Single Crystal Electron Paramagnetic Resonance of

Article pubs.acs.org/JPCC

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field † Mantas Šimeṅ as,*,† Anastasia Kultaeva,‡ Sergejus Balčiunas, Monika Trzebiatowska,§ Daniel Klose,∥ ̅ § ‡ Gunnar Jeschke,∥ Mirosław Ma̧czka,§ Juras ̅ Banys, and Andreas Pöppl †

Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany § Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box-1410, PL-50-950 Wroclaw 2, Poland ∥ Department of Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland ‡

S Supporting Information *

ABSTRACT: We present a continuous wave electron paramagnetic resonance (EPR) study of a Mn2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid dense metal−organic framework (MOF) that exhibits an order−disorder structural phase transition at Tc = 163 K. The W-band EPR measurements of a powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn2+ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn2+ probe ions are susceptible to the phase transition, as the spectrum changes drastically at Tc. The angular dependent EPR spectra of Mn2+ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO6 octahedra in the ordered phase that originate from a severe crystal twinning of the [(CH3)2NH2][Zn(HCOO)3] MOF. The possible ferroelectric origin of the crystalline twins is investigated by single crystal EPR measurements with an applied external electric field. No significant effect of the electric field on the spectra is observed. The EPR results are supported by the measurements of the electric field dependence of the macroscopic electric polarization. Analogous EPR measurements are performed on a single crystal sample of ferroelectric Mn2+ doped [NH4][Zn(HCOO)3] MOF. Contrary to the dimethylammonium framework, the EPR signal and electric polarization of the ammonium compound demonstrate clear ferroelectric behavior.

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INTRODUCTION

The most studied dense MOF family contains dimethylammonium (DMA + ) (CH 3 ) 2 NH 2 + molecular cations: [(CH3)2NH2][M(HCOO)3] (DMAM), where M is Zn, Cu, Ni, Co, Fe, Mn, or Mg.2,3 Various experimental techniques indicate a single first-order structural phase transition in these compounds at Tc = 160−180 K13−18 with the exceptionally high Tc of 270 K for the Mg analogue.19,20 The X-ray diffraction (XRD) studies reveal that the high- and low-temperature phases of these compounds belong to the trigonal R3̅c and the monoclinic Cc (noncentrosymmetric) space groups, respectively.21 The metal−formate frameworks of these materials consist of pseudocuboid cavities, each containing a single DMA+ cation (see Figure 1). The structure of the DMAM compounds belongs to a new class of hybrid AMX3 perovskites,

Recently, the formate-based dense metal−organic frameworks (MOFs) of general chemical formula [A][M(HCOO)3]n attracted significant scientific attention.1−3 These hybrid compounds consist of metal centers M (usually divalent transition metal ions) which are interconnected by the formate HCOO− linkers into anionic frameworks containing MO6 octahedra. The excess electric charge is compensated by the An+ molecular cations (e.g., NH4+) which are confined in the cavities (pores) of the framework. Each cation forms H-bonds with the formate linkers resulting in several preferential positions in the cavity. The common property of the majority formate frameworks are structural order−disorder phase transitions that involve the cooperative ordering of the An+ cations. Some of the transitions result in the noncentrosymmetric phases suggesting potential lead-free ferroelectric4−6 and multiferroic3,7−12 behavior. © 2017 American Chemical Society

Received: June 26, 2017 Revised: July 12, 2017 Published: July 13, 2017 16533

DOI: 10.1021/acs.jpcc.7b06257 J. Phys. Chem. C 2017, 121, 16533−16540

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The Journal of Physical Chemistry C

Figure 1. Low-temperature structure of DMAMn MOF. View along the (a) [012] and (b) pseudo-C3 axes. Structure taken from ref 21.

