A Series of Chiral, Polar, Homospin Topological Ferrimagnets: M3

Article pubs.acs.org/cm

A Series of Chiral, Polar, Homospin Topological Ferrimagnets: M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni) Joshua T. Greenfield,† Colin D. Unger,† Michael Chen,† Nezhueyotl Izquierdo,† Katherine E. Woo,† V. Ovidiu Garlea,‡ Saeed Kamali,§ and Kirill Kovnir*,†,∥ †

Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Mechanical, Aerospace and Biomedical Engineering Department, University of Tennessee Space Institute, Tullahoma, Tennessee 37388, United States ∥ Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States ‡

S Supporting Information *

ABSTRACT: An isostructural series of transition metalformate-chloride-hydrate compounds, M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni), have been synthesized using a solvothermal method. These compounds crystallize in the chiral and polar space group P31 and are comprised of three different types of helical chains of edge-sharing M2+-centered octahedra. All three compounds undergo 3D ferrimagnetic ordering at low temperature, and the iron and cobalt analogues exhibit fieldinduced metamagnetic transitions. The magnetic structure was determined by neutron powder diffraction, revealing ferromagnetic intrachain coupling and antiferromagnetic interchain interactions, with the three chains arranged in a two-up/onedown triangular lattice. As all three chains contain one type of metal in the same spin state, these compounds are rare examples of homospin topological ferrimagnets.

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INTRODUCTION

Prior to the start of our work, the 3d transition metal formate chlorides were essentially unexplored. We have previously reported on a series of transition metal-formate-chlorides, NH4MCl2(OOCH) (M = Fe, Co, Ni), which are characterized by infinite 1D zigzag chains of M2+-centered octahedra linked by μ2-chloride and syn-syn formate bridges. These compounds order antiferromagnetically at low temperature and exhibit anisotropic metamagnetic transitions.23,24 In this report, we describe an alternative structure in which the linear chains are twisted into helices, which can be formed by removing the monovalent cation from the synthesis. Presented here is the synthesis, structure, and characterization of M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni), a series of chiral, polar, homospin topological ferrimagnets.

In recent years, there has been considerable interest in the study of low-dimensional magnetic coordination polymers, specifically in the search for compounds exhibiting high magnetic anisotropy.1−5 One strategy for designing these compounds is to use three-atom organic linkers to connect transition metal atoms in hybrid-inorganic structures; 6 carboxylate groups in particular have been shown to efficiently transmit both ferromagnetic (FM) and antiferromagnetic (AFM) couplings, and a wide variety of bridging modes allow for many different structure types ranging from isolated chains to metal−organic frameworks.7,8 The formate anion has been studied extensively in this capacity, and examples of compounds exhibiting 1D9 to 3D10 magnetic ordering, chirality,11 porosity,12 and ferroelectricity13 are known. There have also been several reports of compounds exhibiting multiple functionalities, including coexistent magnetic and electric ordering in a chiral framework,14 as well as guest-dependent magnetism15,16 and chirality17 in porous frameworks. Different bridging ligands can also be mixed to tune the structure and properties; single-atom bridges such as end-on azide18 and μ2chloride19 are useful for transmitting magnetic coupling, while larger or bifunctional carboxylates can magnetically isolate or cross-link chains of metal atoms across greater distances.9,20−22 © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. Caution: Solvothermal reactions involving transition metal chlorides and formic acid may produce various gaseous species, leading to the generation of extremely high pressures. Suitable high-strength reaction vessels are required to minimize the risk of an explosion. A solvothermal method was used to prepare phase-pure samples of M 3(OOCH) 5Cl(OH2) (M = Fe, Co, Ni). Iron(II) chloride Received: May 1, 2017 Revised: August 23, 2017 Published: August 25, 2017 7716

