Use of Halogen Bonding in a Molecular Solid Solution to

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Use of Halogen Bonding in a Molecular Solid Solution to Simultaneously Control Spin and Charge Genta Kawaguchi,*,†,∥ Mitsuhiko Maesato,*,† Tokutaro Komatsu,†,⊥ Tatsuro Imakubo,‡ Andhika Kiswandhi,§,# David Graf,§ and Hiroshi Kitagawa† †

Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan § National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

ABSTRACT: Halogen-bonding interactions have attracted increasing attention in various fields of molecular science. Here we report the first comprehensive study of halogen-bonding-utilized solid solution for simultaneous control of multifunctional properties. A series of anion-mixed molecular conductors (DIETSe) 2 MBr 4x Cl 4(1−x) [DIETSe = diiodo(ethylenedithio)tetraselenafulvalene; M = Fe, Ga; 0 < x < 1] were synthesized without changing crystal structure utilizing strong halogen bonds between DIETSe molecules and anions. Detailed physical property measurements (T > 0.3 K, H < 35 T) using the single crystals demonstrated simultaneous control of both spin and charge degrees of freedom. The increase in Br content x gradually suppresses a metal−insulator transition attributed to the nesting instability of the quasi-one-dimensional Fermi surfaces. It suggests the dimensionality of π electrons is extended by increasing the anion size, which is opposite of the typical effect of chemical pressure. We found that the “negative” chemical pressure is associated with the characteristic halogen-bonding network. Br substitution also enhances the antiferromagnetic (AF) ordering of d-electron spins in the Fe salts, as indicated by the Néel temperature, AF phase boundary field, and saturation field. Furthermore, we observed hysteresis in both magnetization and resistivity only in halogen-mixed salts at very low temperatures, indicating simultaneous spin and charge manipulation by alloying.

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INTRODUCTION

can give rise to novel physical properties such as superconductivity under uniaxial strain29 or at ambient pressure,30 and the latter type can give hexagonal supramolecular architecture with unique chemical recyclability31 and also controllability of the donor/anion ratio.32 Fourmigué et al. found intriguing properties33−37 including the competition between halogen bond and hydrogen bond33 and chargeassisted halogen bonding.34 Enoki et al. reported anomalous magnetoresistance (MR) in π−d hybrid semiconductors based on brominated TTFs with magnetic anions FeX4− (X = Br, Cl), indicating an interaction between the donor and the anion.38,39 The use of the tetraselenafulvalene (TSF) skeleton instead of TTF skeleton is advantageous to make highly conducting materials. Thus, the iodinated TSFs are promising building units for conducting supramolecular assemblies with strong iodine bonds. Recently, we have investigated π−d hybrid

Crystal engineering is a rapidly developing field of research. Tailor-made supramolecular architectures with desired functions can be widely applicable for modern technology such as molecular scale devices, sensors, and so on. Supramolecular engineering strategies rely on noncovalent intermolecular interactions such as van der Waals interactions, coordination bonds, hydrogen bonds, and halogen bonds. Among them, the halogen bonds have attracted growing attention because of the highly directional nature and tunable bond strength,2−9 and are utilized in a variety of fields such as anion recognitions,10,11 drug design,12−14 organic catalysts,15−17 gel formation,18 molecular rotors,19,20 phosphorescent materials,21 and conducting molecular materials.22−43 Halogenated tetrathiafulvalenes (TTFs) are attractive building blocks for functional molecular conductors.22 One of the authors (T.I.) developed iodinated TTFs cation radical salts25−32 including unique iodine-bonding networks between the donor and counteranion (donor···anion type) or among the donor molecules only (donor···donor type). The former type 1−3

© XXXX American Chemical Society

Received: June 20, 2016 Revised: September 22, 2016

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DOI: 10.1021/acs.chemmater.6b02495 Chem. Mater. XXXX, XXX, XXX−XXX