where X represents the formate linker.1−3 In the disordered phase the DMA+ cation is constantly hopping between the three equivalent positions in a cavity. As the temperature is decreased, the long-range order is established due to the cooperative ordering of the cations at Tc. In the lowtemperature phase each cation occupies a single position in a cavity (alternating, check board arrangement) (see Figure 1), while the metal−formate framework becomes more distorted.22 Several experimental and theoretical studies suggest that the low-temperature phase of the dimethylammonium formate frameworks might be ferroelectric.8,9,23,24 However, a proper ferroelectric hysteresis loop, that would definitely prove such behavior, was obtained only for a deuterated DMACo compound.7 Note that for a similar ammonium formate framework [NH4][M(HCOO)3] ferroelectricity was clearly demonstrated by measuring the hysteresis loops below 190 K.4 In contrast to the perovskite structure of the dimethylammonium MOF, this hybrid compound crystallizes in the less common 49·66 chiral structural topology.25 Among many other experimental methods, electron paramagnetic resonance (EPR) spectroscopy is well-suited to study structural phase transitions.26,27 This method detects the local environment of a paramagnetic center (e.g., local order parameter such as electric polarization) that can be influenced by the structural transformations. In our previous study we employed continuous wave (CW) EPR spectroscopy to successfully investigate the type and order of the phase transition in DMAZn MOF powder doped with a small amount of paramagnetic Mn2+ ions.28 The undoped DMAMn compound was studied by Abhyankar et al.29 We also used CW and more advanced pulse EPR methods to characterize the lowtemperature phases and dynamics of the phase transitions in Mn2+ and Cu2+ doped niccolite [NH3(CH2)4NH3][Zn(HCOO)3]230 and perovskite [CH3NH2NH2][Zn(HCOO)3]31 MOF powders. The EPR measurements of powder samples are frequently used to characterize the paramagnetic centers. Such measurements are less time-consuming; however, they lack information about the orientation and the number of the paramagnetic sites within the crystal lattice.32 Thus, crystalline twins and structural domains, occurring in materials with structural phase transitions, usually are not observed in the powder EPR spectra. On the other hand, such information can be obtained from single crystal EPR experiments. During such measurements one can also apply an external stimulus (e.g., electric field) on a single crystal sample which may influence the domain structure and phase transition properties, hence affecting the EPR spectrum.33−38 In this study we employ CW EPR spectroscopy to further investigate the low-temperature phase of Mn2+ doped DMAZn.

The powder W-band EPR experiments are employed to verify the spin Hamiltonian of the Mn2+ centers. The determined parameters are used to interpret the angular dependence of the single crystal X-band EPR spectra. We also check the influence of the applied external electric field on the low-temperature single crystal spectrum. The EPR experiments are complemented by the measurements of the macroscopic electric field dependent electric polarization. The obtained results are compared with the analogous measurements for the ferroelectric Mn2+ doped [NH4][Zn(HCOO)3] compound revealing a different behavior for both frameworks.

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EXPERIMENTAL METHODS Sample Preparation. ZnCl2 (99%, Fluka), MnCl2 (99%, Sigma-Aldrich), [NH4][HCOO] (99%, Fluka), methanol (99.8%, Sigma-Aldrich), formic acid (98%, Fluka), and 2.0 M solution of dimethylamine in methanol (Sigma-Aldrich) were commercially available and used without further purification. DMAZn:0.1 Mn2+ mol % (1) was synthesized by a slow diffusion method. In a typical experiment, 2.5 mL of 2.0 M solution of (CH3)2NH in methanol and 1 mL of formic acid were added to 10 mL of methanol. This solution was placed at the bottom of a glass tube (20 mm inner diameter). To this solution was layered 2 mL of methanol, followed by 20 mL of methanol solution containing 0.999 mmol of ZnCl2 and 0.001 mmol of MnCl2. The tube was sealed and kept undisturbed. The colorless crystals were harvested after 1 week, washed three times with methanol, and dried at room temperature. A single crystal sample of 1 used for EPR experiments is presented in Figure 2. We assumed a cuboid shape of the crystal, and the axes of the Cartesian reference frame were chosen to coincide with the edges of the crystal. [NH4][Zn(HCOO)3]:0.1 Mn2+ mol % (2) was also obtained by a slow diffusion method. In a typical experiment 20 mL methanol solution containing 20 mmol of [NH4][HCOO] and 20 mmol of formic acid was placed at the bottom of a glass tube (20 mm inner diameter). To this solution was gently added 30

