Nanostructured fayalite (α-Fe 2SiO 4) with a large volume fraction of interfaces is synthesized for the first time via single-step mechanosynthesis, starting from a 2α-Fe 2O 3 + 2Fe + 3SiO 2 mixture. The nonequilibrium state of the as-prepared silicate is characterized by the presence of deformed polyhedra in the interface/surface regions of nanoparticles. © 2012 The Royal Society of Chemistry

Becker, Klaus-Dieter

Da Silva, Klebson Lucenildo

Fabián, Martin

Feldhoff, Armin

Ghafari, Mohammad

Hahn, Horst

Heitjans, Paul

Menzel, Dirk

Myndyk, Maxym

Šepelák, Vladimir

Nanostructured fayalite (α-Fe2SiO4) with a large volume fraction of interfaces is synthesized for the first time via single-step mechanosynthesis, starting from a 2α-Fe2O3 + 2Fe + 3SiO2 mixture. The nonequilibrium state of the as-prepared silicate is characterized by the presence of deformed polyhedra in the interface/surface regions of nanoparticles.Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

Da Silva, Klebson L.

Becker, Klaus D.

Qucosa

Mechanosynthesis of nanocrystalline fayalite, Fe2SiO4

Technische Universität Dresden: Qucosa

 This article is part of the Mechanochemistry: fundamentals and applications in synthesis web themed issue  Guest editors: Stuart James and Tomislav Friščić   All articles in this issue will be gathered together online at www.rsc.org/mechanochemistry    Published on 26 September 2012. Downloaded by SLUB DRESDEN on 26/03/2014 08:25:33. View Article Online / Journal Homepage / Table of Contents for this issueThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 11121–11123 11121Cite this: Chem. Commun., 2012, 48, 11121–11123Mechanosynthesis of nanocrystalline fayalite, Fe2SiO4wVladimir Šepelák,z*ab Maxym Myndyk,c Martin Fabián,d Klebson L. Da Silva,eArmin Feldhoff,bfDirk Menzel,gMohammad Ghafari,aHorst Hahn,aPaul HeitjansbfandKlaus D. BeckerbhReceived 1st September 2012, Accepted 25th September 2012DOI: 10.1039/c2cc36370dNanostructured fayalite (a-Fe2SiO4) with a large volume frac-tion of interfaces is synthesized for the first time via single-stepmechanosynthesis, starting from a 2a-Fe2O3 + 2Fe + 3SiO2mixture. The nonequilibrium state of the as-prepared silicate ischaracterized by the presence of deformed polyhedra in theinterface/surface regions of nanoparticles.Complex oxides of the type M2SiO4 (M is a divalent metal cation)are important rock-forming silicates, as they belong to the mostabundant phases of the Earth’s crust and upper mantle. In parti-cular, the Fe- and Mg-containing members, (Fe1xMgx)2SiO4,have been the subject of many studies.1 In addition to theirimportance in geophysics, geology and materials science, thesecompounds are important due to their application as refractorymaterials, additives in cement concrete, acid-resistant containers,ceramic pigments, etc. Fe2SiO4 is known to exist in two structuralmodifications: orthorhombic a-Fe2SiO4 phase (the so-calledfayalite) with the olivine structure and cubic g-Fe2SiO4 phasewith the spinel structure. Whereas fayalite is stable underambient conditions, the spinel phase, first synthesized in1958 by Ringwood,2 is found to be stable in a wide range ofhigh pressures and temperatures. g-Fe2SiO4 can be obtaineddirectly from its orthorhombic polymorph at about 4 GPa and1000 1C.3The present work is focused on the one-step synthesis of nano-fayalite via mechanochemical processing of the stoichiometricmixture of a-Fe2O3, Fe and SiO2 precursors at ambient tempera-ture. Although in the last few years a surge of investigations in thefield of mechanochemistry has resulted in the preparation of avariety of nanostructured complex oxides by forcing a system toacquire metastable and nonequilibrium configurations,4–6 to thebest of our knowledge there is no report in the literature on thenon-conventional mechanochemical route to synthesize Fe2SiO4.In this communication, the phase evolution during the mechano-synthesis of a-Fe2SiO4 is monitored by X-ray diffraction (XRD)and 57Fe Mössbauer spectroscopy. Quantitative information onthe short-range structure and hyperfine interactions in thefar-from-equilibrium