Phase Transitions and Coexistence of Magnetic and Electric Orders in

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Phase Transitions and Coexistence of Magnetic and Electric Orders in the Methylhydrazinium Metal Formate Frameworks Mirosław Mączka,*,† Anna Gągor,† Maciej Ptak,† Waldeci Paraguassu,‡ Tercio Almeida da Silva,‡ Adam Sieradzki,§ and Adam Pikul† †

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland Faculdade de Física, Universidade Federal do Pará, 66075-110 Belém, Pará, Brazil § Department of Experimental Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland ‡

S Supporting Information *

ABSTRACT: We report the synthesis of four perovskite-type metal formate frameworks, [CH3NH2NH2][M(HCOO)3] (MHyM) with M = Mn, Mg, Fe, and Zn. These compounds exhibit two structural phase transitions. The first transition temperature depends weakly on a type of divalent metal and is observed at 310−327 K on heating. Xray diffraction, DSC, and vibrational studies revealed that it has a second-order character. It is associated with partial ordering of the methylhydrazinium (MHy+) cations and change of symmetry from nonpolar R3̅c to polar R3c. Pyroelectric measurements suggest the ferroelectric nature of the room-temperature phase. The second, lowtemperature phase transition has a first-order character and is associated with further ordering of the MHy+ cations and distortion of the metal formate framework. Magnetic susceptibility data show that MHyMn and MHyFe exhibit ferromagnetic-like phase transitions at 9 and 21 K, respectively. Since the low-temperature phase is polar, these compounds are possible multiferroic materials. MHyFe shows additional magnetic anomaly in the magnetically ordered state, which most likely manifests some blocking of magnetic moments. We also report high-pressure Raman scattering studies of MHyMn that revealed a pressureinduced reversible phase transition between 4.8 and 5.5 GPa. Analysis of the data indicates that the transition leads to significant changes in both the manganese formate framework and the MHy+ structure.

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INTRODUCTION Jain et al. reported in 2009 that dense metal organic-framework (MOF) compound [(CH3)2NH2][Mn(HCOO)3] exhibits multiferroic properties.1 This compound crystallizes in the ABX3 perovskite-like architecture, where A = (CH3)2NH2+, B = Mn2+, and X = HCOO−. This discovery promoted broad interest in studies of ferroelectric, magnetic, dielectric, vibrational, and mechanical properties of this compound as well as its structural phase transition mechanism and possible magnetoelectric coupling.2−7 It also showed new routes for the synthesis of multiferroic materials, i.e., synthesis of metal formate frameworks containing magnetic ions and templated by various protonated amines. In this respect, it has been shown that properties of [(CH3)2NH2][Mn(HCOO)3] can be greatly modified by aliovalent doping at Mn2+ sites or by replacing Mn2+ by other divalent cations.8−13 Furthermore, novel perovskite- and niccolite-like compounds can be prepared by using pairs of metal cations: Na and M3+ or M3+ and M2+, where M3+ = Al, Cr, Fe; M2+ = Fe, Mn, Co, Ni, Mg, Zn, and Cu.14−18 The most often used procedure was, however, replacement of (CH3)2NH2+ by other cations.19−27 Among the synthesized materials, type I multiferroic properties were © 2017 American Chemical Society

reported for ammonium, imidazolium, and hydrazinium metal formates and suggested for ethylammonium and guanidinium analogues.19−25 Recently, magnetic ordering-induced type II multiferroic behavior was also reported for methylammonium cobalt formate.28 It is worth adding that apart from magnetic and ferroelectric properties, some of the metal formate frameworks also exhibit ferroelastic, negative thermal expansion and negative compressibility properties.21,29−33 In the metal formate frameworks, the organic cations are located in channels (for chiral phases) or cavities of the framework (for perovskites and niccolites).1,7,14−21,27,34,35 They interact with the framework via hydrogen bonds (HBs) and Coulomb interactions. These interactions are often weak, leading to disorder at A-sites at elevated temperatures. Upon cooling, the A+ cations may order, leading to ferroelectric or antiferroelectric properties. Thus, properties of these MOFs and related perovskites with N3−, CN−, and N(CN)2− ligands depend strongly on interactions between the A-site cations and Received: December 12, 2016 Revised: February 15, 2017 Published: February 16, 2017 2264

DOI: 10.1021/acs.chemmater.6b05249 Chem. Mater. 2017, 29, 2264−2275

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Chemistry of Materials

Dielectric Properties. The dielectric measurements at ambient pressure were performed using a Novocontrol Alpha impedance analyzer (10°−106 Hz). Since the obtained single crystals were not large enough to perform single-crystal dielectric measurements, pellets made of well-dried samples were measured instead. The pellets were placed between two flat copper electrodes of a capacitor with a gap of approximately 1 mm. A small signal with an amplitude of 1 V was applied across the sample. The temperature was controlled by the Novo-Control Quattro system by using a nitrogen gas cryostat. The measurements were taken every 1 K in the temperature range from 160 to 360 K. The temperature stability of the samples was better than 0.1 K. Pyroelectric Measurements. Pyroelectric measurements were made by regulating the alternating temperature change on the sample and monitoring resultant short circuit pyroelectric current by an electrometer. For the pyroelectric measurements, the pellet samples were poled (with two opposite polarizations) in standard3,11 electric fields of 0.5−5 kV cm−1 in the temperature from 350 to 150 K. After removing the poling electric field and releasing space charges for at least 30 min, the pyroelectric currents were recorded with heating at a constant rate of 1 and 2 K min−1. Single Crystal X-ray Diffraction. X-ray diffraction experiments were carried out on an Xcalibur diffractometer operating with graphitemonochromated Mo Kα radiation (with 22 mA, 50 kV power settings) and a 2D CCD camera (Atlas). Absorption was corrected by multiscan methods in CrysAlis RED, Oxford Diffraction Ltd., version 1.171.33.42 (release 29-05-2009 CrysAlis171.NET). Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm was applied. The structures were solved and refined using the SHELX-2014 program package in two structural phases I (R3̅c) and II (R3c). To ensure the symmetry of the second phase, the refinements of the centrosymmetric R-3c model were also performed at lower temperatures. For all crystals, the solutions obtained in R3̅c symmetry were worse than those in R3c. Because of the complex twinning, the structure of the IIIrd phase has been solved only for MHyMn, for which the dominating domain with well-resolved reflections was found. Indexation in the triclinic cell allowed refinement only separated peaks for which Rint = 0.045. Figure S2 in Supporting Information shows the axial images of diffracted intensities for 001 and 100 direction (with respect to the hexagonal setting) in all three phases in MHyMg. Additionally, axial photos with respect to the primitive rhombohedral cell are given to better illustrate the crystal twinning after the second phase transition (Figure S3). Because of the disorder of MHy+ counterions in both phases I and II, the terminal C and N atoms from MHy+ were refined isotropically. Hydrogen atoms were included in geometric positions (C−H ∼ 0.96 Å, N−H ∼ 0.89) and treated as riding atoms. The details of the crystal, data collection, and refinement, as well as selected geometrical parameters and hydrogen bond geometry, are presented in Tables S1, S2, and S3 in Supporting Information. Table S4 summarizes the refinement results of the centrosymmetric R3̅c structure model in phase II, for comparison. Magnetic Measurements. Magnetic properties of a large number of freely oriented single crystals of MHyFe and MHyMn (about 60 and 65 mg in total, respectively) were studied using a commercial Quantum Design SQUID (superconducting quantum interference device) magnetometer. Temperature and magnetic-field dependences of magnetization were measured in the temperature range 2−30 K and in external magnetic fields up to 50 kOe. AC magnetic susceptibility was measured at the same temperatures using an AC probing magnetic field of 3 Oe and frequencies ranging from 1 to 100 Hz. The background coming from a weakly diamagnetic sample holder (not shown here) was found to be negligible in the temperature range studied in comparison to the total signal measured. Therefore, its subtraction was omitted. No demagnetization corrections were made to the data reported here either. Raman and IR studies. Room temperature Raman spectra of MHyMn, MHyMg, and MHyZn were measured using a Bruker FT 100/S spectrometer with YAG:Nd laser excitation (1064 nm). Because of the strong background for the MHyFe sample under 1064 nm

