A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings
Yun Li, Shu‐Qi Wu, Jin‐Peng Xue, Xiao‐Lei Wang, Osamu Sato, Zi‐Shuo Yao, Jun Tao
Abstract
Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings Yun Li†, Shu-Qi Wu†, Jin-Peng Xue†, Xiao-Lei Wang, Osamu Sato, Zi-Shuo Yao and Jun Tao Yun Li† Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081 , Shu-Qi Wu† Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395 , Jin-Peng Xue† Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081 , Xiao-Lei Wang Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081 , Osamu Sato Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395 , Zi-Shuo Yao *Corresponding author: E-mail Address: [email protected] Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081 and Jun Tao Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081 https://doi.org/10.31635/ccschem.020.202000489 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) complex that exhibits unprecedented two-step magnetic switching accompanied by paraelectric–ferroelectric–paraelectric phase transition. The ferroelectricity of the material is governed by changes in the directionality of the sulfate dianions therein that trigger nonpolar–polar–nonpolar variation of the crystal symmetry and induce slight structural changes in the Co(II) complex. The unquenched orbital angular momentum of the Co(II) ion, which has trigonal antiprismatic coordination geometry, is susceptible to the coordination environment. Accordingly, two-step magnetic switching accompanied by ferroelectric phase transitions is demonstrated, and the detailed mechanism of the paraelectric–ferroelectric–paraelectric phase transitions and the consequent magnetic switching are investigated. Thus, this study presents a unique multifunctional material as well as a viable strategy for the development of superior molecular magnetoelectric materials. Download figure Download PowerPoint Introduction The development of molecular materials that exhibit correlated magnetic and electric properties has attracted significant research attention owing to their potential for application in advanced information processing and storage devices as well as their fundamental utility in research concerning the relationship between magnetic and electric properties.1–10 In the past decade, numerous molecular materials that demonstrate remarkable ferroelectric performance or switchable magnetic properties have been developed.11–24 However, the correlated switching of magnetic and ferroelectric properties in a single-phase molecular material remains to be demonstrated. One promising strategy to achieve this aim involves manipulating the spin–orbit coupling of the magnetic centers in a material to enable ferroelectric switching.25–27 However, the significant structural changes typically required to tune orbital angular momentums obstruct the reverse of spontaneous polarization by an external electric field. Consequently, the development of molecular materials that exhibit correlated magnetic switching and ferroelectric properties is highly desired.28–34 Our previous study on single-crystal [CoII(en)3](ox) (en = ethylenediamine; ox2− = oxalate dianions) revealed that the unquenched orbital angular momentum of [CoII(en)3]2+ cations is highly susceptible to minor changes in coordination configuration.35 In this compound, the order–disorder phase transition of ox2− dianions in cavities encapsulated by five complex cations is reminiscent of the perovskite compounds that exhibit ferroelectric properties, which are derived from the polarization of dynamic molecular cations in their frameworks.36,37 For example, several ferroelectric metal-free perovskite materials that feature ordered–disordered motion of organic cations in cavities constructed by NH4+ and halogen anions have been synthesized.38 Accordingly, we have investigated the concept of imparting ferroelectricity to [CoII(en)3](ox) by replacing the ox2− anions with polarizable anions. We envisaged that the polarization of dynamic counter anions in the cavity would impart ferroelectricity and modulate the coordination configuration of the [CoII(en)3]2+ cations, enabling correlated magnetic switching. Accordingly, we herein present the molecular ferroelectric compound [CoII(en)3]SO4 ( 1), which exhibits two-step magnetic switching accompanied by paraelectric–ferroelectric–paraelectric phase transitions. The ferroelectricity of this material is dependent upon the displacive