nanofayalite, provided by the nuclearspectroscopic technique, is complemented by an investigationof its magnetic behavior by means of SQUID magnetometry.For the mechanochemical synthesis of a-Fe2SiO4, thea-Fe2O3, Fe and SiO2 reactants (Merck) were mixed in a molarratio of 2 : 2 : 3. The mixtures (10 g) were milled for varioustimes tm (up to 4 h) in a high-energy mill Fritsch Pulverisette 6(Fritsch). A grinding chamber (250 cm3 in volume) and balls(10 mm in diameter) made of tungsten carbide were used. Theball-to-powder weight ratio was 40 : 1. Milling experimentswere performed in an argon atmosphere at 550 rpm. The XRDpatterns were measured using Cu Ka radiation. 57Fe Mössbauerspectra were taken at 293 K in transmission geometry using a57Co/Rh g-ray source. The derived isomer shifts are givenrelative to 57Fe in Rh. The morphology of powders wasstudied using TEM (JEOL JEM-2100F). The temperature-dependent magnetic susceptibility was measured in a magneticfield of 0.1 T using a SQUID magnetometer.As seen in Fig. 1, the XRD pattern of the non-milledmixture is characterized by diffraction peaks corresponding tothe reactants a-Fe2O3, Fe and SiO2. For milling times tm Z 1 h,clear evidence of new diffraction features that correspond tothe a-Fe2SiO4 phase is observed. In the XRD pattern of thesample milled for 4 h (a product of the mechanochemicalreaction), all diffraction peaks detected above the backgroundare due to the fayalite phase (JCPDS PDF 34-0178).Representative TEM micrographs of mechanosynthesizeda-Fe2SiO4 at low and high magnifications are shown in Fig. 2.It is revealed that the material consists of nanoparticles with asize distribution ranging from about 5 to 25 nm; the averagea Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen,Germany. E-mail: vladimir.sepelak@kit.edu;Fax: +49-721-60826368; Tel: +49-721-60828929bCenter for Solid State Chemistry and New Materials, LeibnizUniversity Hannover, Callinstr. 3-3a, D-30167 Hannover, GermanycDepartment of Inorganic Chemistry, Dresden University ofTechnology, Mommsenstrasse 6, D-01062 Dresden, Germanyd Institute of Geotechnics, Slovak Academy of Sciences,Watsonova 45, SK-04353 Košice, SlovakiaeDepartment of Physics, State University of Maringá,Av. Colombo 5790, 87020-900 Maringá, Brazilf Institute of Physical Chemistry and Electrochemistry, LeibnizUniversity Hannover, Callinstr. 3-3a, D-30167 Hannover, Germanyg Institute of Condensed Matter Physics, Braunschweig University ofTechnology, Mendelssohnstrasse 3, D-38106 Braunschweig, Germanyh Institute of Physical and Theoretical Chemistry, BraunschweigUniversity of Technology, Hans-Sommer-Str. 10, D-38106Braunschweig, Germanyw This article is part of the ChemComm ‘Mechanochemistry’ webthemed issue.z On leave from the Slovak Academy of Sciences, Košice, Slovakia.ChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONPublished on 26 September 2012. Downloaded by SLUB DRESDEN on 26/03/2014 08:25:33. View Article Online11122 Chem. Commun., 2012, 48, 11121–11123 This journal is c The Royal Society of Chemistry 2012crystallite size is estimated to be 13(2) nm. The nanoparticles arefound to be roughly spherical, consisting of an ordered region(crystalline inner core) surrounded/separated by a structurallydisordered surface shell/interface region. The HRTEM micro-graph shows lattice fringes corresponding to the crystallographicplane (111) (d=3.556 Å) of the a-Fe2SiO4 phase. They cross thewhole particle core demonstrating its single-crystalline character.The surface shell with a thickness of about 1 nm is also evident.To determine the evolution of the product phase during millingin greater detail, the mechanochemical reaction was followed by57Fe Mössbauer spectroscopy (Fig. 3). As seen, with increasingtm, sextets corresponding to a-Fe2O3 and Fe collapse, and aregradually replaced by a central doublet characteristic of Fe2+ions. It indicates that the milling generates a complex series ofheterogeneous solid-state transformations, including redox reaction(2Fe3+ + 2Fe0 - 3Fe2+) and formation reaction (2a-Fe2O3 +2Fe + 3SiO2 - 3a-Fe2SiO4), which are completed after only4 h. In this