Novel Phase-Transition Materials Coupled with Switchable Dielectric

Sep 30, 2014 - Muhammad Adnan Asghar , Jing Zhang , Shiguo Han , Zhihua Sun .... Aurang Zeb , Tariq Khan , Muhammad Adnan Asghar , Zhihua Sun ...
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Novel Phase-Transition Materials Coupled with Switchable Dielectric, Magnetic, and Optical Properties: [(CH3)4P][FeCl4] and [(CH3)4P][FeBr4] Ping-Ping Shi, Qiong Ye,* Qiang Li, Hui-Ting Wang, Da-Wei Fu, Yi Zhang, and Ren-Gen Xiong* Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P.R. China S Supporting Information *

ABSTRACT: Two inorganic−organic hybrid compounds with zerodimensional cryst al structures, tetramethylphosphonium tetrachloroferrate(III) (compound 1, [(CH3)4P][FeCl4]) and tetramethylphosphonium tetrabromoferrate(III) (compound 2, [(CH3)4P][FeBr4]), are discovered as multifunctional materials exhibiting simultaneously switchable dielectric, magnetic, and optical properties. Despite the analogue chemical formulas, compounds 1 and 2 crystallize in the different noncentrosymmetric space groups, that is, P63mc and F4̅3m, and exhibit distinct responses in the three above-mentioned physical channels, especially for the magnetic property. Compound 1 undergoes dielectric anomalies which could be tuned in three distinct dielectric states and switched by the sequential phase transitions around 362 and 436 K, respectively. The symmetry breaking occurring during the first phase transition is confirmed by the switchable temperaturedependent second harmonic generation (SHG) effect. Weak antiferromagnetic interactions are also found in compound 1 below room temperature. In contrast, the continuous phase transitions occur at 353 and 359 K in compound 2, together with the steplike dielectric anomalies which also could be tuned in three distinct dielectric states. Except for the switchable SHG effect and the antiferromagnetic interactions stronger than compound 1, compound 2 displays magnetic bistability in the vicinity of the second phase transition, with a thermal hysteresis of 6 K.

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filled. Recently, beyond the conventional inorganic multiferroics like BiFeO3 and BiMnO3, inorganic−organic hybrid frameworks become excellent candidates for multiferroics by combining metal ions inducing magnetic coupling with polar components resulting in ferroelectricities.24−28 A series of three-dimensional perovskite-type metal−formate frameworks have been discovered as inorganic−organic hybrid multiferroics.29−36 [(CH3)2NH2][M(HCOO)3], (M = Mn, Fe, Co, Ni, and Zn) reported by Jain et al. exhibits ferroelectric ordering and magnetic ordering within 160−185 K and 8−36 K, respectively, which are mainly attributed to the order− disorder transitions of hydrogen bonds.29,30 Wang and Gao and others have documented chiral [NH4][M(HCOO)3] (M = Mn, Fe, Co, Ni, and Zn) as a new class of metal−organic frameworks displaying coexistent magnetic and electric orderings, that is, paraelectric−ferroelectric phase transitions and spin-canted antiferromagnetic interactions within 191−254 K and 8−30 K, respectively.34−36 Another representative work is triethylmethylammonium tetrabromoferrate discovered as the first inorganic−organic hybrid multiferroic exhibiting strong

INTRODUCTION The ability to control and even switch the responses in multiple physical channels like electrical, magnetic, and optical usually provides coupling as magneto-electric, optoelectronic, and magneto-optic, and consequently lead to novel multifunctional properties.1−4 Multifunctional materials in which physical properties coexist in a single phase have attracted considerable attention, owing to their wide applications in multifunctional devices exploiting more than one task at the same time.5−8 However, in spite of persistent explorations in such materials, the conditions of coexistence and coupling mechanisms are still confused. Generally, the physical responses mentioned above always display abrupt changes in the vicinity of the phase transition, affording phase-transition materials potentials to be used in data storage, signal processing, and switchable dielectric devices.9−11 Ferroelectrics, a type of well-known multifunctional material, usually undergo paraelectric−ferroelectric phase transitions accompanied by notable dielectric anomalies, nonlinear optical (NLO) responses, and reversible polarizations.12−20 The coexistence of ferroelectricity and magnetism in a single phase is difficult to be realized in view of the contradictory natures of them, and therefore multiferroics with scientific and technical interest are so rare.21−23 Specifically, ferroelectrics require transition-metal ions having empty d shells, while magnetic materials should have d shells partially © XXXX American Chemical Society