motion of sulfate dianions upon order–disorder structural transition. The motion of the sulfate dianions not only modifies the polarization of the crystal, but also induces a minute structural change in the coordination geometry of the [CoII(en)3]2+ cations through intermolecular interactions. Accordingly, two-step switching is observed in the magnetic susceptibility of the single-crystal form of the material. Detailed structural analyses and theoretical calculations revealed that the paraelectric–ferroelectric–paraelectric phase transition of 1 is due to competing hydrogen-bond and dipole–dipole interactions between the complex cations and sulfate anions. Furthermore, we demonstrate that the accompanying magnetic switching in the single-crystal form of the material is due to modulation of the unquenched orbital angular momentum of high-spin Co(II) ions allowed by minute molecular structural variations. Experimental Methods Synthesis of [CoII(en)3] (SO4) ( 1) Stoichiometric ethanediamine (en 1 mL) was added to an aqueous solution containing CoSO4 (5 mmol, 15 mL). The solution was sealed in a Teflon-lined stainless-steel autoclave and heated at 433 K for 2 days, after which the autoclave was cooled slowly with a programed cooling rate at 0.5 K h−1 ( Supporting Information Figure S1). Crystals of 1 were obtained in 50% yield based on CoSO4 ·7H2O. Elemental analysis: Calcd for [CoII(en)3]SO4: C, 21.49%; H, 7.21%; N, 25.06%. Found: C, 21.34%; H, 6.98%; N, 25.00%. The obtained crystals, whose phase purity was confirmed by powder X-ray diffraction analysis, were stocked in a glovebox to avoid air oxidation (see Supporting Information Figure S2). Magnetic measurements Magnetic measurements were performed using a Quantum Design Superconducting Quantum Interference Device (MPMS-5) magnetometer. The temperature dependence of direct current (DC) magnetic susceptibility was measured for both finely ground microcrystalline powders and single crystals. A rod-shaped single crystal was restrained with Apiezon oil with the long axis parallel or perpendicular to the external field. Alternating current (AC) and M–H measurements were performed on finely ground microcrystalline powders. Data were corrected for the diamagnetic contribution calculated using Pascal's constants. Computational methods The complex cations were adopted from single-crystal X-ray diffraction structures without optimization. Ab initio calculations were performed by the ORCA 4.0.0 package with the complete active space self-consistent field and N-electron valence state perturbation theory approach.39,40 The spin–orbit coupling was calculated with a mean-field spin–orbit coupling operator. Mixing of configuration interaction eigenfunctions and magnetic parameters was calculated by quasi-degenerate perturbation theory. The all-electron def2-TZVPP Ahlrichs basis sets (redefined by the Karlsruhe group) are used for all the atoms. The active space comprises seven electrons of Co(II) ion on the five d orbitals [CAS(7,5)]. All the 10 quartets and 40 doublets have been considered by state-average complete active space self-consistent field, and further corrected by N-electron valence perturbation theory. The ligand-field orbital energies and ligand-field parameters were extracted from the ab initio ligand-field theory (AILFT) module implemented in the ORCA package based on CAS(7,5) calculations for all the model systems. Simulation of magnetic susceptibility was performed based on the effective spin–orbit Hamiltonian using PHI code.41 Detailed experimental information is presented in the Supporting Information. Results and Discussion Crystal structures at different temperatures Single-crystal X-ray diffraction analyses revealed that complex 1 crystallizes in a trigonal space group P 3 ¯ 1 c at 185 K (see Supporting Information Table S1).42 In the crystal, the Co(II) metal center is coordinated by six N atoms from three en ligands with distorted trigonal antiprismatic coordination geometry. The sulfate dianion, which serves as a counter ion, is rotationally disordered in a cavity surrounded by five molecular cations (Figure 1a). The cations and anions connected by N−H···O hydrogen bonds are alternatively arranged along the crystallographic c-axis, forming one-dimensional (1D) columnar structures that are further linked to form a three-dimensional (3D) structure (see Supporting Information Figure S3). The Co(II) and sulfur atoms are positioned on the C3 axes of the