Figure 2. (a) Photographed and (b) portrayed crystal of 1 used for CW EPR measurements. 16534


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The Journal of Physical Chemistry C mL of methanol solution containing 1.998 mmol of ZnCl2 and 0.002 mmol of MnCl2. The tube was sealed and kept undisturbed. The colorless crystals were harvested after 1 week, washed three times with methanol, and dried at room temperature. XRD was used to determine the crystal axes of 1 and 2 single crystal samples. EPR Spectroscopy. The X-band (∼9.4 GHz) CW EPR experiments were performed using a Bruker EMX micro spectrometer equipped with a Bruker ER 4119HS cylindrical cavity resonator. W-band (∼95 GHz) CW EPR spectra were recorded with a Bruker E680 spectrometer. For W-band measurements the magnetic field was calibrated using a Mn2+ doped CaO reference sample. For the CW EPR measurements the strength and frequency of the modulating field were typically set to 6 G and 100 kHz. The angular dependent single crystal EPR measurements were performed at X-band frequency by rotating the sample about three perpendicular directions. Simulations of the CW EPR spectra were performed using EasySpin 5.1.9 software.39 The orientations of the single crystal and magnetic tensors are described by the Euler angles as defined in EasySpin. For X-band EPR measurements with the external electric field, two parallel silver paste electrodes were applied on single crystal samples. The electrodes were connected to a dc voltage supply by two thin copper wires that were introduced into the resonator. We did not observe any parasitic EPR signal from the empty setup. An electric field up to 12 kV/cm was used for poling of the samples. Most of the measurements were performed after cooling the samples in the applied electric field while keeping the crystal at the same position. The quality of the electric contact was checked after and before the EPR measurements by a Fluke PM6304 RCL meter. Electric Polarization Measurements. An Aixact equipment was used to measure the electric field dependence of the electric polarization. The single crystal sample was placed in vacuum and cooled using liquid nitrogen in a custom-made cryostat. The experiments were performed using the periodic triangular signal of 0.1−10 Hz frequency. A high voltage of up to 4 kV was obtained using a Trek 609E6 voltage amplifier.

Figure 3. Experimental and simulated W-band CW EPR spectra of 1 at 20 K. Emphasis on (a) central and (b) outer fs transitions.

transitions (±3/2 ↔ ± 1/2 and ±5/2 ↔ ±3/2) are less intense (Figure 3b). We used the following spin Hamiltonian to simulate the obtained experimental W-band EPR powder spectrum of 1:40 / = βe BgS + SAI + /FS

(1)

The first term in the equation is the electron Zeeman interaction, where g denotes the g-tensor of the Mn2+ ion. βe and B are the Bohr magneton and external magnetic field, respectively. The second term describes the hf interaction characterized by the hf tensor A of the manganese nucleus. The fs of the spectrum is described by the last term, which is expressed in terms of the extended Stevens operators Ô qk(S) (k = 2, 4, 6 and q = +k, ..., −k) as41 /FS =

q

∑ ∑ BkqÔk (S) k

q

(2)

Bkq

Here entities are real coefficients that represent the magnitude of the corresponding zfs. The second-order axial D and orthorhombic E zfs parameters, that form the usual zfs tensor D, are related to these coefficients as D = 3B02 and E = B22. Note that frequently higher-order zfs parameters are also necessary to describe the Mn2+ and Fe3+ EPR spectra.40 The fourth-order cubic a and axial F parameters are expressed as a = 24B44 and F = 180B04 − 36B44. In our case these parameters are defined in a molecular reference frame coinciding with a 4-fold symmetry axis of the cubic system. The expressions of the extended Stevens operators Ô qk(S) are given in ref 41. The best simulation of the experimental W-band EPR spectrum of 1 is also presented in Figure 3. It was obtained using isotropic g and A tensors with the corresponding components g = 2.00053(5) and Aiso = −264 MHz. The determined parameters that describe the fs of the spectrum are D = 424(2) MHz, E = 58(2) MHz, a = 20(1) MHz, F = 18(1) 2 MHz, B−2 2 = 25(2) MHz, and B4 = 0.30(4) MHz. They are in a perfect agreement with the previously reported values obtained from the simulation of the X- and Q-band CW EPR spectra.28 Note that in the current work the g-factor is determined more accurately than in our previous study. The multifrequency (X-, Q-, and W-bands) simulation provides a reliable set of spin Hamiltonian parameters of the Mn2+ center in the low-temperature phase of 1. This data is used for the analysis of the single crystal EPR experiments that are presented further. X-Band EPR of 1 Single Crystal. We investigated the temperature dependence of the X-band EPR spectrum of 1 single crystal sample. The obtained results are presented in