Received: August 15, 2014 Revised: September 30, 2014

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reflections with I > 2σ(I). All H atoms were generated by geometrical considerations and placed at their idealized positions with Uiso(H) = 1.5Ueq(C). The asymmetric units and the packing views were drawn with DIAMOND (Brandenburg and Putz, 2005). Distances and angles were calculated by DIAMOND and SHELXLTL. Crystallographic data and structure refinements for compounds 1 and 2 are given in Table 1.

magneto-dielectric coupling above room temperature by our group.37 One of the recent hot topics is switchable dielectric in which dielectric constant undergoes a transition between the distinct high and low dielectric states in the vicinity of the phase transition. As one class of the most attractive multifunctional materials, molecular switchable dielectrics afford potential applications in phase shifters, data communications, and so on.38−41 Researchers have made great efforts to synthesize and investigate switchable dielectrics and already achieved much progress.42−47 Facts have proven that introducing polar or flexible moieties easily undergoing order−disorder transitions into functional blocks probably results in not only switchable dielectric anomalies but also complex functional responses. Zhang et al. reported a well-designed tunable and switchable dielectric accompanied by an order−disorder phase transition below room temperature, [(CH3)2NH2]2[KCo(CN)6]. Different molecular dynamics of the polar dimethylammonium cations seem to be the main factor driving the phase transition and the switchable dielectric property.45 In this context, we present two multifunctional materials, tetramethylphosphonium tetrachloroferrate(III) (1) and tetramethylphosphonium tetrabromoferrate(III) (2), which undergo switchable anomalies in the dielectric, optical, or magnetic channels accompanied by the reversible sequential phase transitions. Compound 1 exhibits two dielectric phase transitions above room temperature and weak antiferromagnetic behaviors at low temperature. Except for the continuous dielectric phase transitions and stronger antiferromagnetic interactions, notably, compound 2 undergoes a magnetic phase transition above room temperature. Moreover, compounds 1 and 2 also display switchable SHG effects, confirming symmetry breakings during the phase transitions. In consideration of the steplike dielectric anomalies which could be tuned in three distinct dielectric states and switched by the sequential phase transitions, they have the potential to be used as excellent switchable dielectrics.

Table 1. Crystallographic Data for Compounds 1 and 2 (1) moiety formula crystal system, space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z μ (mm−1) Tmin/Tmax F(000) no. of measured, independent, and observed [I > 2(I)] reflections Rint refinement R[F2 > 2(F2)] wR(F2) GOF

(CH3)4P, FeCl4 hexagonal, P63mc 293 7.6677 (11) 7.6677(11) 12.514(3) 90.00(0) 90.00(0) 120.00(0) 637.2(3) 2 1.479 0.658/0.665 280 6208, 595, 556 0.0307 0.0277 0.0652 1.162

(2) (CH3)4P, FeBr4 cubic, F4̅3m 293 11.1330(12) 11.1330(12) 11.1330(12) 90.00(0) 90.00(0) 90.00(0) 1379.9(4) 4 2.207 0.078/0.098 836 1632, 156, 151 0.1729 0.0460 0.1212 1.243

Powder X-ray Diffractions. Powder X-ray diffraction (PXRD) data for compounds 1 and 2 were measured on a Rigaku D/MAX 2000 PC X-ray diffractometer at room temperature and high temperature. Diffraction patterns were collected in the 2θ range of 5°−52° with a step size of 0.02°. PXRD patterns obtained at 294 K match well with the calculated data based on the crystal structures, confirming the purity of as-grown crystals of compounds 1 and 2 (as shown in Figure S2, Supporting Information). DSC Measurements. The differential scanning calorimetry (DSC) analyses of compounds 1 (11.4 mg) and 2 (20.7 mg) were performed using a Perkin-Elmer Diamond DSC instrument in the temperature ranges of 325−450 and 330−410 K, respectively. The crystalline samples were placed in aluminum crucibles that were heated (cooled) at a rate of 5 K min−1 under flowing air at atmospheric pressure. Thermogravimetric (TG) analyses of compounds 1 and 2 were performed on the PerkinElmer Pyris 1 differential scanning calorimeter in the temperature range of 298−973 K with a heating rate of 20 K min−1. SHG Measurements. Powder SHG measurements of compounds 1 and 2 were carried out on FLS 920, Edinburgh Instruments, in the respective temperature ranges of 293−449 and 293−413 K, using an unexpanded laser beam with low divergence (pulsed Nd:YAG at a wavelength of 1064 nm). The laser is Vibrant 355 II, OPOTEK. Dielectric Measurements. The pressed-powder pellets deposited with silver conducting glue were used for the dielectric measurements with a heating/cooling rate of about 10