the framework that are controlled to large extent by the size, shape, and the charge distribution of these cations.34−38 Studies of hydrazinium analogues also showed that [NH2NH3][Zn(HCOO)3] can be obtained in two different structures, perovskite or chiral, depending on the growth conditions.39 This behavior was attributed to entropic and kinetic effects.39 In order to better understand the structure−property relationship in this family of compounds, it is important to search for frameworks templated by novel protonated amines and study such compounds in a broad temperature range. Recently, pressure is emerging as another important parameter that can help to understand properties of these materials.40−46 For instance, it has been shown that pressure may enhance ferroelectric properties or lead to novel phases with potentially useful properties, which are not accessible at ambient conditions.46 In the present article, we report the synthesis and studies of methylhydrazinium metal formates. There are very few reports on the synthesis of complexes with MHy+ cations, and these cations have not been yet employed in the synthesis of any MOF. We will show that in contrast to known hydrazinium, ammonium, formamidinium, imidazolium, and dimethylammonium analogues, the methylhydrazinium compounds exhibit two temperature-induced structural phase transitions. Furthermore, the iron analogue shows the stair-shaped magnetic hysteresis, that indicates presence of two magnetic components.

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EXPERIMENTAL SECTION

Synthesis. MnCl2 (99%, Sigma-Aldrich), ZnCl2 (99%, SigmaAldrich), FeCl2 (98%, Sigma-Aldrich), MgCl2 (98%, Sigma-Aldrich), methanol (99.8%, Sigma-Aldrich), methylhydrazine (98%, SigmaAldrich), and formic acid (98%, Fluka) were commercially available and used without further purification. To obtain single crystals, 10 mL of methanol solution containing 0.5 mL of methylhydrazine and 2 mL of HCOOH was placed at the bottom of a glass tube (9 mm inner diameter). To this solution, 15 mL of methanol solution containing 2 mmol of metal salt was gently added. The tube was sealed and kept undisturbed. Since Fe2+ has a tendency to oxidize to Fe3+, a small amount of ascorbic acid was added to the methanol solution of FeCl2 (this method was reported for the first time by Xu et al. in the synthesis of [NH4][Fe(HCOO)3]19). Furthermore, an additional layer of methanol was added to the FeCl2 methanol solution to completely fill the glass tube with the liquids and prevent contact with air. Light pink (for Mn) and colorless (for Zn, Mg and Fe) crystals were harvested after 5 days, washed with methanol, and dried at room temperature. A good match of their powder XRD patterns with the calculated ones based on the single-crystal data (see Figure S1 in Supporting Information) confirmed the phase purity of the bulk samples. Anal. Calcd for MHyMn (%): C, 20.25; H, 4.22; N, 11.82. Found (%): C, 20.29; H, 4.30; N, 11.74. Anal. Calcd for MHyFe (%): C, 20.18; H, 4.20; N, 11.77. Found (%): C, 20.15; H, 4.26; N, 11.68. Anal. Calcd for MHyZn (%): C, 19.40; H, 4.04; N, 11.32. Found (%): C, 19.35; H, 4.09; N, 11.24. Anal. Calcd for MHyMg (%): C, 23.26; H, 4.84; N, 13.57. Found (%): C, 23.24; H, 4.89; N, 13.48. DSC. Heat capacity was measured using Mettler Toledo DSC-1 calorimeter with high resolution of 0.4 μW. Nitrogen was used as a purging gas, and the heating and cooling rate was 5 K/min. The sample weight was 14.13, 20.97, 18.41, and 15.22 mg for MHyMn, MHyFe, MHyZn, and MHyMg, respectively. The excess heat capacity associated with the phase transitions was evaluated by subtraction from the data the baseline representing variation in the absence of the phase transitions. X-ray Powder Diffraction. Powder XRD pattern was obtained for all samples on an X’Pert PRO X-ray diffraction system equipped with a PIXcel ultrafast line detector and Soller slits for CuKα1 radiation (λ = 1.54056 Å). The powders were measured in the reflection mode, and the X-ray tube settings were 30 mA and 40 kV. 2265


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Chemistry of Materials excitation, for this sample the Raman spectrum was recorded using a Renishaw InVia Raman spectrometer equipped with a confocal DM 2500 Leica optical microscope, a thermoelectrically cooled CCD as a detector, and an argon laser operating at 488 nm. Temperaturedependent Raman spectra for MHyMn in the 3500−50 cm−1 range were recorded with a 488 nm excitation line using a Renishaw InVia Raman spectrometer and Linkam cryostat cell. Additional temperature-dependent Raman measurements in the 350−10 cm−1 range were performed using the same spectrometer but with 830 nm excitation from a diode laser and Eclipse filter. IR spectra were measured using a Biorad 575C FT-IR spectrometer. The spectral resolution of Raman and IR spectra was 2 cm−1. High-Pressure Raman Scattering Studies. The high-pressure Raman spectra were recorded in backscattering geometry using a microscope attached to a triple-grating spectrometer Jobin Yvon T64000 and the 514.4 nm line of an argon laser as excitation. The spectral resolution was 2 cm−1. In order to reach high pressures, a diamond anvil cell microscope DAC HT(S) from Almax easyLab with a diamond of 0.4 mm of culets was used. The sample was loaded into a 100 μm hole drilled in a stainless steel gasket with a thickness of 200 μm using an electric discharge machine from Almax easyLab. Nujol served as the pressure transmitting medium. Pressures were measured based on the shifts of the ruby R1 and R2 fluorescence lines.

