Solid State Communications 141 (2007) 467–470www.elsevier.com/locate/ssc

Synthesis and structural determination of the new oxide fluoride BaFeO2F

Richard Heapa, Peter R. Slatera,∗, Frank J. Berryb, Orn Helgasonc, Adrian J. Wrightd

a Chemistry Division, University of Surrey, Guildford, Surrey GU2 7XH, UKb Department of Chemistry, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

c Science Institute, University of Iceland, Dunhagi, IS-107 Reykjavik, Icelandd School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK

Received 24 October 2006; received in revised form 23 November 2006; accepted 24 November 2006 by H. AkaiAvailable online 18 December 2006

Abstract

Low temperature fluorination of BaFeO3−x using poly(vinylidene fluoride) leads to the formation of the oxide fluoride BaFeO2F. Mossbauerspectroscopy shows that this phase exhibits magnetic ordering at room temperature due to interactions between the Fe3+ ions, with an orderingtemperature of 645 (±5) K. Neutron diffraction studies show that the phase has cubic symmetry and confirm the presence of magnetic ordering(G-type antiferromagnetic) at room temperature.c© 2006 Elsevier Ltd. All rights reserved.

PACS: 71.20.Ps

Keywords: A. Magnetically ordered materials; A. Mossbauer spectroscopy; B. Chemical synthesis; C. Crystal structure and symmetry

1. Introduction

Ceramic transition metal-containing oxides display a widerange of properties (electrical, magnetic, catalytic) which areof vital importance in technological applications. An importantfeature in the development of materials for use in suchapplications is fine control of the properties. In this respect thecontrol of the transition metal oxidation state is crucial andin most cases this is achieved through doping on the cationsites. An alternative route to achieving the same aim is bymodifications on the anion site, such as replacement of O2− byF−, leading to the preparation of oxide fluorides. One problemwith the synthesis of such phases is the high thermodynamicstability of the simple fluoride starting materials, which meansthat most synthesis routes to transition metal containingoxide fluorides have involved low temperature fluorination ofprecursors, e.g. the formation of superconducting Sr2CuO2F2+xby the fluorination of Sr2CuO3 with gaseous fluorine or thefluorination of YBa2Cu3O7−x with gaseous NF3 [1]. Theseinitial groundbreaking studies subsequently paved the way for

∗ Corresponding author. Tel.: +44 1483 686847; fax: +44 1483 686851.E-mail address: p.slater@surrey.ac.uk (P.R. Slater).

0038-1098/$ - see front matter c© 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2006.11.037

the synthesis of a range of perovskite-related transition-metalcontaining oxide fluorides through low temperature synthesisroutes [2–9]. Due to the toxicity and handling problemsassociated with gaseous fluorine, much of this later work hasfocused on the use of alternative fluorinating agents, suchas NH4F and MF2 (M = Cu, Zn, Xe). More recently, wehave demonstrated a new low temperature fluorination routewhich entails heating the precursor oxide with the polymerpoly(vinylidene fluoride) [9]. Initial studies using this routeresulted in the synthesis of Sr2TiO3F2 and Ca2CuO2F2 fromthe fluorination of Sr2TiO4 and Ca2CuO3 respectively. A keyobservation was that the method produced high quality sampleswithout the metal fluoride impurities that were often formed byother low temperature fluorination methods [9]. Subsequentlywe extended this work to successfully fluorinate SrFeO3−x (x ≈

0.12) to give SrFeO2F [10]. In this case, the partial replacementof oxygen by fluorine was accompanied by a reduction in theiron oxidation state from ≈3.76+ to 3+ and the appearanceof magnetic ordering, reflecting the interactions between Fe3+

ions that are not possible in oxides containing Fe4+.In this paper, we extend this work showing the fluorination

of BaFeO3−x to give BaFeO2F. We report on both the structuralproperties of BaFeO2F, as well as its magnetic structure, as

468 R. Heap et al. / Solid State Communications 141 (2007) 467–470

elucidated by neutron diffraction at room temperature and at773 K, and by Mossbauer spectroscopy between 80 and 650 K.

2. Experimental

The oxygen deficient compound BaFeO3−x was preparedby the calcination of appropriate quantities of a well groundmixture of barium (II) carbonate and iron (III) oxide at 1100 ◦Cfor 24 h in air, with an intermediate regrind. Fluorination wasachieved by mixing 1.0 g BaFeO3−x phase with poly(vinylidenefluoride) in a 1:0.6 molar ratio (precursor oxide: CH2CF2monomer unit), constituting an excess of polymer, and heatingthe mixture at 400 ◦C for 24 h in flowing nitrogen. This heattreatment was followed by further heating at 400 ◦C for 24 hin air to ensure that no carbon-containing residues from thepoly(vinylidene fluoride) fluorination agent remained in thefinal product.