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X-ray Crystallography. Single crystal X-ray diffraction data were collected using a Bruker SMART APEX II ULTRA CCD diffractometer employing graphite-monochromated Mo Kα radiation at 293 K. A single crystal of each of (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga; x = 0, 0.25, 0.75, 1] was mounted on a glass fiber. The crystal structures were solved using a direct method (SIR2011)51 and refined on F 2 using a full-matrix least-squares method with the SHELXL2014.52 All calculations were performed using the CrystalStructure software package.53 Band Calculations. Band calculations of (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga; 0 < x < 1] were performed using density functional theory (DFT). Detailed conditions are described in the Supporting Information. Transport and Magnetotransport Measurements. The electrical resistivities of the MCl4, MBrCl3, MBr2Cl2, MBr3Cl, and MBr4 [M = Fe, Ga] salts were measured along the b axis (Figure 1a)

conductors (DIETSe)2FeX4 (X = Br, Cl), where DIETSe represents diiodo(ethylenedithio)tetraselenafulvalene (Chart 1),40 and found anomalous spin−charge interactions, such as spin-flop switching and memory,41,42 and very large hysteresis in MR.43 Chart 1. Molecular Structure of DIETSe

To develop multifunctional magnetic molecular conductors, fine-tuning of both the electronic state of π electrons and the strength of π−d interaction is crucial. For such purpose, making molecular solid solutions is one of the most efficient ways. However, it is usually difficult to make isostructural molecular solid solution crystals because of the weak intermolecular interaction. For example, the electronic state of the magneticfield-induced superconductor λ-(BETS)2FeCl4,44 where BETS represents bis(ethylenedithio)tetraselenafulvalene, can be tuned by the substitutions of Ga for Fe45−47 and of Br for Cl.48,49 The former is possible in the whole Fe/Ga ratio, but the latter is limited to low Br concentration region because of the large difference in ion radius between Cl and Br. Here we report that the robust halogen bond can sustain an isostructure against chemical substitution, which enabled us to simultaneously control the spin and charge degrees of freedom of these magnetic molecular conductors. This is the first comprehensive study of halogen-bonding-utilized molecular solid solutions for controlling multifunctionality. We demonstrate the ground state of π electrons and the strength of π−d interaction can be systematically controlled by changing the halogen ratio in the isostructural series of (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga; 0 < x < 1] compounds. A metal−insulator (M−I) transition is monotonically suppressed by replacing Cl with Br, which is opposite to expected behavior from chemical pressure. The “negative” chemical pressure is deeply connected with characteristic halogen-bonding architecture. We also found nontrivial transport and magnetic properties such as magnetic hysteresis in mixed salts, which is unforeseen in the other π−d molecular conductors and attributable to the random exchange interaction by anion mixing. Our solid solution approach featuring halogen bonds can be a facile and powerful method to control multifunctionality of molecular materials.

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Figure 1. (a) Crystal structure of (DIETSe)2FeCl4 viewed from the a axis. C, gray; I, violet; Se, pink; S, yellow; Fe, brown; and Cl, green. Donor molecules with shaded heterocycles are on the upper side. Red and blue lines indicate short I−Cl and I−S contacts, respectively. (b) Relative lattice constants of (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga] normalized by the values of (DIETSe)2MCl4 [M = Fe, Ga]. Open and filled symbols represent data for the Ga and Fe species, respectively. (c) Fermi surface of (DIETSe)2FeCl4 calculated by DFT.

EXPERIMENTAL SECTION

using a four-probe dc method. Four gold wires (10 μm in diameter) were attached to a single crystal with carbon paste. The b-axis resistivity is larger by a factor of about 104 compared to the a-axis resistivity and coherent transport is observed in this system.41−43,54,55 The b-axis resistivity seems tougher against microcracks by cooling than the a-axis resistivity, especially in the case of mixed salts. To prevent microcracks from forming, we also applied a moderate hydrostatic pressure (13%) than the sum of van der Waals radii (Figure S3a). Therefore, magnetic interactions are mainly based on π−d interactions rather than d−d interactions. It is also noteworthy that the anion tetrahedron is distorted from a regular tetrahedron, which is associated with a strong donor− anion-type halogen bond (Figures S3b,c).

Figure 2. Temperature dependences of the b-axis resistivities of (DIETSe)2GaBr4xCl4(1−x) (a) and (DIETSe)2FeBr4xCl4(1−x) (b). Resistivity is divided by room-temperature value and multiplied by factors (GaCl4, 6; GaBrCl3, 2.4; GaBr2Cl2, 1; GaBr3Cl, 2.6; GaBr4, 0.4, FeCl4, 3.5; FeBrCl3, 9; FeBr2Cl2, 1; FeBr3Cl, 2; FeBr4, 0.4) for clarity. Broken and solid arrows show onsets of spin-density-wave (SDW) transition and antiferromagnetic (AF) ordering of d spins, respectively. Temperature−Br-content (T−x) phase diagrams of (DIETSe)2GaBr4xCl4(1−x) (c) and (DIETSe)2FeBr4xCl4(1−x) (d). Blue squares: onset of SDW transition, defined by the minimum of resistivity ρ. Open red circles: onset of AF transition of d spins from ρ anomaly. Filled red circles: long-range order of AF transition, defined by the peak in d{ln(ρ)}/d(1/T). Open brown triangles: short-range AF orders from magnetic susceptibility χ (see Figure 4 and description). Filled brown triangles: long-range AF orders from χ.

