Tuning the Magnetic Properties of New Layered Iron Chalcogenides

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Tuning the Magnetic Properties of New Layered Iron Chalcogenides (BaF)2Fe2−xQ3 (Q = S, Se) by Changing the Defect Concentration on the Iron Sublattice Mihai Sturza,† Jared M. Allred,† Christos D. Malliakas,†,‡ Daniel E. Bugaris,† Fei Han,† Duck Young Chung,† and Mercouri G. Kanatzidis*,†,‡ †

Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: Effecting and controlling ferromagnetic-like properties in semiconductors has proven to be a complex problem, especially when approaching room temperature. Here, we demonstrate the important role of defects in the magnetic properties of semiconductors by reporting the structures and properties of the iron chalcogenides (BaF)2Fe2−xQ3 (Q = S, Se), which exhibit anomalous magnetic properties that are correlated with defects in the Fe-sublattice. The compounds form in both long-range ordered and disordered polytypes of a new structure typified by the alternate stacking of fluorite (BaF)22+ and (Fe2−xQ3)2− layers. The latter layers exhibit an ordered array of strong Fe−Fe dimers in edge-sharing tetrahedra. Given the strong Fe−Fe interaction, it is expected that the Fe−Fe dimer is antiferromagnetically coupled, yet crystals exhibit a weak ferromagnetic moment that orders at relatively high temperature: below 280−315 K and 240−275 K for the sulfide and selenide analogues, respectively. This transition temperature positively correlates with the concentration of defects in the Fe-sublattice, as determined by single-crystal X-ray diffraction. Our results indicate that internal defects in Fe2−xQ3 layers play an important role in dictating the magnetic properties of newly discovered (BaF)2Fe2−xQ3 (Q = S, Se), which can yield switchable ferromagnetically ordered moments at or above room temperature.

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Ta),23 Na1.9Cu2Se2·Cu2O,24 Bi2YO4Cu2Se2,25 RE2FeSe2O2 (RE = La, Ce)26), and sulfides or selenides (K2xMnxSn3−xS6,27 Rb2Cu2Sn2S6,28 Ae2F2SnX3,29 Ae2Sb2X4F2 (Ae = Sr, Ba; X = S, Se),30 Tl8Sn10Sb16Se48,31 and (AgxPb1−xSe)5(Bi2Se3)3m (m = 1, 2)32). With regard to our interest in new heterostructured layered compounds with physical properties related to the AxFe2−xSe2 (A = alkali metal) superconductors, we have discovered a family of materials (BaF)2Fe2−xQ3 (Q = S, Se) with a surprising layered heterostructure structure where PbO-type Ba2F2 layers alternate with a previously unknown Fe2Q3 layer. The Fe2Q3 layer has considerable phase width arising from the numerous Fe vacancies giving the compositions of (BaF)2Fe2−xQ3. The new compounds are semiconductors that show evidence of a weak ferrimagnetic transition around room temperature. The magnetic properties are very unusual and are strongly linked to the vacancies and change with composition. The Fe2Q3 layers are composed of pairs of edge-sharing Fe2S6 tetrahedra that contain a very short Fe−Fe bond (∼2.4 Å). While the geometry of the

INTRODUCTION Low-dimensional compounds of transition-metal elements continue to be the focus of diverse scientific studies because of a multitude of interesting electrical and magnetic properties, such as high-temperature superconductivity,1−4 colossal magnetoresistance,5−7 and thermoelectricity.8−10 Binary transition-metal compounds with highly polarizable anions, such as transitionmetal chalcogenides, are commonly found to have layered structures.11−13 Compounds close to the metal−semiconductor boundary commonly composed of transition-metal oxides or sulfide layers are particularly attractive. Especially interesting are structural motifs that have different types of alternating layers (heterostructures), where one can act as a charge-balancing component to another layer that is electronically and/or magnetically active. This determines the physical properties of the material, often in a tunable way.14 Many alkaline-earth or rare-earth transition-metal compounds exhibit layered-type heterostructures featuring segregated well-defined layers. Notable examples include oxypnictides (LaOFeP,15 Ba2Ti2Fe2As4O,16 A2MnZn2As2O2 (A = Sr, Ba),17 Na2Ti2Pn2O (Pn = As, Sb)18), oxychalcogenides (A2F2Fe2OQ2,14 (LaO)2Fe2OQ2 (A = Sr, Ba; Q = S, Se),19 SrF1−xOxCuS,20 Ln2Ti2S2O5 (Ln = Nd, Pr, Sm),21 (Cu2S2)(Srn+1MnO3n−1),22 La2LnMS2O5 (Ln = La, Y; M = Nb, © XXXX American Chemical Society