context, it should be noted that the preparationof Fe2SiO4 by a conventional solid-state route requires anumber of stages, including homogenization of the powderprecursors, their compaction, and finally prolonged treatmentat considerably high temperatures under controlled oxygenfugacity.1To separate the surface/interface effects from the core/bulkeffects in the as-prepared nanoparticles and to determine theirlocal ionic configuration, we analyzed in detail Mössbauer spectraof both mechanosynthesized nanofayalite and its bulk counter-part (synthesized by the standard ceramic route), see Fig. 4. Thespectrum of the bulk silicate is well fitted by the superposition oftwo doublets with the hyperfine parameters listed in Table 1.These parameters are consistent with those of the olivine crystalstructure of a-Fe2SiO4, in which Fe2+ cations occupy two non-equivalent octahedral sites, [M1] and [M2], in equal proportions,Fig. 1 XRD patterns of the 2a-Fe2O3 + 2Fe + 3SiO2 mixture milledfor various times (tm). Diffraction peaks of the reaction precursors andthe product phase are denoted by Miller indices.Fig. 2 (a) Bright-field and (b) high-resolution TEM micrographs ofmechanosynthesized a-Fe2SiO4 nanoparticles demonstrating their core–shell configuration with the thickness of the surface shell of about 1 nm.Fig. 3 57Fe Mössbauer spectra of the 2a-Fe2O3 + 2Fe + 3SiO2mixture milled for various times.Fig. 457Fe Mössbauer spectra of (top) bulk a-Fe2SiO4 and (bottom)nanosized mechanosynthesized a-Fe2SiO4. [M1]c, [M2]c and [M1]s,[M2]s denote cation sites of octahedral coordination in the inner coreand the surface shell of a-Fe2SiO4 nanoparticles, respectively.Published on 26 September 2012. Downloaded by SLUB DRESDEN on 26/03/2014 08:25:33. View Article OnlineThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 11121–11123 11123whereas Si4+ cations are coordinated by four O2 ions to formSiO4 tetrahedra (T). Thus, the crystal chemical formulaemphasizing the site occupancy at the atomic level in the bulka-Fe2SiO4 may be written as [Fe][M1][Fe][M2]Si(T)O4.The core–shell morphology of mechanosynthesized nano-particles requires to fit their Mössbauer spectrum by a super-position of four subspectra (Fig. 4, bottom): two componentsaccount for Fe2+ nuclei in the well-crystallized core of Fe2SiO4nanoparticles (denoted [M1]c and [M2]c) and two are associatedwith Fe2+ ions in the surface shell/interface regions of nano-particles (denoted [M1]s and [M2]s). The Mössbauer lines corre-sponding to Fe atoms located at surfaces/interfaces are muchbroader than those belonging to the core of nanoparticles (com-pare s[M1]c, s[M2]c and s[M1]s, s[M2]s in Table 1). This provides clearevidence of a broad distribution of electric field gradients acting atthe Fe2+ nuclei in the near-surface layers/interface regions of thenanomaterial. The distribution reflects the presence of stronglydistorted octahedra around the ferrous ions in the surface/inter-face regions of the nanofayalite. Another striking feature is thatthe electric field gradients which act on Fe nuclei in the shell arefound to be smaller than those experienced by Fe nuclei in thecore of Fe2SiO4 nanoparticles. FromMössbauer measurements, itis concluded that the surface shell regions of mechanosynthesizednanoparticles are disordered due to broadly distorted albeit moresymmetric octahedra ([M1]s and [M2]s octahedra with a reduceddispersion of Fe–O bond lengths). This finding is in agreementwith recent NMR and Mössbauer investigations of mechano-synthesized Sn-containing nanooxides.6From the intensity ratio (I[M1]s + I[M2]s)/(I[M1]c + I[M2]c +I[M1]s + I[M2]s) the volume fraction (w) of the disorderedsurface/interface regions in the nanoolivine is estimatedto be about 40%. Thus, the distribution of Fe ions inthe mechanosynthesized Fe2SiO4 can be represented as[Fe0.6][M1]c[Fe0.6][M2]c[Fe0.4][M1]s[Fe0.4][M2]sSi(T)O4. Consideringthat nanoparticles are spherical with an average diameterD = 13 nm, the thickness of the disordered surface shell canbe estimated to be t= 1.02(1) nm (w= 100 [1  (1  2t/D)3]).This value is in good agreement with the HRTEM resultsshown