination octahedra are listed in Table S1, which are comparable to the results obtained from single-crystal analysis at 298 K. The electron-band structure and density of states (DOS) were calculated for 1 based on the optimized crystal structure. The Cambridge sequential total energy package (CASTEP) module36 was employed in these calculations. The total plane-wave pseudopotential method forms the basis of the CASTEP calculations. The exchange-correlation effects were treated within the generalized gradient approximation with the Perdew−Burke−Ernzerhof func-

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RESULTS AND DISCUSSION Crystal Structure of 1 at Room Temperature. The hybrid crystal of 1 belongs to the monoclinic system with space group P21/c at 298 K. As shown in Figure 1a, an asymmetric unit consists of two crystallographically different Pb2+ ions (labeled as Pb1 and Pb2) and three crystallographically independent I− ions (labeled as I1, I2, and I3), together with one triethylpropylammonium cation. All non-hydrogen atoms in the triethylpropylammonium cation show larger displacement parameters, indicating that cation dynamics are possible in the lattice at elevated temperature. Two different Pb2+ ions locate at an inversion center and are coordinated with six I− ions to form a distorted octahedral coordination sphere, respectively (refer to Figure 1b). The Pb−I bond lengths fall within the ranges of 3.213(1)−3.259(1) Å, which are comparable to the corresponding values in other iodoplumbates. The neighboring PbI6 octahedra connect together via a face-sharing mode to form a ···Pb1Pb2Pb1Pb2··· inorganic chain along the a-axis direction. As illustrated in Figure 1c, the triethylpropylammoniums are incorporated in the space between the inorganic chains. It is found that there are shorter C−H···I contacts (dC5#1−H5B#1···I1#1 = 3.133(1) Å; #1 = 1 − x, −0.5 + y, 0.5 − z) between the −CH2 groups of the alkyl chains 9527


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Figure 2. (a) Plots of the real (ε′) part of the dielectric permittivity versus temperature in the range of 150−473 K, where the anomalous points at ca. 453 K are bad data arising from the instrument, and (b) DSC curves in two heating and cooling runs in the temperature range of 400−480 K for 1.

Figure 3. Plots of the modulus imaginary part versus frequency at selected temperatures of (a) 123−393 and (b) 398−473 K for 1.

in the cations and the I− ions of the [PbI3] chains and C···H contacts (dC9#1···H9A#2 = 2.850(1) Å and dC9#2···H9A#1 = 2.850(1) Å; #1 = 1 − x, −0.5 + y, 0.5 − z, #2 = −1 + x, 0.5 − y, −0.5 + z) between the neighboring cations (see Figure 1d), indicating the presence of weak interatomic interactions between the inorganic chains and organic cations in the lattice. Dielectric Nature and DSC of 1. The plots of the real (ε′) part of dielectric permittivity versus temperature are shown in Figure 2a for 1 at the selected frequencies. The dielectric permittivity is temperature-independent with ε′ ≈ 19.2−21 in the temperature range of 123−353 K, indicating that the thermally activated dipole motion is suppressed at temperatures below 353 K and then increases quickly and depends strongly on the ac frequency when the temperature is above 383 K. This observation demonstrates the existence of dielectric relaxation at elevated temperature. In addition, a remarkable dielectric anomaly appears at ca. 432 K, and the maximum of the dielectric anomaly peak is independent of the ac frequency, revealing that dielectric phase transitions probably happen there. A dielectric anomaly is generally related to the change in the crystal structure, together with a thermal event. Such a phenomenon is often observed in the ferroelectric materials, where the dielectric anomaly is associated with the paraelectricto-ferroelectric phase transition.39−41 DSC measurement is an effective thermodynamic method to detect phase transitions triggered by temperature. Two sequential heating and cooling runs were performed for 1 in the DSC measurement. As displayed in Figure 2b, the endothermic and exothermic events occur in two sequential thermal cycles in the range of 400−480 K; it is worth noting that the exothermic peaks almost coincide with each other in two sequential heating and cooling runs, while the endothermic peak shifts slightly toward the hightemperature side in the second heating run with respect to that