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EXPERIMENTAL SECTION Syntheses. Compound 1, [(CH3)4P][FeCl4], was synthesized by the reaction of tetramethylphosphonium chloride and iron trichloride in the molar ratio 1:1 at room temperature. Yellow block crystals were easily obtained by slow evaporation of the mixture aqueous solution. Compound 2, [(CH3)4P][FeBr4], was easily obtained as rufous block crystals by mixing equimolar tetramethylphosphonium chloride and iron bromide in aqueous solution containing excess hydrobromic acid. IR spectra (as shown in Figure S1, Supporting Information) certified the formations of compounds 1 and 2. Elemental analyses for C, H contents were carried out on the Heraeus CHN-O-Rapid elemental analyzer. Anal. Calcd for compound 1: C, 16.64%; H, 4.19%. Found: C, 16.70%; H, 4.54%. Anal. Calcd for compound 2: C, 10.30%; H, 2.59%. Found: C, 10.41%; H, 2.87%. Single-Crystal X-ray Diffractions. Single-crystal X-ray diffractions were performed on a Rigaku Saturn 924 diffractometer with Mo Kα radiation (λ = 0.71073 Å). The crystals with approximate dimensions of 0.20 × 0.20 × 0.20 mm were used in data collections of compounds 1 and 2 at 293 K. Data were processed by the Crystalclear software package (Rigaku, 2005). The structures of compounds 1 and 2 were solved by direct methods and then refined by the full-matrix least-squares refinements on F2 using the SHELXLTL software package. Non-H atoms were refined anisotropically based on all B

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K min−1. For compounds 1 and 2, the temperature dependencies of dielectric constants were measured on the Tonghui TH2828 analyzer in the temperature ranges of 303−458 and 310−420 K, respectively, within the frequency range of 5 kHz to 1 MHz. Magnetic Characterizations. For compounds 1 and 2, magnetic susceptibilities and field-dependent magnetizations were measured on a Quantum Design SQUID (MPMSXL-7) magnetometer within the temperature range of 2−400 K, using finely ground crystalline powders. Calculations of Rotational Potential Energies. HF/631(d) basis set was used for the calculations of the rotational potential energies for compounds 1 and 2. Both relative calculations were based on the fixed atomic coordinates obtained from the X-ray single-crystal structures at room temperature. For the rotations of P−C, Fe−Cl, and Fe−Br bonds, the corresponding ions and their six nearest-neighboring counterions were included in the calculation models, as depicted in Figure S3, Supporting Information. The rotational potential energies were calculated with the rotational angles changing in the range of 0°−360° with a step size of 30°.

ΔS are estimated to be 16.94 and 18.61 J mol−1 K−1, respectively. Although the anomalies in the heating and cooling modes are not consistent, ΔS based on the endothermic peak is almost equal to the sum of those based on the two exothermic peaks. This fact indicates that two reversible phase transitions occurred in compound 2 with the endothermic peaks merged, probably owing to the so close phase-transition temperatures. To further confirm the unusual thermal property of compound 2, the DSC curves obtained in the second heating−cooling cycle are given in Figure S4 (Supporting Information), in good agreement with those obtained in the first heating−cooling cycle. The sharp peaks usually suggest the first-order features. Moreover, the calculated N values on account of the cooling process are estimated to be 7.67 for the first phase transition around TC1(2) ≈ 353 K and 9.37 for the second phase transition around TC2(2) ≈ 359 K. Similar to those of compound 1, the three phases are also labeled as RTP, ITP, and HTP for convenience. The extremely large values of N suggest the two-step phase transitions as typical order−disorder ones, while compound 2 undergoes severer structural transformations around TC2(2) than that around TC1(2). Moreover, to detect the thermal stabilities, the TG curves as depicted in Figure S5 (Supporting Information) indicate that the deposition temperatures of compounds 1 and 2 are similar, that is, about 630 K. Crystal Structure Discussions. In spite of the analogue chemical formulas, several differences were observed between the crystal structures of compounds 1 and 2, reflected in the DSC results as indicated above. At RTP, compound 1 crystallizes in a hexagonal space group, P63mc (point group 6mm), which is noncentrosymmetric and polar, with cell parameters of a = b = 7.6677(11) Å, c = 12.514(3) Å, α = β = 90.00(0)°, γ = 120.00(0)°, Z = 2, V = 637.2(3) Å3. As a consequence of the replacement of chloride ligands by bromide ligands; however, the space group of compound 2 changes to a noncentrosymmetric cubic one, F43̅ m (point group 43̅ m), with cell parameters of a = b = c = 11.1330(12) Å, α = β = γ = 90.00(0)°, Z = 4, V = 1379.9(4) Å3. As given in Figure 2a, the

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RESULTS AND DISCUSSION Thermal Properties. It is well-known that DSC is one of the most useful methods to confirm phase transitions triggered by temperature. As shown in Figure 1a, compound 1,

Figure 1. DSC curves of compounds (a) 1 and (b) 2 in the heating− cooling cycles.