Phase purity was confirmed by X-ray powder diffraction(Seifert XRD3003 diffractometer with Cu Kα radiation).Neutron diffraction data were collected on the POLARISdiffractometer, ISIS, Rutherford Appleton Laboratory. In orderto determine the magnetic structure, data were collected both atroom temperature and 773 K (i.e. above the magnetic orderingtemperature). All structural refinements used the GSAS suite ofthe Rietveld refinement software [11].

The 57Fe Mossbauer spectra were recorded in a furnacein situ at elevated temperature in transmission geometry usinga constant acceleration spectrometer, a ca. 10 mCi 57Co/Rhsource and a furnace, which has been described in detailelsewhere [12]. All chemical isomer shift data are quotedrelative to the centroid of the metallic iron spectrum at 298 K.

3. Results and discussion

3.1. Structural determination

X-ray powder diffraction indicated that the fluorinationof BaFeO3−x led to the formation of a single phase cubicperovskite (a ≈ 4.06 A). The X-ray powder diffraction patternsalso showed that fluorination induces a general shift in peakpositions to lower angles as compared to those in BaFeO3−x,corresponding to an increase in unit cell size, and similarto the results recorded when SrFeO3−x was fluorinated togive SrFeO2F [10]. This result is consistent with the partialreplacement of O2− by F− leading to a reduction in thetransition metal oxidation state, and consequently longer bonddistances and a larger unit cell.

The structure of the fluorinated phase was subsequentlyexamined in detail by neutron diffraction. The roomtemperature neutron diffraction pattern showed the presenceof extra peaks which were not present in the X-ray powderdiffraction data or the neutron diffraction data collected athigher temperature (773 K). These were assigned as evidence ofmagnetic order and were indexed on a G-type antiferromagneticsupercell (see Fig. 1). The structure of the fluorinated phasewas refined from the neutron diffraction data recorded at bothroom temperature and 773 K, with the inclusion of a magnetic

Fig. 1. G-type antiferromagnetic ordering observed for BaFeO2F. For clarity,only the Fe atoms are shown and the arrows indicate the direction of magneticmoments.

supercell from which the magnetic reflections were refined as asecond phase in the former.

The best fit to the magnetic diffraction data was foundwith a G-type antiferromagnetic order. This suggests thatthe moment on each Fe atom is antiferromagneticallycoupled to its six nearest neighbours via intervening O(or F) atoms. This magnetic structure matches that predictedusing the Goodenough and Kanamori rules for magneticsuperexchange [13]. In this, octahedral Fe3+ (high spin d5)would be expected to interact with the six adjacent Fe atomsvia their singly occupied eg orbitals and the intervening anionp-orbitals. Such 180◦ interactions (e.g. Fe(dx2

−y2)1−O(2px)

2−

Fe(dx2−y2)1) are expected to be antiferromagnetic, and this

agrees with the magnetic structure we observe. The saturatedmoment on each Fe atom was found to be 4.13(3) µB. Thisis slightly lower than expected for Fe3+ (5 µB), even after theusual reductions for covalency and zero point spin deviationsare considered, and it is likely to indicate that some additionallimited magnetic disorder is present.

In both refinements, the O/F site occupancy was initiallyallowed to vary and, since this led to negligible deviation fromfull occupancy, the occupancy of the anion site was fixed at1.0 in the final refinement. The near-identical neutron- andX-ray-scattering factors for O and F mean that it is not possibleto refine unique fractional O and F occupancies for the O/Fsite by either diffraction technique, However, the presence offull occupancy of the anion site, along with the Mossbauer datawhich showed the presence of only Fe3+ (see later), indicatesthat the composition of the fluorinated phase is BaFeO2F.

The refined crystallographic structural parameters for thedata collected at 773 K and at room temperature are shown inTables 1 and 3 respectively, with selected bond distances beinggiven in Tables 2 and 4. The neutron diffraction profiles areshown in Figs. 2 and 3.

One noticeable feature of the refined data is the high thermaldisplacement parameters for the iron site. As a result of thesehigh values, attempts were made to allow the iron to move offsite, but this resulted in no significant improvement in the fit,and so in the final refinement the iron was fixed on the ideal

R. Heap et al. / Solid State Communications 141 (2007) 467–470 469

Table 1Refined structural data for BaFeO2F at 773 K

Atom Site x y z UI (×100)/A2 Site occ.