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RESULTS AND DISCUSSION W-band EPR of 1 Powder. In our previous study we reported X- and Q-band CW EPR measurements of Mn2+ ions in DMAZn MOF powder.28 In the current work we additionally performed W-band CW EPR experiments of the same powder compound to verify the reported Mn2+ spin Hamiltonian parameters that are essential for the interpretation of the single crystal EPR data. The W-band EPR spectrum of 1 powder recorded at 20 K is presented in Figure 3. The observed pattern is typical of Mn2+ ions in the 3d5 electronic configuration and 6S5/2 electronic high-spin ground state.27,40 The total electron spin of this state is S = 5/2 resulting in five ΔmS = ±1 fine structure (fs) transitions (here mS is a magnetic electron spin quantum number). The resonance fields of these transitions differ, if the zero-field splitting (zfs) of the Mn2+ ions is present. The interaction of the electrons with the 55 Mn nucleus (nuclear spin I = 5/2) causes a further splitting of each fs transition into six hyperfine (hf) lines (ΔmS = ± 1 and ΔmI = 0, where mI is a magnetic nuclear spin quantum number). The observed Mn2+ spectrum of 1 (Figure 3a) is dominated by the six hf lines of the central fs transition (mS = −1/2 ↔ 1/2), while the outer 16535


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The Journal of Physical Chemistry C Figure 4 revealing that the phase transition of 1 drastically influences the Mn2+ spectrum. This indicates that the Mn2+ ion

Figure 4. Temperature dependence of the single crystal X-band CW EPR spectra of 1 obtained at an arbitrary orientation of the crystal.

Figure 5. Angular dependent CW EPR spectra of 1 obtained by rotating the magnetic field vector in the (11̅2) plane. Spectra recorded at 0° and 90° are explicitly presented. The zero angle corresponds to the magnetic field vector in the (012) plane. Measurements were performed at 70 K.

probes are susceptible to the structural changes occurring at Tc. The high-temperature phase spectrum shows unresolved outer fs transitions implying a distribution of the zfs parameters. It originates from many different Mn2+ ion environments due to the constant hopping motion of the DMA+ cations. Below the phase transition point this motion significantly slows down, and the long-range order is established, resulting in well-resolved outer fs transitions. As the temperature is further decreased, the spectrum becomes wider indicating a higher value of the axial zfs parameter D due to the increasing distortion of the MnO6 octahedra. Temperature dependent spectral changes were analyzed in more detail in our previous powder EPR study of Mn2+ doped DMAZn.28 We also observed that a rapid cooling of the sample (fast insertion to the cryostat operating at 70 K) results in an EPR spectrum with unresolved outer fs transitions which is similar to the spectrum obtained for the high-temperature phase (see Figure S1 in the Supporting Information (SI)). The spectrum did not transform to the pattern characteristic to the lowtemperature phase even after keeping the sample at 70 K for several hours. This may indicate that the dynamic disorder of the DMA+ cations becomes quenched at a fast cooling rate resulting in many different environments of Mn2+ ions and hence the distribution of the zfs parameters. Note that in an NMR study of DMAZn framework Besara et al.13 observed that the temperature dependence of the proton spin−lattice relaxation rate follows different paths on cooling and warming if the sample is cooled below 40 K. However, in our experiments the cooling memory effect is observed independently on the lowest reached temperature. Note that, at the usual cooling rate (several kelvin per minute), we always obtained the same low-temperature spectrum at a fixed orientation of the crystal. The Mn2+ probe centers were further investigated by recording the angular dependence of the low-temperature CW EPR spectra of 1 single crystal. The sample was rotated around three approximately perpendicular directions in such a way that the magnetic field vector was in the (11̅2), (1̅02), or (012) plane. The obtained spectra are presented in Figure 5 and Figures S2 and S3 revealing several magnetically inequivalent Mn2+ sites. The central part of the spectra is very complicated, since the mS = −1/2 ↔ 1/2 fs transitions have the least expressed angular dependence resulting in a high

