Structure and physical properties of new layered iron oxychalcogenide BaFe2OSe2 Hechang Lei,1, Hyejin Ryu,1,2 John Warren,3 A. I. Frenkel,4 V. Ivanovski,5 B. Cekic5 and C. Petrovic1,2 1Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA 2Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA 3Instrumentation Division, Brookhaven National Laboratory, Upton, New York 11973, USA 4Physics Department, Yeshiva University, 245 Lexington Avenue, New York, New York 10016, USA 5Institute of Nuclear Sciences Vinca, University of Belgrade, Belgrade 11001, Serbia (Dated: June 8, 2018) We have successfully synthesized a new layered iron oxychalcogenide BaFe2OSe2 single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c-axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b-axis and are bridged by oxygen along the a-axis. Physical property measurements indicate that BaFe2OSe2 is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long range antiferromagnetic (AFM) transition at 240 K, corresponding to the peak in specific heat measurement and two glassy transitions at 115 K and 43 K. The magnetic entropy up to 300 K is much smaller than the expected value for Fe2+ in tetrahedral crystal fields and M¨osbauer spectrum indicates that long range magnetic order is unlikely at 294 K. Both results suggest that a short range magnetic correlations exist above the room temperature. PACS numbers: 75.50.Ee, 75.10.Pq, 75.30.Gw, 75.47.Np 2− I. INTRODUCTION The materials with [TM2OCh2] layers exhibit sim- ilar structural diversity to iron-based superconduc- tors. This is understandable since new compounds The discovery of layered iron oxypnictides can be obtained by simply replacing [FeAs]− lay- (Ln(O,F)FeAs, Ln = rare earth elements, 1111-type) 2− 1 ers with [TM2OCh2] layers, such as LnOFeAs → superconductors with Tc up to 56 K, has stimulated Ln2O2TM2OCh2, AEFFeAs (AE = alkali earth metals) great interest in mixed-anion materials. Mixed anions 10−12 → AE2F2TM2OCh2, and AFeAs (A = alkali met- from the same row of the Mendeleev periodic table 13−15 als) → Na2Fe2OSe2. Thus, it is of considerable in- tend to randomly occupy the same crystallographic site terest to explore the structural derivatives of AEFe2As2 (anion disorder) because of the relatively similar sizes. among the oxychalcogenide compounds. On the other hand, if the anions are from different rows Here, we report the detailed synthesis and physi- (oxysulfide or oxyselenide compounds for example), the cal properties of a new layered iron oxychalcogenide distinctive difference of anion sizes and ionic polarization BaFe2OSe2 single crystal. Even though it has the same might lead to ordered occupancy in the different crys- 2− chemical formula and [TM2OCh2] layers, the struc- tallographic sites (anion order), often forming layered 2 ture is different from other iron oxychalcogenides with crystal structure. 2− [TM2OCh2] layers. To the best of our knowledge, The mixed-anion compounds have attracted some in- BaFe2OSe2 is the first layered iron oxychalcogenide with terest during the exploration of novel cuprate high alkali earth metal. It shows a semiconducting behavior temperature superconductors. Halooxocuprates are ex- with possible successive spin-glass transitions at low tem- ample where copper ions are coordinated with four perature and short range antiferromagnetic order above oxygen ions, forming the CuO2 sheet, whereas the the room temperature. halogen ion usually occupies a so-called apical site.3 With proper electron or hole doping, halooxocuprates will become superconductors.3 Besides superconductiv- II. EXPERIMENT arXiv:1206.5788v1 [cond-mat.supr-con] 25 Jun 2012 ity, mixed-anion materials also exhibit diverse physical properties. Copper oxychalcogenides LnCuOCh (Ch = Single crystals of BaFe2OSe2 were synthesized by a S, Se, and Te), isostructural to 1111-type iron-based su- self-flux method. Ba rod, Fe powder, Fe2O3 powder and perconductors, are wide-gap p-type semiconductors with Se shot were used as starting materials. BaSe was pre- transparent p-type conductivity, photoluminescence, and reacted by reacting Ba piece with Se shot at 800 ◦C for large third order optical nonlinearity.4−6 Transition 12 hours. BaSe was mixed with other reagents and inti- metal oxychalcogenides Ln2O2TM2OCh2 (TM = Mn, Fe, mately ground together using an agate pestle and mortar. and Co) contain the layers built up by the edge-shared The ground powder was pressed into pellets, loaded in an 2− octahedral unit [TM2OCh2] . These materials show alumina crucible and then sealed in quartz tubes with Ar strong electron-electron interactions (Mott insulators) on under the pressure of 0.2 atmosphere. The