in Fig. 2 and is also comparable to that observed inother mechanochemically prepared nanooxides.4–6One could expect that the presence of distorted octahedra inthe nanofayalite would result in altered magnetic properties,as it has been observed for mechanosynthesized ferrites.5As seen in Fig. 5, the Néel temperature of the nanofayalite(TN = 65.0(1) K) is not much different from that of theconventionally prepared bulk a-Fe2SiO4 (64.5 K).7 However, theincrease of the ZFC curve above TN reflects a high aggregationlevel of the interacting fayalite nanoparticles. It indicates thatthere is no well-defined Néel temperature for the nanofayalite;rather, there is a distribution of Néel temperatures. We attributethis behavior to the presence of the canted surface spins due tobroadly distorted octahedra as evidenced byMössbauer spectro-scopy. Note that similar findings have recently been reported forthe mechanochemically prepared ilmenite (FeTiO3).8The present communication clearly demonstrates that nano-structured a-Fe2SiO4 can be synthesized at ambient temperaturedirectly from its precursors, without the need for calcination athigh temperatures under a controlled atmosphere, thus makingthe process very simple. The local structure of the as-preparednanoparticles is characterized by deformed M1 and M2 octa-hedra in the surface/interface regions, whose volume fraction isabout 40%. The Néel temperature of the nanofayalite is notsignificantly affected by the mechanochemical preparation route.This work is supported by the DFGwithin the framework of thePriority Program ‘‘Crystalline Nonequilibrium Phases’’ (SPP 1415).Partial support from the APVV (project 0528-11) and the VEGA(2/0174/11) is gratefully acknowledged.M.F. thanks theDAAD forsupporting his work at Braunschweig University of Technology.Notes and references1 M. T. DeAngelis, A. J. Rondinone, M. D. Pawel, T. C. Labotka andL. M. Anovitz, Am. Mineral., 2012, 97, 653; T.-L. Tsai, K.-D. Beckerand R. Dieckmann, Solid State Ionics, 2011, 194, 17; Y. Noël,M. De La Pierre, L. Maschio, M. Rérat, C. M. Zicovich-Wilsonand R. Dovesi, Int. J. Quantum Chem., 2012, 112, 2098.2 A. E. Ringwood, Geol. Soc. Am. Bull., 1958, 69, 129; A. E. Ringwood,Geochim. Cosmochim. Acta, 1958, 13, 303.3 M. H. G. Jacobs, B. H. W. S. De Jong and H. A. J. Oonk, Geochim.Cosmochim. Acta, 2001, 65, 4231.4 V. Šepelák, S. M. Becker, I. Bergmann, S. Indris, M. Scheuermann,A. Feldhoff, C. Kübel, M. Bruns, N. Stürzl, A. S. Ulrich,M. Ghafari, H. Hahn, C. P. Grey, K. D. Becker and P. Heitjans,J. Mater. Chem., 2012, 22, 3117.5 V. Šepelák, A. Feldhoff, P. Heitjans, F. Krumeich, D. Menzel,F. J. Litterst, I. Bergmann and K. D. Becker, Chem. Mater., 2006,18, 3057; K. L. Da Silva, D. Menzel, A. Feldhoff, C. Kübel,M. Bruns, A. Paesano, Jr., A. Düvel, M. Wilkening, M. Ghafari,H. Hahn, F. J. Litterst, P. Heitjans, K. D. Becker and V. Šepelák,J. Phys. Chem. C, 2011, 115, 7209.6 V. Šepelák, K. D. Becker, I. Bergmann, S. Suzuki, S. Indris, A. Feldhoff,P. Heitjans and C. P. Grey, Chem. Mater., 2009, 21, 2518.7 E. Dachs, C. A. Geiger, V. von Seckendorff and M. Grodzicki,J. Chem. Thermodyn., 2007, 39, 906.8 S. Mørup, H. K. Rasmussen, E. Brok, L. Keller and C. Frandsen,Mater. Chem. Phys., 2012, 136, 184.Table 1 Parameters obtained by fitting the Mössbauer spectra ofbulk Fe2SiO4 and nanosized mechanosynthesized Fe2SiO4MaterialOctahedralsite of Fe2+IS(mm s1)QS(mm s1)s(mm s1) I (%)Bulk Fe2SiO4 [M1] 0.99(5) 2.91(4) 0.042(1) 50.1(3)[M2] 1.02(8) 3.02(1) 0.041(1) 49.9(4)Nano Fe2SiO4 [M1]c 1.00(1) 2.69(6) 0.045(2) 30.0(4)[M2]c 1.04(5) 2.89(1) 0.046(2) 30.0(5)[M1]s 0.86(3) 1.86(5) 0.77(2) 20.0(7)[M2]s 1.03(3) 2.41(8) 0.40(1) 20.0(6)IS: isomer shift; QS: quadrupole splitting; s: Gaussian widths of theVoigt lines; I: relative intensity. A Lorentzian line width of 0.26 mm s1resulting from the fit of the spectrum of a bulk Fe2SiO4 was chosen forthe fit of the spectrum of mechanosynthesized Fe2SiO4.Fig. 5 Temperature-dependent magnetic susceptibility (ZFC and FC)for mechanosynthesized a-Fe2SiO4.Published on 26 September 2012. Downloaded by SLUB DRESDEN on 26/03/2014 08:25:33. View Article Online