crystal. Because the three crystallographic C2 axes pass through the Co(II) centers, the complex cations adopt an ideal D3 molecular symmetry at 185 K. Notably, the Co−N distance is 2.167 Å for this high-temperature phase (HTp), suggesting a high-spin state for Co(II). Figure 1 | Single-crystal structures of 1 in different phases. (a) In the HTp (185 K), the sulfate dianions are rotationally disordered over six positions. (b) In the ITp (160 K), the sulfate dianions become partially ordered and orientate in the same direction with one S−O bond coinciding with the C3 axis of crystal. The green arrows denote the relative displacement of dianions during phase transitions from the central positions (black points) of two adjacent complex cations along the crystallographic c-axis. The spontaneous polarization of the crystal is indicated by the black arrow in the left bottom of the figure. (c) In the LTp (140 K), the thermodynamic motion of sulfate anions is further restricted, and the opposite orientation between adjacent 1D columns cancels out the spontaneous polarization of single crystal in the ITp. Co, orange; N, blue; O, red; C, gray. The thermal ellipsoids of the O atoms are drawn at a 50% probability level, and the hydrogen atoms are omitted for clarity. Download figure Download PowerPoint Differential scanning calorimetry (DSC) analysis was performed on the crystal of 1 to investigate its phase transitions. As shown in Figure 2a, two distinct exothermic peaks at 169 and 148 K are observed upon cooling from room temperature to 100 K with corresponding enthalpy changes of 0.58 and 0.27 kJ mol−1, respectively. The corresponding sharp endothermic peaks along with distinct thermal hysteresis loops (ca. 7 K for both the two-phase transitions) upon heating indicate that the compound undergoes two reversible phase transitions with first-order characteristics. Figure 2 | The paraelectric–ferroelectric–paraelectric phase transitions of 1. (a) The DSC curve of 1. (b) The temperature-dependent SHG signal. (c) The temperature-dependent dielectric constant of 1 along the crystallographic c-axis. (d) The ferroelectric hysteresis loops measured for different phases. The P-E hysteresis loop is only observed for the ITp. Download figure Download PowerPoint To investigate the structural transformations that occur during phase transition, we analyzed the single-crystal structures for each phase and found that the phase transitions are primarily actuated by motion of the sulfate dianions. As shown in Figure 1b, the sulfate dianions, which are heavily disordered in the HTp, adopt an ordered state in the intermediate-temperature phase (ITp) with one S−O bond lying on the crystallographic C3 axis. The directional change of the sulfate dianions leads to a relative displacement of the cations and anions. As shown in Figures 1a and 1b, the sulfate anion, which is located at a central position between two neighboring complex cations along the crystallographic c-axis for the HTp (dCo−S = 4.771 Å), shifts to one side and forms a shorter dCo−S (4.674 Å) and a longer dCo−S (4.928 Å). The unidirectional displacement of the sulfate dianions disrupts the central symmetry of the crystal structure and leads to spontaneous polarization along the crystallographic c-axis (Figure 1b). Correspondingly, the space group of crystal 1 changes to a polar trigonal space group P31c in the ITp. During this phase transition, the symmetry elements of 1 reduce from 12 (E, 3C2, 2C3, 2S6, i, 3σv) in the HTp to 6 (E, 2C3, 3σv) in the ITp (see Supporting Information Figure S4), which is in accordance with a ferroelectric phase transition with an Aizu notation of 3 ¯ mF3m.43 Upon further cooling of the sample to below 148 K, the crystal of 1 undergoes a second structural variation. For the low-temperature phase (LTp), the thermodynamic motion of the sulfate anions is further restricted, as evidenced by the small thermal ellipsoids for the O atoms. Furthermore, during this phase transition, half the sulfate anions perform a 180° rotation along the crystallographic c-axis. As shown in Figure 1c, sulfate anions in adjacent columns point in opposite directions in the LTp, cancelling out the spontaneous polarization of the single crystal. Consequently, the crystal symmetry of 1 changes to a nonpolar space group P 3 ¯ . Notably, the crystal structures for both the ITp and LTp were refined with a ca. 50% twin component. Paraelectric–ferroelectric–paraelectric properties The