DOI: 10.1021/acs.chemmater.7b01755 Chem. Mater. 2017, 29, 7716−7724

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Chemistry of Materials tetrahydrate, cobalt(II) chloride hexahydrate, nickel(II) chloride hexahydrate (all Alfa Aesar, 98%), formic acid (Acros Organics, 99%), and ethanol (Koptec, >99.5%) were used as received. In a typical synthesis, 1−2 mmol of MCl2·nH2O (M = Fe, Co, Ni), 10 mL (0.27 mol) of HCOOH, and 20 mL of ethanol were added to a 45 mL polytetrafluoroethylene (PTFE)-lined stainless steel acid digestion vessel (Parr Instrument Company) equipped with two 0.2 mm stainless steel rupture discs. The vessel was sealed tightly and heated in a drying oven at 200 °C for 24 h, followed by removal from the oven for natural cooling. Solid products were filtered and washed with ethanol, yielding a large mass of extremely fine needle-like crystals that resembled glass wool. Crystals of the iron, cobalt, and nickel compounds were colorless, bright pink, and pale green, respectively. All operations involving the iron compound were performed in an argon-filled glovebox to prevent oxidation, and the cobalt and nickel analogues were also stored in a glovebox to prevent slow decomposition to the starting hydrated metal chlorides. Characterization. The phase-purity of each sample was confirmed by powder X-ray diffraction (PXRD) using a Rigaku MiniFlex 600 diffractometer with Cu Kα radiation (Figure SI 1). Single-crystal X-ray diffraction data sets were collected using a Bruker D8 Venture with a Photon CMOS detector for the iron compound and a Bruker Apex II for the cobalt compound. As the largest crystal of the nickel compound only measured 2 × 2 × 6 μm, it could not be measured with a laboratory diffractometer and data collection was therefore performed at beamline 11.3.1 at the Advanced Light Source (ALS) at Lawrence Berkeley National Lab (LBNL). All three compounds could be indexed to similar trigonal unit cells, and integration was performed using the Bruker SAINT software package.25 The structures for all three compounds were solved in space group P31 (no. 144) using the SHELX suite of programs;26 multiple measurements were performed on different crystals of the iron and cobalt analogues, several of which also crystallized in space group P32. For the iron and cobalt compounds, all non-hydrogen atoms were refined with anisotropic thermal displacement parameters; hydrogen atoms in the water moiety were fully refined with isotropic thermal parameters, while formate hydrogens were refined positionally with thermal parameters set to 1.2·Ueq of the bonded carbon atom. Due to the weak diffraction from the small crystal of the nickel compound, only heavy elements (Ni, Cl) were refined anisotropically, while light atoms were kept isotropic and hydrogen atoms were constructed geometrically. Additionally, analysis with the software Rotax27 revealed that the crystal of the nickel compound was a merohedral twin, with one component (37%) rotated by 180° about the [1−100] direction. Further details about the data collection, structure refinement, unit cell parameters, and selected interatomic distances are summarized in Table 1. Solid-state UV−vis diffuse reflectance spectroscopy was performed using a Thermo Scientific Evolution 220 spectrophotometer. Samples were prepared by pressing finely ground material onto a piece of white filter paper, which was taped onto a glass slide wrapped in white PTFE-tape; diffuse reflectance data were collected in Kubelka−Munk mode from 190 to 1100 nm. 57 Fe Mössbauer spectra were collected using a conventional constant-acceleration spectrometer with a 57Co/Rh source held at room temperature. A powdered sample of Fe3(OOCH)5Cl(OH2) was diluted in hexagonal boron nitride to reduce preferred orientation, and spectra were collected at room temperature and at 6 K. Least-squares data fitting was performed using the Recoil software package,28 and all centroid shifts (δ) are given with respect to metallic α-Fe at room temperature. A Quantum Design MPMS-XL SQUID magnetometer was used to measure temperature-dependent magnetic susceptibility (2−300 K, 0.01 T applied field) and isothermal magnetization (2 K, 0−7 T applied field). Measurements were performed on finely ground powder samples that were held between two long strips of Kapton tape to minimize diamagnetic contributions from the sample holder. The magnetic structure was determined for Co3(OOCH)5Cl(OH2) from low-temperature powder neutron diffraction data collected on beamline HB-2A at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Diffraction patterns were

Table 1. Single Crystal Data Collection and Structure Refinement Parameters for M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni)a metal Fe space group temp. (K) λ (Å) a (Å) c (Å) V (Å) Z size (μm) ρ (g·cm−3) μ (mm−1) θ (deg) data/param. flack x R1/wR2 (%) GoF M1−M1 M2−M2 M3−M3 M1−M2 M1−M3 M2−M3

Co

P31 (no. 144) 100 (2) 90 (2) 0.71073 (Mo Kα) 12.2116(3) 12.090(1) 7.5739(2) 7.533(1) 978.13(5) 953.7(2) 3 13 × 14 × 98 29 × 38 × 444 2.27 2.31 3.55 4.35 3.31−30.53 1.95−30.50 4006/204 3876/205 −0.006(12) 0.006(11) 2.80/5.08 2.62/5.42 1.04 1.05 Intrachain Metal−Metal Distance (Å) 3.21 3.17 3.26 3.20 3.33 3.29 Interchain Metal−Metal Distance (Å) 5.15 5.15 5.28 5.27 5.72 5.68