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Figure 1. Change in (a) Cp and (b) S related to the phase transitions in the studied compounds.

RESULTS AND DISCUSSION DSC. The DSC measurements show the presence of two heat anomalies for each compound (Figure S4 and Table 1).

to be R ln 2 = 5.8 Jmol−1K−1. X-ray diffraction, Raman, and IR data suggest that the transition at T2 is associated with loss of trigonal disorder, and thus, the change in entropy at this transition is expected to be R ln 3 = 9.1 Jmol−1K−1. Large experimental values of ΔS suggest that the phase transition at T1 is associated with partial ordering of MHy+ cations, followed by their further ordering at T2. It is worth noting, however, that the ΔS values are smaller than expected (see Table 1). Similar behavior was reported for related manganese formate frameworks. For instance, the change of entropy associated with loss of the trigonal disorder in [(CH3)2NH2][Mn(HCOO)3] and [NH2CHNH2][Mn(HCOO)3] was found to be 4.7 Jmol−1K−1 and 0.97 Jmol−1K−1, respectively.8,27 As discussed in the literature, some residual entropy is always left over if a phase transition has some relaxor character.47 Thus, our results showing less than expected entropy change indicate that much residual entropy is left over, implying the relaxor character of the phase transitions. Inspection of the DSC data shows that whereas T1 weakly depends on the type of divalent cation, T2 decreases in the order Mg > Mn > Fe > Zn. This shift is large, about 70 K when going from MHyMg to MHyZn. It cannot be attributed to the mass of divalent cations since although these masses are very similar for Fe2+ and Mn2+, both compounds have significantly different transition temperatures. The size of the divalent cation also cannot explain this behavior since the ionic radius of Mg2+ is only slightly smaller (0.72 Å) than that of Zn2+ (0.74 Å).48 The highest T2 value for MHyMg can be, therefore, explained in the same way as that proposed by Pato-Doldán et al. for related [(CH3)2NH2][M(HCOO)3] compounds.9 That is, the harder Lewis acidity of the Mg2+ ion (its lower Pauling electronegativity) compared with that of other divalent metal ions gives the Mg−O bonds more ionic character and localizes the negative charge on the formate oxygens. This makes the transition temperature of MHyMg higher compared to that of MHyMn, MHyZn, and MHyFe. Recent Bader charge analysis for [(CH3)2NH2][M(HCOO)3] confirms further the conclusion that the phase transition temperature increases with increasing ionic character of the M-O bonds. Namely, it showed

Table 1. Summary of Phase Transitions and Thermal Properties of the Studied Compoundsa compound

MHyMn

MHyZn

MHyMg

Tc1 (K) ΔH (KJmol−1) ΔS (Jmol−1K−1) N, by ΔS = RlnN Tc2 (K) ΔH (KJmol−1) ΔS (Jmol−1K−1) N, by ΔS = RlnN

309d, 310u 0.53d, 0.63u, 0.58av 1.82d, 2.17u, 1.99av 1.3

309d, 310u 0.67d, 0.60u, 0.635av 2.34d, 2.07u, 2.20av 1.3

MHyFe

321d, 324u 0.79d, 0.63u, 0.71av 2.64d, 2.09u, 2.36av 1.3

326d, 327u 0.70d, 0.68u, 0.69av 2.23d, 2.18u, 2.20av 1.3

213d, 224u 0.69d, 0.62u, 65av 3.25d, 2.76u, 3.00av 1.4

180d, 183u 0.38d, 0.47u, 0.42av 2.15d, 2.68u, 2,41av 1.4

165d, 168u 0.24d, 0.27u, 0.25av 1.47d, 1.66u, 1.56av 1.2

230d, 243u 0.76d, 0.77u, 0.76av 3.37d, 3.17u, 3.27av 1.5

a

Superscripts u, d, and av denote heating, cooling, and average, respectively.

The first anomaly, observed in the 309−327 K range, is strongly asymmetric, and the change in entropy at T1 extends down to about 250−270 K (Figure 1). This type of behavior suggests a second-order character of the phase transition. The associated changes in enthalpy ΔH and entropy ΔS were estimated to be ∼0.58−0.71 KJmol−1 and ∼1.99−2.36 Jmol−1K−1, respectively (Table 1). The second anomaly is observed in a much broader temperature range, i.e., from T2 = 168 to T2 = 243 K upon heating, depending on the sample (Table 1). The strongly symmetric shape of this anomaly, the very sharp change of entropy, and large thermal hysteresis (up to 13 K for MHyMg, see Table 1, Figures 1 and S4) point to the first-order character of this transition. The associated changes in entropy are between 1.56 and 3.27 Jmol−1K−1. According to our X-ray diffraction data, the MHy+ cations are disordered within six symmetrically equivalent positions above 309−327 K and trigonally disordered at room temperature (see next paragraph). Thus, the change in entropy at T1 is expected 2266


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example. Interestingly, when the sample is poled from 350 K, which is above both phase transition temperatures, to 150 K, two distinct anomalies appear (Figure 3). First, upon heating a

that the M-O bonds become more ionic when going from Zn to Fe and Mn,5 and our results show that T2 also increases in the same order (see Table 1). Dielectric and Pyroelectric Measurements. Since the thermal data revealed the presence of two phase transitions in each sample, we decided to perform dielectric studies to see if these transitions are related to some dielectric anomalies. Figure 2 shows the temperature dependence of the dielectric

Figure 3. Pyroelectric current as a function of temperature after poling MHyMn from 350 to 150 K with ±2 kV/cm, during heating with the temperature rate of 1 K/min. The inset shows the estimated change of the spontaneous polarization of the polycrystalline sample as a function of temperature.