quartz tubes the two dimensional (2D) frustrated antiferromagnetic were heated up to 600 ◦C in 10 h, kept at 600 ◦C for 12 (AFM) checkerboard spin-lattice.7−9 h, ramped again to 1100 ◦C in 12 h, kept at 1100 ◦C for 2 12 h, and then cooled slowly at a rate of 3 ◦C/h to 800 ◦C, finally the furnace was shut down and the sample was cooled down to room temperature naturally. Black plate-like crystals with typical size 2×2×0.5 mm3 can be grown. Except for heat-treatment, all of processes for sample preparation were performed in glove boxes filled with argon. The crystal structure of BaFe2OSe2 crystal was iden- tified by single crystal x-ray diffraction (XRD). The data were collected using the Bruker APEX2 software package16 on a Bruker SMART APEX II single crystal x-ray diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 A)˚ at room temperature. Data processing and an empirical absorption correction were also applied using APEX2 software package. The structures were solved (direct methods) and refined by full-matrix least-squares procedures on |F 2| using Bruker SHELXTL program software.17 Obtained lattice param- eters of BaFe2OSe2 are given in Table 1. Atomic coor- dinates, isotropic displacement parameters and selected interatomic bond distances and angles are listed in Table 2. Phase identity and purity were confirmed by powder X-ray diffraction carried out on a Rigaku miniflex X-ray machine with Cu Kα radiation (λ = 1.5418 A).˚ Structural refinements of powder BaFe2OSe2 sample was carried out by using Rietica software.18 The average stoichiometry was determined by exami- nation of multiple points using an energy-dispersive X- ray spectroscopy (EDX) in a JEOL JSM-6500 scanning FIG. 1. (a) Crystal structure of BaFe2OSe2. The biggest electron microscope. The presence of oxygen was con- orange, big red, medium blue and small purple balls represent firmed for BaFe2OSe2, but the exact amount could not Ba, Fe, O and Se ions, respectively. (b) and (c) Powder and be quantified because of experimental limitations. The single crystal XRD patterns of BaFe2OSe2. average atomic ratios determined from EDX are Ba:Fe:Se = 1.00(7):2.2(2):2.0(2), close to the ratio of stoichiomet- ric BaFe2OSe2. different from those in K Fe − Se and BaFe Se . In M¨osbauer spectrum was taken in transmission mode x 2 y 2 2 3 BaFe OSe , double chains of edge-shared Fe-Se(O) tetra- with 57Co(Rh) source at 294 K and the parameters were 2 2 hedra propagate along the Se atoms parallel to b-axis. obtained using WinNormos software.20 Calibration of the The Fe-Se(O) double chains are bridged by oxygen along spectrum was performed by laser and isomer shifts were the a-axis (Fig. 1(a)). The bond distances of Fe-Se (∼ given with respect to α-Fe. 2.5 A)˚ and Fe-O (∼ 1.9 A)˚ are very similar to distances in The X-ray absorption spectra of the Fe and Se K-edges compounds where Fe is also in tetrahedral coordination were taken in transmission mode on powder samples of (BaFe2Se3: dF e−Se ∼ 2.4 A,˚ Fe3O4: dF e−O ∼ 1.9 A).˚ Be- BaFe2OSe3 at the X19A beamline of the National Syn- cause of different bond distances between Fe-Se and Fe-O, chrotron Light Source. Standard procedure was used to Fe atoms are located at the highly distorted tetrahedral extract the extended x-ray absorption fine-structure (EX- 19 environment when compared to other materials with pure AFS) from the absorption spectrum. Fe-Se tetrahedra. This distortion is also reflected in the Electrical transport, heat capacity, and magnetiza- significant deviation of bond angles from the value for tion measurements were carried out in Quantum Design the ideal tetrahedron (109.5◦). The bond angles range PPMS-9 and MPMS-XL5. ◦ up to 130.38(10) in BaFe2OSe2, much larger than the 22,23 values in BaFe2Se3 and KxFe2−ySe2. Moreover, the different connection of Fe-Se(O) leads to the larger near- III. RESULTS AND DISCUSSION est neighbor Fe-Fe distances (dF e−F e (∼ 3.13 A))˚ when 22,23 compared to BaFe2Se3 and KxFe2−ySe2. Using ob- 21,22 Similar to KxFe2−ySe2 and BaFe2Se3, the struc- tained Fe-Se and Fe-O bond lengths as shown in Table ture of BaFe2OSe2 is built up by stacking the Ba cations 2, the valence of Fe ions in BaFe2OSe2 can be calcu- and Fe-Se(O) layers alternatively along the c axis (Fig. lated using the bond valence sum (BVS) formalism in 1(a)). However, within the Fe-Se(O) layers, the connec- which each bond with a distance dij contributes a va- tion of Fe-Se(O) tetrahedra in BaFe2OSe2 is distinctive lence vij = exp[(Rij − dij )/0.37] with Rij as an empir- 3 TABLE I. Crystallographic Data for BaFe2OSe2. Chemical Formula BaFe2OSe2 Formula Mass (g/mol) 422.94 Crystal System orthorhombic Space Group Pmmn (No.