Ni

0.77490b 11.9286(5) 7.3887(4) 910.5(1) 2×2×6 2.49 6.17 2.15−28.88 2491/102 0.02(3) 5.72/12.10 1.06 3.10 3.12 3.21 5.12 5.24 5.63

a

Further details are available from the Cambridge Crystallographic Data Centre by quoting the depository numbers CCDC 1530342, 1530343, and 1530344. bData for Ni3(HCOO)5Cl(OH2) were collected at beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Lab.

collected at two wavelengths (Ge115, λ = 1.53618 Å; Ge113, λ = 2.40608 Å) and two temperatures (1.5 and 10 K) with the powder sample loaded into a vanadium can. Additional intensity from magnetic scattering appeared in the 1.5 K pattern, and the magnetic peaks could be indexed using the chemical unit cell, implying the trivial propagation vector k = (0,0,0). Symmetry-allowed magnetic models were generated for the space group P31 using the SARAh Representational Analysis program.29 The Fullprof Suite program30 was used in combined mode for simultaneous Rietveld refinement of both data sets (two neutron wavelengths) at each temperature. The 10 K patterns were used to refine the nuclear structure with isotropic thermal displacement parameters. To account for the additional intensity from magnetic diffraction in the 1.5 K pattern, a phase compatible with the magnetic space group P31 (#144.4) was added; the symmetry operators and magnetic ordering sequence for the cobalt sites are (x, y, z|μa, μb, μc), (−y, x − y, z + 1/3|−μb, μa − μb, μc), and (−x + y, −x, z + 2/3|−μa + μb, −μb, μc). The μa and μb components were constrained to be equal, as powder averaging prevents the determination of the moment direction in the basal plane; the μa=b and μc components of the magnetic moment were refined with equal magnitudes for the three different sites, and the Co1 moments were chosen to be antiparallel to those of Co2 and Co3.

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RESULTS AND DISCUSSION Synthesis. The compounds presented here were initially discovered as a side product in a solvothermal reaction between elemental iron and selenium in dilute formic acid, with ammonium chloride present as a mineralizing agent. Removing the selenium from this mixture avoided the unfortunate formation of H2Se gas but also did not afford the same 7717


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Chemistry of Materials

Figure 1. Crystal structure of M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni). (A) Helical chains of metal atoms related by 31 screw axes along c. (B) Side view of the three types of helical chains. (C and D) General view of unit cell along c and a axes. M1: magenta; M2: cyan; M3: yellow; Cl: green; O: red; C: black; H: white.

product. Subsequent optimization showed that FeCl2·4H2O was a more suitable precursor, and its solubility also made the ammonium chloride unnecessary. The target compound was successfully synthesized by dissolving FeCl2·4H2O in a 2:1 mixture of ethanol and formic acid, followed by heating in a sealed vessel at 200 °C for 24 h and natural cooling; this procedure was also successful with CoCl2·6H2O and NiCl2· 6H2O. Both the iron and cobalt compounds form very large masses of long, thin needles that occupy roughly half the volume of the autoclave, while the nickel compound forms a similar quantity of extremely small needles that do not exceed 10 μm in length; this difficulty in growing large crystals of the nickel analogue has also been reported for Ni3(OOCH)6.31 As crystals were observed to form in either of the space groups P31/P32, we assume that the reaction products crystallize in both space groups with equal frequency, with each crystal containing only one helical enantiomer. The synthetic conditions necessary to form phase-pure M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni) are quite flexible, with reaction temperature and water concentration having the most significant effects. All reactions performed at temperatures below 180 °C yielded no solid products, whereas those heated between 180 and 220 °C produced only the target compounds. Reactions at the lower end of this range had smaller yields, while reactions at 220 °C were significantly more likely to overpressurize and burst despite the use of two 0.2 mm rupture discs. The concentration of water must be carefully controlled as well; while the esterification of formic acid and ethanol under reaction conditions should in theory generate enough water to form the product, the use of anhydrous metal chlorides only produced polycrystalline samples of M2(OH)3Cl. However, the use of 96% ethanol also prevented the formation of the target phase, placing an upper limit on the acceptable water concentration. Factors with less significant effects include the reaction time, the cooling rate, and the concentration of formic acid. There was no observable difference in products for reactions heated longer than 24 h, but if left too long (>7 days), the reaction vessels will overpressurize due to the decomposition of formic acid into various gaseous species.10,32