[(CH3)4P][FeCl4], displayed the first reversible phase transition around TC1(1) ≈ 362 K. Both sharp peaklike anomalies and a large thermal hysteresis of 16 K are indicative of a firstorder phase transition. Then the second phase transition appeared around TC2(1) ≈ 436 K, with a thermal hysteresis of 8 K. In addition, the entropy changes (ΔS) for the first and second phase transitions are estimated to be 36.2 and 0.60 J mol−1 K−1, respectively. According to the Boltzmann equation ΔS = R ln N, where R is the gas constant and N represents the ratio of possible configurations, here N is approximately calculated as 78.26 around TC1(1) and 1.08 around TC2(1). The large value of N suggests an order−disorder phase transition and extremely severe structural transformation. However, the second phase transition is more of a displacive type than of an order−disorder type. Conveniently, we labeled the phase below TC1(1) as the room-temperature phase (RTP), the phase between TC1(1) and TC2(1) as the intermediate temperature phase (ITP), and the phase above TC2(1) as the high-temperature phase (HTP). For compound 2, [(CH3)4P][FeBr4], upon heating, a single anomaly appeared at 377 K, with ΔS = 35.01 J mol−1 K−1, as displayed in Figure 1b. However, two continuous anomalies occurred at 353 and 359 K on cooling, while the corresponding

Figure 2. For compound 1: at RTP, (a) molecular structure showing the atom-labeling scheme with 50% probability thermal ellipsoids, and (b) packing view of the unit cell viewed along the c axis with hydrogen atoms omitted.

asymmetric unit of compound 1 consists of one-half [(CH3)4P]+ cation and one-half isolated [FeCl4]− anion. The [(CH3)4P]+ cation lies on a threefold rotation axis along the P1−C1 bond, while the [FeCl4]− anion lies on a sixfold rotation axis along the Fe1−Cl1 bond. For compound 2, the asymmetry unit is composed of a quarter of each [(CH3)4P]+ cation and [FeBr4]− anion, as displayed by Figure 3a. The [(CH3)4P]+ cation and the [FeBr4]− anion lie on threefold rotation axes along the P−C and Fe−Br bonds, respectively. The geometries of ions in both compounds 1 and 2 could be C

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respectively. Each Fe atom is surrounded by six P atoms and vice versa. The P···Fe distances are different, that is, 5.376, 5.466, 5.377, 5.466, 5.377, and 5.465 Å with an average value of 5.421 Å, while the corresponding P···Fe···P angles vary in the range of 89.08°−90.97°. Moreover, the closest P···P and Fe···Fe distances found in compound 1 are 7.665 and 6.257 Å, respectively. In the case of compound 2, in the unit cell, the positions (1/4, 3/4, 3/4), (3/4, 3/4, 1/4), (3/4, 1/4, 3/4), and (1/4, 1 /4, 1/4) are occupied by the Fe atoms, and the other positions as (1/4, 3/4, 1/4), (3/4, 3/4, 3/4), (3/4, 1/4, 1/4), and (1/4, 1/4, 3/4) are occupied by the P atoms. Distinct from its chloride analogue, the P and Fe centers in compound 2 are alternately arranged along the c-axis direction to form antiparallel arrays. Each Fe atom is also surrounded by six P atoms like that observed in compound 1. However, the P···Fe distances show an identical value of 5.567 Å, and the P···Fe···P angles are both 90.00°. Besides, in comparison with compound 1, the closest P···P and Fe···Fe distances of 7.872 Å in compound 2 are longer. Consequently, for compounds 1 and 2, the abovementioned notable dissimilarities in the P···Fe distances and P···Fe···P angles result in the different lattices and space groups and maybe further make impacts on their physical properties. Because of the difficulties of obtaining single-crystal structures at HTPs, variable-temperature PXRD measurements were performed on compounds 1 and 2 to further reveal the phase transitions. In the case of compound 1, as illustrated in Figure 5, upon heating, the PXRD pattern recorded at 353 K

Figure 3. For compound 2: at RTP, (a) molecular structure showing the atom-labeling scheme with 50% probability thermal ellipsoids, and (b) packing view of the unit cell viewed along the c axis with hydrogen atoms omitted.