density of the EPR lines. In addition, there is a significant overlap between the allowed and forbidden hf transitions in this spectral region. The rotation patterns of the outer transitions are better resolved, especially of the mS = ± 5/2 ↔ ± 3/2 transitions that exhibit apparent angular dependences at all three orientations of the single crystal. Note that the observed maximum spans of the angular patterns are close to the widths of the powder spectra.28 The initial XRD study of the low-temperature structure of DMAMn perovskite revealed two MnO6 octahedra that are tilted with respect to each other (Figure 1).21 However, the experimental angular dependent EPR spectra of 1 clearly show more than two Mn2+ sites indicating crystal twinning or ferroelectric domain phenomena. We performed spectral simulations of the obtained angular dependences to determine how many differently oriented MnO6 octahedra are in the system. The angular dependent resonance fields of the EPR lines are presented in Figure 6 for all three orientations of the single crystal. We simulated only the positions of the outermost hf lines (mI = ±5/2) of the mS = ±5/2 ↔ ±3/2 fs transitions that show well pronounced angular dependence. Simulations were performed using the spin Hamiltonian parameters determined from the powder EPR experiments in the current and previous studies.28 To simplify the simulation procedure, we neglected the higher-order zfs parameters and used only the second-order parameters D and E which have the main influence on the angular patterns. Note that the g and A tensors of Mn2+ ions in 1 are isotropic and therefore provide no additional angular dependence. Six magnetically inequivalent Mn2+ sites were required to simulate the determined angular dependences (see Figure 6). The simulation agrees rather well with the experimental data taking into account a nonideal crystal shape, alignment, and simplified spin Hamiltonian. The corresponding Euler angles of the six D tensors of Mn2+ ions are also indicated in Figure 6. The Euler angles are defined in a reference frame depicted in Figure 2b. Note that the third angle was impossible to determine accurately, since each Mn2+ center exhibits clear 16536


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Figure 6. Experimental (above) and simulated (below) angular dependent EPR signal of 1 obtained by rotating the magnetic field vector in the (11̅2), (1̅02), and (012) planes.

Figure 7. (a, b) X-band CW EPR spectra of 1 measured at arbitrary orientation after cooling to 130 K with and without the applied external electric field. (c) Electric field dependence of the electric polarization of DMAZn. The sample was poled along the [012] direction.

orientation, but without the electric field, is presented for comparison. No clear and reliable differences between both spectra are observed. We performed the measurements for several other crystal orientations, and we tried to pole the crystal along the two other directions, but the outcome was the same. This suggests that the low-temperature phase of DMAZn is either not a proper ferroelectric or the applied electric field is too small to significantly influence the domain structure. We also measured the electric field dependence of the electric polarization of a DMAZn single crystal sample. The obtained results are presented in Figure 7c revealing no hysteresis loop, despite the much stronger electric field used in this experiment. We were not able to use the stronger fields due to the electrical breakdown of the samples. To support our experiments on 1, we performed the same type of measurements with the applied electric field for a single crystal sample of manganese doped [NH4][Zn(HCOO)3] MOF (sample 2) which is known to be ferroelectric.4 Figure 8 shows the CW EPR spectra of 2 recorded at 150 K (lowtemperature phase) with and without the external electric field. A clear redistribution of the EPR line intensities can be seen when the electric field is present indicating that it influences the volumes of the domains in 2 as expected for a proper

