Polymorphism in Hybrid Organic−Inorganic Bilayered Magnetic

Dec 11, 2009 - *To whom correspondence should be addressed. (B.Z.) E-mail: [email protected]; phone: 86-10-62558982; fax: 86-10-62559373; (D.Z.) ...
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DOI: 10.1021/cg9011718

Polymorphism in Hybrid Organic-Inorganic Bilayered Magnetic Conductors (BEDT-TTF)3(FeIIICl4)2, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene )

2010, Vol. 10 782–789

Bin Zhang,*,† Mohamedally Kurmoo,§ Takehiko Mori,‡ Yan Zhang, Francis Laurence Pratt,# and Daoben Zhu*,† †

)

Organic Solid Laboratory, BNLMS & CMS, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, §Laboratoire de Chimie de Coordination Organique, CNRS-UMR7140, Universit e de Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg Cedex 1, France, ‡Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-Okayama, Tokyo 152-8552, Japan, Department of Physics, Peking University, Beijing 100871, P. R. China, and #Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K.

Received September 23, 2009; Revised Manuscript Received November 25, 2009

ABSTRACT: Two polymorphic hybrid organic-inorganic bilayered magnetic conductors (BEDT-TTF)3(FeCl4)2 (I and II) (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) were obtained by the diffusion method. I crystallizes in a monoclinic system with cell parameters: a = 55.9187(7) A˚, b = 6.7424(1) A˚, c = 14.9057(3) A˚, β = 101.056(1)°, V = 5515.5(2) A˚3, Z = 4, and C2/c at 290 K, and it transforms to a triclinic system at 220 K: a = 13.4440(2) A˚, b = 14.6761(2) A˚, c = 28.1529(5) A˚, R = 100.738(1)°, β = 96.875(1)°, γ = 90.017(1)°, V = 5416.8(2) A˚3, Z = 4 and P1 which is stable to 110 K. II crystallizes in a triclinic system with cell parameters: a = 6.7396(3) A˚, b = 9.9640(5) A˚, c = 21.132(1) A˚, R = 79.536(2)°, β = 81.032(2)°, γ = 81.066(3)°, V = 1367.3(1) A˚3, Z = 1, and P1 at 290 K. They both contain two donor layers having formal charge of BEDT-TTFþ0.5 in layerA and BEDT-TTFþ1.0 in layer-B. The donor arrangements in layer-A are δ-type in I and β0 -type in II, while the donor arrangements in layer-B and anion arrangements in I and II are also different. Layer-A is separated from layer-B by FeCl4-. The energy band calculation indicates a Peierls-like metal-insulator transition in the transverse direction at 220 K in I (σ300K = 120 S 3 cm-1), and a Mott insulating state in II (σ300K = 10-2 S 3 cm-1 and ER = 0.2 eV). The presence of π-d interactions between donors and anions results in the magnetic anomalies observed at 4.8 K in I and 2.7 K in II.

Introduction In the process of engineering crystals of molecular conductors consisting of organic-inorganic hybrids, polymorphism is occasionally encountered. Although its presence at times poses problems in the characterization of the phases in bulk, it has overall been beneficial in developing a comprehensive structure-property relationship. Consequently, synthetic control has succeeded in the construction of molecular conductors with various electronic ground states.1-6 For example, when BEDT-TTF crystallizes with I3-, more than 10 types of charge-transfer complexes of (BEDT-TTF)/I3 have been identified including polymorphism in four phases: R-(BEDT-TTF)2I3, β-(BEDT-TTF)2I3, γ-(BEDT-TTF)2I3, and κ-(BEDT-TTF)2I3. They show different conductivity behaviors, such as a metal-insulator transition at 135 K for R-(BEDT-TTF)2I3, and superconductivity below 10 K for β-(BEDT-TTF)2I3, γ-(BEDT-TTF)2I3, and κ-(BEDTTTF)2I3.3-5 Polymorphism has also been observed for organicinorganic hybrids in the process of constructing magnetic molecular conductors. For example, three polymorphic phases have been observed for charge-transfer salts of bis(ethylenedithio)tetraselenafulvalene (BETS) and FeCl4-: κ-(BETS)2FeCl4, κ0 -(BETS)2FeCl4, and λ-(BETS)2FeCl4.7-10 Their conductivity and magnetism are influenced by π-d interactions between the π electrons of BETS and the d-electron of Fe3þ of FeCl4- through S 3 3 3 Cl contacts between donors and anions. *To whom correspondence should be addressed. (B.Z.) E-mail: zhangbin@ iccas.ac.cn; phone: 86-10-62558982; fax: 86-10-62559373; (D.Z.) E-mail: [email protected]; phone: 86-10-62544083; fax: 86-10-62544083. pubs.acs.org/crystal