Received: January 22, 2015 Revised: April 10, 2015

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DOI: 10.1021/acs.chemmater.5b00287 Chem. Mater. XXXX, XXX, XXX−XXX

a

B

328/0/25 0.957 Robs = 0.0166, wRobs = 0.0436 Rall = 0.0238, wRall = 0.0448 1.689 and −0.843 e Å−3

Ba2F2Fe1.96(6)S3 518.32 293(2) K 0.71073 Å tetragonal I41/acd a = 6.1446(9) Å, c = 38.675(8) Å 1460.2(4) Å3 8 4.736 g/cm3 15.329 mm−1 1840 0.0478 mm × 0.0374 mm × 0.0124 mm 2.21°−24.96° −7 ≤ h ≤ 7, −7 ≤ k ≤ 7, −45 ≤ l ≤ 45 3915 328 [Rint = 0.0269] 100%

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Ba2F2Fe1.72(6)S3 Ba2F2Fe1.98(5)S3 505.2 519.42 293(2) K 293(2) K 0.71073 Å 0.71073 Å orthorhombic tetragonal Immm I4/m a = 4.3075(9) Å, b = 4.3832(9) Å, c = 19.189(4) Å a = 4.3464(6) Å, c = 19.344(4) Å 362.30(13) Å3 365.42(10) Å3 2 2 4.629 g/cm3 4.731 g/cm3 −1 14.917 mm 15.313 mm−1 446 460 0.0232 mm × 0.0156 mm × 0.0094 mm 0.0450 mm × 0.0355 mm × 0.0118 mm 4.25°−29.11° 2.11°−31.82° −5 ≤ h ≤ 5, −6 ≤ k ≤ 5, −25 ≤ l ≤ 26 −6 ≤ h ≤ 6, −6 ≤ k ≤ 6, −28 ≤ l ≤ 28 2027 2251 311 [Rint = 0.0201] 334 [Rint = 0.0340] 100% 100% full-matrix least-squares on F2 311/0/25 334/0/19 1.26 1.161 Robs = 0.0181, wRobs = 0.0530 Robs = 0.0291, wRobs = 0.0790 Rall = 0.0192, wRall = 0.0533 Rall = 0.0297, wRall = 0.0791 1.93 and −0.68 e Å−3 1.267 and −1.107 e Å−3

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R = ∑||F0| − |Fc||/∑|F0|, wR ={∑[w(|F0|2 − |Fc|2)2]/∑[w(|F0|4)]}1/2, and w = 1/(σ2(F) + 0.0001F2).

empirical formula formula weight temperature wavelength crystal system space group unit-cell dimensions volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to θ = 29.15° refinement method data/restraints/parameters goodness-of-fit final R indices [>2σ(I)] R indices [all data]a largest diff. peak and hole

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283/0/19 1.316 Robs = 0.0201, wRobs = 0.0494 Rall = 0.0219, wRall = 0.0498 1.055 and −1.409 e Å−3

Ba2F2Fe1.78(5)Se3 661.26 293(2) K 0.71073 Å tetragonal I4/m a = 4.4929(6) Å, c = 19.548(4) Å 394.60(11) Å3 2 5.565 g/cm3 27.224 mm−1 568 0.082 mm × 0.055 mm × 0.019 mm 2.08°−29.06° −6 ≤ h ≤ 6, −5≤ k ≤ 6, −26 ≤ l ≤ 26 1931 283 [Rint = 0.0709] 100%

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Table 1. Summary of Crystallographic Data and Structure Refinement for Ba2F2Fe1.96(6)S3 (1), Ba2F2Fe1.72(6)S3 (2), Ba2F2Fe1.98(5)S3 (3), and Ba2F2Fe1.78(5)Se3 (4) at 293 K

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Chemistry of Materials

Table 2. Representative Bond Lengths and Bond Angles of (a) Ba2F2Fe1.96(6)S3 (1), (b) Ba2F2Fe1.72(6)S3 (2), (c) Ba2F2Fe1.98(5)S3 (3), and (d) Ba2F2Fe1.78(5)Se3 (4) at 293(2) K (a) Ba2F2Fe1.96(6)S3

(b) Ba2F2Fe1.72(6)S3

atom−atom

bond length (Å)