English

HSSS - Hochschulschriftenserver der SLUB

Vladimir Šepelák

Maxym Myndyk

Martin Fabián

Klebson L. Da Silva

Armin Feldhoff

Dirk Menzel

Mohammad Ghafari

Horst Hahn

Paul Heitjans

Klaus D. Becker

Crossref

Chemical Communications

Institutionelles Repositorium der Leibniz Universität Hannover

 This article is part of the Mechanochemistry: fundamentals and applications in synthesis web themed issue  Guest editors: Stuart James and Tomislav Friščić   All articles in this issue will be gathered together online at www.rsc.org/mechanochemistry    Published on 26 September 2012. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 08:02:28. View Article Online / Journal Homepage / Table of Contents for this issueThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 11121–11123 11121Cite this: Chem. Commun., 2012, 48, 11121–11123Mechanosynthesis of nanocrystalline fayalite, Fe2SiO4wVladimir Sˇepela´k,z*ab Maxym Myndyk,c Martin Fabia´n,d Klebson L. Da Silva,eArmin Feldhoﬀ,bf Dirk Menzel,g Mohammad Ghafari,a Horst Hahn,a Paul Heitjansbf andKlaus D. BeckerbhReceived 1st September 2012, Accepted 25th September 2012DOI: 10.1039/c2cc36370dNanostructured fayalite (a-Fe2SiO4) with a large volume frac-tion of interfaces is synthesized for the ﬁrst time via single-stepmechanosynthesis, starting from a 2a-Fe2O3 + 2Fe + 3SiO2mixture. The nonequilibrium state of the as-prepared silicate ischaracterized by the presence of deformed polyhedra in theinterface/surface regions of nanoparticles.Complex oxides of the type M2SiO4 (M is a divalent metal cation)are important rock-forming silicates, as they belong to the mostabundant phases of the Earth’s crust and upper mantle. In parti-cular, the Fe- and Mg-containing members, (Fe1xMgx)2SiO4,have been the subject of many studies.1 In addition to theirimportance in geophysics, geology and materials science, thesecompounds are important due to their application as refractorymaterials, additives in cement concrete, acid-resistant containers,ceramic pigments, etc. Fe2SiO4 is known to exist in two structuralmodiﬁcations: orthorhombic a-Fe2SiO4 phase (the so-calledfayalite) with the olivine structure and cubic g-Fe2SiO4 phasewith the spinel structure. Whereas fayalite is stable underambient conditions, the spinel phase, ﬁrst synthesized in1958 by Ringwood,2 is found to be stable in a wide range ofhigh pressures and temperatures. g-Fe2SiO4 can be obtaineddirectly from its orthorhombic polymorph at about 4 GPa and1000 1C.3The present work is focused on the one-step synthesis of nano-fayalite via mechanochemical processing of the stoichiometricmixture of a-Fe2O3, Fe and SiO2 precursors at ambient tempera-ture. Although in the last few years a surge of investigations in theﬁeld of mechanochemistry has resulted in the preparation of avariety of nanostructured complex oxides by forcing a system toacquire metastable and nonequilibrium conﬁgurations,4–6 to thebest of our knowledge there is no report in the literature on thenon-conventional mechanochemical route to synthesize Fe2SiO4.In this communication, the phase evolution during the mechano-synthesis of a-Fe2SiO4 is monitored by X-ray diﬀraction (XRD)and 57Fe Mo¨ssbauer spectroscopy. Quantitative information onthe short-range structure and hyperﬁne interactions in thefar-from-equilibrium nanofayalite, provided by the nuclearspectroscopic technique, is complemented by an investigationof its magnetic behavior by means of SQUID magnetometry.For the mechanochemical synthesis of a-Fe2SiO4, thea-Fe2O3, Fe and SiO2 reactants (Merck) were mixed in a molarratio of 2 : 2 : 3. The mixtures (10 g) were milled for varioustimes tm (up to 4 h) in a high-energy mill Fritsch Pulverisette 6(Fritsch). A grinding chamber (250 cm3 in volume) and balls(10 mm in diameter) made of tungsten carbide were used. Theball-to-powder weight ratio was 40 : 1. Milling experimentswere performed in an argon atmosphere at 550 rpm. The XRDpatterns were measured using Cu Ka radiation. 