Published on Web 12/11/2009

κ-(BETS)2FeCl4 shows superconductivity at 0.4 K, and λ-(BETS)2FeCl4 shows metal-insulator transition at 7 K and field-induced superconductivity below 0.2 K under 14 T. However, polymorphism for BEDT-TTF and FeCl4- is not known, and only two phases have been reported, the semiconductor δ-(BEDT-TTF)2FeCl4 and the insulator (BEDTTTF)FeCl4.11-13 Here, we report the first observation of polymorphism in BEDT-TTF/FeCl4 with the phases, δ-(BEDT-TTF)3(FeCl4)2 (I) and β0 -(BEDT-TTF)3(FeCl4)2 (II). The two polymorphs have been characterized by crystal structure determinations, band structure calculations, infrared reflectivity, Raman spectroscopy, electrical conductivity measurements, and SQUID magnetometry. Both consist of bilayered charge-transfer salts containing BEDT-TTF charged 0.5þ and 1.0þ in alternate layers separated by FeCl4. I is a metal with a metal-semiconductor transition at 220 K while II is a semiconductor. Both exhibit magnetic anomalies at low temperatures. Experimental Section FeIII2FeIIO(CCl3COO)6(THF)3

was synthesized by the reported method and recrystallized twice from THF.14 100 mg of FeIII2FeIIO(CCl3COO)6(THF)3 was dissolved in 30 mL of CH2Cl2 and placed in one arm of an H-tube and 10 mg of BEDT-TTF dissolved in 30 mL of CH2Cl2 in the other arm. Brown blocks of (BEDT-TTF)2(Fe3O(CCl3COO)5), black plates of δ-(BEDT-TTF)2FeCl4, black needles of (BEDT-TTF)FeCl4, black plates of δ-(BEDT-TTF)3(FeCl4)2 (I), and black needles of β0 -(BEDT-TTF)3(FeCl4)2 (II) were obtained at the same time at the bottom of the H-tube after 2 weeks. r 2009 American Chemical Society

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Table 1. Crystallographic Data of δ-(BEDT-TTF)3(FeCl4)2 (I) and β0 -(BEDT-TTF)3(FeCl4)2 (II) color morphology temperature (K) crystal system space group a, A˚ b, A˚ c, A˚ R, ° β, ° γ, ° V, A˚3 Z Dcalc, g/cm3 μ, mm-1 θ, ° no. total reflns no. unique reflns (Rint.) no. obs [I g 2σ(I0)] no. params R1, wR2 [I g 2σ(I0)] R1, wR2 (all data) GOF shift/error ΔF, e/A3 CCDC

I

I

I

II

black plate 110 triclinic P1 13.3935(2) 14.5251 (2) 28.1362(5) 100.556(1) 96.840(1) 90.028(1) 5341.1(1) 4 1.927 1.912 27.5 69085 23279(0.0779) 10315 1153 0.0534, 0.1128 0.1405, 0.1312 0.953 0.001/0.000 1.834/-1.094 698223

black plate 200 triclinic P1 13.4440(2) 14.6761(2) 28.1529(5) 100.738(1) 96.875(1) 90.017(1) 5416.8(2) 4 1.900 1.885 27.5 71160 23639(0.084) 8057 1153 0.0454, 0.0888 0.1777, 0.1137 0.815 0.002/0.000 1.141/-1.009 642828

black plate 290 monoclinic C2/c 55.9187(7) 6.7424(1) 14.9057(3) 90 101.056 (1) 90 5515.5(2) 4 1.866 1.852 27.5 35326 6298(0.0792) 3102 289 0.0408, 0.0891 0.1138, 0.1037 0.932 0.001/0.000 -0.471/0.549 642827

black needle 290 triclinic P1 6.7396(3) 9.9640(5) 21.132(1) 79.536(2) 81.032(2) 81.066(3) 1367.3(1) 1 1.882 1.867 25.0 17138 4738(0.1523) 1803 289 0.0569, 0.0671 0.2123, 0.0887 0.854 0.000/0.000 -0.441/0.416 698222