Fe−S(1) Fe−S(2) Ba−S(2) Ba−S(1) Ba−F Ba−F Fe−Fe

2.352(3) 2.3783(8) 3.3529(1) 3.3412(7) 2.635(3) 2.644(5) 2.415(3)

atom−atom

atom−atom−atom

bond angle (deg)

Fe(1)−S(1) Fe(2)−S(1) Fe(1)−S(2) Fe(2)−S(2) Fe(1′)−S(1) Fe(1′)−S(2) Ba−F Fe(2)−Fe(2) atom−atom−atom

S(1)−Fe−S(1) S(1)−Fe−S(2) S(2)−Fe−S(2) S(1)−Fe−Fe S(2)−Fe−Fe Fe−S(1)−Fe Fe−S(2)−Fe Fe−S(1)−Ba Fe−S(2)−Ba Ba−S(2)−Ba Ba−S(1)−Ba Ba−F−Ba

118.41(5) 102.025(7) 131.97(8) 59.21(2) 114.015(2) 61.59(5) 180.00 90.27(5) 90.00 180.00 133.71(5) 109.11(8)

S(1)−Fe(1)−S(1) S(1)−Fe(1)−S(2) S(2)−Fe(2)−S(2) S(1)−Fe(1′)−S(2) S(1)−Fe(1)−Fe(1) S(2)−Fe(1)−Fe(1) Fe(2)−S(1)−Fe(2) Fe(1′)−S(1)−Fe(2) Fe(1′)−S(1)−Fe(1′) Fe(2)−S(1)−Fe(2) Ba−S(1)−Ba Ba−F−Ba

(c) Ba2F2Fe1.98(5)S3 atom−atom

bond length (Å) 2.547(5) 2.374(1) 2.145(3) 2.332(2) 2.298(3) 2.416(3) 2.651(2) 2.482(4) bond angle (deg) 139.14(2) 134.32(3) 115.76(4) 101.83(3) 110.58(3) 67.17(7) 45.22(1) 87.82(1) 139.14(2) 180.0(5) 180.0(4) 108.842(4) (d) Ba2F2Fe1.78(5)Se3

bond length (Å)

atom−atom

bond length (Å)

Fe(1)−S(1) Fe(1)−S(2) Ba(1)−F(1) Ba(1)−F(1) Ba(1)−S(2) Ba(1)−S(1) Fe(1)−Fe(1) atom−atom−atom

2.3789(19) 2.358(2) 2.639(3) 2.6406(4) 3.3359(8) 3.3411(9) 2.413(4) bond angle (deg)

Fe(1)−Se(1) Fe(1)−Se(2) Fe(1)−Se(2) Ba(1)−F(1) Ba(1)−Se(1) Ba(1)−Se(2) Fe(1)−Fe(1) atom−atom−atom

2.4460(12) 2.4731(17) 2.4759(17) 2.6641(4) 3.4436(5) 3.4550(8) 2.418(4) bond angle (deg)

S(1)−Fe(1)−S(1) S(1)−Fe(1)−S(2) S(2)−Fe(1)−S(1) S(1)−Fe(1)−Fe(1) S(2)−Fe(1)−Fe(1) Fe(1)−S(1)−Fe(1) Fe(1)−S(2)−Fe(1) Fe(1)−S(1)−Ba(1) Fe(1)−S(2)−Ba(1) Ba(1)−S(2)−Ba(1) Ba(1)−S(1)−Ba(1) Ba(1)−F(1)−Ba(1)

132.04(9) 101.99(5) 102.02(5) 113.96(6) 59.22(5) 90.00 42.43(6) 90.24(4) 90.00 81.15(3) 133.64(2) 108.824(10)

Se(1)−Fe(1)−Se(1) Se(1)−Fe(1)−Se(2) Se(2)−Fe(1)−Se(2) Fe(1)−Fe(1)−Se(1) Fe(1)−Fe(1)−Se(2) Fe(1)−Se(1)−Fe(1) Fe(1)−Se(2)−Fe(1) Fe(1)−Se(1)−Ba(1) Fe(1)−Se(2)−Ba(2) Ba(1)−Se(2)−Ba(1) Ba(1)−Se(1)−Ba(1) Ba(1)−F(1)−Ba(1)

120.77(8) 102.00(4) 130.42(8) 60.38(4) 114.71(10) 59.23(8) 180.00 90.77(4) 90.00 180.00 134.61(3) 106.795(9)

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Fe2S6 dimer has been observed previously in molecular complexes,33,34 the short Fe−Fe bond length is more characteristic of the metallic bonds of intermetallic compounds instead of the coordinated covalent bonds observed here. In fact, to our knowledge, this is the first example to exhibit such a strong, ligand-supported Fe−Fe bond in this type of extended solid. The Fe2S6 dimers share their four outside vertices with in-plane neighbors, giving rise to another unusual structural feature: square-planar coordinated sulfur atoms. We report the synthesis, structure, and the unusual magnetic properties of (BaF)2Fe2−xQ3 (Q = S, Se).