in the first thermal process. The analogous thermal behavior is normally observed in the ion-liquid crystals42,43 and related to the fact that the thermal motion of the alkyl chains cannot follow the temperature change during the heating−cooling cycle. In the first heating process, the onset and peak temperatures of the endothermic event locate at 440 and 445 K, respectively. The critical temperature of the phase transition obtained from DSC (440 K) is a little bit higher than that achieved from dielectric measurements, and this is due to the different thermal rates used in the DSC and dielectric measurements. On the basis of the DSC measurement, the entropy (ΔS) and enthalpy (ΔH) changes are estimated to be 32.5 J mol−1 K−1 and 14.4 kJ mol−1 in the first heating run. The dielectric relaxation behavior of 1 at elevated temperature is further analyzed. Generally, the dielectric relaxations arise probably from the processes of ion migration, molecular dipole orientation, electrode polarization, and space charge injection. The ion migration and molecular dipole orientation are the intrinsic nature of a dielectric material. However, the electrode polarization and space charge injection belong to extrinsic effects.44 A typical electrode polarization effect was caused by the double-layer capacity of the nonhomogenous surface.45 As the ac field was applied, most carriers injected at the electrodes during a half-cycle were ejected during the next half-cycle and, thus, the net balance of charge on a cycle is practically zero. However, a small fraction of carriers can be trapped at levels deep enough for them to be retained when the field is inverted. The amount of charge increases slower in the case of an ac frequency than in the case of a direct current (dc) and becomes observable after longer periods of time, which is known as space-charge injection. The electrode polarization and space-charge-injection effects occur in the case of low frequency and high temperature, and as a result, these extrinsic effects are significantly reducible using dielectric modulus 9528


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Inorganic Chemistry analysis.46,47 The electric modulus (M*) was calculated using eq 1.46,47 M *(ω) =

ε ′ + jε ″ 1 = 2 = M′ + jM″ ε*(ω) ε′ + ε″2

lattice of 1 is thermally activated at elevated temperatures. It is noted that the liquidlike fast transportation of organic cations (1-ethyl-3-methylimidazoliums) has been observed in the robust crystal structure, which is comprised of one-dimensional zinc sulfate chains.48 The impedance plots were fitted using an equivalent circuit, where each impedance semicircle can be represented by a resistor (R) and capacitor (C) in the parallel mode. Two such parallel units, representing respectively the crystal boundary and bulk contribution to the impedance spectrum, are connected together with an addition resistor (peripheral circuit) and a constant-phase element (abbreviated as CPE, which stands for the interface capacitor between electrodes and the sample pellet) in the series mode. The resistances at selected temperatures are obtained from nonlinear fits using the Zview program, and the corresponding resistances are summarized in Table S2. The values of the resistance fall within the range of 2.85 × 104−1.35 × 109 Ω in the temperature range of 383−473 K, and the corresponding values of the conductivity, calculated by the equation of σdc = L/RS, where the symbols L, S, and R represent respectively the thickness, area, and resistance of the sample pellet, span from 9.43 × 10−10 to 4.47 × 10−5 S cm−1. It is worth mentioning that the ionic conductivity in 1 reaches higher values (∼10−5 S cm−1) when the temperature is over 438 K (refer to Table S2) and increases by 4 orders of magnitude from 383 to 473 K. The temperature-dependent conductivity σdc against temperature is plotted in the form of ln(σdcT) versus 1000/T and shown in Figure 5d. Such a plot shows a linear relationship in the temperature ranges of 383−428 and 433−473 K, respectively; the corresponding activation energy is estimated to be 1.05 in the temperature region of 383−428 K and 0.87 eV in the temperature range of 433−473 K. The activation energies of ionic conduction are comparable to these activation energy values obtained from dielectric relaxation analysis. Origin of the Dielectric Phase Transition and Dielectric Relaxation in 1. The dielectric phase transition is probably related to the phase transition of the crystal structure, and the dielectric relaxation generally concerns the dynamics of the dipole of molecules or migration of ions. The temperature-dependent single-crystal structure analyses could provide direct messages to better understand the abovementioned two issues. The single-crystal X-ray diffraction data were collected at 100, 298, and 373 K (refer to Table 1) and 413 K.34 The refinement parameters of single-crystal X-ray diffraction, such as Rint, Rσ, R1, and wR2, become rather large when the temperature is over 373 K, and this situation is due to the thermally activating dynamics of organic cations in the lattice, which results in the cations being heavily disordered at elevated temperatures. Unfortunately, it is unavailable to determine the crystal structure at temperatures above the dielectric phase transition. Therefore, the variable-temperature PXRD measurements, as an auxiliary means, were further performed in the temperature range of 303−493 K (refer to Figure S4), and the PXRD profiles are shown in Figure 6a for 1 at selected temperatures. At first glance, the position of each diffraction peak is almost unchanged within the range of 2θ = 5−50° between the low- and high-temperature phases. However, the small variation can be found between the PXRD patterns in the low- and high-temperature phases by careful inspection. As exemplified, Figure 6b displays variabletemperature PXRD patterns within the range of 2θ = 8−16°. The diffractions (0, 2, 0) with 2θ = 8.955° and (0, 1, 1) with 2θ = 9.145° at 303 K are close to each other; these diffractions