Similarly, modification of the cooling rate had no effect on the yield or crystal size. The ratio of formic acid to ethanol only affected the outcome of the reaction at the extremes; less than 5 mL of formic acid (with a constant solvent volume of 30 mL) reduced the yield of solid products, while more than 25 mL led to overpressurization. One special case is the iron analogue, for which at least 10 mL of formic acid was necessary to prevent the formation of α-Fe2O3, though this was subsequently avoided by performing the synthesis under inert atmosphere. The cobalt and nickel compounds were stable in dry air, while the iron compound slowly oxidized under ambient conditions, turning a light gray color. In moist air, all three compounds slowly degraded back to hydrated metal chloride salts, and this conversion was markedly accelerated when the crystals were ground into powders. Crystal Structure. M3(OOCH)5Cl(OH2) is of particular interest because all three of its isostructural analogues crystallize in one of the enantiomorphic space group pairs, P31/P32; the chiral and polar nature of these groups allows for any of the primary ferroic order parameters, leading to the possibility of multiferroic behavior. The structure itself is built around three crystallographically distinct metal atoms (M1, M2, and M3) that each form helical chains via a 31 screw axis along [0001], as can be seen in Figure 1A. These chains are made up of distorted edge-sharing octahedra, similar to the linear zigzag chains in NH4MCl2(OOCH) (M = Fe, Co, Ni),24 but in this case, the shared edges include one trans-vertex and one cisvertex. Despite the lack of any chiral components or structuredirecting agents, each shared edge shifts in the same direction along all of the chains, thus making the structure chiral. Each of the three metal centers has a different octahedral environment: M1 is coordinated by oxygen from six formates, M2 by oxygen from five formates and one water, and M3 by oxygen from four formates and two chlorine atoms (Figure 1B). In the iron compound, the octahedral angles vary from 77.9° to 101.6°, with Fe−O bonds ranging from 2.06 to 2.15 Å and Fe−Cl bonds of 2.50 Å; the cobalt and nickel analogues show a similar trend with slightly reduced bond lengths. All of the formate groups bond to three different metal atoms in a μ37718


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Chemistry of Materials (syn, syn, anti)-coordination mode, with three formates forming intrachain linkages and the remaining two connecting separate chains. For a helical chain with the sequential atoms M, M′, and M″, one oxygen of the intrachain formate is located on an unshared vertex of M, while the other oxygen forms one-half of the shared edge between M′ and M″. This causes the octahedra to tilt toward each other with dihedral angles ∠M−O−O−M in the range of 148−154°, effectively making each chain polar; when generated by a 31 screw axis, any deviation from 180° precludes rotational symmetry normal to the direction of chain propagation. A convenient marker for the unique direction of a given chain is the C−H bond of the intrachain formate, which bisects the angle between adjacent octahedra; as can be seen in Figure 1B,D, this bond points downward for Chain 1 and upward for Chains 2 and 3. While all three chains have an identical configuration of intrachain formates occupying three of the six vertices surrounding each metal atom, the remaining three vertices differentiate the chains and control the overall connectivity of the framework. In Chain 1, a second formate group occupies the other unshared vertex and links to the remaining shared edge in Chain 2, while a third completes the shared edge in Chain 1 and links to the unshared vertex in Chain 3. In Chain 2, the last unshared vertex is occupied by a covalently bound water molecule, while in Chain 3 two of the shared vertices are occupied by chlorine atoms. As shown in Figure 1C, the chains pack in a hexagonal array where each chain only neighbors chains of the other two types; Chain 1 has formate bridges connecting it to both Chains 2 and 3, but the sites on these chains that would allow them to be linked to each other instead contain the water and chlorine, forming a relatively weak hydrogen bond (O−H···Cl = 2.34 Å) instead of a covalent formate bridge. Note that only chains with opposite directionalities are linked by formate bridges. The overall geometry of the helical chains in M3(OOCH)5Cl(OH2) is similar to those found in several other transition metal formates, such as Fe(OOCH)2· 1/3HCOOH 3 3 and β-Mg(OOCH) 2 . 3 2 Fe(OOCH) 2 · 1/3HCOOH shares the same intrachain bonding as Chain 1, but the chains are generated by a 21 screw axis and are crosslinked by additional metal centers to form a porous framework structure; β-Mg(OOCH)2 is comprised of 3-fold helical chains nearly identical to Chain 1, but they each contain multiple crystallographically distinct magnesium atoms and are interconnected in a hybrid square-hexagonal pattern. There are exceptionally few helical compounds that show mixed bridging similar to what occurs in Chain 3,34 and we were unable to find an example of a helical chain compound with edge-sharing octahedra that contained both mixed and nonmixed bridging along the chains. UV−Vis and Mö ssbauer Spectroscopy. The UV−vis diffuse reflectance spectra for M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni) are shown in Figure 2. All three compounds appear to be transparent to UV below 200 nm but show a single large peak in the range of 230−270 nm. The visible region is populated with the expected number of absorption bands based on Tanabe-Sugano diagrams for d6, d7, and d8 atoms in an octahedral environment; the spectra are also in good agreement with previously reported transition metal formate compounds.35 The colorless iron compound has a broad peak centered at 975 nm and an extremely weak peak at 455 nm; the pink cobalt compound has two overlapping peaks at 480 and 525 nm with a third weak peak at 650 nm, and the pale green