57Fe Mo¨ssbauerspectra were taken at 293 K in transmission geometry using a57Co/Rh g-ray source. The derived isomer shifts are givenrelative to 57Fe in Rh. The morphology of powders wasstudied using TEM (JEOL JEM-2100F). The temperature-dependent magnetic susceptibility was measured in a magneticﬁeld of 0.1 T using a SQUID magnetometer.As seen in Fig. 1, the XRD pattern of the non-milledmixture is characterized by diﬀraction peaks corresponding tothe reactants a-Fe2O3, Fe and SiO2. For milling times tm Z 1 h,clear evidence of new diﬀraction features that correspond tothe a-Fe2SiO4 phase is observed. In the XRD pattern of thesample milled for 4 h (a product of the mechanochemicalreaction), all diﬀraction peaks detected above the backgroundare due to the fayalite phase (JCPDS PDF 34-0178).Representative TEM micrographs of mechanosynthesizeda-Fe2SiO4 at low and high magniﬁcations are shown in Fig. 2.It is revealed that the material consists of nanoparticles with asize distribution ranging from about 5 to 25 nm; the averagea Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen,Germany. E-mail: vladimir.sepelak@kit.edu;Fax: +49-721-60826368; Tel: +49-721-60828929bCenter for Solid State Chemistry and New Materials, LeibnizUniversity Hannover, Callinstr. 3-3a, D-30167 Hannover, GermanycDepartment of Inorganic Chemistry, Dresden University ofTechnology, Mommsenstrasse 6, D-01062 Dresden, Germanyd Institute of Geotechnics, Slovak Academy of Sciences,Watsonova 45, SK-04353 Kosˇice, SlovakiaeDepartment of Physics, State University of Maringa´,Av. Colombo 5790, 87020-900 Maringa´, Brazilf Institute of Physical Chemistry and Electrochemistry, LeibnizUniversity Hannover, Callinstr. 3-3a, D-30167 Hannover, Germanyg Institute of Condensed Matter Physics, Braunschweig University ofTechnology, Mendelssohnstrasse 3, D-38106 Braunschweig, Germanyh Institute of Physical and Theoretical Chemistry, BraunschweigUniversity of Technology, Hans-Sommer-Str. 10, D-38106Braunschweig, Germanyw This article is part of the ChemComm ‘Mechanochemistry’ webthemed issue.z On leave from the Slovak Academy of Sciences, Kosˇice, Slovakia.ChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONPublished on 26 September 2012. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 08:02:28. View Article Online11122 Chem. Commun., 2012, 48, 11121–11123 This journal is c The Royal Society of Chemistry 2012crystallite size is estimated to be 13(2) nm. The nanoparticles arefound to be roughly spherical, consisting of an ordered region(crystalline inner core) surrounded/separated by a structurallydisordered surface shell/interface region. The HRTEM micro-graph shows lattice fringes corresponding to the crystallographicplane (111) (d=3.556 A˚) of the a-Fe2SiO4 phase. They cross thewhole particle core demonstrating its single-crystalline character.The surface shell with a thickness of about 1 nm is also evident.To determine the evolution of the product phase during millingin greater detail, the mechanochemical reaction was followed by57Fe Mo¨ssbauer spectroscopy (Fig. 3). As seen, with increasingtm, sextets corresponding to a-Fe2O3 and Fe collapse, and aregradually replaced by a central doublet characteristic of Fe2+ions. It indicates that the milling generates a complex series ofheterogeneous solid-state transformations, including redox reaction(2Fe3+ + 2Fe0- 3Fe2+) and formation reaction (2a-Fe2O3 +2Fe + 3SiO2 - 3a-Fe2SiO4), which are completed after only4 h. In this context, it should be noted that the preparationof Fe2SiO4 by a conventional solid-state route requires anumber of stages, including homogenization of the powderprecursors, their compaction, and ﬁnally prolonged treatmentat considerably high temperatures under controlled oxygenfugacity.1To separate the surface/interface eﬀects from the core/bulkeﬀects in the as-prepared nanoparticles and to determine theirlocal ionic conﬁguration, we analyzed in detail Mo¨ssbauer spectraof both mechanosynthesized nanofayalite and its