variations in crystal symmetry during phase transitions were verified by variable-temperature second-harmonic generation (SHG) measurements. As shown in Figure 2b, the temperature-dependent SHG signal, which is zero for the HTp, presents a significant increase in the vicinity of the HTp transition point upon cooling, and then decreases abruptly upon further cooling to zero at ca. 148 K. The nonzero intensity of the SHG in the ITp unambiguously confirms the nonpolar–polar–nonpolar phase transitions and thus evidences the ferroelectricity of 1 in the ITp.44,45 To investigate the electric property of 1 upon phase transition, we first measured the variable-temperature dielectric constant (ɛ′) of its single crystal. As shown in Figure 2c, the value of ɛ′ along the crystallographic c-axis presents significant anomalies in the vicinity of the phase-transition points. Upon cooling, ɛ′ increases abruptly at ca. 164 K, then, after a slow decrease in a narrow-temperature range, falls off abruptly at ca. 148 K. Therefore, a dielectric plateau is observed for the ITp. The abrupt changes in ɛ′ and the intensity of the SHG are consistent with the first-order characteristics of the phase transitions identified by DSC analysis. The ɛ′ value of the single crystal is highly anisotropic. As shown in Supporting Information Figure S5, the ɛ′ value along the direction perpendicular to the long axis changes very little at the phase-transition points. The anisotropic ɛ′ value is consistent with the spontaneous polarization of 1 emerging in the crystallographic c-axis. The ferroelectric hysteresis loop for the single crystal was measured along the crystallographic c-axis for different phases to provide direct evidence of the ferroelectricity of 1. As shown in Figures 2d and Supporting Information Figure S6, P-E hysteresis loops for the single crystal are only observed in the ITp, confirming the atypical paraelectric–ferroelectric–paraelectric phase transitions of 1. The spontaneous polarization (Ps) is ca. 0.7 μC cm−2 at ca. 152 K, which is close to the value of 1.2 μC cm−2 calculated using the point-charge method (see Supporting Information Table S2). The Ps value of 0.7 μC cm−2 for 1 is lower than that of triglycine sulfate (Ps = 2.3 μC cm−2), the ferroelectricity of which originates from the concurrent displacements of the sulfate dianions and the terminal ammonium groups of the triglycine cations,46 but is comparable with that of 4-(cyanomethyl)anilinium perchlorate (Ps = 0.75 μC cm−2), the spontaneous polarization of which can be attributed to the deviation of the perchlorate anion from its central position.47 The paraelectric–ferroelectric–paraelectric phase transitions of 1 are also supported by a pyroelectric current measurement. As shown in Supporting Information Figure S7, two sharp current peaks with opposite charges are detected along the crystallographic c-axis at the phase-transition points upon cooling. The peak at 168 K is due to the paraelectric-to-ferroelectric symmetry breaking of the crystal upon the HTp transition, while the peak at 146 K is attributed to the ferroelectric-to-paraelectric structural change involved in the phase transition in the low-temperature range. Magnetic switching involved in the ferroelectric phase transitions The rotation of the sulfate dianions not only modifies the symmetry of the single crystal, but also actuates minute structural changes in the complex cation through hydrogen-bond interactions (see Supporting Information Figure S8). As shown in Supporting Information Figure S9, the six Co−N coordination bonds, which are equivalent in the HTp with a bond length of 2.167 Å, are divided into two groups in the ITp with coordination bond length of 2.170 and 2.165 Å. Correspondingly, the molecular symmetry of the complex cation changes from the D3 point group at the HTp to the C3 point group at the ITp. At the LTp, the bond lengths are further slightly changed to 2.167 and 2.183 Å. Because the orbital angular momentum of high-spin Co(II) coordinated in a distorted trigonal antiprismatic coordination geometry is sensitive to this structural variation, the magnetic property of the single crystal is potentially switched by the minute structural change in the complex cations upon the ferroelectric structural change of the single crystal.35,48–50 Magnetic susceptibility was measured on both the single crystal and powder samples of 1 to examine the potential magnetic switching during phase