Figure 2. UV−vis spectra for M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni) collected between 190 and 1100 nm. Insets: photographs of finely ground powder samples of each compound (Fe: colorless/gray; Co: pink; Ni: pale green) and an expanded view of the d−d transitions in the 300−1100 nm region.

nickel compound has one peak at 405 nm and two overlapping peaks at 680 and 740 nm. 57 Fe Mö s sbauer spectroscopy was performed on Fe3(OOCH)5Cl(OH2) to confirm the oxidation and spin states of the three distinct metal centers (Figure 3). Spectra were collected at room temperature and at 6 K to probe for any structural transitions or magnetic ordering that might occur in

Figure 3. 57Fe Mössbauer spectra of Fe3(OOCH)5Cl(OH2) collected at 293 K (top) and 6 K (bottom). Experimental data: black circles; calculated spectrum: black lines; component spectra: magenta, cyan, and yellow lines. 7719


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Chemistry of Materials this temperature range, though no changes were observed; the following discussion of hyperfine parameters refers only to the values derived from the room-temperature data. The spectra were best fit using three nonmagnetic doublets which assumed values close to one-third of the total intensity when freely refined; on the basis of knowledge of the crystal structure, these components were fixed at equal intensities for the final refinement. All of the doublets have centroid shifts characteristic of high-spin (S = 2) Fe2+,36 and the relatively large and temperature-dependent electric quadrupole splittings (ΔEQ) suggest high electric field gradients (EFG) caused by the distortion of the octahedral environments.33 Of the three components, Q1 and Q2 are so similar to each other that their doublets mostly overlap, while Q3 is better resolved due to its larger ΔEQ. As increasing ΔEQ values correspond to an increasing EFG around the iron atoms, it is therefore possible to assign each doublet to one of the three types of helical chains based on differences in local coordination: Q1 (ΔEQ = 0.995 mm/s) corresponds to Chain 1, which contains metal centers coordinated by six formate oxygens; Q2 (ΔEQ = 1.305 mm/s) corresponds to Chain 2, where one formate oxygen is replaced by a neutral water oxygen, leading to a slightly more distorted environment and a larger EFG; Q3 (ΔEQ = 1.999) corresponds to Chain 3, which is by far the most distorted with four short Fe−O bonds and two long Fe−Cl bonds. This value is also similar to what was observed for FeCl4(OOCH)2 octahedra connected in zigzag chains.23,24 These measurements confirm that none of the iron atoms are in the +3 oxidation state, as would be expected if the covalently bound water in Chain 2 was instead a hydroxide group, and this distinction is further supported by the results of neutron powder diffraction (see below). Magnetism and Magnetic Structure. As the crystal structure of M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni) permits any of the primary ferroic order parameters, we investigated the magnetic properties of all three compounds by SQUID magnetometry; the temperature-dependent magnetic susceptibility and isothermal magnetization for each analogue can be seen in Figure 4. All three compounds undergo 3D magnetic ordering at low temperature, though the type of ordering is not as simple as it first appears. The susceptibility of the iron compound begins to substantially deviate from Curie-Weiss behavior below 3 K, though the transition does not appear to be complete and the true ordering temperature is likely to be lower than the base temperature of 1.9 K. This assumption is supported with the Mössbauer spectroscopy results where no hyperfine splitting or line broadening were observed at 6 K, indicating that magnetic ordering should be at lower temperatures. The cobalt compound more clearly shows ferro- or ferrimagnetic ordering with the zero-field cooled (ZFC) and field-cooled (FC) measurements diverging at 3 K, while the nickel compound orders in a similar fashion at 6 K. However, the isothermal magnetization reveals that these materials are not simple ferromagnets. In the iron compound, the moment initially saturates around 1 μB/mol Fe at 1 T, but between 2 and 3 T, there is a clear inflection point and the moment increases to 2.9 μB/mol Fe at 7 T. Even though the compound was not fully ordered at the measurement temperature of 2 K, the step in the magnetization is indicative of a metamagnetic transition; similar behavior has been observed for Fe(OOCH)2·1/3HCOOH, which exhibits partial magnetic saturation at 20 K, slightly above its ordering temperature of 16 K.33 The cobalt compound behaves similarly,