(1)

In eq 1, the symbols M′ and M″ represent the real and imaginary parts of the complex modulus M*, respectively. The plots of M″ versus frequency (f) are respectively shown in Figure 3 for 1. In the temperature region below 273 K, the M″ values are almost independent of the temperature and ac frequency. With an increase of the temperature, the M″ value, especially in the low-frequency region, increases rapidly and the dielectric relaxation peak comes into view at temperatures above 333 K; moreover, the maximum of the dielectric relaxation peak shifts toward the higher-frequency side as the temperature increases. We estimate the activation energy for the dielectric relaxation process using the empirical Arrhenius expression ⎛ E ⎞ fmax = f0 exp⎜ − a ⎟ ⎝ kBT ⎠

(2)

In eq 2, f max is the ac frequency at the maximum of the relaxation peak in the M″−f plot and selected temperature, f 0 is the preexponential factor, and Ea represents the activation energy or potential barrier. The plot of ln( f max) versus 1000/T is displayed in Figure 4 for the dielectric data in 333−473 K. It

Figure 4. Fits of the Arrhenius equation for the dielectric relaxation activation energy of 1.

is worth noting that the plot of ln( f max) versus 1000/T shows two discontinuous straight lines and the temperature at the break point is ca. 433 K, which almost coincides with the critical temperature of the dielectric phase transition. The activation energy of the dielectric relaxation is estimated as 0.65 eV in the high-temperature phase versus 1.09 eV in the low-temperature phase using a linear fit for the dielectric data in the corresponding temperature region. Impedance Spectra of 1. The impedance spectra were further analyzed for 1, and the Nyquist plots at selected temperatures are shown in Figure 5a−c. In the temperature range of 368−383 K, a series of arc-shaped impedance plots reveal that the sample shows large resistance and small conductivity at temperatures below 383 K. However, as the temperature increases, two overlapped arcs are visible in the Z″−Z′ plots at temperatures above 393 K, which are probably contributed by the crystal boundary and bulk resistances. The arc radii decrease with increasing temperature, and this is due to the decrease in the resistance with rising temperature. These results revealed that the dynamic motion of the cations in the 9529


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Figure 5. (a−c) Impedance plots at selected temperatures from 368 to 473 K. (d) Plot of ln(σdcT) versus 1000/T and the fit using the Arrhenius equation for 1.

Figure 6. Variable-temperature PXRD profiles of 1 in the ranges of 2θ = (a) 5−50° and (b) 8−16°.

In eq 3, the symbols R, N h i g h ‑ t e m p e r a t u r e p h a s e , and Nlow‑temperature phase represent the ideal gas constant and microscopic state numbers in the high- and low-temperature phases, respectively. On the other hand, the main diffractions, such as (0, 1, 1), (0, 2, 0), (0, 3, 1), (1, 3, −2), (2, 2, 0), (2, 1, −3), etc., are clearly observed, and the diffraction intensities show no sizable change in the high-temperature phase versus those in the low-temperature phase, revealing that the lattice built from inorganic [PbI3] chains remained unchanged in the hightemperature phase. As a consequence, it is suggested that the dielectric phase transition undergone in 1 arises from the disorder-to-order transformation of cations. Analysis of the atomic displacement parameters could further give the message of molecular motion in crystals. An asymmetric unit of 1 is shown in Figure 7 at 100, 298, 373, and 413 K, where all non-hydrogen atoms show thermal ellipsoids at the 50% probability level and all hydrogen atoms in the cation are omitted for clarity. Obviously, the thermal ellipsoids of the carbon and nitrogen atoms in the cation increase significantly with rising temperature owing to the motion of the atoms being thermally activated at elevated temperature. The shapes of the ellipsoids of carbon and nitrogen atoms indicate that the alkyl chains show a rocking

