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Molecular Ferroelectric Pyridin-4-ylmethanaminium Perchlorate Undergoes Paraelectric-Ferroelectric and Ferroelectric-Ferroelectric Phase Transitions Zepeng Cui, Kaige Gao, Chuang Liu, Yan Yin, Da-Wei Fu, Hong-Ling Cai, and XiaoShan Wu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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The Journal of Physical Chemistry

Molecular Perchlorate

Ferroelectric Undergoes

Pyridin-4-ylmethanaminium Paraelectric-Ferroelectric

and

Ferroelectric-Ferroelectric Phase Transitions Zepeng Cui1, Kaige Gao1, Chuang Liu1, Yan Yin1, Da-Wei Fu2, Hong-Ling Cai1* and X. S. Wu1* 1

Collaborative Innovation Center of Advanced Microstructures, Lab of Solid State

Microstructures, School of Physics, Nanjing University, Nanjing 210093, P. R. China 2

Ordered Matter Science Research Center, College of Chemistry and Chemical Engineering,

Southeast University, Nanjing 211189, P.R. China

ACS Paragon Plus Environment

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ABSTRACT: Molecule-based ferroelectrics are a class of highly desirable intelligent materials for their rich switchable physical properties, easy and environmentally friendly processing, especial light weight and mechanical flexibility. In the current work, a new molecule-based ferroelectrics pyridin-4-ylmethanaminium perchlorate was discovered undergoing paraelectricferroelectric phase transition at Tc = 258.40(8) K and ferroelectric-ferroelectric phase transition at T1 = 255.93(2) K. It crystallizes in monoclinic crystal system with a centrosymmetric space group of C2/c at 293 K and also crystallizes in monoclinic crystal system but with a polar space group of Cc at 223 K. The spontaneous polarization can reach 1.25 µC/cm2 and the coercive field is about 2.6 kV/cm below 254 K. The paraelectric-ferroelectric phase transition belongs to displacive phase transition and the nearby ferroelectric-ferroelectric phase transition contains order-disorder features, which consistent with the dynamic process including deformations of each pyridin-4-ylmethanaminium cation and the resulting difference for -NH2 to be protonated. Due to the high pyroelectric coefficient values and small-amplitude anomalies of the ε′ values in the vicinity of T1 temperature, pyridin-4-ylmethanaminium perchlorate shows two ultrahigh pyroelectric figures of merit (FOMs) with M1 ≈ 0.16 cm2/µC and M2 ≈ 0.21 cm3/2·J-1/2. The ultrahigh FOMs could make pyridin-4-ylmethanaminium perchlorate a potential element of the sensitive small-area pyroelectric detectors or pyroelectric vidicons.


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INTRODUCTION Ferroelectrics are polar substances, in which spontaneously generated electric polarization can be reversed by external alternating electric field.1 As multifunctional electroactive materials, many ferroelectrics have been important components of optoelectronic devices, such as electric random access memory (FeRAM), ferroelectric field-effect transistors, piezoelectric devices, nonlinear optical devices, capacitors, pyroelectric infrared detectors and pyroelectric vidicons.2-5 Ferroelectrics own pyroelectric effect which refers to the change of internal polarization due to small variation of temperature. It is an important research topic that how to improve the detectivity of minimum detectable power and how to achieve a fast frame rate particularly for pyroelectric infrared detectors.6 To examine whether or not the pyroelectric materials are suitable for target materials of small-area pyroelectric detectors and pyroelectric vidicons, two critical pyroelectric figures of merit (FOMs) associated with dielectric phase transition are described as M1 = pe/ε′Cp and M2 = pe/ε′1/2Cp, where pe is the pyroelectric coefficient, Cp is the specific heat, ε′ is real part of the dielectric constant.6-8 The parameter M1 denotes the gain bandwidth and intrinsic ability to reproduce the pulse shape for the small-area detectors, the M2 determines the signal-to-noise ratio of the pyroelectric vidicon.7 Dielectric constants generally increase sharply in the vicinity of the Curie temperature Tc in proper ferroelectrics, which probably leads to the decrease of the FOMs. Therefore, more attention has been focused on the high pe improper ferroelectrics in which the ε′ value displays two plateaus with a quite small-amplitude increment upon heating, for example, the recently discovered Di-n-Butylaminium Trifluoroacetate with maximum FOMs (pe ≈ 0.6 µC·cm-2·K-1, M1 ≈ 0.056 cm2·µC-1, M2 ≈ 0.16 cm3/2·J-1/2).7 In addition, to take advantage of the properties of ferroelectric-ferroelectric phase transition and to develop new-style composite materials are two basic methods to seek for high-performance pyroelectric