Ba 1a 0 0 0 1.71(3) 1Fe 1b 0.5 0.5 0.5 4.73(3) 1O/F 3c 0.5 0.5 0 2.48(2) 1

Pm–3m, a = 4.09139(2) A, Chi2 = 1.696, Rwp = 0.0114, Rp = 0.0203.

Table 2Selected bond distances for BaFeO2F at 773 K

Bond Bond distance (A)

Fe–O/F 2.04569(1) (×6)Ba–O/F 2.89305(2) (×12)

Table 3Refined structural data for BaFeO2F at room temperature

Atom Site x y z UI (×100)/A2 Site occ.

Ba 1b 0.5 0.5 0.5 0.59(2) 1Fe 1a 0 0 0 3.59(2) 1O/F 3c 0 0 0.5 1.052(8) 1

Pm–3m, a = 4.05884(3) A, Chi2 = 3.398, Rwp = 0.0339, Rp = 0.0454. Femagnetic moment =4.13(3) µB.

Table 4Selected bond distances for BaFeO2F at room temperature

Bond Bond distance (A)

Fe–O/F 2.02942(1) (×6)Ba–O/F 2.87004(2) (×12)

Fig. 2. Observed, calculated and difference neutron diffraction profiles forBaFeO2F at 773 K.

site. Furthermore, the occupancy of the iron site was allowed tovary, but no deviation from full occupancy was observed.

These high thermal displacement parameters for ironare interesting and require further discussion. One possibleexplanation is that there is a series of random displacements.Such displacements could be related to the distributionof fluorine atoms within the material, e.g. different irondisplacements for axial fluorine atoms compared to equatorialfluorine atoms, or indeed some iron atoms having three fluorineatoms or only one fluorine atom within their coordination

Fig. 3. Observed, calculated and difference neutron diffraction profiles forBaFeO2F at room temperature (lower tick marks represent the crystallographicstructure, while the upper tick marks represent the magnetic structure).

spheres. Another potential origin for the high thermaldisplacement parameters is the presence of ferroelectric-typedisplacements similar to those observed in BaTiO3, due to the Bsite being too large to comfortably accommodate iron. Possibleevidence in support of this latter interpretation comes frombond valence calculations which give a low value of 2.7 forthe iron bond valence sum in BaFeO2F at room temperature.The fact that the sample possesses cubic symmetry, however,implies that if such displacements are present, there is nounique direction for them. It does, however, raise the questionof whether, at lower temperatures, a phase transition to anon-centrosymmetric ferroelectric cell may occur, leading tothe coexistence of magnetic- and ferroelectric-order, and thiswarrants further investigation.

3.2. Mossbauer spectroscopy results

The 57Fe Mossbauer spectra recorded in situ from BaFeO2Fbetween 80 and 650 K are shown in Fig. 4. The spectra recordedat 80 and 285 K were amenable to fitting to a narrow (ca.2–3 T) distribution of magnetic hyperfine fields. The isomershift, δ = 0.39 (2) mm s−1 was characteristic of Fe3+. Theresult is consistent with the refinement of the diffraction data,which showed clear magnetic peaks, which were refined to adoubled antiferromagnetic supercell (Fig. 1).

The spectra recorded in situ at increasingly highertemperatures (Fig. 4) showed the lines to broaden and themagnitude of the magnetic hyperfine fields to decrease until,between 600 and 650 K, the sextet patterns characteristicof magnetic order collapsed to doublets indicative of aparamagnetic state. The variation of the average magnetichyperfine field with increasing temperature (Fig. 5) showed amagnetic ordering temperature of 645 (±5) K. The variation ofthe average magnetic hyperfine field with temperature is alsocompared in Fig. 5 with that calculated from the expressionB = Bo (1 − T/Tc)

β , where B is the magnetic hyperfinefield, Bo is the magnetic hyperfine field at 0 K (60 T), Tc is themagnetic ordering temperature, and β is usually in the range of0.25–0.33 and, in this calculation, was taken as 0.3.

470 R. Heap et al. / Solid State Communications 141 (2007) 467–470

Fig. 4. The 57Fe Mossbauer spectra recorded in situ from BaFeO2F between80 and 650 K.

Fig. 5. Variation of the magnetic hyperfine field with temperature.

4. Conclusions

Fluorination of BaFeO3−x with poly(vinylidene fluoride)leads to the formation of BaFeO2F, which shows G-typeantiferromagnetic ordering at room temperature. The workfurther demonstrates that this low temperature fluorinationroute is a powerful method for the formation of novel transitionmetal containing oxide fluorides.

