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Impact of Postsynthetic Modification on the Electrical and Magnetic Properties of Materials K. S. Asha,† Niyaz Ahmed,‡ Ramesh Nath,‡ Denis Kuznetsov,§ and Sukhendu Mandal*,† †

School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695016, India ‡ School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram, Kerala 695016, India § “MISIS” Department of Functional Nanosystems and High Temperature Materials, National University Science and Technology, Lenisky pr. 4, Moscow 119049, Russia S Supporting Information *

were crystallized in the same orthorhombic system with polar space group Pna21. In the present work, we report the postsynthetic ion addition of Co2+ ions to the structure of the parent compound without altering the original structural architecture. We have also explored the effect of the incorporation of Tb3+ and Co2+ ions toward their electric and magnetic behaviors. Compound 1 exhibits piezo- and ferroelectric behavior at room temperature.11 The ferroelectricity is switched off, and the compound behaves nonferroelectrically when the Ba2+ ion is exchanged by the Tb3+ ion. On the other hand, the polarization field of the parent compound is increased after Co2+ (Co@1) ion incorporation. Compound 1 was synthesized via the solvothermal method,11 and then metal exchange/addition in a N,N-dimethylformamide (DMF) solvent was carried out at room temperature. For this, we immersed single crystals of compound 1 in the respective metal nitrate solution for a definite period. Complete metal exchange by Tb3+ ions produced a new compound (Tb@1) with the same structural features as that of the parent compound. The daughter compound was characterized using microscopy, scanning electron microscopy−energy-dispersive X-ray (SEM−EDX), thermogravimetric analysis (TGA), differential scanning calorimetry, single-crystal X-ray diffraction (SCXRD), and powder X-ray diffraction (PXRD) techniques (see Figures S1−S8 and Table S1).12 Similarly, we performed Co2+ ion incorporation, and interestingly we observed that the Co2+ ion was not exchanged; instead, it was incorporated in the channel of the framework. However, both ions (Co2+ and Ba2+) were divalent. This must be due to the smaller ionic radii of the Co2+ ion compared to the Ba2+ ion, and, moreover, the coordination environment that the Ba2+ ion possesses is not very common for the Co2+ ion. Postsynthetic metal addition using Co2+ ions was executed in a single crystal as well as in a powder sample. The addition of cobalt ions could be examined under visible light because the color of the crystals after incorporation changed from colorless to light pink (Figure 1). Due to the lack of long-range ordering (LRO), the crystals after cobalt addition were not suitable for

ABSTRACT: Postsynthetic modification is a promising tool for introducing multifunctional properties in metal− organic frameworks (MOFs). The effects of postsynthetic metal addition/exchange in a barium-based MOF have been well examined toward their magnetic and electrical properties. The rattling motion of the extraframework organic cation is responsible for the ferroelectric behavior. The strong magnetic frustration in Tb@1 is found to arise from the nearly triangular arrangement of Tb3+ ions in its secondary building unit along the chain direction.

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t is well-known that pure organic and inorganic molecules have their advantages and disadvantages, so it is highly desirable to combine their properties into a single-phase hybrid material. Thus, the synthesis and design of inorganic−organic hybrid materials [like metal−organic frameworks (MOFs) or coordination polymers] have been found to have very rapid and extensive growth in the past years.1 Many desired features can be achieved in MOFs by accurate control of the structure through the appropriate ligand design and proper selection of a metal node.2 However, total control over the MOF structure is always difficult because of the many factors that affect their assembly.3 The MOFs can easily swap their constituents with the surrounding medium through postsynthetic modification and can create novel structures with induced properties.4 Cation and/or ligand exchange is a powerful technique for designing new materials when conventional direct synthesis fails.5−7 The materials exhibiting electric and magnetic ordering are of great significance because of their technological importance. There are reports on MOFs unveiling the perovskite structure that exhibit multiferroic properties due to the order−disorder transition of either an organic cation or a solvent trapped inside the pore of the framework.8−10 Here, we focus on the effect of postsynthetic metal exchange/addition in a MOF toward its electrical and magnetic properties. Recently, we have reported a three-dimensional porous MOF, [H2N(CH3)2][Ba(H2O)(BTB)] (1), constructed from an alkaline-earth-metal ion and a 1,3,5-benzenetribenzoic acid ligand.11 We have also shown that the barium compound undergoes complete metal exchange with the Tb3+ ion at the metal node in a single crystal (SC)-to-SC fashion to form Tb(H2O)(BTB) (Tb@1).12 Both compounds © XXXX American Chemical Society

Received: April 6, 2017

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DOI: 10.1021/acs.inorgchem.7b00862 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Photographs of crystals of (a) compound 1 (under visible light), (b) Co@1 (under visible light), (c) 1 (under UV light), and (d) Tb@1 (under UV light).