Figure 4. Temperature-dependent magnetic susceptibility for M3(OOCH)5Cl(OH2) (M = Fe, Co, Ni). Insets: isothermal magnetization for each compound and Curie-Weiss fitting parameters from high-temperature data (see Figure SI 2 for additional information). Zero-field cooled: open symbols; field cooled: closed symbols; Fe: red circles; Co: blue triangles; Ni: magenta squares.

though the effect is much more pronounced; the initial saturation reaches a moment of 0.93 μB/mol Co at 1.7 T, but after a transition between 3.5 and 4.5 T, the final moment is 2.75 μB/mol Co. This value is slightly lower than the expected value of 3 μB, though in this case the compound appears to be fully ordered at the measurement temperature and it is much more likely that the discrepancy is due to either noncollinearity of the magnetic moments or a deviation of the g-factor to be slightly less than 2. It is also apparent that in both the iron and cobalt compounds the first saturation reaches only one-third of the total moment, while the remaining two-thirds are achieved through the metamagnetic transition; this strongly suggests that the three types of helical chains are arranged such that two have net magnetizations in one direction and the third is pointed in the opposite direction. While a distinct metamagnetic transition 7720


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Figure 5. Neutron powder diffraction and magnetic structure of Co3(OOCH)5Cl(OH2). (A) Comparison of 10 and 1.5 K diffraction patterns showing magnetic peaks at 1.5 K. Inset: order parameter for the (1,0,1̅,0) reflection, compared to the temperature dependence of the magnetic susceptibility. (B) Rietveld refinement of the 1.5 K data. Tick marks show the calculated peak positions for the nuclear (black) and magnetic (blue) phases. (C) General view of the magnetic structure viewed along [0001] with intrachain formates and nonwater hydrogens omitted for clarity. (D) Side view along [112̅0] showing only the metal atoms. (E) Diagram showing the net magnetic moment of each chain and interchain connectivity. Magnetic moments are shown as magenta, cyan, and yellow arrows for M1, M2, and M3, respectively.

Co, slightly above the reported value for Co(OOCH)2·1/ 3HCOOH·1/2H2O33 and equal to that of NH4CoCl2HCOO.24 This value is typical for octahedral Co2+, as significant spin-orbit coupling leads to higher moments than the spin-only value of 3.87 μB/mol Co.37,38 The inverse magnetic susceptibility of the nickel compound exhibited a non-linear temperature-dependence above the ordering temperature, a behavior that is typical for materials with a significant temperature-independent paramagnetic component; this is best described by the modified Curie-Weiss law,37 and fitting in the range of 50-300 K resulted in a temperature-independent component (χ0) of 0.0178 emu/ mol. The derived value μEff = 3.22 μB/mol Ni is typical of Ni2+ compounds37,38 and is slightly lower than what was reported for NH4NiCl2HCOO.24 All of the compounds had negative θ values, indicating that the short-range magnetic interactions are predominantly antiferromagnetic. For the iron compound, the absolute value of θ = −5.4 K is at least two to three times the ordering temperature, indicating some degree of magnetic frustration; for the cobalt compound (θ = −23.1 K), this ratio is closer to eight, suggesting a very high degree of frustration. The nickel compound (θ = −2.2 K) does not follow the trend, but this value was found to be highly correlated to the other parameters; the values presented are from a free refinement of θ, C, and χ0, but even slight changes can produce larger