EXPERIMENTAL SECTION

Reagents. The following reagents were used as received: barium metal dendritic pieces, purified by distillation (99.99%, Sigma−Aldrich), barium fluoride (99.9%, Sigma−Aldrich), sulfur chunks (99.999%, Spectrum Chemical Mfg. Corp.), selenium powder (99.99%, Sigma− Aldrich), and iron metal (99.9%, Sigma−Aldrich). Synthesis of (BaF)2−xFe2Q3 (Q = S, Se). Synthetic preparations were carried out under a dry argon atmosphere in an M-Braun glovebox. The phase-pure binary compounds BaS and BaSe were first prepared via direct combination of the elements, followed by heating at temperatures of 400 °C (24 h) and 850 °C (48 h). Polycrystalline samples of (BaF)2Fe2−xQ3 (x = 0, 0.1, 0.2, 0.3; Q = S, Se) were synthesized by conventional solid-state reaction by using a stoichiometric mixture of BaS, BaSe, BaF2, iron metal, and elemental sulfur or selenium. The C


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Chemistry of Materials ground powders of these starting materials were loaded into graphite crucibles to avoid the reaction of BaF2 with the glass. The graphite crucible was then placed inside a fused silica tube and flame-sealed under a pressure of 0 imply oxidation of the Fe/Q network, with some of the Fe atoms becoming formally +3. All Fe−S bonds fall in the narrow range between 2.352(3) Å and 2.3798(8) Å, which is standard for this bond type. The Fe− Fe bond in the Fe2S6 tetrahedral dimer is 2.415(3) Å, which represents a single covalent bond, and is significantly shorter than it is in iron metal (2.6 Å).45 The angles around Fe and S are close to those expected for distorted tetrahedral coordination in the ranges of 102.040(1)°−118.23(12)° for S(1)−Fe(1)−S(2) and S(1)−Fe(1)−S(2) (see Table 2). A projection of the [Fe2−xS3]2− block in the ab-plane is shown in Figure 2b. Although for both the [Ba2F2]2+ and [Fe2−xS3]2− layers there is only one crystallographically independent layer, the 41 screw axis in I41/acd requires that there are four of each per unit cell. Only the Fe site distribution necessitates the long c-axis through the loss of the xy mirror plane; the Ba, F, and S atoms are at high symmetry sites and have a shorter translation symmetry in both the c-axis and the basal plane. This point can be clarified by analogizing this new structure type to the known structure type (LaO)2(Fe2Se2O).19 In this family, the [La2O2]2+ layer is equivalent to the [Ba2F2]2+ here, and the Se2O arrangement in the other layer is equivalent to the S sites here. This equivalency is illustrated in Figure 3a by removing the Fe sites; the resulting cell is qualitatively equivalent for both structure types. The subcell symmetry is I4/mmm meaning that there are only two layers of each type per cell in the conventional setting. This arrangement is the a′ = a/√2, c′ = c/2 subcell of the (BaF)2Fe2S3 structure reported here.

Figure 1. Calculated and observed XRD patterns of the Rietveld refinement for (a) (BaF)2Fe1.78(3)S3, (b) (BaF)2Fe1.80(8)S3, and (c) (BaF)2Fe1.88(1)Se3 (λ = 1.5406 Å). [Bragg R-factor = ∑|Iko − Ikc|/∑Iko; Rf-factor = [(N − P)/∑wi·yio2]1/2.]

(BaF)2Fe1.88(1)Se3, the tetragonal I4/m space group was used (see Figure 1c). The results are in good agreement with the structure refined by single-crystal XRD. Despite its incongruent melting behavior (described below), the growth of micrometersized single crystals by slow cooling from the melt was successful. Differential thermal analysis (DTA) experiments up to 1100 °C at a rate of 5 °C/min on polycrystalline (BaF)2Fe2−xQ3 (Q = S, Se) samples show a single endothermic melting point at ∼975 °C for the sulfide and ∼960 °C for the selenide compounds and exothermic crystallization points at ∼960 and 935 °C, respectively (see Figures S2(a) and S2(b) in the Supporting Information). The samples after DTA were examined by XRD and showed the presence of BaFe2S3, BaFe2Se3, and BaF2, indicating that the (BaF)2Fe2−xQ3 (Q = S, Se) compounds melt incongruently (see Figures S3(a) and S3(b) in the Supporting Information). Crystal Structure. The single-crystal structures of (BaF)2Fe2−xQ3 (Q = S, Se) were determined at room temperature (293 K). The crystal structures of the two layered E