bulk counter-part (synthesized by the standard ceramic route), see Fig. 4. Thespectrum of the bulk silicate is well ﬁtted by the superposition oftwo doublets with the hyperﬁne parameters listed in Table 1.These parameters are consistent with those of the olivine crystalstructure of a-Fe2SiO4, in which Fe2+ cations occupy two non-equivalent octahedral sites, [M1] and [M2], in equal proportions,Fig. 1 XRD patterns of the 2a-Fe2O3 + 2Fe + 3SiO2 mixture milledfor various times (tm). Diﬀraction peaks of the reaction precursors andthe product phase are denoted by Miller indices.Fig. 2 (a) Bright-ﬁeld and (b) high-resolution TEM micrographs ofmechanosynthesized a-Fe2SiO4 nanoparticles demonstrating their core–shell conﬁguration with the thickness of the surface shell of about 1 nm.Fig. 3 57Fe Mo¨ssbauer spectra of the 2a-Fe2O3 + 2Fe + 3SiO2mixture milled for various times.Fig. 4 57Fe Mo¨ssbauer spectra of (top) bulk a-Fe2SiO4 and (bottom)nanosized mechanosynthesized a-Fe2SiO4. [M1]c, [M2]c and [M1]s,[M2]s denote cation sites of octahedral coordination in the inner coreand the surface shell of a-Fe2SiO4 nanoparticles, respectively.Published on 26 September 2012. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 08:02:28. View Article OnlineThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 11121–11123 11123whereas Si4+ cations are coordinated by four O2 ions to formSiO4 tetrahedra (T). Thus, the crystal chemical formulaemphasizing the site occupancy at the atomic level in the bulka-Fe2SiO4 may be written as [Fe][M1][Fe][M2]Si(T)O4.The core–shell morphology of mechanosynthesized nano-particles requires to ﬁt their Mo¨ssbauer spectrum by a super-position of four subspectra (Fig. 4, bottom): two componentsaccount for Fe2+ nuclei in the well-crystallized core of Fe2SiO4nanoparticles (denoted [M1]c and [M2]c) and two are associatedwith Fe2+ ions in the surface shell/interface regions of nano-particles (denoted [M1]s and [M2]s). The Mo¨ssbauer lines corre-sponding to Fe atoms located at surfaces/interfaces are muchbroader than those belonging to the core of nanoparticles (com-pare s[M1]c, s[M2]c and s[M1]s, s[M2]s in Table 1). This provides clearevidence of a broad distribution of electric ﬁeld gradients acting atthe Fe2+ nuclei in the near-surface layers/interface regions of thenanomaterial. The distribution reﬂects the presence of stronglydistorted octahedra around the ferrous ions in the surface/inter-face regions of the nanofayalite. Another striking feature is thatthe electric ﬁeld gradients which act on Fe nuclei in the shell arefound to be smaller than those experienced by Fe nuclei in thecore of Fe2SiO4 nanoparticles. FromMo¨ssbauer measurements, itis concluded that the surface shell regions of mechanosynthesizednanoparticles are disordered due to broadly distorted albeit moresymmetric octahedra ([M1]s and [M2]s octahedra with a reduceddispersion of Fe–O bond lengths). This ﬁnding is in agreementwith recent NMR and Mo¨ssbauer investigations of mechano-synthesized Sn-containing nanooxides.6From the intensity ratio (I[M1]s + I[M2]s)/(I[M1]c + I[M2]c +I[M1]s + I[M2]s) the volume fraction (w) of the disorderedsurface/interface regions in the nanoolivine is estimatedto be about 40%. Thus, the distribution of Fe ions inthe mechanosynthesized Fe2SiO4 can be represented as[Fe0.6][M1]c[Fe0.6][M2]c[Fe0.4][M1]s[Fe0.4][M2]sSi(T)O4. Consideringthat nanoparticles are spherical with an average diameterD = 13 nm, the thickness of the disordered surface shell canbe estimated to be t= 1.02(1) nm (w= 100 [1  (1  2t/D)3]).This value is in good agreement with the HRTEM resultsshown in Fig. 2 and is also comparable to that observed inother mechanochemically prepared nanooxides.4–6One could expect that the presence of distorted octahedra inthe nanofayalite would result in altered magnetic properties,as it has been observed for mechanosynthesized ferrites.5As seen in Fig. 5, the Ne´el temperature of the nanofayalite(TN = 65.0(1) K) is not much diﬀerent