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materials. At present, both the scientific advances and applications are limited to be based on the conventional oxide materials such as BaTiO3, PbTiO3 or PbZrxTi1-xO3 because of their high spontaneous polarization and thermal stability. However, the expensive productive process and the environmentally unfriendly elements can’t cater to the necessary of next-generation devices.9 Compared with conventional inorganic ferroelectrics, molecular ferroelectrics have additional advantages including lightness, low-cost, flexibility, solution processibility and non-toxicity.10 It is worth to mention that the connection between ferroelectrics and organic molecules started in 1920 with the discovery of the first ferroelectric crystal, Rochelle salt (NaKC4H4O6·4H2O), containing organic tartrate ions.11 Thus, much of the attention in this field has been focused on finding more molecular ferroelectrics in recent years. Reactions of amine and acid represent a successful design of molecular ferroelectrics. One of the most representative molecular ferroelectrics is DIPAX (DIPA = Diisopropylammonium, X = Cl12, Br13-16, ClO417-18). Particularly, DIPAB crystal plays surprisingly strong ferroelectric properties, with spontaneous polarization (Ps) of 23 µC/cm2 (close to that of BaTiO3), high Curie temperature (Tc) of 426 K, large dielectric constant and low dielectric loss. DIPAC and DIPAP are high Tc (Tc = 440 K) ferroelectrics and improper ferroelectrics respectively. Ferroelectric properties only exist in ferroelectric phase (below Curie temperature Tc), in which the lattice must adopt one of 10 polar point groups: C1, C2, C3, C4, C6, Cs, C2v, C3v, C4v and C6v. When temperature exceeds Tc, ferroelectrics will meet reversible structural phase transition from ferroelectric phase to paraelectric phase, accompanied by heat flow, dielectric anomaly, domain wall motion, dielectric hysteresis loops.19 Generally, ferroelectrics transform into ferroelectric phase through either a displacive phase transition and/or an order-disorder phase transition. In displacive ferroelectric crystals, displacement of ions or molecules from their higher temperature symmetry positions


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can accumulate dipole moments to macroscopic polarization.20 For example, BaTiO3, CMAP21, Ca(NO3)2(15-crown-5)22. For order-disorder phase transition, the dipoles are randomly oriented but then spontaneously align, in a correlated manner below Tc.10 Therefore, it is effective to design and construct crystals containing atomic fluctuation motion that can be frozen by decreasing temperature. In solid ferroelectric salts PyX ( Py = Pyridinium and X = ClO423, BF424, ReO425, IO426 ), orientational disorder of Py cations were disclosed by NMR as in-plane 60

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reorientational jumps about its pseudohexad C6 axis. At lower temperature, a considerable electric dipole moment is held rigidly in the crystal via cation-anion hydrogen privileging to a specific direction. This is why the previously reported pyridinium based dielectric materials and ferroelectrics are mainly of order-disorder type.

Figure 1. Molecular structure of compound 1 at (a) 293 K and (b) 223 K. The hydrogen atoms of pyridine groups and -CH2- groups are not depicted for clarity. Recently we have found a new molecular ferroelectrics pyridin-4-ylmethanaminium perchlorate with the chemical formula of ( H+ya-py · ClO4– )3 ·ya-Py, [ ya = 4-ylmethanamine,