broad pyroelectric current peak with an amplitude of 0.7 pA appears at 219 K, and the pyroelectric current does not decrease to zero after the phase transition. Second, a sharp pyroelectric current peak around 312 K is observed. The nonzero pyroelectric current and large width of the peaks can be explained by the polycrystalline nature of the sample. More importantly, both features are reversible by reversing the poling field and fully repetitive for different heating rates and poling electric fields (see Figures S7 and S8). Such behavior suggests the ferroelectric nature of the observed phase transitions. The estimated positive spontaneous polarization values (see inset in Figure 3) clearly indicate the polar character of the roomtemperature (RT) and LT phases. Single Crystal X-ray Diffraction. To obtain information on the symmetries of the observed phases, we employed the Xray diffraction method. X-ray diffraction shows that the hightemperature phase I of MHyM crystals is trigonal, space group R3c̅ . The same symmetry was reported for the paraelectric phase of [(CH3)2NH2][M(HCOO)3] multiferroics (where M = Mn,Co,Fe,Ni).1,7,49 Metal nodes and formate linkers form a perovskite-like framework with pseudocubic holes adopting MHy+ templates; see Figure 4a. Each metal site is connected with six neighbors by formate linkers being in the anti-anti mode. The divalent metal ions occupy the C3i symmetry sites and are octahedrally coordinated by formate-oxygen ligands. The M-O distances are equal to 2.0916(9) Å (Mg), 2.1125(8) Å (Zn), 2.1385(8) Å (Fe), and 2.1859(9) Å (Mn). The Mn−O bonds are equal within the error limit to the Mn−O in [(CH 3 ) 2 NH 2 ][Mn(HCOO) 3 ] and [NH 2 CHNH 2 ][Mn(HCOO)3].27,49 The MHy+ counterions locate in perovskite-like cavities. In phase I, they are dynamically disordered within six symmetrically equivalent positions. Similarly to [(CH3)2NH2][M(HCOO)3] compounds, the MHy+ ions rotate around the 3fold (3.) axis. There are, however, additional thermally activated rotations along the 2-fold (0.2) axis, which demands higher activation energy. The mean picture, which comes from the

Figure 2. Real part of dielectric permittivity vs temperature traces for the four compounds. Lines depict high frequency (0.1 MHz) and open circles low frequency (1 Hz) regimes. Arrows indicate the phase transition temperatures obtained from DSC traces.

permittivity for the MHyM compounds at 1 Hz and 0.1 MHz frequencies. Figure 2 shows that the real part of the dielectric permittivity is relatively small (less than 10) for all compounds. The dielectric responses (ε′ and ε″) of the materials strongly depend on the chemical composition (Figures S5 and S6). Nevertheless, the temperature-dependent dielectric permittivity shows noticeable anomalies in the vicinity of the phase transition temperatures for all compounds. The low-temperature (LT) phase transition is well depicted for all frequencies, but the high-temperature (HT) one is strongly affected by the relaxation process. Furthermore, some conductivity processes appear at high temperatures (Figure S6). Dielectric data also show that replacement of one metal cation by another one has a significant impact on the electric properties. This is well depicted by the changes of the phase transition character, i.e., the smeared bell shaped peak is observed at T2 for MHyMg and MHyMn, while the step-like change occurs for MHyFe and MHyZn (Figure 2). On the basis of the character of the observed anomalies for the measured polycrystalline samples, it is impossible to clearly indicate the type of the observed phase transitions. In particular, due to the strong frequency dispersion, anomalies associated with the HT phase transition are difficult to detect. The exception is the MHyMn compound, for which a distinct anomaly appeared at the HT phase transition at low frequencies. In order to better clarify the nature of these dielectric transitions, we have also performed the pyroelectric current measurements for the MHyMn sample as a representative 2267


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Figure 4. (a) Perovskite-like metal-formate framework templated by disordered MHy+; (b) disordering of the cation in phase I, space group R3̅c, MHy+ symmetry − C2, T = 330 K; (c) disordering in phase II, polar arrangement of MHy+ gives resultant spontaneous polarization in the cdirection, space group R3c, MHy+ symmetry − C1, T = 230 K. Displacement ellipsoids are drawn with 50% probability.

Figure 5. (a) MHyMn, one of three equivalent arrangements of MHy+ highlighted in the perovskite-like cavity in phase II, space group R3c, T = 230 K; (b) the final arrangement of MHy+ in low-temperature phase III, two inequivalent settings, space group P1, T = 100 K; dashed lines stand for hydrogen bonds. Ellipsoids are drawn with 50% probability.

with donor-to-acceptor distances of ∼3.1 Å stabilize the methyl groups. As a result, the displacement parameters of the terminal nitrogen atoms are larger than those of the carbon atoms. Figure 5 illustrates one of the three positions available for MHy+ in phase II with appropriate HBs in MHyMn. The detailed HBs geometry is given in Tables 2 and S3. The I−II phase transition does not influence the metalformate framework. Its symmetry remains R3c̅ , and the M-O and C−O distances in the both phases are equal within the 3σ limit (where σ is the standard uncertainty). The pure order− disorder nature of this transition and the phase situation differs from those encountered for [(CH3)2NH2][M(HCOO)3] multiferroics, where the direct R3̅c to Cc symmetry breaking associated with ordering of the template ions and distortion of the framework is observed;2 it differs also from the R3̅c to C2/c phase transition in [NH2CHNH2][Mn(HCOO)3], which is also of mixed order−disorder and displacive character.27 The framework distortion in MHyM occurs during the II−III phase transition accompanied by a radical symmetry lowering to P1. The new unit cell is reduced to triclinic, which is a distorted primitive rhombohedral cell. However, the symmetry of the metal-formate framework seems to be higher than the symmetry of the framework with templates. On the basis only of the atomic positions of the Mn(HCOO)3− substructure, the monoclinic I2 relationships may be found between atoms in a ∼ 8.93 Å, b ∼ 8.19 Å, c ∼ 12.22 Å, and β ∼ 97.338° unit cell. Refinement of the crystal structure in pseudo-orthohexagonal axes (a ∼ 14.17 Å, b ∼ 8.18 Å, c ∼ 22.93 Å, and β ∼ 90°), in Cc space group (a maximal subgroup of the R3c), gave very poor R

diffraction experiment, is presented in Figure 4b. In phase I the terminal CH3 and NH2 groups are indistinguishable and symmetrically equivalent, which gives the same C−N and N−N distances and partial occupancy (0.167) of the available sites. Xray diffraction data show that the RT structure (phase II) remains trigonal. The structure could be solved assuming both the centrosymmetric R3̅c or R3c symmetry. However, for all crystals the solutions obtained in R3c̅ symmetry were worse than these in R3c (see Tables S1 and S4). Furthermore, the pyroelectric current measurements show strong evidence that the structure of phase II is noncentrosymmetric. The phase transition from phase I to phase II is associated with partial ordering of the template ions. In phase II, the HB interactions win over the thermally induced 0.2 rotations, prevent them, and organize MHy+ ions in a polar arrangement. All MHy+ are set toward the same direction, which gives resultant nonzero spontaneous polarization in the c-direction; see Figure 4c. The geometry of the MHy+ in phase II, with distinctly longer C−N distances over N−N ones in all crystals measured, clearly confirms the new assembly (for details, see Table S2). The easy rotations along the 3-fold axis are still present. The symmetry of the whole crystal is reduced from R3̅c to R3c, whereas the point symmetry of MHy+ is reduced to C1. It appears that the main role in the ordering of MHy+ cations is played by N−H···O interactions between the central nitrogen and the formateoxygen atoms. In all materials, these bonds have the shortest donor−acceptor distance (∼2.8−2.9 Å) and determine the placement of the entire molecule. The terminal NH2 groups do not find the acceptors, whereas the C−H···O hydrogen bonds 2268