respectively. In the Ga salts (Figure 2a), M−I transitions are observed at low temperature, which is inherent in the π system and is attributed to the nesting instability of the Q1D Fermi surfaces. The Q1D metal is known to be highly susceptible to the 2kF periodic potential and undergoes a transition to a charge-density-wave (CDW) state or a spin-density-wave (SDW) state due to electron−phonon or electron−electron interaction, respectively, where kF is Fermi wave vector.60 The C

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Figure 3. (a) Interdonor distances along the b and c axis. (b) Interdonor distances along the a axis and definitions of overlap integrals between donors. (c) Interdonor distances plotted versus Br content x. For comparison, the values are normalized by those of (DIETSe)2MCl4 [M = Fe, Ga] (see also the Supporting Information). Open and filled symbols represent the data for the Ga and Fe species, respectively. (d) The relation between interdonor distances and anion size through halogen bonds (see description).

Table 1. Overlap Integrals (× 10−3) in (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga] sA sB sC |sA/sC|

GaCl4

GaBrCl3

GaBr2Cl2

GaBr3Cl

GaBr4

FeCl4

FeBrCl3

FeBr2Cl2

FeBr3Cl

FeBr4

−16.25 −1.50 0.28 58.5

−16.16 −1.49 0.32 50.0

−16.27 −1.47 0.34 47.2

−16.18 −1.46 0.35 46.5

−16.04 −1.44 0.35 46.3

−16.33 −1.50 0.32 51.8

−16.21 −1.49 0.34 47.2

−16.33 −1.48 0.38 43.5

−16.10 −1.46 0.39 41.2

−16.28 −1.46 0.40 40.5

dimensionality of π electrons as with physical pressure, while involving a lattice expansion (Figure 1b), which is the opposite effect of applying pressure. Thus, we call this unusual phenomena in (DIETSe)2MBr4xCl4(1−x) [M = Fe, Ga; 0 < x < 1] as “negative” chemical pressure effect. Origin of “Negative” Chemical Pressure Effect. To clarify the unusual SDW−metal crossover by Br substitution, interdonor distances of the mixed-anion salts are investigated. Figure 3a,b illustrate four kinds of interdonor distances, d1, d2, d3, and d4. d1 corresponds to separation of neighboring columns along the c axis, equal to c/2. d2 is the relative displacement of TSF skeletons of neighboring columns along the b axis. d3 is the distance of π−π stacking along the a axis and equivalent to a/2. d4 indicates the mismatch of π−π stacking periods between neighboring columns along the a axis. These interdonor distances are plotted against Br content x in Figure 3c and Figure S6. Only d4 shows a large decrease with increasing x, whereas d1, d2, and d3 slightly increase. This indicates Br substitution strengthens side-by-side interactions of donors with negligibly small changes in intracolumn interactions, thus extending the dimensionality. Indeed, the overlap integrals obtained by extended Hü ckel methods62 support this. Calculated values of overlap integrals sA, sB, and sC (see Figure 3b) are summarized in Table 1. Since sC is nearly perpendicular to sA, the ratio |sA/sC| is a good measure of anisotropy. The |sA/ sC| decreases with x, indicating an increase in the two dimensionality of the π system. We also found the above donor displacements are associated with characteristic halogen-bonding interactions. Figure 3d