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py = pyridine] (Compound 1) based on pyridin-4-ylmethanamine and perchlorate acid. Compound 1 undergoes paraelectric-ferroelectric phase transition (ferroelectric phase transition) at Tc = 258 K and ferroelectric-ferroelectric phase transition at T1 = 256 K. The molecule structures of compound 1 at 293K and 223K are showed in Figure 1a and Figure 1b, respectively. The high pyroelectric coefficient pe values and the small-amplitude anomalies of dielectric constant ε′ and heat capacity Cp make compound 1 display two ultrahigh FOMs with M1 ≈ 0.16 cm2/µC and M2 ≈ 0.21 cm3/2·J-1/2 in the ferroelectric-ferroelectric phase transition. The ferroelectricity of Compound 1 were well confirmed by thermal analysis, variable-temperature crystal structure, second harmonic generation (SHG) measurement, dielectric anomalies, pyroelectricity and P-E hysteresis loops. EXPERIMENTAL METHODS All reagents and solvents in the syntheses were of reagent grade and used without further purification. Concentrated perchloric acid (71 wt%, 4.25 g, 0.03 mol) was added dropwise to pyridin-4-ylmethanamine (98 wt%, 4.41 g, 0.04 mol) in methanol (70 mL). Green block crystals of compound 1 were obtained through slow evaporation after three weeks. Crystals of compound 2 were also prepared through slow evaporation of aqueous. Differential Scanning Calorimetry (DSC) and heat capacity measurements of the single crystals were performed on NETZSCH DSC 200 F3 under nitrogen protection in aluminum crucibles with a heating or cooling rate of 10 K/min. Variable-temperature single-crystal X-ray diffraction analysis was carried out using a Rigaku Saturn 724+ CCD diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å). Data collection, cell refinement, and data reduction were performed using Rigaku Crystalclear 1.3.5. The structures of crystals were solved by direct methods and refined by the full-matrix method based on F2 using the SHELXLTL software package. All non-hydrogen atoms were refined


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anisotropically, and the positions of all hydrogen atoms were generated geometrically. For second-harmonic generation (SHG) measurements, an unexpanded laser beam with low divergence (pulsed Nd:YAG at a wavelength of 1064 nm, 5 ns pulse duration, 1.6 MV peak power, 10 Hz repetition rate) was used. The numerical values of nonlinear optical coefficient for SHG have been determined by comparison with a KDP reference. For dielectric, pyroelectric, PE hysteresis loops measurement, the single crystals were cut into thin plate. Silver conductive paste deposited on the plate surfaces was used as the electrodes. The measurement axis is perpendicular to the biggest crystal plane of natural growth (Figure S3) of compound 1. The complex permittivity measurement was using Tonghui TH2828A LCR meter at the frequency from 20 Hz to 1 MHz with an applied electric field of 0.5 V. The pyroelectric current was measured using Keithley 6517B electrometer. The P-E hysteresis loops were recorded on a Precision Premier Ⅱ (Radiant Technologies, Inc).

RESULTS AND DISCUSSION Compared with stable reference materials, ferroelectric crystals will absorb or release latent heat during ferroelectric-to-paraelectric and/or ferroelectric-to-ferroelectric phase transitions, which can be recorded by DSC measurements. As shown in Figure 2a, two coupled peaks at T1 = 255.93(2) K and Tc = 258.40(8) K in heating process as well as T1′ = 252.87(1) K and Tc′ = 255.99(3) K in cooling process are observed by DSC measurements, which indicates two reversible phase transitions. Specific heat capacity (Cp), which is calculated based on DSC data compared with a sapphire, also reveals two characteristic anomalies at 256.08(1) K and 258.43(7) K. The narrow thermal hysteresis (≈ 3 K and ≈ 2.4 K) between heating and cooling process reveals a sharp transition and small thermal potential barrier. The entropy changes of phase transitions can be calculated as ∆S1 = ∆H1/T1, ∆S2 = ∆H2/Tc, where the values of enthalpy


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change ∆H can be integral from the peak areas of DSC anomalies. The enthalpy changes are ∆H1 = 563.1 J/mol and ∆H2 =101.7 J/mol for the transitions at T1 and Tc, respectively. This corresponds to the entropy changes of ∆S1 = 2.2 J mol-1 K-1 and ∆S2 = 0.394 J mol-1 K-1. From the Boltzmann equation (∆S = R lnN, where R is gas constant and N represents ratio of possible configurations),27 it is found that N1= 1.303 and N2 = 1.048, indicating that the phase transition at T1 contains order-disorder feature and the phase transitions at Tc is belong to displacive transition.

Figure 2. (a) The temperature dependence of DSC and Cp measurement of compound 1, revealing the phase transitions at 255.93(2) K and 258.40(8) K. (b) The temperature dependence of second-order nonlinear coefficient measured on polycrystals of compound 1.