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radius of the divalent ion B, and hX,eff. denotes the effective height of the organic cation X.51,52 Kieslich et al. showed that perovskites with the formate ligands in the anti-anti coordination mode are expected to form when 0.8 < TF < 1.51,52 The effective ionic radius of the methylhydrazinium cation has not yet been reported, but it can be calculated as rA,eff = rmass + rion (2)

Table 2. Selected Hydrogen-Bond Parameters for MHyMn in the Three Observed Phases DH···A

DH (Å)

H···A (Å)

D···A (Å)

DH···A (deg)

Phase I, T = 330 K 0.89 2.00 2.878 (3) 170.4 0.89 2.57 3.2033 (9) 128.6 0.89 2.57 3.2033 (9) 128.6 0.89 2.00 2.878 (3) 170.4 Phase II, T = 230 K N1H1A···O1ii 0.89 2.62 3.14 (2) 118.3 N1H1A···O2v 0.89 2.07 2.888 (18) 151.9 N1H1B···O1iv 0.89 1.99 2.838 (18) 157.5 N1H1B···O2vi 0.89 2.47 3.23 (2) 144.0 N2H2C···O1vii 0.85 2.63 3.095 (14) 115.6 C2H2D···O2 0.96 2.26 3.14 (2) 152.3 symmetry code(s): (i) −x + 5/3, −y + 4/3, −z + 1/3; (ii) −y + 5/3, −x + 4/3, z − 1/6; (iii) x − y + 2/3, x − 2/3, −z + 1/3; (iv) −x + y + 2/3, y − 2/3, z − 1/6; (v) −x + y + 1, −x + 1, z; (vi) −y + 1, x − y, z; (vii) x − 1/3, x − y + 1/3, z − 1/6. Phase III, T = 100 K N1H1A···O3i 0.89 1.89 2.76 (2) 166.3 N1H1B···O8ii 0.89 1.87 2.745 (19) 165.5 N2H2C···O11iii 0.90 2.42 3.20 (2) 145.7 N3H3A···O12iv 0.89 2.57 3.25 (2) 134.3 N4H4A···O5 0.89 1.95 2.822 (18) 166.0 N4H4B···O1iii 0.89 1.87 2.75 (2) 170.4 symmetry code(s): (i) x + 1, y, z − 1; (ii) x, y + 1, z − 1; (iii) x − 1, y + 1, z − 1; (iv) x, y − 1, z. N1H1B···O1i N1H1B···O1ii N1H1A···O1iii N1H1A···O1iv

with rmass being the distance between the center of mass of the molecule and the atom with the largest distance to the center of mass, and rion is the corresponding ionic radius of this ion.51 Using our X-ray diffraction data, we estimate rA,eff for the MHy+ cation as 264 pm, that is, between imidazolium (258 pm,51) and dimethylammonium (272 pm,51). The TF values can be then calculated as 0.923, 0.938, 0.951, and 0.957 for MHyMn, MHyFe, MHyZn, and MHyMg, respectively. These values are within the stability field of the perovskite structure, and they are significantly larger than that reported for hydrazinium analogues (TF = 0.814 for [NH2NH3][Mn(HCOO)3]),39 slightly larger than that for formamidinium analogues (TF for [NH2CHNH2][Mn(HCOO)3] is 0.897) but smaller than that for dimethylammonium metal formates (TF for [(CH3)2NH2][Mn(HCOO)3] is 0.941). Magnetic Properties. Since the studied compounds exhibit electric order, we further investigated if the Mn and Fe analogues also exhibit magnetic order at low temperatures. Figure 6 displays temperature dependence of the magnetization

factors and several negative isotropic displacement parameters for formate oxygen and carbon atoms. Despite the drastic symmetry decrease, the volume of the perovskite-like cavities remains almost the same in both II and III phases, and equals 75 and 74 Å3, respectively. In phase III, distinct changes are recorded in the cationic arrangement. The rotations of MHy+ are blocked, and in each cavity, template ions are ordered. They are anchored by HBs formed both between central and terminal NH2 groups and formate oxygen atoms. Figure 5b presents two independent MHy+ ions in phase III. Similarly to phase II the strongest HBs involve central NH2 groups, with a donor−acceptor distance of 2.75(2)− 2.82(2) Å; the donor−acceptor distances in bonds containing terminal NH2 groups are longer (3.20(2)−3.25(2) Å). Comparison of the HBs in the three phases shows that N···O distances change weakly upon phase I−phase II transition, indicating weak changes in the HB strength. These distances become, however, much shorter in phase III (see Table 2), implying significant increase of the HB strength. In both rhombohedral phases, the tilt of the oxygen octahedra about their triad axis can be described as a−a−a−, according to Glazer’s notation.50 In the triclinic phase, the octahedra are further distorted, and the tilt can be described as a−b−c−. The structural analysis shows that the studied compounds crystallize in the perovskite-like structures. In that context, Kieslich et al. extended the classical concept of ionic tolerance factor (TF) to hybrid organic−inorganic material. TF can be calculated using the formula: TF = (rA,eff + rX )/ 2 (rB + 0.5hX,eff )

Figure 6. Temperature dependences of magnetization M of MHyFe divided by magnetic field H measured in zero-field-cooling (ZFC, open symbols) and field-cooling (FC, closed symbols) regimes in various magnetic fields; for the sake of clarity, the experimental curves were multiplied by the factors indicated on the right side. Solid lines serve as guides for the eye, and the arrows mark the ordering temperature Tm and the blocking temperature Tb.