described as tetrahedrons. But the difference between them is that the ions of compound 1 adopt slightly distorted tetrahedral coordination, but those of compound 2 adopt ideal geometries. Specifically, in the case of compound 1, the P−C bond distances and C−P−C bond angles vary in the respective ranges of 1.780−1.800 Å and 108.99−109.94°. Meanwhile, the Cl−Fe−Cl bond angles differ from 108.58° to 110.35°, and all Fe−Cl bond distances have an equal value of 2.191 Å. By contrast, in the case of compound 2, the P−C and Fe−Br bond lengths elongate to 1.830 and 2.336 Å, respectively, accompanied by the regular C−P−C and Br−Fe−Br bond angles of 109.50°. As shown in Figure 2b and 3b, each unit cell in compound 1 contains two [(CH3)4P][FeCl4] molecules, while that in compound 2 contains four [(CH3)4P][FeBr4] molecules. For compound 1, the [(CH3)4P]+ cations exhibit two orientations, which are almost parallel to the two [FeCl4]− anions. That is, viewed along the P···Fe direction, the angles between the corresponding P−C and Fe−Cl bonds range in 0.00°−0.42°. With regard to compound 2, only one kind of orientation is observed for either the [(CH3)4P]+ cations or the [FeBr4]− anions; in addition, the cationic and anionic tetrahedrons are completely parallel to each other. In view of the lack of hydrogen bonds and unusual short intermolecular contacts, the two compounds display zero-dimensional cation−anion packing structures. The simple schematic presentations of the positioning of the P and Fe centers in compounds 1 and 2 are illustrated in Figure 4, with all of the C, H, Cl, and Br atoms

Figure 5. Variable-temperature PXRD patterns of compounds (a) 1 and (b) 2 measured in the heating modes during 294−403 and 294− 453 K, respectively.

matches well with that recorded at 294 K, that is, RTP. Over the temperature range of 363−423 K, only five new diffraction peaks at 11.98°, 17.28°, 24.82°, 40.22°, and 45.38° were observed, corresponding to the ITP. Generally, the sharp decrease of the number of diffraction peaks suggests a transition from low symmetry to high symmetry. Above 423 K, the diffraction peak at 40.22° disappeared, and this may be related to a slight variation of the crystal structure, corresponding to the HTP. As expected, these results show obvious phase transitions agreeing well with the DSC measurement. For compound 2, variable-temperature PXRD patterns depicted in Figure 5 reveal clear phase transitions within the temperature range of 294−403 K, in good agreement with the DSC results. Upon heating, all of the present diffraction peaks at 353 K were completely consistent with those found at RTP. At 363 K, those diffraction peaks were still observed, but three new diffraction peaks appeared at 10.62°, 12.20°, and 17.40°, suggesting coexistence of the RTP and ITP. Above 373 K, in addition to the three diffraction peaks mentioned above, only new diffraction peaks at 21.24°, 24.64°, and 39.54° were observed, corresponding to the HTP. In addition, for

Figure 4. Schematic presentations of the positioning of the cations and anions in compounds (a) 1 and (b) 2, while all of the C, H, Cl, and Br atoms are omitted for clarity.

omitted for clarity, to discover clearly the packing structures in compounds 1 and 2. For compound 1, in the unit cell, the two P centers occupy the positions as (1/3, 2/3, 0.70539) and (2/3, 1 /3, 0.20539), while the Fe centers occupy (0, 0, 0.9616), (0, 0, 0.4616), (0, 1, 0.96158), (0, 1, 0.46158), (1, 1, 0.96158), (1, 1, 0.46158) and (1, 0, 0.96158), (1, 0, 0.46158). Along the c-axis direction, the P and Fe atoms are arrayed in parallel lines, D

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six nearest-neighboring anions or anion ([FeCl4]− or [FeBr4]−) and six nearest-neighboring [(CH3)4P]+ cations, for evaluating steric hindrances. It is worth mentioning that, for compound 2, the four P−C bonds or the four Fe−Br bonds are equivalent and located in the same environments, and thus the selection of any bond makes no difference in the calculation. By contrast, for compound 1, despite the slightly distinct geometries and environments of the P−C (or Fe−Cl) bonds, the selection of the bond makes very little influence on the calculation result (Figure S7, Supporting Information). The rotation angle (i.e., Φ) dependencies of potential energies for compounds 1 and 2, from 0° to 360° with 30° per step, are outlined in Figure 7 (the

compounds 1 and 2, upon heating and cooling, the PXRD patterns recorded at 294 K match well with each other, indicating the reversible phase transitions (Figure S2). SHG Characterizations. As a widely used technology for confirming symmetry breaking even polar phenomenon, SHG signal is very sensitive to the transition from a centrosymmetric phase to a noncentrosymmetric one because it exists only in materials lacking inversion symmetries. Explorations of NLO materials with striking SHG responses afford not only understanding of the relationships between structures and properties but also applications in highly technical fields like laser systems. Consequently, for compounds 1 and 2, the temperature-dependent SHG effects during the heating processes are plotted in Figure 6, to investigate the details of