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Figure 3. Comparison of crystal structures of compound 1 between HTP and LTP. (a) The packing diagram is viewed along b axis at 293 K. (b) The packing diagram is viewed along b axis at 223 K. The hydrogen atoms are not labeled for clarity. To understand the origin of ferroelectricity, single-crystal X-ray diffraction was performed to determine the structures of compound 1 at various temperatures. The structure determined at 293 K (Figure 3a) can be taken as the average structure of high-temperature phase (HTP), and the structure at 223 K (Figure 3b) is taken as that of low-temperature phase (LTP). The intermediatetemperature phase can be labeled as ITP for convenience. The crystal structure of ITP was not determined because the temperature range of ITP is very narrow (≈ 2 K). The cations denoted with A (or B) and A′ (or B′) are centrosymmetry in HTP. In fact, one of these four pyridin-4ylmethanamine molecules in HTP is not protonated, though we can’t determine which one is not protonated due to the disorder. However, in LTP it can be determined that the -NH2 groups of pyridin-4-ylmethanamine molecules denoted as B are not protonated. The centrosymmetry between B and B′ disappeared in the low temperature because that one is protonated while the other is not. According to Fourier hydrogenation, the suitable protonated H locations have been determined by specifying the residual Q peak direction. Compound 1 crystallizes in a monoclinic


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crystal system with a centrosymmetric space group C2/c (point group: C2h) at 293 K and the structure contains four symmetric operation elements (E, C2, σh, i). As shown in Figure 3a, the locations and the shapes of pyridin-4-ylmethanaminium cations B and B′ are equivalent, which can be applied to the couple of cations A and A′. The centrosymmetric HTP structure generates equivalent circumstances for two couples of pyridin-4-ylmethanaminium cations A and A′, B and B′ respectively, giving rise to the nature of disorder that all of the -NH2 are protonated shown as Figure 1a. The polarization is zero in HTP because the dipolar moments (from ClO4– anions to NH3+ cations) related by inversion centers are opposite in direction and equal in value. At low temperature of 223 K, compound 1 still crystallizes in a monoclinic crystal system but with a polar space group Cc (point group: Cs) and the number of symmetric operation elements decreases to 2 (E, σh). The geometry details (Table S1) reveal that the angle of C8-C9-C12 decreases from 128.3(4)º of HTP to 124.3(3)º of LTP with variation of -4.0(1)º and the angle of C8′-C9′-C12′ changes from 128.3(4)º to 119.2(4)º with variation of -9.1º. The angle of C10-C9C12 increases from 114.4(4)º to 118.8(3)º with change of 4.3(9)º, and the angle of C10′-C9′-C12′ increases from 114.4(4)º to 122.0(4)º with change of 7.6º. The angle of C8-C9-C10 decreases from 117.3(4)º to 116.8(3)º with change of -0.5(1)º, and the angle of C8′-C9′-C10′ increases from 117.3(4)º to 118.7(4)º with change of 1.4º. In addition, the angle of C9-C12-N3 demonstrates obvious change (≈ -3.5º) from 117.1(4)º to 113.6(3)º, while the angle of C9′-C12′N3′ performs a relatively small change (≈ -0.1º) from 117.1(4)º to 117.0(4)º. The most visualized difference between HTP structure and LTP structure is the large displacement of -NH3+ in pyridin-4-ylmethanaminium cation B′ which is derived from the torsion of C9′-C12′-N3′ around the axis C9′-C12′. However, the -NH2 of pyridine-4-ylmethanamine B doesn’t perform such displacement. It reveals the obvious difference between B and B′ in LTP structure. Each pyridin-