Selected single crystals of the two compounds were mounted on glass fibers for X-ray diffraction experiments using a Nonius-Kappa CCD equipped with graphite-monochromated Mo KR (λ = 0.71073 A˚) radiation. The intensity data was corrected for Lorentz and polarization and finally for absorption using a numerical method.15 The structures were solved by the direct method and all the non-hydrogen atoms were refined anisotropically by the full-matrix method.16 The crystallographic data of I and II are listed in Table 1. Additional data (CIFs) have been deposited at the Cambridge Data Center (CCDC642827, CCDC642828, CCDC 698222, CCDC698223). Raman spectra on single crystals were recorded on a Renishaw inVia Raman microscope using λ = 514.5 nm excitation at room temperature. The measurements of electrical resistance as a function of temperature from 2 to 300 K were performed on single crystals using a Quantum Design PPMS 9XL. A four-probe dc-method employing 15 μm gold wires attached to the crystals by gold paste was used. Magnetization measurements were performed on a Quantum Design MPMS SQUID in various applied fields up to (70 kOe and temperature range 1.9-300 K on a polycrystalline sample of I and a single crystal of II. The as-measured data were corrected for the contributions from the signal of the aluminum background and sample holder and for the diamagnetism of the elements using Pascal’s constants (7.7243  10-4 cm3 mol-1).17

Results and Discussion Synthesis. When BEDT-TTF and Fe2IIIFeIIO(CCl3COO)6(THF)3 were allowed to diffuse slowly in CH2Cl2 partial oxidation of the organic by the inorganic takes place, resulting in the crystallization of several organic-inorganic hybrid charge-transfer salts. Although the main product after 2 weeks is the block brown crystals of (BEDT-TTF)2Fe3O(CCl3COO)5, the others contain FeCl4- as counteranion, viz: black plate of δ-(BEDT-TTF)2FeCl4, black needle of (BEDT-TTF)FeCl4, black plate of δ-(BEDT-TTF)3(FeCl4)2, and black needle of β0 -(BEDT-TTF)3(FeCl4)2. The longer periods of crystallization resulted in more crystals containing FeCl4- and principally δ-(BEDT-TTF)2FeCl4 and (BEDT-TTF)FeCl4. The latter two salts are also the principal products during the electrocrystallization of BEDT-TTF in the presence of R4N(FeCl4) or the diffusion with FeCl3 3 6H2O in CH2Cl2. These results suggest that the latter two salts are likely to be the most

Figure 1. ORTEP drawing with 30% ellipsoid of I. Symmetry code: A: -x, -y, 1 - z.

thermodynamically and kinetically stable. It is to be noted that the solvent is also involved in the formation of the salts containing FeCl4 where in the present case the FeCl4 is produced in the reaction cell. A similar mechanism has also been experienced for the formation of κ0 -(BETS)2FeCl4 from BETS and (Me4N)3FeIII(C2O4)3 in C6H5Cl under 2.0 V.8 Chlorinated alkanes have also been the source of chloride in the formation of (BEDTTTF)3Cl2 3 2H2O.18 Crystal Structures. The key feature in the structure of the two phases of (BEDT-TTF)3(FeCl4)2, I and II, is the presence of two different alternating layers of BEDT-TTF separated by the anionic sheet of FeCl4- (Scheme S1, Supporting Information). The two layers of BEDT-TTF molecules, labeled as layer-A and layer-B, contain a 2:1 ratio of donor-A (A) to donor-B (B), thus are differently packed and have different thickness. One and a half donors are crystallographically independent, in which donor-A is located on a general position and donor-B is on an inversion center as shown in Figure 1. Thus, these compounds can be classified as being bilayered. The formal charges of A and B at 290 K are assigned to BEDT-TTF0.5þ and BEDTTTF1.0þ, respectively, using established correlation of the bond-lengths of the TTF-core (Table 2).19 I crystallizes in the monoclinic space-group C2/c at 290 K. The packing diagram of I along the b axis is shown in Figure 2. There are two layer-A, two layer-B, and four anion sheets in a

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Table 2. Overlap Integral ( 10-3 eV) of Molecule A and B in I and II at 290 K