rotation patterns only in two out of three planes (in one plane these patterns overlap with the central part of the spectra). The obtained six Mn2+ sites are in agreement with the later XRD study that indeed revealed the same number of MnO6 octahedra in the low-temperature phase of the DMAMn framework.15 The six MnO6 octahedra arise from the three crystalline twins each containing two centers (see Figure 1). The crystal twinning is observed only in the low-temperature phase, and thus, each twin contains the ordered arrangement of the DMA+ cations. They are mapped to each other by a rotation around the crystallographic pseudo-C3 axis by 120° which is related to the initial 3-fold disorder of the DMA+ cations.15 Single Crystal EPR with External Electric Field and Polarization Measurements. We performed the X-band EPR measurements of 1 single crystal with the applied electric field to investigate whether the detected crystalline twins correspond to the ferroelectric domains. If the domains are ferroelectric, the electric field should change their volumes causing the redistribution of the Mn2+ line intensities in the EPR spectrum. The obtained low-temperature single crystal spectrum of 1 with the applied external electric field of 12 kV/ cm is presented in Figure 7. The spectrum recorded at the same 16537


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The W-band powder EPR measurements were performed to characterize the Mn2+ centers in the low-temperature phase. The simulation of the experimental powder pattern recorded at 20 K revealed the Mn2+ spin Hamiltonian that is in a perfect agreement with our previous X- and Q-band CW EPR study.28 Such a multifrequency approach provided a reliable set of parameters further used for the interpretation of the single crystal EPR data. The temperature dependence of the single crystal X-band EPR spectrum revealed that the Mn2+ probes are susceptible to the local structural changes occurring at Tc. We observed the unresolved fs of the high-temperature spectrum due to the hopping motion of the DMA+ cations. This motion significantly slows down at Tc providing well-defined fs parameters. The effect of the phase transition on the single crystal EPR spectrum of 1 corresponds well with our previous powder EPR study.28 We also observed that a rapid cooling of the sample through the phase transition point results in the spectrum similar to the high-temperature phase. This indicates a new state of the dimethylammonium formate frameworks which occurs due to the quenching of the initial dynamic disorder of the DMA+ cations. The angular dependent single crystal EPR experiments were performed to determine the number of the magnetically inequivalent Mn2+ sites in 1. The EPR spectra were recorded by rotating the single crystal sample around the three distinct directions. The determined rotation patterns displayed high density of the EPR transitions in the central part of the spectrum. We were able to successfully simulate the angular dependence of the outermost hf lines of the outer fs transitions. Simulations revealed six magnetically inequivalent Mn2+ sites in 1 independently confirming the crystal twinning model proposed by the XRD study.15 The origin of the crystalline twins in 1 was further investigated by performing single crystal EPR experiments with the applied electric field. We did not detect any reliable spectral differences between the measurements with and without this external stimulus below Tc. The EPR results were complemented by the direct measurement of the macroscopic electric polarization. We also did not observe any ferroelectric hysteresis loop in the low-temperature phase of DMAZn despite much stronger electric fields used in this experiment. The experimental results obtained for the dimethylammonium MOF were further supported by the similar EPR and polarization measurements of the Mn2+ doped ferroelectric [NH4][Zn(HCOO)3] single crystal. We observed a clear effect of the external electric field on the low-temperature EPR spectrum of this compound. The electric field dependence of the relative EPR intensity provided the ferroelectric hysteresis loop in agreement with the macroscopic polarization measurements. This result is independent evidence of the ferroelectric behavior of the ammonium formate framework. It also demonstrates that the single crystal EPR spectroscopy can be used to investigate the polar nature of the low-temperature phases in various dense MOFs. Such microscopic experiments are highly attractive, since the usual measurements of the electric polarization involve difficulties related to the size and the conductivity of the sample. Our study also suggests that the nature of the ordered phases in the dimethylammonium and ammonium zinc formates may be different. The negligible effect of the electric field on the EPR spectrum of 1 raises serious doubts about the proper

Figure 8. (a) X-band CW EPR spectra of 2 measured at arbitrary orientation after cooling to 150 K with and without the applied external electric field. The right-wing of the spectrum is magnifiend in part b. The sample was poled along the c-axis.