from that of theconventionally prepared bulk a-Fe2SiO4 (64.5 K).7 However, theincrease of the ZFC curve above TN reﬂects a high aggregationlevel of the interacting fayalite nanoparticles. It indicates thatthere is no well-deﬁned Ne´el temperature for the nanofayalite;rather, there is a distribution of Ne´el temperatures. We attributethis behavior to the presence of the canted surface spins due tobroadly distorted octahedra as evidenced byMo¨ssbauer spectro-scopy. Note that similar ﬁndings have recently been reported forthe mechanochemically prepared ilmenite (FeTiO3).8The present communication clearly demonstrates that nano-structured a-Fe2SiO4 can be synthesized at ambient temperaturedirectly from its precursors, without the need for calcination athigh temperatures under a controlled atmosphere, thus makingthe process very simple. The local structure of the as-preparednanoparticles is characterized by deformed M1 and M2 octa-hedra in the surface/interface regions, whose volume fraction isabout 40%. The Ne´el temperature of the nanofayalite is notsigniﬁcantly aﬀected by the mechanochemical preparation route.This work is supported by the DFGwithin the framework of thePriority Program ‘‘Crystalline Nonequilibrium Phases’’ (SPP 1415).Partial support from the APVV (project 0528-11) and the VEGA(2/0174/11) is gratefully acknowledged.M.F. thanks theDAAD forsupporting his work at Braunschweig University of Technology.Notes and references1 M. T. DeAngelis, A. J. Rondinone, M. D. Pawel, T. C. Labotka andL. M. Anovitz, Am. Mineral., 2012, 97, 653; T.-L. Tsai, K.-D. Beckerand R. Dieckmann, Solid State Ionics, 2011, 194, 17; Y. Noe¨l,M. De La Pierre, L. Maschio, M. Re´rat, C. M. Zicovich-Wilsonand R. Dovesi, Int. J. Quantum Chem., 2012, 112, 2098.2 A. E. Ringwood, Geol. Soc. Am. Bull., 1958, 69, 129; A. E. Ringwood,Geochim. Cosmochim. Acta, 1958, 13, 303.3 M. H. G. Jacobs, B. H. W. S. De Jong and H. A. J. Oonk, Geochim.Cosmochim. Acta, 2001, 65, 4231.4 V. Sˇepela´k, S. M. Becker, I. Bergmann, S. Indris, M. Scheuermann,A. Feldhoﬀ, C. Ku¨bel, M. Bruns, N. Stu¨rzl, A. S. Ulrich,M. Ghafari, H. Hahn, C. P. Grey, K. D. Becker and P. Heitjans,J. Mater. Chem., 2012, 22, 3117.5 V. Sˇepela´k, A. Feldhoﬀ, P. Heitjans, F. Krumeich, D. Menzel,F. J. Litterst, I. Bergmann and K. D. Becker, Chem. Mater., 2006,18, 3057; K. L. Da Silva, D. Menzel, A. Feldhoﬀ, C. Ku¨bel,M. Bruns, A. Paesano, Jr., A. Du¨vel, M. Wilkening, M. Ghafari,H. Hahn, F. J. Litterst, P. Heitjans, K. D. Becker and V. Sˇepela´k,J. Phys. Chem. C, 2011, 115, 7209.6 V. Sˇepela´k, K. D. Becker, I. Bergmann, S. Suzuki, S. Indris, A. Feldhoﬀ,P. Heitjans and C. P. Grey, Chem. Mater., 2009, 21, 2518.7 E. Dachs, C. A. Geiger, V. von Seckendorﬀ and M. Grodzicki,J. Chem. Thermodyn., 2007, 39, 906.8 S. Mørup, H. K. Rasmussen, E. Brok, L. Keller and C. Frandsen,Mater. Chem. Phys., 2012, 136, 184.Table 1 Parameters obtained by ﬁtting the Mo¨ssbauer spectra ofbulk Fe2SiO4 and nanosized mechanosynthesized Fe2SiO4MaterialOctahedralsite of Fe2+IS(mm s1)QS(mm s1)s(mm s1) I (%)Bulk Fe2SiO4 [M1] 0.99(5) 2.91(4) 0.042(1) 50.1(3)[M2] 1.02(8) 3.02(1) 0.041(1) 49.9(4)Nano Fe2SiO4 [M1]c 1.00(1) 2.69(6) 0.045(2) 30.0(4)[M2]c 1.04(5) 2.89(1) 0.046(2) 30.0(5)[M1]s 0.86(3) 1.86(5) 0.77(2) 20.0(7)[M2]s 1.03(3) 2.41(8) 0.40(1) 20.0(6)IS: isomer shift; QS: quadrupole splitting; s: Gaussian widths of theVoigt lines; I: relative intensity. A Lorentzian line width of 0.26 mm s1resulting from the ﬁt of the spectrum of a bulk Fe2SiO4 was chosen forthe ﬁt of the spectrum of mechanosynthesized Fe2SiO4.Fig. 5 Temperature-dependent magnetic susceptibility (ZFC and FC)for mechanosynthesized a-Fe2SiO4.Published on 26 September 2012. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 08:02:28. View Article Online

Mechanosynthesis of nanocrystalline fayalite, Fe 2SiO 4

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Mechanosynthesis of nanocrystalline fayalite, Fe 2SiO 4

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