M of MHyFe divided by magnetic field H, measured under zero-field-cooling (ZFC) and field-cooling (FC) conditions. As seen, in all applied magnetic fields and in both regimes M(T) exhibits a pronounced anomaly at Tm = 21 K, which manifests the onset of a long-range magnetic order. In the intermediate fields H = 1 and 10 kOe, it is followed by an additional anomaly on the ZFC curves at Tb = 9 and 8 K, respectively. While the position of the anomaly at Tm is nearly field independent, its shape strongly evolves with H. In particular, in H = 0.1 kOe the magnetization curve shows a sharp, cusp-like maximum just below Tm followed by saturation below about 18 K with some bifurcation of M(T). Upon increasing the magnetic field, the

(1)

where rA,eff. denotes the effective ionic radius of the protonated amine A, rX is the ionic radius of the anion X, rB is the ionic 2269


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Chemistry of Materials cusp broadens and evolves into a Brillouin-like curve in H = 10 kOe. In 45 kOe a distinct (yet broadened) cusp is observed, and the difference between the ZFC and FC curve is hardly visible. M(T) of MHyFe reminds the behavior of [(CH3)2NH2][Fe(HCOO)3].11,53−55 The dominant ferromagnetic anomaly observed in the latter compound at 18.5 K was interpreted as a manifestation of a transition from paramagnetic to spin-canted antiferromagnetic state, i.e., weak ferromagnetism. Such behavior was already found in many metal−organic frameworks, and it is commonly interpreted as resulting from an antisymmetric exchange interaction or single-ion anisotropy,56,57 which are very likely in MOFs, including those with the perovskite-like crystal structure.7,20,49 In turn, the low-temperature drop of M of [(CH3)2NH2][Fe(HCOO)3] in the ordered region (8.8. K) was interpreted as manifestation of a blocking of magnetic moments, which is often observed in single-molecule or single-ion magnets, like, e.g., the archetypal Mn12 system.58,59 One can suppose that in MHyFe the anomaly visible at Tb has a similar origin. Field variations of the magnetization measured for MHyFe (Figure 7) seem to corroborate the latter hypothesis. In

One of the features of the single-ion magnets is a stair-shaped hysteresis loop in M(H) below the blocking temperature, which comes from resonant quantum tunneling of magnetization.54 Following the approach presented by Tian et al.54,55 we have the subtracted linear (i.e., antiferromagnetic) contribution from M(H) of MHyFe and obtained a similar, stepped shape of the remaining contribution ΔM (see the solid lines in Figure 7a), which could be another evidence for close similarity of MHyFe to multiferroic [(CH3)2NH2][Fe(HCOO)3]. Temperature dependence of the AC susceptibility of MHyFe measured in zero magnetic field is displayed in Figure S9. A sharp peak at Tm in the real part of the susceptibility (χ′, Figure S9a) and pronounced anomaly just below Tm in the imaginary part (χ″, Figure S9b) confirm our hypothesis on the ferromagnetic character of the ordering at Tm. However, in contrast to the [(CH3)2NH2][Fe(HCOO)3] compound, the AC susceptibility of MHyFe has not shown any clear frequencydependent features in the ordered region, which could be associated with the resonant quantum tunneling of magnetization.54 The coexistence of the two magnetic components in [(CH3)2NH2][Fe(HCOO)3], i.e., the spin-canted antiferromagnetism (weak ferromagnetism) and the single-ion quantum magnetism, were explained by Tian et al. by assuming the presence of long-distance superexchange interaction.54 That interaction involves at least three atoms of ligands L between the transition-metal cations T (i.e., T-L-L-L-T) and is a variation of the superexchange (T-L-T) and supersuperexchange (T-L-L-T) interactions. Such a scenario might be suitable to MHyFe not only because of the similar magnetic properties of these two compounds but also because one can distinguish in both crystal structures the necessary, relatively long O−C−O bridges of formate ligands between the Fe2+ cations. However, although MHyFe seems to possess two magnetic components, the AC susceptibility does not show any evidence for the resonant quantum tunneling of magnetization in this compound. It is worth noting that other known perovskites ([NH2CHNH2][Fe(HCOO)3] and [NH2NH3][[Fe(HCOO)3]),60,61 chiral ([NH4][Fe(HCOO)3),19 or niccolite ([NH3(CH2)4NH3][Fe2(HCOO)6])62 formate frameworks do not show features characteristic for the presence of two magnetic states and quantum tunneling of magnetization, although these compounds also possess similar O−C−O bridges. Magnetic properties of the second phase studied, i.e., MHyMn, are presented in Figure 8. In contrast to MHyFe, the compound with Mn exhibits in the M(T) curve only one and a relatively small anomaly at about 9 K (see Figure 8a). It vaguely reminds us of a Brillouin-like curvature, but distinct differences between the ZFC and FC curves allow us to ascribe it tentatively to a long-range ferromagnetic-like phase transition. Moreover, the magnetic ordering at Tm = 9 K can be observed in MHyMn only in very weak magnetic fields. Also, the magnetic hysteresis loop is hardly visible (if any) in M(H) of MHyMn; only a small deviation from the dominating linearity in M(H) is visible at the lowest magnetic fields studied (see Figure 8b). The behavior of the MHyMn compound is, therefore, similar to that reported for [NH2NH3][Mn(HCOO)3], which was interpreted as a weak ferromagnet but with negligible canting between two antiferromagnetic lattices (which is responsible for the small ferromagnetic component).20 Hence, one can conclude that MHyMn is almost a pure antiferromagnet.

Figure 7. M of MHyFe as a function of H measured at various, constant temperatures upon increasing and decreasing fields (open and closed symbols, respectively); thin solid lines and arrows serve as guides for the eye; thick solid lines display ΔM, which is the magnetization after subtraction of the linear component.

particular, M(H) measured in the ordered region below (Figure 7a) and above Tb (Figure 7b) shows the qualitatively different shape of the ferromagnetic hysteresis, which is characteristic of the blocking behavior in single-ion or single-molecule magnets.54 In the paramagnetic region (Figure 7c), the M(H) curve becomes a linear function with no trace of magnetic hysteresis, as expected. 2270


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Figure 9. Details of the Raman spectra of MHyMn at different temperatures in a heating run. Figure 8. (a) Temperature variation of the magnetization of MHyMn measured in ZFC and FC regimes (open and closed symbols, respectively); solid lines serve as guides for the eye, and the arrow marks the ordering temperature Tm. (b) M vs H measured at constant temperature upon increasing and decreasing fields (open and closed symbols, respectively); the thin solid line and arrows serve as guides to the eye.