previous 77Se nuclear magnetic resonance (NMR) study of the GaCl4 salt indeed confirmed the SDW ground state,61 because the peak in the spin−lattice relaxation rate 1/T1 and the broadening of the spectra characteristic to incommensurate SDW have been observed. With increasing Br content x, the SDW transition is suppressed and metallic behaviors are maintained down to 1.9 K in the GaBr4 salt.40,54 The temperature−Br-content (T−x) phase diagram (Figure 2c) clearly shows the SDW phase shrinks with increasing x. It indicates Br substitution suppresses the nesting instability of Q1D Fermi surfaces by increasing the Fermi surface warping, in other words, enhancing the dimensionality of π electrons, which is quite similar to the application of pressure.41,42,54,55 Such a SDW suppression is also observed in the Fe salts (Figure 2b,d). By comparing previous high pressure studies,41,55 we estimated the chemical pressure by Br substitution. Chemical pressure refers to virtual physical pressure generated by chemical modifications such as atomic substitution or changing anion species, usually discussed in terms of dimensionality changes in a conducting system. Assuming that the chemical pressure is proportional to Br content x, the chemical pressure relative to the most one-dimensional (DIETSe)2MCl4 [M = Fe, Ga] is expressed as Pchem/kbar ∼ 7.2 x, where x denotes Br content [0 < x < 1] (Figure S5). It follows that the SDW critical Br content xC, above which SDW transition is completely suppressed, of the Fe and Ga species are 0.76 and 0.90, respectively (Figure 2c,d and Figure S5). It is very interesting to note that Br substitution increases the D

DOI: 10.1021/acs.chemmater.6b02495 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials represents molecular coordinates in the ac plane, where redcolored donors in neighboring columns are connected to the same anion colored blue by I−X [X = Br, Cl] halogen bonds. Through this donor−anion−donor motif, the anion size decides the distance along the a axis between red donors, which equals d3 − d4. Considering d3 = a/2 is almost constant with respect to x, the anion size is approximately − d4 + const. Therefore, Br substitution increases the anion size and decreases d4, leading to the increase in the dimensionality of π electrons. This “negative” chemical pressure effect can also explain the slightly smaller SDW critical Br content xC or pressure PC of the Fe species than that of the Ga species. The ion radius of Fe3+ (0.49 Å) is larger than that of Ga3+ (0.47 Å), 63 leading to smaller d 4 (Figure S6) and higher dimensionality in the FeBr4xCl 4(1−x) salts than in the GaBr4xCl4(1−x) salts. (DIETSe)2MBr4xCl4(1−x) [M = Ga, Fe] are the first systems, where the halogen-bonding interactions are revealed to be closely associated with an unusual volume− dimension relationship. Magnetic Properties. In addition to the SDW transition, the π−d conductors (DIETSe)2FeBr4xCl4(1−x) exhibit a rapid resistance increase (Figure 2b), which is not observed in nonmagnetic analogues, (DIETSe)2GaBr4xCl4(1−x), suggesting a π−d-coupled transition. The temperatures of the resistance anomaly coincide with AF ordering temperatures of d spins from magnetic susceptibility χ (Figure 2d), as discussed below.40−43,54,55 The increase in resistance at the Néel temperature TN is associated with reconstruction of the Fermi surface induced by a periodic AF potential of Fe3+ d spins. The magnetic susceptibilities χ of (DIETSe)2FeBr4xCl4(1−x) obey Curie−Weiss behavior at high temperature (Figures S7− S11). Curie constants C are 4.2−4.6 emu K/mol, comparable to the value C = 4.4 emu K/mol expected for d spins (S = 5/2) of Fe3+. The absolute value of the Weiss temperature |θ| increases with increasing x, suggesting enhancement of AF interactions (Figure S12). At low temperature, anisotropic behaviors were observed in χ, where Br substitution increases the Néel temperature TN and rotates the magnetic easy axis. Figure 4a−c displays the temperature dependences of χ along the a, b, and c axes, thereafter denoted by χa, χb, and χc, respectively. Broken arrows indicate short-range AF orders where χb or χc starts to decrease rapidly with cooling. In contrast, χa is large compared to the others at low temperature, suggesting the a axis corresponds to hard axis. Therefore, the cusp-like anomaly in χa is indicative of three-dimensional longrange order (solid arrows in Figure 4a,d). Since the FeCl4 salt exhibits a negligibly small anomaly in χa, the long-range AF ordering is estimated by a rapid decrease of χb (Figure 4b). The long-range-ordering temperature obtained from χ is in good agreement with the TN from resistivity anomaly in the T−x phase diagram (Figure 2d). TN becomes higher with increasing Br content x, indicating the enhancement of π−d interactions. It is markedly different from the slight increase in TN by applying physical pressure.41−43,54,55 Br has larger electron orbitals (4p) than Cl (3p), which gives more effective interactions. It is also suggested that the energy levels of Br 4p orbitals are closer to those of Fe 3d orbitals than to Cl 3p orbitals, which increases d−p mixing to strengthen magnetic interactions.64,65 Br substitution also changes the easy-axis direction from the b to the c axis. The FeCl4 salt shows a large decrease in χb, which is consistent with the easy axis determined by angle-