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Figure 2. (a) Perspective view of the layered structure of (BaF)2−xFe2Q3 (Q = S, Se) along the crystallographic b-axis. Ba atoms are gray, Fe atoms are brown, F atoms are green, and chalcogenides atoms (Q = S, Se) are yellow. Projections of the [Fe2Q3]2− block in the (ab) plane: (b) Q = S and (c) Q = Se (Fe atoms are brown, S atoms are orange, and Se atoms are red). Also shown are SEM micrographs of typical (d) (BaF)2Fe2S3 and (e) (BaF)2Fe2Se3 crystals.

polytypes from (LaO)2(Fe2Se2O) and each other. Figure 3c shows that, in (LaO)2(Fe2Se2O), the Fe atoms only go into octahedral sites. In the I41/acd polytype of (BaF)2Fe2S3, the Fe2+ ions only fill half of the tetrahedral sites (Figure 3d), although they do this in an alternating fashion. This is the same as moving the Fe atom off of the octahedral site in order to form the Fe−Fe bond in the Fe2Q6 dimers. The ordered filling of the tetrahedral sites to give the Fe2S6 dimer described above is what gives rise to a = √2a′ cell in the I41/acd polytype. There are two different choices for filling the tetrahedral sites, so when each of the two configuration are alternately realized in an ordered fashion, this results in the cell’s c = 2c′. Another related material is the mixed valence iron compound (BaF)2Fe1.5S3.46 Similar to the title compound, this one can also be compared to the (LaO)2(Fe2Se2O) structure; however, in this case, one-quarter of the octahedral sites remain filled and only one-quarter of the tetrahedral sites are filled. The building block here is an Fe3S10 trimer, with an octahedron placed between two tetrahedra. The presence of the octahedron precludes the formation of any Fe−Fe bonds in this compound. The I4/m disordered structure is observed in both sulfide and selenide compounds. The selenide compound is the example used here to illustrate the features of the disorder structure. The black platelike crystal of (BaF)2Fe2−xSe3 (depicted in Figure 2e) crystallizes in the tetragonal space group I4/m, with cell parameters a = 4.4929(6) Å, c = 19.548(4) Å, and Z = 2. The structure of (BaF)2Fe2Se3 is almost equivalent to (BaF)2Fe2S3, except that Fe site disorder (Figure 2c) folds in the translational symmetry. The disordered structure can be seen as a superposition of the two types of tetrahedral filling (see Figure 3e). The superposition of both types of Fe-site ordering give a crystallographic average that destroys the supercell, which is why

Figure 3. Comparison of (BaF)2Fe2Q3 and (LaO)2Fe2Se2O. Panel (a) shows a view down the b-axis; the Fe sites are removed to show that the remaining atomic sites are equivalent for both structures. Panels (b)− (e) show a canted top-down view of the Fe2Q2Q′ layer: (b) potential octahedral sites (green) and tetrahedral sites (sites); (c) (LaO)2Fe2Se2O-type filling of octahedral sites; (d) the ordered variant (I41/acd) of (BaF)2Fe2Q3, which only has half of the tetrahedral sites filled (alternating layers have an opposite pattern of filled and empty sites, giving the long c axis); and (e) the disordered I4/m variant of (BaF)2Fe2Q3, where stacking faults cause the Fe-site distribution to be a superposition of the alternating layers sequence observed in the I41/acd variant. The Immm disordered variant is equivalent, except that one Fe subsite is split into two sub-subsites along the y-axis.