Figure 2. Packing diagram of I along the b-axis at room temperature. Dashed lines are S 3 3 3 S, S 3 3 3 Cl, C-H 3 3 3 S, and C-H 3 3 3 Cl contacts. There are hydrogen bonds between donor and anion: C9-H9B 3 3 3 Cl30 3.904(5) A˚/156.7°; C10-H10A 3 3 3 Cl10 3.704(4) A˚/121.2°; C10-H10A 3 3 3 Cl2 3.879(5) A˚/153.1°; C10-H10B 3 3 3 Cl30 3.889(5) A˚/139.8°; H14-H14B 3 3 3 Cl1’ 3.880(7) A˚/162.9°; C15-H15 3 3 3 Cl20 3.702(6) A˚/139.9°; C15-H15B 3 3 3 Cl40 3.601(5) A˚/131.4°, and S 3 3 3 Cl contacts between B and anion: S9 3 3 3 Cl4 3.308(2) A˚.

unit cell. The arrangement of the BEDT-TTF molecules within layer-A is similar to δ-(BEDT-TTF)2FeCl4 having a 12,13,20 Thus, we adopted the phase 3 3 3 XYYX 3 3 3 sequence. definition and labeled it δ-(BEDT-TTF)3(FeCl4)2,21,22 in which adjacent molecules stack face-to-face but with an in-plane displacement along the b axis of layer-A. In layer-B the adjacent molecules also stack face-to-face with the longmolecule axis of the molecules rotated relative to each other making a dihedral angle of 8.56(2)°. There are intermolecular C 3 3 3 S contacts along the c-axis and S 3 3 3 S side-by-side contacts along the b-axis resulting in the formation of layer-A along the bc-plane (Figure S1, Supporting Information). In layer-B the molecules stack side-by-side along the c-axis and extend in the bc-plane without intermolecular contacts. Layer-B incorporates with the anion sheet through several S 3 3 3 Cl contacts: S7 3 3 3 Cl10 3.635(2) A˚, and S9 3 3 3 Cl4 3.308(2) A˚. Along the c-axis the planes of B molecules are not parallel to the neighboring one and make a dihedral angle of 59.60(3)° (Figure S2, Supporting Information). The dihedral angles between molecules A and B are 29.39(3)° and 37.14(2)°. One of the ethylene groups in A is disordered on two positions. Apart from the ethylene groups all the atoms are coplanar with deviations of 0.110 A˚ in A and 0.037 A˚ in B. The temperature-dependent single crystal X-ray diffraction experiment of I shows a clear anomaly in the cell

parameters around 220 K (Figure 3a). This transition is accompanied by an additional set of Bragg reflections along the b-axis (Figure 3b) due to doubling of the b-axis and halving of the a-axis. The unit cell transforms from monoclinic C2/c above 220 K to triclinic P1 below 220 K and remains so until 110 K. This is accompanied by ordering of the ethylene groups of A and B, and appearance of additional S 3 3 3 S contacts within layer-A and layer-B. There are four A (A1, A2, A3, A4), one (B1) and two half B (B2, B3), four anions in an independent unit (Figure 4). There are one layerA, one layer-B, and two anion sheets in a unit cell. The formal charges assigned from bond lengths of the TTF-core on A and B remain the same as 290 K: A0.5þ and B1.0þ (Table S1, Supporting Information). In layer-A, S 3 3 3 S contacts exist between neighboring BEDT-TTF molecules and dimerization along the b-axis is observed (Figure S3, Supporting Information), similar to δ-(BEDT-TTF)2FeCl4. Below 220 K S 3 3 3 Cl contacts between donor and anion are now observed for both layers, while they were absent for layer-A at 290 K. The structure of II is closely related to that of I, having two different layers of BEDT-TTF separated by the anions, but it differs in the packing of the molecules. It is, however, isostructural to the structures reported for β0 -(BEDT-TTF)3(MnCl4)2, β0 -(BEDT-TTF)3(ZnCl4)2, and β0 -(BEDT-TTF)3(MII1-xMIIIxCl4)2 (Figure 5).21-25 Molecules A stack face-to-face

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along the b-axis with intermolecular C 3 3 3 S contacts and sideby-side S 3 3 3 S contacts along the a-axis of 3.513(2)-3.618(3) A˚, resulting in the propagation along the ab-plane. The plane and long axis of the molecules are parallel to each other. The mode of packing is as found in the β0 -phase. Molecules B stack along the a-axis with S 3 3 3 S contacts S9 3 3 3 S90 3.632(3) A˚, S9 3 3 3 S100 3.233(3) A˚, and S10 3 3 3 S110 3.644(3) A˚ and no interaction along the b-axis as observed for (BEDT-TTF)FeCl4 and (BEDT-TTF)FeBr4.11,13 Apart from the ethylene groups all atoms are coplanar with deviation of 0.105 A˚ in A and 0.033 A˚ in B. The long axes and planes of molecules B are parallel to each other. The dihedral of angle of A and B is 48.63(4)°. There are extensive short contacts between donors and anions that may favor π-d interaction.