Figure 4. Temperature dependences of the magnetic susceptibilities χ of (DIETSe)2FeBr4xCl4(1−x) along the a axis (a), b axis (b), and c axis (c). The applied magnetic fields are 0.1 T for the FeCl4 and FeBr4 salts and 0.5 T for the FeBrCl3, FeBr2Cl2, and FeBr3Cl salts. Broken and solid arrows show short- and long-range AF orderings, respectively. (d) Extended figure of χ along the a axis of the FeBr3Cl and FeBr4 salts. (e) χ along the b and c axis of the FeBr3Cl salt.

dependent MR (±16° from the b to c axis in the bc plane).42 The drop of χb becomes smaller with increasing Br content x (Figure 4b), while the decrease in χc becomes more dominant (Figure 4c). In the FeBr3Cl salt, χb and χc decrease to the same degree (Figure 4e). The FeBr4 salt shows significant decrease in χc, in good agreement with the easy axis confirmed by angledependent magnetic torque measurement (±54° from the b to c axis in the bc plane).43 Similar rotation of the easy axis by anion mixing is observed in λ-(BETS)2FeBryCl4−y48 and κ-(BETS)2FeBrzCl4−z66 [0 < y < 2, 0 < z < 4]. However, these BETS salts have Br-preferred halogen sites in the anions and the origins of change in magnetic anisotropy were not clarified. It is noteworthy that (EDO-TTFBr2)2FeX4 [EDO-TTFBr2 = 4,5-dibromo-4′,5′ethylenedioxotetrathiafulvalene; X = Br, Cl] show different magnetic anisotropies according to halogen X,38 which is similar to (DIETSe)2FeX4. As seen in the EDO-TTFBr2 salts, FeX4− tetrahedron anions are uniaxially elongated by halogenbonding interactions in the DIETSe salts (Figure S3b,c). Such a distortion to lower symmetry can give rise to single-ion anisotropy due to spin−orbit coupling.38 The effect of covalency on the spin−orbit coupling has also been discussed in (PPh4)FeCl4 by Solomon et al.67 Single-ion anisotropy is considered to play a significant role in the FeBr4 salt because of the large spin−orbit coupling. Indeed, the magnetic easy axis of (DIETSe)2FeBr4 is within the bc plane and the a axis is the hard axis. In the FeCl4 salt, on the other hand, dipole−dipole interaction also plays an important role due to relatively small spin−orbit coupling. Therefore, it is concluded that the balance between the single-ion anisotropy and dipole−dipole interaction is gradually changed in (DIETSe)2FeBr4xCl4(1−x). We E

DOI: 10.1021/acs.chemmater.6b02495 Chem. Mater. XXXX, XXX, XXX−XXX

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torque result (Figure 5b). MR of the FeBr4 salt is similar to that of the FeBr3Cl salt, though field-induced AF−paramagnetic (PM) transition is not observed below 12 T at low temperature due to strong π−d interactions (Figure 5c). As seen in Figure 2b, the FeBr4 and FeBr3Cl salts show M−I transition at TN, which is considered to be an AF-induced SDW state. By applying high magnetic fields, AF ordering is destroyed and a low-resistance state is restored. Therefore, a MR minimum should appear at HAF. When a magnetic field is applied near the easy axis, a firstorder spin-flop transition occurs, which is a spin reorientation from an AF to a canted AF (CAF) state (green arrows at the top of Figure 6). Therefore, a discontinuous magnetization

also note that the magnetic anisotropy energy is small compared to the exchange energy in our salts, since we have not observed a metamagnetic transition.68 Magnetic-Field-Induced Transitions and π−d Interactions. To explore the effect of high magnetic field, we measured magnetic torque and MR in (DIETSe)2FeBr4xCl4(1−x) [0 < x < 1], where the direction of the magnetic field was along the b axis. First of all, we describe the magnetization process when the easy axis is far from the b axis. Figure 5a,b shows magnetic