For this arrangement of anions, there are many different potential (octahedral and tetrahedral) sites for metal cations, and the ordered filling of these sites is what distinguishes the various F


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Figure 4. Electrical resistivity data as a function of temperature for single crystals of (BaF)2Fe2−xQ3 (x = 0, 0.2; Q = S, Se): low-temperature region measured on a PPMS ((a) Ea = 0.39 eV and (c) Ea = 0.32 eV) and high-temperature region measured on MMR ((b) Ea = 0.39 eV and (d) Ea = 0.32 eV), showing semiconducting behavior.

series of ordered defect phases in NiAs-type Fe1−xS and Fe1−xSe.47,48 Physical Properties. Electrical Resistivity. Temperaturedependent electrical resistivity measurements were carried out on single crystals of (BaF)2Fe2−xQ3 (Q = S, Se) from 140 K to 700 K. The electrical resistivity of these compounds shows a typical thermally activated behavior for a semiconductor over the temperature range studied. As shown in Figures 4a and 4b, the resistivity of (BaF)2Fe2S3 is ∼2.9 Ω cm at room temperature and decreases to ∼0.12 Ω cm at 700 K. The overall resistivity of (BaF)2Fe2Se3 at 300 K, ∼43.5 Ω cm, is almost one order of magnitude higher than that of (BaF)2Fe2S3 and decreases to ∼1.6 Ω cm at 700 K (see Figures 4c and 4d). For both compounds, the electrical resistivity exhibits an activated behavior that can be described by an Arrhenius temperature dependence,

this cell has the same metric as the (LaO)2(Fe2Se2O) cell, although the displaced Fe sites lower the symmetry to I4/m. Physically, the disorder is most likely due to stacking faults along the c-axis, since the ordered stacking along the c-axis is not coupled by direct bonds. Indeed, in the disordered crystals, scattering diffraction rods are observed that coincide with the ordered cell, which supports this interpretation. As for bonding environment, the Fe−Fe bond length in the selenide is almost exactly the same as in the sulfide (2.418(3) Å), and the Fe−Se bond lengths are also standard (between 2.446 Å and 2.476 Å). This justifies the designation of the Fe−Fe bond bridging the tetrahedra as the primary structural unit of this layer, since the geometry of the tetrahedra compensates for the different ligand sizes in order to keep the Fe−Fe bond intact. The reported bond lengths are based on the presumption that, despite the long-range crystallographic disorder, the local structure looks like the nondisordered cell. Therefore, certain interatomic distances are not reported, since they are interpreted to be artifacts of the superposition of two conformations. Finally, note that we found a disordered crystal in (BaF)2Fe2−xS3 (x = 0.27) that is orthorhombic (Table 1), indicating that sufficient defects can introduce new, symmetrybreaking effects. Compared to the I4/m polytype, the b-axis shortens and the a-axis lengthens. The disorder giving rise to this distortion comes from one of the partially occupied Fe sites (Fe1), which splits along the b-axis into two subsites. This likely comes from the rather large concentration of vacancies (x/2 = 14% of sites), and possibly one site corresponds to a dimerized (formally) Fe2+ ion, and the other to an undimerized (formally) Fe3+ ion. The fact that they cooperatively choose a single axis in the crystal along which to disorder implies that the defect structure actually is well-ordered in the ab-plane. Unfortunately, the presence of scattering rods from stacking faults obscures the ordering, which makes resolving the ordering nontrivial. Nevertheless, this implies that there are ordered line compounds in the compositional phase diagram analogous to the complex

⎛ E ⎞ ρ(T ) = ρ0 exp⎜ a ⎟ ⎝ kBT ⎠

where ρ0 is a pre-exponential factor, Ea the activation energy, and kB the Boltzmann constant. The Ea values were estimated to be 0.39 eV in the temperature range of 170−250 K for sulfides and 0.32 eV in the temperature range of 165−230 K for selenide compounds. For nonmagnetic semiconductors, replacing Se atoms with more electronegative S atoms increases the band gap, and this feature is widely used to tune the band gaps of semiconductors such as CuIn1−xGaxS2−ySey (CIGS)49 used for solar cells. As will be shown in the next section, these chalcogenides are magnetic semiconductors with unpaired spins at the Fe2+ sites. Magnetic Properties. The unusual behavior in these two compounds is exhibited in their magnetic properties. A different magnetic response is observed from crystal to crystal and from batch to batch, which is attributed to the difference in the value of x. The values of x for the each measured sample were determined by single-crystal X-ray crystallography. Summary of crystalloG


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Figure 5. Temperature dependence of DC magnetic susceptibility measured at 0.1 T for single crystals of (a) (BaF)2Fe1.8S3, (b) (BaF)2Fe1.78Se3, (c) (BaF)2Fe1.73S3, and (d) (BaF)2Fe1.76S3.