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4-ylmethanaminium cation performs different deformations in aspect of bond length and bong angle more or less. The displacement of -NH3+ in cation B′ and the deformations of each pyridin4-ylmethanaminium cation reveal the displacive mechanism of the paraelectric-ferroelectric phase transition. As depicted in Figure 3b, the various-degree deformations destroy the equivalence of A and A′, B and B′, which gives rise to the difference for -NH2 groups of B and B′ to be protonated and the loss of inversion center. The obvious displacement of -NH3+ of cation B’ and the deformations of each cation can cause the H-bond changes in N-H···O and N-H···N from HTP to LTP. However, the H-bonds are chaotic and can’t form clear network. The polarization is the sum of dipole moments from ClO4- to -NH3+ that can’t cancel each other out. The paraelectric-to-ferroelectric phase transition is triggered by the collective ordering of protons upon lowering the temperature. In LTP, the protons are ordered to protonate the specific pyridine-4-ylmethanamine molecules denoted with A, A’ and B’. This process reveals the orderdisorder mechanism of the ferroelectric-ferroelectric phase transition. The cations of high symmetry in HTP transform to ordering protons collective in LTP is also consistent with the conclusion. Finally, all these differences lead to the symmetry broken from HTP to LTP, which indicates the paraelectric-ferroelectric phase transition. This is consistent with the conclusion revealed by DSC measurement that with the decrease of temperature, the compound 1 undergoes displacive phase transition and the phase transition containing order-disorder feature in succession. In LTP, for the obvious displacive of -NH2 groups contribute to the noncoincidence of positive and negative charges centers, the sum of the dipolar moments generates the macroscopic polarization. It is very interesting or confusing why the ferroelectricity exists in the case where three quarters of pyridine-4-ylmethanamine molecules are protonated. We try to make all the pyridine-


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4-ylmethanamine molecules protonated by adding more perchloric acid in the solution. When the mole ratio of pyridin-4-ylmethanamine to perchlorate acid is 1:2, another transparent block crystals are obtained (compound 2) with the chemical formula of (H+ya-H+py) · 2ClO4–, [ ya = 4-ylmethanamine, py = pyridine]. Single crystal XRD determines that compound 2 crystallizes in a monoclinic crystal system with a centrosymmetric space group P21/c and all of the nitrogen atoms of compound 2 are protonated. The crystal data and crystal structure of compound 2 are depicted in Table S2 and Figure S2. However, the physical properties of compound 2 are largely distinguished from compound 1. No phase transition was observed in compound 2 from DSC and dielectric constant. The SHG measurement is a sensitive tool to probe the symmetry-breaking process of ferroelectric phase transition,28-29 since that only non-centrosymmetric materials are SHG-active. The loss of the inversion center of compound 1 in the structural phase transition from C2/c to Cc is revealed by the temperature-dependent second-order nonlinear optical coefficient χ(2) (Figure 2b). The χ(2) is close to zero except some noise signal when the temperature is above Tc′ in the cooling process, indicating that the crystal structure is centrosymmetric. As temperature decreases, χ(2) performs obvious increase at around Tc′ and keeps a finite value approximately in a large temperature range, suggesting that the crystal structure is non-centrosymmetric below Tc′. The SHG curve in heating process is similar with that in cooling process except for the thermal hysteresis of 2 K, which is consistent with the DSC analysis. The SHG results are consistent with the structural phase transition from space group C2/c to Cc revealed from XRD, although the structure of ITP can’t be determined from SHG.


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Figure 4. Temperature-dependence of real part (ε′) of complex dielectric constant of compound 1 measured in (a) cooling process and (b) heating process. Inset of (a): the reciprocal real part of dielectric constant 1/ε′ at 500 Hz as a function of temperature in cooling process. Inset of (b): the imaginary part of dielectric constant as a function of temperature in heating process. Appearance of large dielectric anomalies in the vicinity of ferroelectric-paraelectric or ferroelectric-ferroelectric phase transition is an essential characteristic of proper ferroelectrics. Measurement of the temperature-dependent permittivity ε′ ( real part of complex dielectric constant ε﹦ε′–iε′′, where ε′′ is imaginary part ) in cooling and heating process are depicted in Figure 4a and 4b, respectively. The permittivity-temperature curves display obvious anomalies at Tc′ = 256 K in cooling process and reveal relatively small anomalies at Tc = 258 K in heating


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process, consistent with the DSC curves. For ferroelectric phase transition, the dielectric response could be fitted with the Curie-Weiss law, ε′ ﹦ε0 + Cpara/(T-Tc) (T>Tc) or ε′﹦ε0 + Cferro/(Tc-T) (T

Molecular Ferroelectric Pyridin-4 ... - ACS Publications

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