Figure 5. Magnetic torque of the FeBr4 (a) and FeBr3Cl salts (b) up to 35 T. Magnetic field is applied along the b axis. Inset in part a is a picture of a cantilever with a crystal. Longitudinal magnetoresistance (MR) of the FeBr4 (c) and FeBr3Cl salts (d) up to 12 T. Inset in part c indicates the measurement configuration against a crystal. Solid arrows in the figures indicate AF phase boundary HAF.

torque of the FeBr4 and FeBr3Cl salts up to 35 T, respectively. Since the easy axis is far from the magnetic-field direction (i.e., b axis) in these salts, magnetic moments continuously tilt toward the b axis with increasing field. The FeBr4 salt shows continuous torque curves with saturation at about 22 T at 0.3 K (Figure 5a). With increasing temperature, torque saturation becomes unclear due to thermal fluctuation. The torque of the FeBr3Cl salt shows a maximum at a high field of about 17 T, which shifts to a higher field with increasing temperature, indicative of saturation (Figure 5b). Besides, the torque minimum appears around 10 T below TN in the FeBr3Cl salt, as shown by the arrow in Figure 5b. This anomaly is less pronounced at higher temperature and vanishes above TN, suggesting the AF phase boundary HAF. Interestingly, we observed anomalous MR in a series of (DIETSe)2FeBr4xCl4(1−x), unseen in the Ga species, indicating significant π−d interactions. MR of the FeBr4 and FeBr3Cl salts are shown in Figure 5c,d, respectively. The FeBr3Cl salt exhibits positive MR at low field before showing a sharp decrease in MR around 7 T at 0.8 K (Figure 5d). The large MR changes are suppressed at higher temperature and disappear above TN (∼6 K), indicating the MR anomaly is related to the AF phase. An offset of anomalous MR suggests the AF phase boundary HAF is located at ∼10 T (solid arrow), in good agreement with the

Figure 6. Magnetic torque of the FeCl4 (a), FeBrCl3 (b), and FeBr2Cl2 salts (c) up to 12 T. Magnetic field is applied along the b axis. Green arrows outside the figures indicate schematic configurations of d spins. Longitudinal MR of the FeCl4 (d), FeBrCl3 (e), and FeBr2Cl2 salts (f) up to 12 T. The measurement configuration is the same as Figure 5c. Broken and solid arrows indicate spin-flop field Hsf and AF phase boundary HAF, respectively.

process is observed. The FeCl4 salt shows a steep change in torque around 1.5 T below TN (∼2.5 K), indicative of the spinflop transition (broken arrow in Figure 6a), before saturation around 6 T. The spin-flop transition is also observed in the FeBrCl3 and FeBr2Cl2 salts (broken arrows in Figure 6b,c), though it is not as sharp as in the FeCl4 salt. This is probably because the mixed salts with nonuniform anions have some distribution of the magnetic anisotropy. Besides, as the susceptibility data suggests, the easy axis tends to tilt from the b axis with increasing Br content x, which might smear the spin-flop transition in the mixed salts. The FeBrCl3 and FeBr2Cl2 salts also show a minimum of torque around 6 T, corresponding to HAF (solid arrows). The torque maximum of the FeBr2Cl2 suggests saturation field HS of about 15 T (Figure S13). F

DOI: 10.1021/acs.chemmater.6b02495 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 7. (a) Temperature−magnetic-field (T−H) phase diagrams of the (DIETSe)2FeBr4xCl4(1−x). SDW phase is omitted here for simplicity. PM and FM denote paramagnetic and field-induced ferromagnetic phases, respectively. Orange inverted triangles: spin-flop field Hsf estimated from torque anomalies on upsweep. Red triangles: Hsf suggested by MR dip structure on upsweep. Orange diamonds: AF boundary HAF estimated from torque minimum or slope change. Red circles: HAF from MR minimum. Open light blue squares: saturation field HS from torque. Light blue crosses: HS estimated from magnetization extrapolation (see the Supporting Information). (b) HAF and HS of (DIETSe)2FeBr4xCl4(1−x). The lowesttemperature data of part a are plotted. Magnetization: 1.8 K (x = 0), 2 K (x = 0.25, 0.5, 0.75, 1). Torque: 0.6 K (x = 0, 0.25), 0.3 K (x = 0.5, 0.75, 1). MR: 0.5 K (x = 0, 0.25, 0.5), 0.8 K (x = 0.75, 1). (c) Up- and down-sweep torque of (DIETSe)2FeBr4xCl4(1−x) at very low temperatures (