Figure 7. Potential energy curves for the rotations of P−C bond (black line) and Fe−Cl bond (blue line) in compound 1 and P−C bond (red line), and Fe−Br bond (orange line) in compound 2.

Figure 6. Temperature dependencies of SHG effects of compounds 1 and 2 in the heating modes.

detailed ΔE−Φ curves for the rotations of P−C bonds are shown in Figure S7, Supporting Information). At Φ = 0°, the first potential energy minimums defined as zero coincided with the initial atomic coordinates based on the X-ray crystal structural analyses at RTP. For both rotations of P−C and Fe− Cl bonds in compound 1, the second and third potential energy minima appeared at Φ = 120° and 240°, respectively. The ΔE−Φ curve of the rotation of P−C bond exhibited a nonsymmetrical feature; that is, the potential energy barriers were observed at Φ = 60°, 180°, and 300°, with ΔE = 168.16, 143.60, and 168.29 kJ mol−1, respectively. In the case of the rotation of Fe−Cl bond, with Φ changing in the range of 0°− 360°, ΔE displayed three almost symmetrical peaks with values of 878.21, 874.80, and 877.76 kJ mol−1 at Φ = 60°, 180°, and 300°, respectively. On the other hand, with regard to the rotations of P−C and Fe−Br bonds in compound 2, the corresponding second and third minima of ΔE were also observed at Φ = 120° and 240°. The nonsymmetrical ΔE−Φ curves indicated that the potential energy barriers appeared at Φ = 60°, 180°, and 300°, with ΔE of 156.21, 154.30, and 154.90 kJ mol−1 for the rotation of the P−C bond, and ca. 617.60 kJ mol−1 for the rotation of Fe−Br bond. Given all that, the relative small magnitudes of ΔE barriers less than 200 kJ mol−1 mean that the rotations of the P−C bonds in both compounds 1 and 2 are easily to be realized at HTPs.48 In contrast, for the rotations of the respective Fe−Cl bond and Fe−Br bond in compounds 1 and 2, the high potential energy barriers result in the fact that such rotations are almost impossible above room temperature. Consequently, for compounds 1 and 2, the rotations of the [(CH3)4P]+ cations should be frozen at RTPs and then be easily activated, triggered by the increasing temperature. That is to say, different from the anions, the rotations of the [(CH3)4P]+ cations are probably related to the phase transitions.

the phase transitions and further evaluate their potential as NLO materials. At RTP, compound 1 displayed a SHG efficiency which is 2.5 times that of potassium dihydrogen phosphate (KDP), and this strong SHG response suggests a noncentrosymmetric phase. In the vicinity of TC1(1), the fact that SHG efficiency is approximately zero indicates symmetry breaking; that is, the ITP belongs to a space group with centrosymmetry. Then upon heating up to 450 K, the SHG efficiency still remained at zero, meaning the HTP is centrosymmetric, too. In the case of compound 2, the SHG efficiency at RTP is almost equivalent to that of KDP. Around 382 K (above TC2(2)), the SHG efficiency decreased sharply down to about zero and then remained unchanged until 410 K. This phenomenon is indicative of symmetry breaking accompanied by the appearance of centrosymmetry. That is to say, RTP and ITP are noncentrosymmetric, while HTP is centrosymmetric. For compounds 1 and 2, the variable temperature can switch the active or nonactive states, and the consequent steplike changes of SHG effects are characteristics of first-order phase transitions. Moreover, the SHG curves recorded in the cooling modes correspond well with those obtained in the heating processes, while the thermal hysteresis is in good agreement with the DSC results of compounds 1 and 2 (Figure S6, Supporting Information). In brief, these results of SHG measurements are well coincident with the single-crystal structures, DSC and PXRD measurements, revealing compounds 1 and 2 as interesting switchable NLO materials. Discussions about Phase Transition Mechanisms. To confirm whether the rotations of the cations or the anions make significant contributions to the phase transitions, the potential energies (i.e., ΔE) for the uniaxial rotations of the P−C, Fe−Cl bonds in compound 1 and P−C, Fe−Br bonds in compound 2 were evaluated on the RHF/6-31(d) basis set. All of the four calculation units are composed of either [(CH3)4P]+ cation and E