Figure 3. Phase-voltage hysteresis of compounds (a) Tb@1 and (c) Co@1 and corresponding “butterfly” loops for (b) Tb@1 and (d) Co@ 1.

SCXRD. Le Bail fitting on PXRD of the Co@1 sample was performed using the FULLPROF software package,13 and a goodness-of-fit (χ2) of 4 was obtained. The lattice parameters extracted from the fit are a = 7.28(9) Å, b = 19.36(7) Å, and c = 26.03(3) Å (Figure S5). PXRD, IR, and TGA show that Co@1 has the same structure (retains the same organic moieties) and crystallinity as that of 1 (see Figures S4−S6). Co2+ ion addition must be taking place via diffusion of these ions to the channel by a DMF solvent and exchange with a dimethylamine (DMA) cation. Postsynthetic metal addition was confirmed using SEM− EDX and inductively coupled plasma atomic emission spectroscopy (ICP-AES) techniques (Figure 2 and Table S1). The

DMA cation in Tb@1 might be responsible for the suppression of phase switching. Ferroelectric hysteresis indicates the reversal of polarization, which includes electronic, atomic, ionic displacement, or switching of the permanent dipole moment under an electric field. According to the structure of compound 1, the polarization reversal could be due to the displacement of a DMA cation, and the displacement occurred along the c direction (Figure S10). This was studied in more detail by collecting the SCXRD of compound 1 at room temperature as well as at 150 K, respectively (Figure S11). The difference in the bond lengths and bond angles of a DMA cation at different temperatures suggests restriction toward the free motion of the cation in the pore at low temperature. At room temperature, the DMA molecule is away from the coordinated water molecule and the motion is not hindered by a hydrogen-bonding interaction (Table S2). Like compound 1, for Co@1, a clear hysteretic 180° phase switching in response to a sweeping direct-current (dc) voltage was observed in the PFM response. The phase switches sharply at Vdc = 140 V by ∼180° with coercive voltages of ±75 V. The “butterfly loop” in amplitude versus Vdc plot is a significant feature of a piezoelectric material. Thus, the observations of hysteretic phase switching and butterfly loops for Co@1 indicate that the compound possesses ferroelectric and piezoelectric properties (Figure 3c,d). In contrast, in the case of compound 1, the phase switches sharply at Vdc = 20 V by ∼180° with coercive voltages of ±4 V (Figure S9).11 Notably, the phase of the PFM response signal is directly related to the direction of the electric polarization of the microscopic region of the surface monitored under the tip. The direction of polarization was switched when the orientation of the electric field was changed. The ferroelectric material with a single domain and a strong depolarizing field may not be energetically favorable, and therefore it may contain a multidomain state. When we applied the field, the domain wall was reduced or removed.14,15 The increase of the voltage below which the system remains nonpolarized in the case of Co@1 compared to 1 might be due to the combined effect of Co2+ ions and DMA cations in the pore.

Figure 2. SEM−EDX analysis of Tb@1 and Co@1.

maximum amount of Co2+ ion incorporation acquired was 40%, and the reaction time was fixed to 6 h for a 1:1 reaction mixture (i.e., a Co2+/Ba2+ ratio). The crystallinity of the parent compound was lost when the reaction proceeded beyond 6 h (Figure S4c), and no Ba2+ ions were leached out from compound 1 during 6 h of reaction. We calculated an empirical formula for Co@1 based on the ICP analysis, and it was [(DMA)0.28(Co2+)0.36][Ba(BTB)(H2O)]. Our group recently reported the ferroelectric properties of compound 1.11 The local ferroelectricity was measured at room temperature after metal exchange/addition. Surprisingly, we could not observe any hysteresis in the piezoresponse force microscopy (PFM) phase response in the case of Tb@1, while there was an enhancement of the coercive voltage in the case of Co@1 (Figure 3). The structural architecture for all of the compounds is similar except the presence/absence of a cation in the pore. Then we realized that the motion of the extraframework cation in the channel might be responsible for the reversal of net polarization in the case of compound 1 and Co@1, respectively, when the electric field is applied. Therefore, the absence of a B