Figure 6. Field-dependence magnetization at different temperatures in the range of 2−380 K for crystals of (a) (BaF)2Fe1.8S3, (b) (BaF)2Fe1.78Se3, (c) (BaF)2Fe1.73S3, and (d) (BaF)2Fe1.74Se3.

graphic data and structure refinement at 293 K for the specimens used to collect the magnetic data are shown in Table S3 in the Supporting Information. The temperature dependence of the magnetic susceptibility data for (BaF)2Fe2−xQ3 (Q = S, Se; x = 0−0.3) single crystals is shown in Figure 5 and in Figure S4 in the Supporting Information. The magnetic susceptibility in both the sulfide and the selenide is quite anisotropic, and the magnetic

transition temperature is extremely sensitive to stoichiometry. When the field is aligned in the basal plane the samples exhibit a large peak in the ZFC in-plane susceptibility near room temperature (at ∼310 K for (BaF)2Fe1.8S3, ∼315 K for (BaF)2Fe1.73S3, ∼290 K for (BaF)2Fe1.76S3 (Figures 5a, 5c, and 5d), and ∼275 K for (BaF)2Fe1.78Se3 (Figure 5b, respectively), a response that is maintained down to 2 K in the FC data. Using a H


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Chemistry of Materials 10 or 100 Oe applied field (not shown) has almost no effect on these measured properties. Oriented field-dependent measurements of (BaF)2Fe1.8S3 (Figure 6a) show that, at 2 K and low field, there is an antiferromagnetic-like signalweak, field-independent susceptibilitythat transitions into a hard ferromagnetic-like regime at higher fields (∼3 and 2 T for the S and Se analogues, respectively). At higher temperatures, the transition moves to lower field, so that by the critical temperature (TC), it occurs at zero field, which corresponds to the peak in the low-field χ(T) data. Also note that, when going from low temperature to high temperature, the ferromagnetic portion of the response softens. At temperatures above the peak M(T) data, the material appears virtually nonmagnetic in the field-dependent data. At all temperatures, the effective moment per Fe atom is much smaller than expected for either a high-spin or low-spin tetrahedral Fe2+ (d6) ion. Crystals with refined compositions of (BaF)2Fe1.78Se3 (Figure 6b) and (BaF)2Fe1.73S3 (Figure 6c) also show a fieldinduced transition at 2 K (∼2 and 2.5 T, respectively), but unlike (BaF)2Fe1.8S3, the high-field, ferromagnetic-like state is much softer. The field dependence of these compounds at the peak in the M(T) data (from Figure 5) is qualitatively the same as in (BaF)2Fe1.8S3. Above the transition, (BaF)2Fe1.78Se3 is also apparently diamagnetic, but the (BaF)2Fe1.73S3 sample shows a small net paramagnetic component. (BaF)2Fe1.74Se3 (Figure 6d) shows rather different behavior in its field-dependent magnetization. At all temperatures, there is the same linear term dominating the magnetization, with only small perturbations, corresponding to the regimes observed in the other compounds. At 2 K, the high-field phase transition that was obvious in the other compounds is now signified by just a minor increase in slope, barely visible above 3 T, and only a small amount of hysteresis is observed. The 220 K data also are almost linear, only showing a small enhancement, which is indicative of a tiny, soft ferromagnetic component. By 300 K, the M(H) curve is just a featureless line. Overall, the trend is that the magnetic properties weaken with decreasing iron content, and a weaker response is seen in the selenides than in the sulfides. The ferrimagnetic-like component could be explained by cooperative canting of the otherwise antiferromagnetically ordered Fe−Fe dimer sublattice. However, it seems unlikely that such a situation would transition into what appears to be a nonmagnetic state at higher temperatures without some form of transformation of the local bonding, which is not observed. Similarly, this should result in some form of feature in the electrical conductivity in this temperature range, which is also not observed. This implies that the magnetic properties are not strictly intrinsic. Yet, it is also unlikely that they are from an inclusion phase (e.g., NiAs-type Fe1−xS or Fe1−xSe), given the sensitivity to crystal orientation, lack of reflections corresponding to intergrown phases in the single-crystal diffraction data, and reproducible properties in single crystals within a batch. The most consistent conclusion is then that the magnetic features are due to vacancies in the Fe sublattice, and indeed the transition temperatures co-vary with compositions obtained from singlecrystal refinements. This scenario assumes that the primary functional unit is the Fe2Q3 dimer, and that its electronic ground state is strongly ordered, so that they do not disorder at any temperature below 700 K, given the lack of features in the electrical conductivity. The presence of an Fe vacancy would convert a Fe−Fe dimer to an undimerized Fe atom that has a moment associated with an Fe(III) ion (S = 5/2). Still, it can couple to the five surrounding dimers through the exchange