is not observed in the nickel analogue, its ordering temperature of 6 K and initial saturation of the magnetization at 0.5 T are significantly higher than the values for the cobalt compound (3 K and 0.05 T, respectively); this suggests that the magnetic coupling in the nickel compound is significantly stronger and a higher field strength than the available 7 T might be required to induce the metamagnetic transition. As spin−orbit coupling causes the typical value of g to be 2.25 for octahedral Ni2+ compounds,37,38 the first saturation moment of 0.75 μB/mol Ni is exactly one-third of the expected 2.25 μB and fits well with a two-up/one-down arrangement of the chains. Curie-Weiss fitting was performed on the high-temperature data for each compound (Figure SI 2) to determine the asymptotic Curie temperature (θ), the Curie constant (C), and the effective magnetic moment (μEff). The details of the fitting procedures are provided in the Supporting Information. Fitting for the iron compound was performed from 50 to 100 K as there was a significant deviation from Curie-Weiss behavior at higher temperatures, likely due to the population of several excited states.37 The value derived from the linear region (μEff = 5.99 μB/mol Fe) is in good agreement with what has been reported for Fe(OOCH)2·1/3HCOOH.33 The cobalt analogue showed no deviations from standard Curie-Weiss behavior from 50−300 K. Fitting this range yielded μEff = 5.22 μB/mol 7721


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Chemistry of Materials negative θ values (e.g., fixing χ0 = 0.019 emu/mol gives θ = −11 K). Overall, these values suggest that the 3D ordering must be ferrimagnetic, even though each compound only contains one type of transition metal in one oxidation state. While the shortest metal−metal interactions are between consecutive atoms in the helical chains with M−M distances of 3.10−3.33 Å, there is no simple magnetic structure with antiferromagnetic intrachain coupling that would also produce the observed metamagnetic transitions. The next-nearest interactions are between M1−M2 and M1−M3 with distances of 5.12−5.28 Å; any magnetic coupling between these atoms would be significantly weaker as they are only connected by a single bridging ligand, but superexchange through the three-atom formate linker is possible.6 Given the helical nature of the chains and the fact that the magnetization data suggest both some degree of frustration and noncollinear ordering of the moments, a magnetic structure with mostly ferromagnetic interactions along the chains and antiferromagnetic interactions between them appears to be more plausible and would be in line with the predicted magnetic structure for Fe(OOCH)2·1/ 3HCOOH33 and similar chain-type metal-carboxylate compounds.20,39 This type of magnetic structure was observed in NH4FeCl2(HCOO), which exhibited a negative θ for powder samples but a positive θ for a single crystal when the magnetic field was aligned parallel to the direction of chain propagation.23,24 With ferromagnetic coupling within chains (+θ∥) and antiferromagnetic coupling between chains (−θ⊥), the value for a powder measurement (θpowder) will be the average of (+θ∥) and 2 × (−θ⊥), making negative θpowder values possible even for systems with ferromagnetic nearest-neighbor interactions. Additionally, powder samples of needle-like crystallites tend to pack in a preferred orientation with the direction of chain propagation perpendicular to the magnetic field, which may also result in a negative θpowder. Due to similar morphologies between NH4FeCl2HCOO and the closely related compounds discussed in this work, it not unreasonable to expect similar results from the magnetic measurements. However, we have shown in the past that relying solely on magnetization and Mössbauer data may not be sufficient to construct an accurate model.23,24 To conclusively determine the magnetic structure, the cobalt analogue was selected for low temperature neutron powder diffraction experiments. The pattern collected at 10 K was used to refine the nuclear phase, and all peaks were well described using the structure determined by single crystal X-ray diffraction at 90 K (Figure SI 3); this also confirmed the presence of two hydrogen atoms with 100% occupancy on the water molecule in Chain 2, ruling out the possibility of a hydroxyl group at this position. At 1.5 K, additional intensity from magnetic scattering was observed, most notably on the (1,0,1̅,0) and (2,0,2̅,0) peaks (Figure 5A); the intensity of the (1,0,1̅,0) peak was tracked with temperature, and as can be seen in the inset of Figure 5A, the order parameter matches well with the ordering temperature derived from the magnetic susceptibility data. The 1.5 K pattern was refined using the nuclear phase from the 10 K refinement and an additional magnetic phase (Figure 5B), resulting in the magnetic structure shown in Figure 5C,D. The magnetic moments have components μa=b = 0.5(1) μB and μc = 1.9(1) μB, placing them in a noncollinear ferromagnetic arrangement along the helical chains with each moment angled ∼14.7° away from the [0001] direction. As was predicted from the magnetization data, two of the chains have moments aligned in the same direction while the third is