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cooling modes match well with those of the heating processes, confirming the reversible phase transitions. Importantly, the dielectric responses which could be tuned in three dielectric states and switched by the stepwise phase transitions as well as the large changes between the corresponding high and low dielectric states afford the two compounds potentials to be excellent candidates for molecular switchable dielectrics. Magnetic Properties. Because of the layered crystal structures containing flexible organic cation, [(CH3)4P]+, and paramagnetic inorganic anions, [FeCl4]− or [FeBr4]−, compounds 1 and 2 are expected to display interesting magnetic responses except for the thermal, optical, and dielectric responses. Therefore, the temperature-dependent magnetic susceptibilities were measured under an applied direct-current field of 1000 Oe to investigate their magnetic properties. However, in the temperature range of 2−400 K, no magnetic phase transition was observed in compound 1, as shown in Figure 9a. The χmT at RTP with a value of 4.42 cm3 K mol−1 is

On the other hand, for compounds 1 and 2, the possibilities of displacement-type phase transitions also should be considered. In the unit cell, the positions of cations and anions are presumed to be located at the respective P and Fe centers. On the basis of the above-mentioned atomic coordinates of P and Fe centers, the centers of the positive and negative ions are located at (0.5, 0.5, 0.45539) and (0.5, 0.5, 0.71158) for compound 1, (0.5, 0.5, 0.5) and (0.5, 0.5, 0.5) for compound 2. Therefore, at RTP, there is a remnant dipole moment in compound 1, but no remnant dipole moment in compound 2.49 However, in view of the SHG results, both of the HTPs of the two compounds belong to centrosymmetric space groups in which no remnant dipole moment will be observed. Generally speaking, because of the variation of the remnant dipole moment in compound 1, the relative motion of positive and negative ions should be indispensable for the phase transition (namely, a displacement type). In contrast, despite the absence of remnant dipole moment at both RTP and HTP, the displacement type should also be included in the potential phase transition mechanisms of compound 2 owing to the obvious symmetry breaking. Anyhow, the final conclusions of the phase transition mechanisms for compounds 1 and 2 are in progress. Dielectric Properties. Variety of physical properties usually displays abrupt changes in the vicinity of the phase transition, and the variable magnitude is correlated to the characteristic feature of the phase transition. Thus, temperature dependencies of the dielectric constants of compounds 1 and 2 were measured in the heating−cooling cycles to detect dielectric responses accompanied by the phase transitions. For compound 1, as illustrated in Figure 8, the real part of

Figure 9. (a) Temperature dependencies of χmT for compounds 1 and 2. Inset: detailed plot of temperature-dependent χmT below 11 K for compound 1. (b) For compound 2, temperature dependence of χmT measured in a heating−cooling cycle during 340−390 K.

slightly larger than the calculated value of 4.38 cm3 K mol−1 (for a spin-only high-spin Fe3+ ion, S = 2.5, g = 2), probably resulting from the spin−orbit coupling interactions. Upon cooling, χmT decreased smoothly until approaching 40 K and then decreased sharply down to 1.44 at 4.9 K, suggesting antiferromagnetic interactions. It is noteworthy that below 4.9 K, χmT first increased to a maximum value of 1.98 and then decreased to 1.49 at 2 K, as depicted in the inset of Figure 9a. This small anomaly usually suggests the spin-canted feature of the antiferromagnetic behavior. What is more, the magnetic susceptibility was fitted by the Curie−Weiss law with χm−1 = (T−θ)/C in the temperature range of 2−300 K (Figure S8, Supporting Information). The calculated Curie constant C and Weiss constant θ are 4.60 cm3 K mol−1 and −7.77 K, respectively. The negative value of θ, the linear field-dependent magnetization at 2 K, and the bifurcate field cooling/zero-field cooling (FC/ZFC) traces (Figure S9 and S10, Supporting Information) further confirm the existence of weak antiferromagnetic interactions between the adjacent Fe3+ ions. For compound 2, during 2−300 K, the temperaturedependent magnetic susceptibility also displays weak antiferromagnetic interactions at low temperature. At RTP, χmT is approximately 4.18 cm3 K mol−1, slightly lower than the calculated value, indicating that there is no contribution of spin−orbit coupling. Below 40 K, χmT decreased more rapidly until it reached 0.18 cm3 K mol−1 at 2 K. The calculated C and θ are 4.93 cm3 K mol−1 and −50.63 K, respectively, confirming the antiferromagnetic interactions which are stronger than those of compound 1 (Figure S8, Supporting Information).