pathways of each of the two in-plane square planar Q atoms. Even if these defects are dilute and/or random, they can significantly affect the emergent properties.50,51 One would also expect unpaired Fe atoms to be much more susceptible to coercive fields. This explains the temperature dependence of the ferrimagnetic state: at low temperatures, there is not enough thermal energy for the moment to return to the ground state orientation set by the surrounding lattice. Similarly, a lower field is needed to flip the spin, because of the existing thermal energy. Above the magnetic transition temperature (TC), the bulk ordered state remains intact, but the ordering energy of the magnetic defects has been overcome, so they are no longer ordered. Notably, the ostensibly diamagnetic signal at high temperatures could imply a singlet-like state of the Fe−Fe dimer, possibly from the extremely strong coupling arising for the very short 2.418 Å Fe−Fe bond. On the basis of these experiments, we made a correlation plot of TC and unit-cell parameters with x the (BaF)2Fe2−xS3 system. Figure 7 shows the magnetic transition temperature (TC) versus

Figure 7. Correlation of magnetic transition (TC) and unit-cell parameters with Fe vacancies in (BaF)2Fe2−xS3 system. TC (red axis) increases with increasing Fe vacancies and lattice dimension c (blue axis) chaotically depends on x.

the Fe vacancies for many of the single-crystal samples that came from different synthetic batches in this system. The occupancy of Fe atoms in (BaF)2Fe2−xS3 was quantified by refinement of single-crystal XRD data. The refinement was performed using the disorder model structure and Fe atoms exhibit vacancies creating nonstoichiometry. Our data indicate that the magnetic transition, the stoichiometry, and the crystal structure are strongly correlated in the (BaF)2Fe2−xQ3 phases. Samples with a higher concentration of Fe vacancies appear to display the highest magnetic transition temperatures and exhibit a reduced value of lattice dimension c. A lower concentration of Fe vacancies yield higher values for cell parameter c and reduced TC values. Neutron Diffraction. The neutron powder diffraction data of the (BaF)2Fe2−xS3 (x = 0) compound shows many extra peaks that are not seen in the X-ray data (see Figure 8). The most obvious are those near the limits of our collected data (near 0.7 Å−1). A √2 × √2 supercell (e.g., (1,1,0) (−1,1,0) (0,0,1) basis set), relative to the ordered 6.2 Å × 6.2 Å × 38.7 Å cell is sufficient to index the new reflections. This confirms the presence of magnetic order. However, the data show evidence of stacking faults, enough so that refining even the nuclear component is not possible. This precludes the possibility of determining a magnetic model. Also, based on the previous discussion, it is not known whether the template iron lattice remains ordered above the observed transition, and whether or not this magnetic wave vector corresponds to a weakly ordered defect sublattice, or if it is I



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ACKNOWLEDGMENTS

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REFERENCES

The work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Use of the Center for Nanoscale Materials, including resources in the Electron Microscopy Center and Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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Figure 8. Neutron diffraction pattern of (BaF)2Fe2−xS3 (x = 0) collected on POWGEN at 220 K. Upper panel shows the full scan (note the extra background starting at ∼2.3 Å−1 that is due to diffuse scattering rods). Lower panels are detailed views of the magnetic reflections compared to XRD data (11BM) at the same temperature.

involves the entire Fe lattice. However, generally, it can be said that the magnetic k-vector means that below the transition not all dimer pairs are translationally equivalent, so there is longer range ordering than just between pairs.

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CONCLUDING REMARKS The new compounds (BaF)2Fe2−xQ3 (Q = S, Se) adopt a new crystal structure type that is related to the (LaO)2Fe2Se2O structure, except that (a) the Fe atoms occupy tetrahedral sites instead of octahedral sites and form strong Fe2S6 dimers with extremely short Fe−Fe distances and (b) the Fe sites are subject to forming massive vacancy defects in the lattice. The physical properties are a direct result of this structural feature, as both compounds appear to be semiconductors. A small ferrimagnetic component is observed below 280−315 K and 240−275 K for the sulfide and selenide analogues, respectively, which is attributed to different concentrations of vacanies on the Fe sites. Hence, the magnetic properties can be fine-tuned by controlling the defect chemistry. This type of property is important to fields such as spintronics, where semiconducting phases with controllable ferromagnetic moments are of interest.52−54

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information includes the following: crystallographic data in CIF format; further details are given in Tables S1−S3 and Figures S1−S4. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00287.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. J


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Tuning the Magnetic Properties of New Layered Iron Chalcogenides

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