aligned in the opposite direction. Although the model used for the magnetic phase cannot distinguish which of the three chains has moments aligned antiparallel to the others, it is very likely to be Chain 1. The strongly negative θ from Curie-Weiss fitting of the magnetic susceptibility indicates that the dominant magnetic couplings are antiferromagnetic, and it is clear from the magnetic structure that the intrachain interactions are largely ferromagnetic; as Chains 1−2 and 1−3 are connected by interchain formate bridges and the M1−M2 and M1−M3 distances are both 0.4−0.5 Å shorter than the M2−M3 distance, it is reasonable to assume that these bridges are transmitting an antiferromagnetic coupling via a superexchange pathway. However, this magnetic model is somewhat limited by the quality of the diffraction pattern; incoherent scattering from hydrogen atoms in the sample gave a noisy background, and the magnetic diffraction only contributed a significant amount of intensity to a very small number of peaks. The refined moment of 2.0(1) μB is significantly smaller than the expected 3 μB, though we were only able to refine the μa=b and μc components of the magnetic moments while constraining them to be the same magnitude for each of the three crystallographically distinct cobalt atoms. Attempts at free refinement of all six variables were unsuccessful, and therefore, it is possible that the magnetic structure may be more complex than the one presented here. With the current magnetic model, even though the individual magnetic moments are not aligned in a collinear fashion, each helical chain has a net magnetic moment directed either parallel or antiparallel to the [0001] direction; if each chain is treated as a whole, one arrives at the diagram shown in Figure 5E. This is essentially a ferrimagnetic triangular lattice, though one of the connecting legs is broken due to the lack of a formate bridge between Chains 2 and 3. The large negative value for θ in the cobalt compound shows that there is some magnetic frustration, but the missing bridge effectively breaks the magnetic coupling between the two chains with parallel moments and allows all of the nearest-neighbor chains to couple antiferromagnetically. This “satisfied” triangular lattice is the basis for both the ferrimagnetic behavior and the metamagnetic transitions; with a two-up/one-down arrangement of the chains, even a material that contains only one type of metal center in a single oxidation and spin state can act as a homospin topological ferrimagnet.40 Although only one-third of the total magnetic moment is apparent in the ordered state, the metamagnetic transition at high field flips the direction of the third chain, leading to a three-up ferromagnetic arrangement that triples the net magnetization.

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CONCLUSION We have shown that through a simple solvothermal synthesis it is possible to create a series of helical chain structures in which transition metal atoms are linked by both μ3-formate and μ2chloride bridges; furthermore, these compounds crystallize in a chiral and polar space group despite the absence of any chiral components or structure-directing agents. All three compounds are homospin topological ferrimagnets, and the magnetic structure suggests that the moments are aligned in the same direction as the polarity of the helical chains. These compounds could potentially exhibit multiferroic behavior and warrant further investigation, though the growth of large single crystals for the measurement of certain properties will be challenging. We are currently investigating other transition metals to assess 7722


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potential changes in the magnetic structure due to differences in the electronic structure.

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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.chemmater.7b01755. Powder X-ray diffraction data and additional magnetic and neutron powder diffraction refinement plots (PDF) Crystallographic data for Fe3(HCOO)5Cl(OH2) (CIF) Crystallographic data for Co3(HCOO)5Cl(OH2) (CIF) Crystallographic data for Ni3(HCOO)5Cl(OH2) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1-530-752-5563. Fax: +1-530-752-8995. E-mail: [email protected]. ORCID

Kirill Kovnir: 0000-0003-1152-1912 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank X. B. Powers for collecting the single-crystal dataset for the nickel compound at the Advanced Light Source, P. Klavins for help with the magnetic measurements, and F. Osterloh for access to the solid-state UV−vis spectrophotometer. The University of California, Davis and UC Davis ChemEnergy NSF REU Grant #CHE-1004925 are gratefully acknowledged for financial support. J.T.G. acknowledges the ARCS fellowship. We thank the National Science Foundation MRI program, Grant 1531193, for the funding for the Bruker D8 Venture single crystal X-ray diffractometer. The work at the Oak Ridge National Laboratory was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences (OBES), U.S. Department of Energy (DOE). The work at the Advanced Light Source was supported by the Director, Office of Science, OBES, U.S. DOE under Contract No. DEAC02-05CH11231.

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REFERENCES

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A Series of Chiral, Polar, Homospin Topological Ferrimagnets: M3

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