transitions. As shown in Figure 3 and Supporting Information Figure S10, the magnetic susceptibility of 1 is highly anisotropic, where the χMT value of single-crystal perpendicular to the crystal long axis (χMTperp) is much larger than that parallel to the single-crystal long axis (χMTpara). Because the principal z-axis of the g tensor (gzz) coincides with the molecular C3 axis parallel to the crystal long axis, the lower value of χMTpara indicates an easy-plane magnetic anisotropy of the complex cations.35 At the phase-transition points, two stepwise magnetic anomalies are observed in the χMTpara curve. As shown in Figure 3, the χMTpara, which shows an abrupt decrease at ca. 171 K upon cooling, increases at ca. 151 K upon further cooling. The two-step magnetic switching is consistent with the paraelectric–ferroelectric–paraelectric phase transitions of 1. It is worth mentioning that the changes in χMTpara are accompanied by ca. 4 K thermal hysteresis loops for each phase, which can be attributed to a cooperative effect deriving from extensive hydrogen-bond interactions between the complex cations and anions. In contrast to the substantial changes found in χMTpara, no significant variation was observed for χMTperp. Consequently, a slight change is observed in the χMT curve of the power sample as a result of the average effect (see Supporting Information Figure S11). As significant magnetic coupling between metal centers is precluded (dCo−Co > 8.592 Å) and no spin crossover occurs during the phase transitions, the variation of χMTpara should be attributed to the adjustment in spin–orbit coupling corresponding to the minute structural change of the molecular cations. Figure 3 | The temperature dependence of the anisotropic χMT of 1. Two-step magnetic switchings of the single crystal with thermal hysteresis loops were detected at the ferroelectric phase-transition points when the magnetic field was applied parallel to the crystal long axis. Download figure Download PowerPoint Mechanism discussion To investigate the atypical paraelectric–ferroelectric–paraelectric of 1, we carefully compared its crystal structures at the ITp and LTp. As shown in Figures 1b and 1c, the frozen sulfate anions in the LTp undergo a further displacive motion where the distance between the Co(II) ions and the S atoms along the crystallographic c-axis changes from 4.674 and 4.928 Å at the ITp to 4.609 and 5.055 Å at the LTp. The larger deviation of the sulfate dianion from the central position at the LTp increases the local dipole moment, leading to an enhancement of the dipole–dipole interactions between adjacent columns. Therefore, the spontaneous polarization, which should be stabilized by the strong hydrogen-bond interactions in the ITp (see Supporting Information Figure S8), is cancelled out in the LTp. The competition between the strong hydrogen-bond interactions and the dipole–dipole interactions between adjacent columns accounts for the atypical paraelectric–ferroelectric–paraelectric properties of 1. Notably, although paraelectric–ferroelectric–paraelectric phase transitions have been observed for several molecular ferroelectrics, the distinct structural explanation offered here is novel.51–53 The ferroelectric structural changes of the single crystal are associated with slight variations in the coordination configuration of the Co(II) complex cations through hydrogen-bond interactions. To gain further insight into the influence of these minute structural variations on the magnetic properties of the crystal, we performed the state-of-the-art ab initio calculations using the CASSCF/NEVPT2 approach.40 As shown in Figures 4a and 4b and Supporting Information Figure S9, the angle between the Co−N coordination bonds and molecular C3 axis (θ) shifts from 57.20° for the HTp to 56.20° and 58.59° for the ITp, and then changes to 55.92° and 58.91° for the LTp. These values are higher than that of an ideal octahedral structure (54.74°), confirming that the complex cation has a slightly compressed trigonal antiprismatic coordination environment. Consequently, the ground orbital triplet (4T2g) in the ideal octahedral ligand field is further split into an orbital singlet, 4A1g, and an orbital doublet, 4Eg. Due to the presence of the t2g-eg hybridization effect, the 4A1g term is stabilized as the ground state. As shown in Figure 4c, the triply degenerate t2g orbitals in the ideal octahedron are split into doubly degenerate eg′ orbitals and an a1g