Acknowledgements

We would like to thank EPSRC for funding (studentshipto R.H.) and ISIS, Rutherford Appleton Laboratory for theirprovision of neutron diffraction time. We would also like tothank Ron Smith (ISIS, Rutherford Appleton Laboratory) forhelp with the collection of the neutron diffraction data.

References

[1] C. Perrin, A. Dinia, O. Pena, M. Sergent, P. Burlet, J. Rossatmignod, SolidState Commun. 76 (1990) 401;M. Al-Mamouri, P.P. Edwards, C. Greaves, M. Slaski, Nature (London)369 (1994) 382.

[2] R.L. Needs, M.T. Weller, Chem. Comm. (1995) 353;R.L. Needs, M.T. Weller, Dalton Trans. (1995) 3015;R.L. Needs, M.T. Weller, U. Scheler, R.K. Harris, J. Mater. Chem. 6(1996) 1219;R.L. Needs, M.T. Weller, J. Solid State Chem. 139 (1998) 422;G.S. Case, A.L. Hector, W. Levason, R.L. Needs, M.F. Thomas,M.T. Weller, J. Mater. Chem. 9 (1999) 2821.

[3] T. Kawahima, Y. Matsui, E. Takayama-Muromachi, Physica C 257 (1996)313;M. Isobe, J.Q. Li, Y. Matsui, F. Izumi, Y. Kanke, E. Takayama-Muromachi, Physica C 269 (1996) 5.

[4] P.R. Slater, P.P. Edwards, C. Greaves, I. Gameson, J.P. Hodges,M.G. Francesconi, M. Al-Mamouri, M. Slaski, Physica C 241 (1995) 151;P.R. Slater, J.P. Hodges, M.G. Francesconi, P.P. Edwards, C. Greaves,I. Gameson, M. Slaski, Physica C 253 (1995) 26;P.R. Slater, J.P. Hodges, M.G. Francesconi, C. Greaves, M. Slaski,J. Mater. Chem. 7 (1997) 2077;G.B. Peacock, I. Gameson, M. Slaski, J.J. Capponi, P.P. Edwards, PhysicaC 289 (1997) 153;C. Greaves, J.L. Kissick, M.G. Francesconi, L.D. Aiken, L.J. Gillie,J. Mater. Chem. 9 (1999) 111;L.D. Aikens, R.K. Li, C. Greaves, Chem. Commun. (2000) 2129.

[5] E.I. Ardashnikova, S.V. Lubarsky, D.I. Denisenko, R.V. Shpanchenko,E.V. Antipov, G. Van Tendeloo, Physica C 253 (1995) 259;E.V. Antipov, S.N. Putilin, R.V. Shpanchenko, V.A. Alyoshin,M.G. Rozova, A.M. Abakumov, D.A. Mikhailova, A.M. Balagurov,O. Lebedev, G. Van Tendeloo, Physica C 282 (1997) 61;A.M. Abakumov, J. Hadermann, G. Van Tendeloo, R.V. Shpanchenko,P.N. Oleinikov, E.V. Antipov, J. Solid State Chem. 142 (1999) 440.

[6] P. Lightfoot, S. Pei, J.D. Jorgensen, X.X. Tang, A. Manthiram,J.B. Goodenough, Physica C 169 (1990) 15;J.L. Yang, J.K. Liang, G.H. Rao, Y.L. Qin, Y. Shi, W.H. Tang, Physica C270 (1996) 35.

[7] M.G. Francesconi, C. Greaves, Supercond. Sci. Techol. 10 (1997) A29;C. Greaves, M.G. Francesconi, Curr. Opin. Solid State Mater. Sci. 3(1998) 132.

[8] P.R. Slater, R.K.B. Gover, J. Mater. Chem. 11 (2001) 2035.[9] P.R. Slater, J. Fluorine Chem. 117 (2002) 43.

[10] F.J. Berry, X. Ren, R. Heap, P.R. Slater, M.F. Thomas, Solid StateCommun. 134 (2005) 621.

[11] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory, Report.No LA-UR-86-748, 1987.

[12] O. Helgason, H.P. Gunnlaungsson, K. Jonsson, S. Steinthorsson,Hyperfine Interact. 91 (1994) 59.

[13] J.B. Goodenough, Phys. Rev. 100 (1955) 564;J. Kanamori, J. Phys. Chem. Solids 10 (1959) 87.