shift toward the lower 2θ angle side with increasing temperature and finally combine into a single diffraction peak when the temperature is over ca. 423 K. In addition, the diffraction of (0, 0, 2) with 2θ = 15.972° disappears at temperatures above 423 K, and the diffraction of (0, 3, 1) with 2θ = 15.624° shifts toward the lower 2θ angle side when temperatures are over 423 K. These results indicate that small crystal structure changes occur around the critical temperature of the dielectric phase transition. It is also noted that most weak diffractions in the range of 2θ = 20−45° disappear when the temperature approaches 463 K. These observations are indicative of the existence of heavy disorder for the cations in the lattice of 1 at higher temperature (≥463 K); such a speculation is also confirmed by the fact that the dielectric phase transition possesses bigger entropy changes (ΔS = 32.5 J mol−1 K−1), and on the basis of the ΔS value, the ratio of the numbers of different microscopic states between the low- and high-temperature phases is estimated as ca. 50 using eq 3, demonstrating that the crystal structure of 1 shows heavy disorder in the high-temperature phase. ΔS = R ln

Nhigh‐temperature phase Nlow‐temperature phase

(3) 9530


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electric field owing to the steric hindrance in the lattice, the rocking motion of alkyl chains could result in a change of the permanent dipole moment to a certain extent. Therefore, it is concluded that dielectric relaxation occurring at the ac frequency below 107 Hz is attributed to ionic displacement polarization and molecular dipole orientation of the cations. UV−Vis Absorption and Fluorescence Spectra of 1. Figure 8a shows the optical absorption spectrum of 1 in the solid state at 300 K, which displays intense absorption below 450 nm. The absorption bands below 450 nm contribute from the electron transition between the valence and conduction bands within the one-dimensional inorganic {PbI3}∞ chains. From the absorption spectrum, the optical band gap is estimated to be ca. 2.79 eV (refer to Figure S6). The emission spectra at selected temperatures upon excitation of λex = 365 nm are shown in Figure 8b. The inorganic−organic hybrid emits the weak luminescence in the solid state at 300 K, and the maximum of the broad emission band locates at ca. 584 nm. As displayed in Figure 8c, the intensity of the emission band rapidly increases upon cooling, and the intensity of the emission band at 10 K is ca. 150 times as strong as that at 300 K. On the other hand, the maximum of the emission band readily shifts from 584 nm at 300 K to 607 nm at 10 K, corresponding to the chromaticity coordinates moving from (0.486, 0.475) at 300 K to (0.559, 0.437) at 10 K. As a result, this inorganic−organic hybrid shows a thermochromic luminescence feature. The electron-band structures are further analyzed for 1, and the calculated bands along the several high-symmetry directions of the Brillouin zone are displayed in Figure 9a. It is observed that the valence and conduction bands near the Fermi level show a small dispersion along the directions from Z to G, G to Y, A to B, B to D, and D to E in k space, which is indicative of the existence of weak orbital interactions along these directions. While both the valence and conduction bands are relatively

Figure 7. Asymmetric unit in 1 where all atoms show thermal ellipsoids at the 50% probability level and temperatures of (a) 100, (b) 298, (c) 373, and (d) 413 K.

motion and the nitrogen atom displays a straight displacement motion in the cation at elevated temperature. This observation demonstrates the existence of ionic displacement polarization for the cations in the lattice of 1 owing to the dynamic motion at elevated temperature. As a consequence, the dielectric relaxation process in 1 is certainly related to the displacement polarization of cations, especially at elevated temperature. With respect to the cation, the thermal ellipsoids of the Pb2+ and I− ions in an inorganic {PbI3}∞ chain show slight variation and their shapes remain approximately spherical from 298 to 413 K, demonstrating that the rigid inorganic {PbI3}∞ chains still hold well-ordered structures at elevated temperature. Generally, the inherent dielectric relaxation processes result from ion migration and/or molecular dipole orientation in a dielectric material. The triethylpropylammonium cation shows an asymmetric molecular structure and possesses a permanent dipole moment, and as shown in Figure S5, the DFT calculation revealed that the permanent dipole moment of triethylpropylammonium is 1.8268 D and the direction of the dipole moment is approximately along the propyl chain to the nitrogen atom. Although it is impossible that dipole inversion occurs for the triethylpropylammonium cation under the ac