Room-Temperature Raman and IR Studies. Further information on the mechanism of the phase transitions can be obtained by analyzing Raman and IR spectra. Room temperature Raman and IR spectra of the studied samples are presented in Figures S10 and S11. The observed Raman and IR modes and their assignments are listened in Table S5. Since internal vibrations of HCOO− ions are observed in similar wavenumber ranges for all metal formates,7,8,10,26,27,61 the assignments of these modes are straightforward. Assignment of vibrational modes of MHy+ has not been reported in the literature, but we propose an assignment of these modes based on experimental and theoretical data reported for methylhydrazine.63,64 It is worth noting that the strong IR band of methylhydrazine, observed at 813 cm−1 and attributed to the N−H bending mode,63 is not observed. This result confirms that the MHy+ cation has the CH3NH2NH2+ form. The Raman modes observed below 230 cm−1 can be assigned mainly to HCOO− vibrations.7,8,10,26,27,61 Raman and IR spectra show that a majority of modes exhibit weak dependence on a type of divalent cation. However, the ν4, ν3, and vibrational modes exhibit significant upshifts when going from MHyMn to MHyMg. This behavior can be attributed to the decreased size of the unit cell and significantly smaller mass of Mg compared to those of the other metal cations. Temperature-Dependent Raman Scattering and IR Studies. Temperature-dependent Raman and IR spectra are presented in Figures 9, 10, and S12−S14, while Figure 11 shows plots of wavenumbers and full-width-at-half-maximum (fwhm) values as a function of temperature for a few selected modes. Table S6 lists wavenumbers of the observed modes at three temperatures within the stability field of the three phases, i.e., at 380, 240, and 80 K. When temperature decreases from 400 to 230 K, the spectra in the internal modes region do not change significantly, i.e., no additional bands appear, and there are no significant changes in

Figure 10. Details of the IR spectra of MHyMn in a heating run.

the bands’ intensities that could indicate symmetry change at T1 = 310 K. Detailed analysis of Raman and IR wavenumbers as a function of temperature for a few selected modes shows only weak changes of slopes near T1 (Figure 11). This behavior indicates that the HT and intermediate phases have very similar structures, i.e., the manganese formate framework is not distorted, and the trigonal symmetry is preserved at this transition. It is also worth adding that the low wavenumber Raman spectra do not show the presence of any soft mode (Figure S13). Therefore, a displacive character of the phase transition at T1 can be ruled out. The only evident change in the spectra is significant narrowing of many internal bands (Figures 9−11 and S12−S14). Closer inspection of the spectra shows that the most affected are bands related to the vibrations of the C−H group in the HCOO− ion (ν1, ν5, and ν6 modes; see Figures 9a,b and 10). Significant narrowing is also observed for the vibrational modes of the HCOO− ions observed in the 100−250 cm−1 range (Figure S13). The observed narrowing suggests that the second-order phase transition might be related to some continuous change in the dynamics of the MHy+ cations. The first-order phase transition at T2 is very clearly observed in the spectra. First, some internal modes of the HCOO− split or exhibit sudden shifts (Figures 9a,b, 10b, and 11c). This behavior indicates that the transition leads to distortion of the framework and lowering of symmetry. The observed changes 2271


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Pressure-Dependent Raman Scattering Studies. The pressure-dependent Raman spectra of MHyMn are presented in Figure 12. Figure S15 shows pressure dependence of the

Figure 12. Pressure-dependent Raman spectra of MHyMn recorded during the compression experiment.

wavenumbers, described using a linear function ω(P) = ω0 + αP. Table S7 summarizes values of wavenumber intercepts at zero pressure (ω0) and pressure coefficients (ω = dω/dP), obtained from fitting of the experimental data by linear functions. Figure 12 shows that Raman spectra look similar each to other up to 4.8 GPa. It is worth noting that all modes related to MHy+ exhibit strong pressure dependence (Table S7). This result indicates that pressure leads to a strong increase of the amine−cavity interactions. Very large pressure coefficients α of about 9 cm−1 GPa−1 are also observed for the C−H stretching modes of the HCOO− ion (the ν1 modes, Table S7), suggesting strong decrease of the C−H bond length upon compression. However, the vibrational and remaining internal modes of the HCOO− ions show much weaker pressure dependence. In particular, the pressure coefficients α for the ν6(HCOO−), ν3(HCOO−), and L(HCOO) modes of MHyMn are smaller compared to those of [(CH3)2NH2][Mg(HCOO)3] and [(CH3)2NH2][Mn(HCOO)3], and much smaller than those for [(CH3)2NH2][Cd(HCOO)3].65,66 This result suggests that MHyMn is less flexible than its dimethylammonium analogues. When the pressure reaches 5.5 GPa, the spectra exhibit sudden changes, indicative of the onset of a first-order pressureinduced phase transition. First, the intense band observed at 802.7 cm−1 (value at 4.8 GPa) shifts to 793.6 cm−1 and its intensity decreases (Figure 12b). Furthermore, the band near 1370 cm−1 narrows, and that near 1390 cm−1 disappears. Since these bands correspond to vibrations of the HCOO− ions, the observed behavior indicates that the phase transition is related to deformation of the manganese formate framework. This conclusion is further supported by significant changes in the lattice modes region (Figure 12a). In particular, the appearance of a few new lattice modes points to the decrease of symmetry of the high-pressure phase. Our data also show that pressureinduced transition leads to a very large shift of the δ(CNN) mode (by 25.5 cm−1) and significant shifts of the νs(CNN) (by 10.2 cm−1) and νas(CNN) (by 6.7 cm−1) modes. This behavior shows that phase transition is also associated with a significant change of the MHy+ structure, i.e., changes in the C−N and N−N bonds as well as the CNN angle. The lack of splitting for

Figure 11. Temperature dependence of Raman (a, b, c, and e) and IR (d) wavenumbers for ν(NH2) (a), νs(CH3) (b), ν1(HCOO−) (c), ρ(NH2) (d), and νs(CNN) (e) modes. Panel f shows the temperature dependence of fwhm values for three selected modes. Vertical dashed lines correspond to the phase transition temperatures.

are, however, less pronounced than reported for the dimethylammonium analogue,7 suggesting weaker distortion of the MHyMn framework. Second, many new bands appear in the lattice modes region (Figure S13), supporting the lowering of the crystal symmetry. Third, many bands show strong narrowing below T2, especially in the lattice modes region (Figure S13). This behavior proves that the phase transition has an order−disorder character. In the internal modes region, the largest change in fwhm at T2 is observed for the ρ(NH2) mode (Figures 10c and 11f). This observation proves that the dynamics of this group plays a major role in the mechanism of the phase transition. Fourth, the ρ(NH2) mode exhibits a large upshift at T2 (Figure 11d), indicative of increased HB strength. Furthermore, the band near 3200 cm−1 that can be attributed to the stretching mode of the second, outer NH2 group, showed a wavenumber decrease on cooling from 400 to 230 K followed by sudden upshift at T2 (Figure 11a). The first feature indicates increased N−H bond length on cooling the HT and intermediate phases, whereas the second one points to abrupt shortening of this bond in the LT phase. Fifth, the νs(CNN) modes do not exhibit any discontinuous shift at the phase transition (Figure 8e) indicating that the C−N and C−C bonds are weakly affected by the phase transition. Different behavior was observed for the dimethylammonium cation, i.e., clear upshift of the νs(CNC) mode at Tc.7 2272


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Chemistry of Materials the discussed modes proves, however, that the high-pressure phase has still one distinct MHy+ cation. Raman spectra of MHyMn during the decompression are presented in Figure S16. The observed changes in the Raman spectra are similar to those observed upon compression indicating reversibility of the phase transition.