The Fe-Cl bond lengths in I, 2.171(1)-2.186(1) A˚ and II, 2.177(2)-2.198(2) A˚ at room-temperature are in the range expected for FeCl4-.7,8,11,13,26-31 As observed in the polymorphic salts κ-BETS2FeCl4 and κ0 -BETS2FeCl4,7,8 the difference between I and II is not only in the donor arrangement, but also in the anion arrangements. The anions in one anionic sheet in I are arranged in a zigzag chain along the c-axis, as in κ0 -BETS2FeCl4 and in II they are arranged along the b-axis, as in κ-BETS2FeCl4 (Figure 6).8 Within the known bilayered compounds of BEDT-TTF, the present compounds (I and II) have two different charged donors (0.5þ and 1.0þ) segregated in the two different layers (A and B) while they are 1.0þ and 2.0þ for the β0 -(BEDTTTF)3(MIICl4)2 series, where M = Mn, Co, or Zn.21,22 It is

Figure 3. (a) Temperature-dependent cell parameters from 290 to 110 K of I. (b) X-ray diffraction pattern on single crystal of I around 220 K.

Figure 5. Packing diagram of II along the a-axis. Dashed lines are S 3 3 3 S, S 3 3 3 Cl, C-H 3 3 3 S, and C-H 3 3 3 Cl contacts. There are hydrogen bonds between donor and anion: C7-H7B 3 3 3 Cl3 3.704(9) A˚/138.6°; C8-H8A 3 3 3 Cl4 3.817(8) A˚/151.5°; C9H9A 3 3 3 Cl4 3.646(8) A˚/142.3°; C10-H10B 3 3 3 Cl1 3.898(7) A˚/ 176.8°, C14-H14A 3 3 3 Cl1 3.743(7) A˚/150.5°; C15-H15A 3 3 3 Cl2 3.615(7) A˚/127.3° and S 3 3 3 Cl contacts between donor and anion: S6 3 3 3 Cl3 3.596(3) A˚, S9 3 3 3 Cl2 3.563(2) A˚, S10 3 3 3 Cl4 3.615(3) A˚, S11 3 3 3 Cl3 3.437(3) A˚.

Figure 4. Packing diagram of δ-(BEDT-TTF)3(FeCl4)2 (I) at 200 K along the a-axis. Dashed lines are S 3 3 3 S, S 3 3 3 Cl, C-H 3 3 3 S, and C-H 3 3 3 Cl contacts.

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Figure 7. Raman spectra of I and II at room-temperature with λ = 514.5 nm. Figure 6. Anions arrangement in an anion sheet on I (top) and II (bottom) in room temperature.

interesting to note that partial substitution of CoIICl42- by GaIIICl4- in β0 -(BEDT-TTF)3(CoIICl4)2 up to the level of 44% has been achieved, thus controlling continuously the band-filling.24,25 Furthermore, the presence of π-d interaction is evidenced by the observation of high conductivity for β0 -(BEDT-TTF)3(MnCl4)2, while its isostructural salt β0 -(BEDT-TTF)3(ZnCl4)2 is a Mott insulator.21,22 In contrast, for the other known bilayered salts of BEDTTTF, (BEDT-TTF)2C(SO2CF3)3, R,β00 -(BEDT-TTF)4NH4M (C2O4)3 3 G (M/G = Ga/PhN(CH3)COH, Ga/ PhCH2CN, Fe/PhCOCH3), (BEDT-TTF)5Fe(C2O4)3(H2O)2CH2Cl2, all the donors are charged 0.5þ, and there are two different charged donors (0.5þ and 1.0þ) coexisting in two layers in (BEDT-TTF)5Fe(C2O4)3(H2O)2(CH2Cl2).32-34 Raman Spectra. Four bands were observed in the Raman spectra at ν3: 1496 cm-1 (A), 1469 cm-1 (B); ν4: 1458 cm-1 (A), 1423 cm-1 (B) in I and ν3: 1508 cm-1 (A), 1450 cm-1 (B); ν4: 1467 cm-1 (A), 1422 cm-1 (B) in II as shown in Figure 7. The relationship between charge and Raman band energy confirms the presence of two kinds of donor molecules with the formal charge BEDT-TTFþ0.5 and BEDT-TTFþ1.0 in agreement with the formal charges assigned from bond lengths of the TTF-core.35 Band Structure and Physical Properties. The conducting states of I and II were initially estimated from IR spectra as a conductor on I and poor conductor on II (Figure S4, Supporting Information).8,36 This was confirmed by band calculation and conductivity measurements. The detailed electronic structures of I and II were studied by band structure calculations. The transfer integrals of donors in layer-A and layer-B were calculated separately