DOI: 10.1021/acs.inorgchem.7b00862 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

presence of LRO. The increase of the applied field results in a slight evolution of TN toward low temperatures, indicating the AFM nature of the transition. The Co@1 compound, on the other hand, does not show any signature of magnetic LRO down to 0.35 K. The parameter f (=θCW/TN),19 which is used as a quantitative measure of frustration in the spin systems, is calculated to be ∼40 for Tb@1, while for Co@1, it has a higher value. Such a large value of the frustration parameter (f) clearly reflects the strong magnetic frustration in [email protected]−22 Careful analysis of the structural data revealed (see Figure S13) that Tb3+ ions are interacting via oxygen with nearestneighbor (NN) interaction J1 to form zigzag chains in the crystallographic a direction. The reduced distance ∼4.25 Å between Tb3+ ions along the chain suggests a strong NN interaction. The chains are well separated from each other and are weakly coupled via ligands. Also, a significant next-nearestneighbor (NNN) interaction (J2) via a Tb−O−C−O−Tb linkage is also envisaged. Thus, in each chain, the AFM J1 and J2 interactions compete in a triangular unit, leading to strong frustration in [email protected] For Co@1, because the position of Co2+ in the crystal structure is not known, we are unable to explain the origin of frustration. In summary, we synthesized cobalt- and terbium-based MOFs (Tb@1 and Co@1) via postsynthetic metal addition to compound 1, without altering the structure of the parent MOF. The structures of both compounds were characterized using various techniques like SCXRD, PXRD, EDX, ICP-AES, etc. We then investigated the effect of this metal exchange/ addition in 1 toward its magnetic and electrical properties. The ferroelectricity of compound 1 is caused by the ordered− disordered transition of an extraframework cation. For Tb@1, the strong magnetic frustration results in the suppression of LRO down to 0.8 K, arising from the competition between NN and NNN interactions in the Tb3+ chain. Under an external magnetic field, it displays a plateau state at a third of the saturation magnetization, further reflecting the strongly frustrated nature of the spin lattice in Tb@1.

The variable-temperature magnetic susceptibilities measured for compounds Tb@1 and Co@1 are shown in parts a and b of Figure S12, respectively. The compounds do not show any significance of magnetic LRO down to 2K. The magnetic parameters were deduced by performing the Curie−Weiss (CW) fit (see the Supporting Information, eq 1) to the 1/χ versus T data at high temperatures (see Figure 4 and Table S3). The

Figure 4. 1/χ versus T of Tb@1 (a) and Co@1 (b). The lower insets show the M versus H curve at T = 2.1 K, while the upper insets demonstrate the corresponding CP versus T data.

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calculated effective moments (μeff ≃ 9.45 and 3.83 μB for Tb@1 and Co@1, respectively) are in close agreement with the expected value of μeff ≃ 3.87 μB for Co2+ (S = 3/2) and μeff ≃ 9.72 μB for Tb3+ (J = 6). The positive value of θCW (θCW ≃ 31.6 and 32.8 K for Tb@1 and Co@1, respectively) for both compounds suggests a dominant antiferromagnetic (AFM) superexchange interaction between magnetic ions. The lower insets of Figures 4a and 4b show the magnetization isotherm (M versus H) at T = 2.1 K measured up to H = 9 T for compounds Tb@1 and Co@1, respectively. For both compounds, M increases smoothly with H and tends to saturate at higher values of H. The y intercept of the linear fit of the highfield magnetization data gives saturation magnetization of MS ≃ 3.4 μB/Tb3+ and 2.9 μB/Co2+, respectively. For Co@1, this value of MS is close to the expected saturation value MS = gSμB = 3 μB (S = 3/2 and g = 2). On the other hand, for Tb@1, the obtained value of MS is almost a third of the expected saturation magnetization value MS = gJμB = 9 μB (J = 6 and g = 1.5) for Tb3+ spins. This reflects a magnetization plateau at a third of MS for Tb@1, typically observed for highly frustrated magnets.16,17 The slow linear rise of M in the saturation and plateau regimes is attributed to the temperature-independent Van Vleck paramagnetic contribution.18 The heat capacity CP(T) measured for compound Tb@1 under zero field shows a weak kink at TN ≃ 0.8 K, confirming the

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00862.

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Methods and materials, experimental section, characterization, and structural figures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sukhendu Mandal: 0000-0002-4725-8418 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We acknowledge the Science and Engineering Research Board, Government of India, for funding through Grant SB/S1/IC-14/ 2013 and the National University of Science and Technology, MISIS, Russia, for financial support through Grant K3-2016-029. C

DOI: 10.1021/acs.inorgchem.7b00862 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b00862 Inorg. Chem. XXXX, XXX, XXX−XXX