ferroelectric material.42 The measurements were verified several times at different orientations of the single crystal. Note that we did not observe any reliable effect of the electric field on the EPR spectrum of 2 in the disordered (paraelectric) phase. The redistribution of the ferroelectric domains upon the application of the electric field should polarize the single crystal sample. The electric polarization is proportional to the change of the domain volumes that also determine the intensities of the EPR signals of the corresponding Mn2+ centers. We recorded how the EPR signal of 2 changes as the electric field is swept from −10 to 10 kV/cm. The obtained dependence of the relative EPR signal is presented in Figure 9a revealing a typical

Figure 9. (a) Electric field dependence of the EPR signal at 338.2 mT. (b) Electric polarization hysteresis loop of 2. The sample was poled along the c-axis, and the measurements performed at 150 K. The error bars in part a are smaller than data points.

ferroelectric hysteresis loop. Our EPR results of 2 are also supported by the electric measurements of the electric polarization presented in Figure 9b (see Figure S4 in SI for the field dependent electric current). Note that the similar electric-field-induced changes of the EPR spectrum (including the ferroelectric hysteresis loops) were previously thoroughly studied in the well-known KD2PO4 and related ferroelectrics.33−38 A more detailed EPR study of 2 will be presented in a forthcoming contribution.

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CONCLUSIONS In this work we reported the CW EPR study of DMAZn dense MOF doped with small amount of paramagnetic Mn2+ ions that act as local probes in the crystal structure. This compound exhibits the order−disorder structural phase transition at Tc = 163 K which involves the cooperative ordering of the DMA+ cations and the deformation of the formate−metal framework. 16538