uckel without consideration of FeCl4- using the extended H€ method for determining the transfer integrals to be used in the tight-binding band structure calculations as shown in Table 2.37 Using the average charge of 1/2þ per BEDT-TTF (A) results in a 3/4 filling of the bands, and the average charge of 1þ per BEDT-TTF (B) results in a 1/2 filling of the bands. I: Similar to other δ-phase conductors, the layer-A has strong dimerization along the stacking (c) axis, while the transverse (a) direction is uniform.38 Consequently, the Fermi surface shows one-dimensional character with respect to the transverse (a) direction (Figure 8). It should be noted that the energy bands on the ZM zone boundary are degenerated on account of the nonsymmorphic space group (C2/c), so that the Fermi surface is regarded as a single waving plane. It has been known that this type of compound undergoes a metal-insulator transition accompanied by the lattice doubling in the transverse direction.39,40 This is exactly what we have observed at 220 K in the present compound. There are two types among this group of twisted stack structures; the δ-phase contains four molecules in a unit cell along the stacking axis, and the R0 -phase involves two molecules.22 The present crystal obviously falls into the δ-phase category. Accordingly, the low-temperature insulating state is expected to be nonmagnetic, and regarded as a kind of Peierls state, although it is not certain from the magnetic measurements due to the magnetism of FeCl4. II: Owing to the quarter filling, the band structure of the layer-A is quite similar to other β0 -phase compounds (Figure 9).41,42 There is very strong dimerization. The effectively half-filled situation is believed to be the origin of the insulating property. Therefore, this compound is regarded as a Mott insulator. The structure is basically the same as β0 -(BEDTTTF)3(MnCl4)2 and β0 -(BEDT-TTF)3(ZnCl4)2.21,22 Because

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Figure 10. Temperature-dependent resistance on single crystal of I.

Figure 8. Band Structure and Fermi surface of A (top) and B (bottom) in I at 290 K.

Figure 11. χT vs T and 1/χ vs T plot of I under an applied field of 10 kOe. Red solid lines are fits to the Curie-Weiss law. Inset: FC under different field from 2 to 20 K.

Figure 9. Band structure and Fermi surface of A (top) and B (bottom) of II.

the Fermi surface is controlled by band-filling with M2þ replaced by Fe3þ, a two-dimensional electron pocket and one-dimensional electron column appear in layer-A and layer-B as expected from a reported paper on the hypothetical parent compound of β0 -(BEDT-TTF)3(CoCl4)2-x(GaCl4)x for x = 2.0.24 Electrical Properties. The electrical resistivity of the crystals I is very anisotropic with values of σa = 21.2, σb = 4.6, and σc = 120 S 3 cm-1 at room temperature. Upon cooling the resistance along the more conducting axis (c-axis) decreases gradually to 220 K where a sudden change is observed transforming to an increase upon further cooling. At around 80 K a much faster increase takes place. The results suggest a metallic behavior above 220 K and metalsemiconductor transition at 220 K that is accompanied by a