Figure 8. (a) UV−vis absorption spectrum in the solid state. (b) Emission spectra at selected temperatures. (c) Temperature-dependent relative emission intensity for the band. (d) CIE chromaticity diagram showing the fluorescence color of 1 at selected temperatures. 9531


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Figure 9. (a) Dispersion relationships in 1 with the k points: G, (0, 0, 0); Y, (0, 0.5, 0); Z, (0, 0, 0.5); A, (−0.5, 0.5, 0); B, (−0.5, 0, 0); C, (0, 0.5, 0.5); D, (−0.5, 0, 0.5); E, (−0.5, 0.5, 0.5). (b) Plots of the total and partial DOSs of the Pb2+ and I− ions.

sharp along the directions from Y to A and from E to C in kspace, these directions in k space are approximately parallel to the inorganic {PbI3}∞ chains, indicating that there are stronger orbital interactions within an inorganic chain. These results are in well agreement with the crystal structure analysis finding that the neighboring inorganic chains are separated by the cations. The calculated energy gap between the highest occupied and lowest unoccupied bands is 3.315 eV, which is larger than the value of the optical band gap obtained from the UV−vis absorption spectrum by ca. 0.5 eV, and this is due to the limitations of the DFT methods.49−51 As shown in Figure 9b, the total DOS of 1 and partial DOS of the Pb2+ and I− ions demonstrate that the valence band is mainly comprised of 5p orbitals of the I− ions, while the conduction band consists of 6p orbitals of the Pb2+ ions. This further demonstrates that the emission in 1 arises from the interband transitions in the visible spectroscopy region, which correspond to the charge-transfer bands between the Pb2+ and I− ions.

Variable-temperature PXRD plots of 1 in the temperature range of 303−493 K and experiemental and simulated PXRD patterns of 1, FT-IR spectrum in 4000−400 cm−1 and thermogravimetric analysis plot in 300−900 K of 1, UV−vis absorption spectrum of 1 with band-edge analysis, fits of the impedance spectra of 1 using the Zview program and the equivalent circuit at selected temperatures, optimized molecular structure of the triethylpropylammonium cation together with the Cartesian coordinate system, Brillouin zone and highsymmetry k points in the band-structure calculation of 1, lists of bond lengths and angles in an asymmetric unit of 1, lists of parameters of impedance from fits for 1 at selected temperatures, and lists of CIE chromaticity of 1 at different temperatures (PDF) Accession Codes

CCDC 1535129−1535132 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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CONCLUSION In summary, we have investigated the crystal structure, dielectrics, impedance, and photoluminescence properties of a hybrid, [triethylmethylammonium][PbI3]. The inorganic− organic hybrid shows a dielectric phase transition at ca. 432 K, which is related to the disorder-to-order transformation of triethylmethylammonium cations in the lattice, the dielectric relaxation, and the single-ion-conducting nature. The dielectric relaxation arises from the ionic displacement polarization and molecular dipole orientation of the cations, and the single-ion conductance is due to migration of the cations that fall in the spaces of rigid inorganic [PbI3]∞ chains. Interestingly, the phase transition gives rise to the hybrid displaying switchable ion-conducting behavior; such a fascinating nature has potential applications in ion-conducting devices. In addition, this hybrid exhibits a thermochromic luminescence feature, and a matter with a smart thermochromic luminescence property could be used in a temperature-sensing device. The emission probably corresponds to the electron transition between the valence and conduction bands of the inorganic [PbI3]∞ chain. This study gives a fresh impetus to achieving multifunctional inorganic− organic hybrid materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-R.C.). Tel.: +86 25 58139476. Fax: +86 25 58139481. *E-mail: [email protected] (X.-M.R.). ORCID

Xiao-Ming Ren: 0000-0003-0848-6503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Natural Science Foundation of the Jiangsu Higher Education Institutions (Grant 15KJB150019), and the National Nature Science Foundation of China (Grants 91122011 and 21671100).

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ASSOCIATED CONTENT

S Supporting Information *

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Phase Transition, Dielectrics, Single-Ion Conductance, and

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