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(9) Tian, Y.; Stroppa, A.; Chai, Y.; Yan, L.; Wang, S.; Barone, P.; Picozzi, S.; Sun, Y. Cross Coupling between Electric and Magnetic Orders in a Multiferroic Metal-Organic Framework. Sci. Rep. 2015, 4, DOI: 10.1038/srep06062. (10) Tian, Y.; Cong, J.; Shen, S.; Chai, Y.; Yan, L.; Wang, S.; Sun, Y. Electric Control of Magnetism in a Multiferroic Metal-Organic Framework. Phys. Status Solidi RRL 2014, 8, 91−94. (11) Maczka, M.; Pasinska, K.; Ptak, M.; Paraguassu, W.; da Silva, T. A.; Sieradzki, A.; Pikul, A. Effect of Solvent, Temperature and Pressure on the Stability of Chiral and Perovskite Metal Formate Frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn). Phys. Chem. Chem. Phys. 2016, 18, 31653−31663. (12) Maczka, M.; Gagor, A.; Ptak, M.; Paraguassu, W.; da Silva, T. A.; Sieradzki, A.; Pikul, A. Phase transitions and Coexistence of Magnetic and Electric Orders in the Methylhydrazinium Metal Formate Frameworks. Chem. Mater. 2017, 29, 2264−2275. (13) Besara, T.; Jain, P.; Dalal, N. S.; Kuhns, P. L.; Reyes, A. P.; Kroto, H. W.; Cheetham, A. K. Mechanism of the Order-Disorder Phase Transition, and Glassy Behavior in the Metal-Organic Framework [(CH3)2NH2]Zn(HCOO)3. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 6828−6832. (14) Asaji, T.; Ashitomi, K. Phase Transition and Cationic Motion in a Metal-Organic Perovskite, Dimethylammonium Zinc Formate [(CH3)2NH2]Zn(HCOO)3. J. Phys. Chem. C 2013, 117, 10185− 10190. (15) Sanchez-Andujar, M.; Gomez-Aguirre, L. C.; Pato Doldan, B.; Yanez-Vilar, S.; Artiaga, R.; Llamas-Saiz, A. L.; Manna, R. S.; Schnelle, F.; Lang, M.; Ritter, F.; et al. First-Order Structural Transition in the Multiferroic Perovskite-Like Formate [(CH3)2NH2][Mn(HCOO)3]. CrystEngComm 2014, 16, 3558−3566. (16) Ma̧czka, M.; Ga̧gor, A.; Macalik, B.; Pikul, A.; Ptak, M.; Hanuza, J. Order-Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3 ) 2 NH 2 ][M(HCOO)3] and [(CH3)2NH2][M(HCOO)3] (M = Ni, Mn). Inorg. Chem. 2014, 53, 457−467. (17) Ma̧czka, M.; Ptak, M.; Macalik, L. Infrared and Raman Studies of Phase Transitions in Metal-Organic Frameworks of [(CH3)2NH2][M(HCOO)3] with M = Zn, Fe. Vib. Spectrosc. 2014, 71, 98−104. (18) Abhyankar, N.; Kweon, J. J.; Orio, M.; Bertaina, S.; Lee, M.; Choi, E. S.; Fu, R.; Dalal, N. S. Understanding Ferroelectricity in the Pb-Free Perovskite-Like Metal-Organic Framework [(CH3)2NH2]Zn(HCOO)3: Dielectric, 2D NMR, and Theoretical Studies. J. Phys. Chem. C 2017, 121, 6314−6322. (19) Pato-Doldan, B.; Sanchez-Andujar, M.; Gomez-Aguirre, L. C.; Yanez-Vilar, S.; Lopez-Beceiro, J.; Gracia-Fernandez, C.; Haghighirad, A. A.; Ritter, F.; Castro-Garcia, S.; Senaris-Rodriguez, M. A. Near Room Temperature Dielectric Transition in the Perovskite Formate Framework [(CH3)2NH2][Mg(HCOO)3]. Phys. Chem. Chem. Phys. 2012, 14, 8498−8501. (20) Asaji, T.; Yoshitake, S.; Ito, Y.; Fujimori, H. Phase Transition and Cationic Motion in the Perovskite Formate Framework [(CH3)2NH2][Mg(HCOO)3]. J. Mol. Struct. 2014, 1076, 719−723. (21) Sánchez-Andújar, M.; Presedo, S.; Yánez-Vilar, S.; CastroGarcía, S.; Shamir, J.; Senarís-Rodríguez, M. A. Characterization of the Order-Disorder Dielectric Transition in the Hybrid Organic-Inorganic Perovskite-Like Formate Mn(HCOO)3[(CH3)2NH2]. Inorg. Chem. 2010, 49, 1510−1516. (22) Duncan, H. D.; Dove, M. T.; Keen, D. A.; Phillips, A. E. Local Structure of the Metal-Organic Perovskite Dimethylammonium Manganese(ii) Formate. Dalton Trans. 2016, 45, 4380−4391. (23) Šimėnas, M.; Balciunas, S.; Ma̧czka, M.; Banys, J.; Tornau, E. E. Structural Phase Transition in Perovskite Metal-Formate Frameworks: a Potts-Type Model with Dipolar Interactions. Phys. Chem. Chem. Phys. 2016, 18, 18528−18535. (24) Jain, P.; Stroppa, A.; Nabok, D.; Marino, A.; Rubano, A.; Paparo, D.; Matsubara, M.; Nakotte, H.; Fiebig, M.; Picozzi, S.; et al. Switchable electric polarization and ferroelectric domains in a metalorganic-framework. Npj Quantum Materials 2016, 1, 16012.

ferroelectric origin of the DMAZn framework. However, we also cannot completely discard the possibility that the electric fields used in our study are too weak to significantly influence the domain structure and hence affect the EPR spectra. Note that recently Jain et al.24 reported a second-harmonic generation study in which an observation of the ferroelectriclike domains was demonstrated in the related DMAMn compound. This raises an open question about the effect of the metal centers on the nature and properties of the lowtemperature phase in the dimethylammonium and similar formates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06257. Additional EPR and electric polarization measurement data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: mantas.simenas@ff.vu.lt Phone: +370 5 2234537. Fax: +370 5 2234537. ORCID

Mantas Šimėnas: 0000-0002-2733-2270 Mirosław Ma̧czka: 0000-0003-2978-1093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Council of Lithuania (Project TAP LLT-4/2017) and the Deutsche Forschungsgemeinschaft (DFG priority program SPP 1601). The authors thank M. Ivanov for extensive discussion.

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REFERENCES

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