change in structure (as discussed above). The activation energy of the semiconducting phase is 0.02 eV between 100 and 220 K and below 80 K it increases to 0.08 eV (Figure 10). Several temperature cycles in resistance measurements indicate no hysteresis and therefore suggest both transitions are second-order, which was also confirmed for the phase transition at 220 K by X-ray diffraction on a single crystal. II behaves as a semiconductor for the whole temperature range studied, 130-300 K (Figure S5, Supporting Information). It is less conducting, σrt = 0.01 S 3 cm-1, and its activation energy ER = 0.2 eV is larger than that of I. The room-temperature conductivity is not much different from the isostructural compounds with different band fillings: 0.04 S 3 cm-1 for β0 -(BEDT-TTF)3 (MnCl4)2 and 0.4 S 3 cm-1 for β0 -(BEDT-TTF)3(ZnCl4)2.21,22 Magnetic Properties. The magnetic susceptibility of I was measured on a polycrystalline sample from 2 to 300 K. At 300 K, the χT value is 9.06 cm3 K mol-1; it is close to the sum expected for two isolated spin-only Fe3þ with S = 5/2, and g = 2.0, 8.75 cm3 K mol-1, and a contribution from the Pauli susceptibility of the metallic state of BEDT-TTF.24 The χT value decreased continuously upon lowering temperature. The data above 10 K can be fitted to the Curie-Weiss law well with C = 9.69(2) cm3 K mol-1, θ = -11.2(3) K, and R = 8.575  10-5 (R = Σ(χo - χc)2/Σχο2, Figure 11). The negative Weiss constant indicates antiferromagnetic interaction between Fe3þ. At 4.8 K, an anomaly in the low-field magnetic susceptibility (inset of Figure 11) is observed. If the anomaly was a transition to a long-range Neel state, we could

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Figure 12. Isothermal magnetization plot of I at 1.9 (black), 4.5 (red), and 8.0 K (blue).

Zhang et al.

organic layers, having segregated BEDT-TTF0.5þ (A) and BEDT-TTF1.0þ (B), are separated by FeCl4- and make S 3 3 3 Cl contacts. Layer-A of I has the δ-phase structure, which consists of twisted dimers, and the Fermi surface is one-dimensional in the transverse direction. Ι displays two transitions at 220 and 80 K, transforming to two different semiconductor states. The former transition is associated with the lattice doubling in the transverse direction, so that it can be interpreted as a kind of Peierls transition. The insulating behavior of II is ascribed to the β0 -like structure of layer-A, and to a Mott insulating state in an effectively half-filled band. Although band-structure calculations predict both to be metallic at 290 K, electrical resistivity measurements found I to be an anisotropic metal and II a semiconductor. Both compounds exhibit magnetic anomalies at low temperatures, 4.8 K for I, 2.7 K for II, suggesting π-d interaction is involved. Acknowledgment. The authors thank Dr. Prof. Zheming Wang on X-ray experiment, Dr. Prof. Song Gao on magnetic measurement, Renishaw Company (China) on Raman experiment. This work was supported by CMS-CX200809, CMS-LX200906, NSFC (20673120, 20873154), MOST (2006CB601001, 2006CB932102) in China, MEXT (18350095) in Japan, EPSRC in U. K., Universite de Strasbourg and CNRS in France. Supporting Information Available: Additional figures and cif files. This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 13. Anisotropic temperature-dependent magnetization of II with an applied field of 10 kOe.

assume that π-d interaction between BEDT-TTF and FeCl4- is present. The shortest Fe 3 3 3 Fe distance (6.74 A˚) is longer than the sum of van der Waals distance for possible d-d interaction. Antiferromagnetic coupling is dominant, as seen by the Weiss constant and also by the linear dependence of the isothermal magnetization (Figure 12). The exchange interactions estimated from the extended H€ uckel method are Jπd(Fe-B) = 4.07 K, Jπd(Fe-A) = 0.57 K, and Jdd = 0.02 at 290 K.43 The magnetic susceptibility measurements of II were carried out on a single crystal at low temperatures. Thus, we are unable to give absolute values. When the magnetic field was applied parallel to the c-axis, an antiferromagnetic transition was observed at 2.7 K (Figure 13). Because the shortest Fe 3 3 3 Fe (6.74 A˚) and Cl 3 3 3 Cl (3.83 A˚) distances are longer than the sum of van der Waals distances for possible d-d interactions, we believe that the magnetic transition may originate from π-d interactions between BEDT-TTF and FeCl4-. The exchange interactions between moments estimated by the extended H€ uckel method are Jπd(Fe-A) = 3.43 K, Jπd(Fe-B) = 25.82 K, and Jdd = 0.12 K. J(Fe-B) is one of the highest values for charge-transfer salts, being almost 10 K higher than for β0 -(BEDT-TTF)3(MnCl4)2 and λ-(BETS)2FeCl4.44 Conclusion Two polymorphic bilayered organic-inorganic hybrid magnetic conductors, δ-(BEDT-TTF)3(FeCl4)2 (I) and β0 -(BEDTTTF)3(FeCl4)2 (II), were obtained and characterized. The two

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