Figure 8. Temperature-dependent dielectric constants of compounds (a) 1 and (b) 2 measured in the heating−cooling cycles at 1 MHz. Inset: detailed plot of ε′ versus temperature around TC1(2).

dielectric constant remained at about 10 at RTP and then exhibited a prominent increase with an abrupt slope around TC1(1). The dielectric constant at ITP (corresponding to a high dielectric state) is almost triple that at RTP (corresponding to a low dielectric state). Subsequently, the dielectric constant increased very slowly until approaching TC2(1) and then displayed the second steplike anomaly around TC2(1), while the dielectric constant at HTP is 1.5 times larger than that at ITP. For compound 2, upon heating to TC1(2), the dielectric constant at ITP is approximately 1.5 times that at RTP, corresponding to the high and low dielectric states, respectively. Then the dielectric constant displayed a gradual increase until approaching TC2(2) and increased sharply from 15 at ITP to 70 at HTP around TC2(2). The dielectric constant in the high dielectric state (HTP) is almost 4.7 times that in the low dielectric state (ITP), displaying a notable steplike change. Moreover, for compounds 1 and 2, the curves obtained in the F

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Moreover, the linear curve of field-dependent magnetization measured at 2 K and the bifurcation of the FC/ZFC traces around 4.5 K further confirm antiferromagnetic interactions (Figure S9 and S10, Supporting Information). Upon heating and cooling during 340−390 K, different from its analogue, the temperature-dependent χmT of compound 2 exhibits magnetic bistability with a thermal hysteresis of about 6 K, as shown in Figure 9b. During the cooling process, χmT remained stable at 4.47 above 368 K and then decreased sharply to 4.27 around 368 K, affording a steplike decrease. Upon heating, a similar anomaly appeared around 374 K. In view of the relatively small anomalies, the mechanism of this switchable magnetic phase transition may be attributed to orbital quenching of the angular momentum more than the change of spin state. Generally speaking, the increase of the distance between the magnetic centers will result in diamagnetic dilution and thus weaker magnetic interaction. However, in the cases of compounds 1 and 2, despite the elongated Fe···Fe distances, the antiferromagnetic interactions in [(CH3)4P][FeBr4] (2) are stronger than those in [(CH3)4P][FeCl4] (1). Such phenomena have also been observed in the other tetrahalogenferrates(III) since the substitution of the chloride ligand by the bromide ligand has more influences on the antiferromagnetic interactions than the Fe···Fe distance.50−52 The results mentioned above reveal the two title compounds as potential molecular magnetic materials, especially for compound 2 exhibiting rare coupled dielectric and magnetic phase transitions above room temperature.

Cambridge CB2 1EZ, U.K. Fax: (+44) 1223-336-033. E-mail: [email protected].

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*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Project 973 (2014CB848800), National Natural Science Foundation of China (21471032 and 21422101). We also thank Dr. Cai-Ming Liu from Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences, for help in magnetic characterizations.

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REFERENCES

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CONCLUSION In conclusion, a novel class of inorganic−organic hybrid complexes, [(CH3)4P][FeX4] (X = Cl, Br), has been obtained and characterized as multifunctional materials which exhibits simultaneously switchable responses in dielectric, magnetic, and optical channels. They undergo sequential reversible phase transitions above room temperature, accompanied by the steplike dielectric anomalies as well as switchable SHG effects. Calculations of the rotational potential energies demonstrate that the phase transitions found in the two compounds may be relevant to the rotations of the [(CH3)4P]+ cations. Notably, for both title compounds, the dielectric constants could be switched by the continuous phase transitions and tuned in three distinct dielectric states, indicating potential switchable dielectrics. Furthermore, they also exhibit weak antiferromagnetic interactions between neighboring Fe3+ ions at low temperature. However, the exchange of chloride ligands for bromide ligands results in not only increased antiferromagnetic interactions but also an obvious magnetic phase transition above room temperature. It may open a new avenue for the design of multifunctional materials with coexistent switchable multiple functionalities.

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AUTHOR INFORMATION

Corresponding Authors

ASSOCIATED CONTENT

* Supporting Information S

For compounds 1 and 2, IR spectra and PXRD patterns measured at room temperature, calculation units of the rotational potential energies, and magnetic measurements. This material is available free of charge via the Internet at http://pubs.acs.org/. Crystallographic data are available from the Cambridge Crystallographic Data Centre, with CCDC No. 981191, for compound 1, and No. 997142 for compound 2. Copies of these data can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, G

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