orbital in complex 1. The energy difference between the eg′ orbitals and the a1g orbital dominates the contribution of orbital angular momentum to the magnetic susceptibility.48 According to the calculation results, the minute structural change in the complex cation raises the energy difference between the eg′ orbitals and the a1g orbital from 13 cm−1 for the HTp to 43 cm−1 for the ITp (Figure 4c and Supporting Information Figure S12). Consequently, the energy splitting between the ground state (4A1g) and the first electronic excited state (4Eg) increases from 431 cm−1 in the HTp to 533 cm−1 in the ITp. The larger energy splitting observed for the ITp increases the axial ligand-field splitting parameter ( υ ), resulting in a lower orbital angular momentum contributing to the magnetic susceptibility of 1 in the ITp (see Supporting Information Table S3). Therefore, a decrease in the χMTpara is observed during the paraelectric-to-ferroelectric phase transition upon cooling. The temperature-dependent anisotropic magnetic susceptibilities of compound 1 are well reproduced by the theoretical calculations. As shown in Supporting Information Figures S13 and S14, a significant change in the χMTpara is observed for the results of ab initio calculations based on the molecular structures for the ITp and HTp. The variation trends of χMT for both the single crystal and powder samples in the theoretical calculations are consistent with the experimental results, confirming the validity of the ab initio calculations. Figure 4 | The minute structural variations of complex cation during ferroelectric phase transition and the corresponding changes in the ligand-field splitting energy. The molecular symmetry of the complex cations changes from D3 in the HTp (a) to C3 in the ITp and the angle between Co−N coordination bonds and the molecular C3 axis shifts from 57.20° in the HTp to 56.20° and 58.59° in the ITp. (c) The energy difference between a1g and eg′ increases from 13 cm−1 in the HTp to cm−1 in the ITp. Download figure Download PowerPoint Magnetic switching accompanied by structural transition has been observed for several However, ferroelectricity accompanied by magnetic switching in a single-phase molecular material remains to be demonstrated. In the compound in this the ferroelectric phase transition as actuated by the motion of sulfate anions is due to a minute structural change in the Co(II) complex cations. The orbital angular momentum of Co(II) in trigonal antiprismatic coordination geometry is sensitive to the coordination Thus, two-step magnetic switchings are by minute molecular structural changes in to ferroelectric phase transitions. These structural changes lower the energy of polarization rotation along the crystallographic c-axis. Therefore, correlated magnetic and ferroelectric switchings are by this The two-step magnetic switchings accompanied by atypical paraelectric–ferroelectric–paraelectric phase transitions the structural between magnetic switching and ferroelectric properties in this material. the structural relationship between magnetic switching and ferroelectric properties is for the development of superior magnetoelectric materials. Furthermore, the strong magnetic anisotropy of compound 1 to its temperature-dependent magnetic susceptibility an applied field of 2 from 2 to K. As shown in Supporting Information Figures and its peak susceptibility shifts to higher temperature as the its slow magnetic The easy-plane magnetic anisotropy of 1 that its slow magnetic is by direct and (see Supporting Information Figures of order–disorder of sulfate dianions encapsulated in the cavities by [CoII(en)3] complex cations, we have a ferroelectric material that atypical paraelectric–ferroelectric–paraelectric phase transitions that can be to competition between strong hydrogen-bond interactions and dipole–dipole interactions between the molecular cations and sulfate dianions. The motion of the sulfate dianions induces minute structural changes in the coordination geometry of the complex cations. As Co(II) ions in trigonal antiprismatic coordination geometry remarkable orbital angular momentum that is sensitive to molecular structural two-step magnetic switchings are involved in the ferroelectric phase transitions. The of correlated magnetic and ferroelectric along with the slow magnetic observed at low-temperature the magnetic and electric properties of this the structural between magnetic and ferroelectric properties