DSC traces, pyroelectric current of MHyMn after different poling electric fields and temperature rates, AC magnetic susceptibility of MHyFe as a function of temperature, temperature-dependent traces of the complex dielectric permittivities, IR and Raman spectra at different temperatures, wavenumber vs pressure plots of the Raman modes observed in the MHyMn crystal for compression experiments, and Raman spectra recorded during the decompression experiment (PDF) Compound MHyZn at 350, 300 and 180 K (CIF) Compound MHyFe at 330, 280 and 200 K (CIF) Compound MHyMg at 345, 280 and 240 K (CIF) Compound MHyMn at 330, 290 and 230 KX (CIF) Compound MHyMn at 100 K (CIF)

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CONCLUSIONS Four novel compounds, containing MHy+ cations as templating agents, were successfully grown by the slow diffusion method. In the high-temperature R3̅c phase, the MHy+ cations are dynamically disordered, i.e, similarly to [(CH3)2NH2][M(HCOO)3] multiferroics, the MHy+ ions rotate around the 3fold axis. However, there are additional, thermally activated rotations along the 2-fold axis. Below 310−327 K, rotations along the 2-fold axis are hindered leading to the polar arrangement of MHy+ ions, which gives resultant nonzero spontaneous polarization in the c-direction. Pyroelectric current measurements confirm the ferroelectric nature of the roomtemperature phase. It is worth adding here that for any application, ferroelectric properties at room temperature or above are desired. The MHyM compounds discovered here fulfill this condition, whereas the most famous [(CH3)2NH2][M(HCOO)3] multiferroics exhibit ferroelectric order well below 300 K. The R3c phase undergoes another phase transition at low temperatures. X-ray diffraction and vibrational studies indicate that this transition is associated with a further reduction of symmetry and ordering of MHy+ ions as well as distortion of the metal formate framework. MHyMn and MHyFe also exhibit magnetic order at 9 and 21 K, respectively, i.e., these compounds are possible new members of metal formate frameworks exhibiting the coexistence of magnetic and electric orders in a single phase. It is worth adding that MHyFe shows additional magnetic anomalies in the magnetically ordered state, which is tentatively interpreted as manifesting some blocking of magnetic moments, often observed in weak ferromagnets. In addition to the temperature-dependent studies, we also performed high-pressure Raman studies of MHyMn. These studies revealed higher stiffness of this compound compared to that of the dimethylammonium analogue and the onset of a pressure-induced phase transition between 4.8 and 5.5 GPa. Analysis of the high-pressure data indicates that the phase transition has a first-order nature and is associated with significant changes in both the manganese formate framework and the MHy+ structure.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48-713954161. Fax: +48-713441029. E-mail: m. [email protected]. ORCID

Mirosław Mączka: 0000-0003-2978-1093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Brazilian National Research Council (CNPq) for a fellowship and grant 401849/2013-9. REFERENCES

(1) Jain, P.; Ramachandran, V.; Clark, R. J.; Zhou, H. D.; Toby, B. H.; Dalal, N. S.; Kroto, H. W.; Cheetham, A. K. Multiferroic Behavior Associated with an Order−Disorder Hydrogen Bonding Transition in Metal−Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture. J. Am. Chem. Soc. 2009, 131, 13625−13627. (2) Sánchez-Andújar, M.; Presedo, S.; Yáñez-Vilar, S.; Castro-Garcia, S.; Shamir, J.; Señaris-Rodriguez, M. A. Characterization of the OrderDisorder Dielectric Transition in the Hybrid Organic-Inorganic Perovskite-Like Formate Mn(HCOO)3[(CH3)2NH2]. Inorg. Chem. 2010, 49, 1510−1516. (3) Wang, W.; Yan, L.-Q.; Cong, J.-Z.; Zhao, Y.-L.; Wang, F.; Shen, S.-P.; Zhou, T.; Zhang, D.; Wang, S.-G.; Han, X.-F.; Sun, Y. Magnetoelectric Coupling in the Paramagnetic State of a MetalOrganic Framework. Sci. Rep. 2013, 3, 2024. (4) Jain, P.; Stroppa, A.; Nabok, D.; Marino, A.; Rubano, A.; Paparo, D.; Matsubara, M.; Nakotte, H.; Fiebig, M.; Picozzi, S.; Choi, E. S.; Cheetham, A. K.; Draxl, C.; Dalal, N. S.; Zapf, V. S. Switchable Electric Polarization and Ferroelectric Domains in a Metal-Organic-Framework. NPJ. Quantum Mater. 2016, 1, 16012. (5) Kosa, M.; Major, D. T. Structural Trends in Hybrid Perovskites [Me2NH2]M[HCOO]3 (M = Mn, Fe, Co, Ni, Zn): Computational Assessment Based on Bader Charge Analysis. CrystEngComm 2015, 17, 295−298. (6) 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. (7) Mączka, M.; Gągor, 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)2ND2][M(HCOO)3] (M = Ni, Mn). Inorg. Chem. 2014, 53, 457−467. (8) Mączka, M.; Sieradzki, A.; Bondzior, B.; Dereń, P. J.; Hanuza, J.; Hermanowicz, K. Effect of Aliovalent Doping on the Properties of Perovskite-Like Multiferroic Formates. J. Mater. Chem. C 2015, 3, 9337−9345.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05249. Crystal data and selected geometrical parameters for the studied compounds at different temperatures, summary of the refinement results of the centrosymmetric R-3c structure model in phase II, Raman and IR wavenumbers together with the proposed assignments and wavenumber intercepts at zero pressure (ω0), and pressure coefficients (α = dω/dP) for the two phases of MHyMn observed in the high pressure experiment; powder XRD patterns, axial photos for different directions in MHyMg, 2273


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