spectroscopy study of composition, structure and magnetic properties of iron oxide phases in iron ores

w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original Article

XRD, internal field-NMR and Mössbauer

spectroscopy study of composition, structure and magnetic properties of iron oxide phases in iron ores

Mushtagatte Manjunatha

^a

, Rajeev Kumar

^b

, Aroli V. Anupama

^b

, Vijay B. Khopkar

^b

, Ramakrishna Damle

^a

, Karalapura P. Ramesh

^c,∗

, Balaram Sahoo

^b,∗

aDepartmentofPhysics,BangaloreUniversity,Bangalore560056,India

bMaterialsResearchCentre,IndianInstituteofScience,Bangalore560012,India

cDepartmentofPhysics,IndianInstituteofScience,Bangalore560012,India

a r t i c l e i n f o

Articlehistory:

Received29August2018 Accepted25January2019 Availableonline11April2019

Keywords:

Ironores

Phasecomposition Magneticproperties InternalfieldNMR Mössbauerspectroscopy

a bs t r a c t

Wereportthephase-composition,structureandmagneticpropertiesoftworepresentative samplesofnaturallyavailableiron-oxidecontainingores/soilscollectedfromtwodifferent regionsinKarnataka,India.PresenceofelementssuchasFe,Si,AlandOwereidentifiedin boththesamplesusingenergydispersiveanalysisofX-rays(EDAX).X-raydiffractionresults confirmedthatdifferentceramicphasessuchascrystallineironoxidephases(Fe3O4,␥-Fe2O3

and␣-Fe2O3),aluminosilicates(Al2SiO5)andlow-quartz(SiO2)phasesconstitutethesoil.

Thepresenceofferrimagneticphases(Fe3O4,␥-Fe2O3)makesthesoilrespondtoaperma- nentmagnet.Separationbetweenthemagneticandnon-magneticphaseswasperformed usingapermanentmagnet.Thenon-magneticpartofthesamplecontainsahighamount of␣-Fe₂O₃ phasealongwithaluminosilicatesandlow-quartz.Themagneticphaseswere furthercharacterizedandquantified.ThepresenceofFe3O4phaseinthesampleswascon- firmedfromtheVerweytransitionobservedbyinternal-fieldNMRspectroscopy.Ourresults demonstratethatinternal-fieldNMRandMössbauerspectroscopyarecomplementarytools forcharacterizingironcontainingsoils.Furthermore,wefoundthatthesoilcollectedfrom thelowtemperatureregion(Sandur)containsmoreamountsofferrimagneticoxide(Fe₃O₄,

␥-Fe2O3)phases,whereasthehightemperatureregion(Hospete)containsmore␣-Fe2O3

phase.Hence,ourresultsconfirmthatthephasecompositionofthesoilisintimatelyrelated tothelocaldailytemperature.

©2019TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

∗ Correspondingauthors.

E-mails:kpramesh@iisc.ac.in(K.P.Ramesh),bsahoo@iisc.ac.in(B.Sahoo).

https://doi.org/10.1016/j.jmrt.2019.01.022

2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1. Introduction

Hematiteandmagnetitearetheprimaryironoresusedforthe extractionofmetallicironbythesteelindustries.Ironoresthat areusedbythesteelindustryrequirestocontain≥58wt%iron compounds,suchasironoxidesbeforebeingconsidered as commerciallyusable[1].Apartfromextractionofmetalliciron, theironoxides(Fe3O4,␣-Fe2O3and␥-Fe2O3)havenumerous directapplications,suchasinelectromagneticinterference shielding[2–4],magnetorheology[5,6],magneticrecording[7], highfrequencyopticalmodulators[8]andbiomedicalappli- cations[9,10]etc.

Theanalysis ofthe characterizationresults ofdifferent minerals and their development provides enormous infor- mationaboutunderstandingofthechangesoftheminerals throughdifferentnaturalprocesses[11,12].Duetotheinven- tionofvariousadvancedscientifictechniques,thein-depth characterization ofvarious unknown minerals interms of their elemental and phase composition became feasible.

Thereare manyreportsonthe studyofironoresindiffer- entpartsofworld.Forexample:Longworthetal.conducted Mössbauerspectroscopicstudyonthemagneticstateofiron oxidesinthesoilscollectedfromtheregionsaroundEngland [13].Theyfoundthatthepercentageofnon-magneticcom- poundinthesoilsvariesbetween50%and63%.Resendeetal.

reportedontheironoxideoresfoundinthetropicalregionsof Braziliansoilsthatcontains∼63%ofhematiteore[14]andFar- bisetal.reportedthattheBraziliansoilsalsocontainhigher amount (∼17%) of magnetite [15]. The iron soils collected fromtheNewZealandcontainsalmost20–40%ofmagnetite [16].

Therearemanyparameterssuchastemperature,quality ofair,pressurebelowthegroundetc.thatdecidethereten- tion ofthe amount ofdifferent iron oxidephases insoils.

Furthermore,thereare many environmentalparametersor combinationofparameterssuchastemperature,mechanical pressure(belowearthcrust),moisture,oxygengas(air)along withmanyotherpollutantsthatinfluencethetransformation ofelementaliron/iron-oxideintootherironoxidephases.The aimofthisstudyistodemonstratetheinfluenceoflocaldaily temperatureandmoisture(duetorainintheforestareas)on theironoxidephasetransformationinthesoilscollectedfrom twonearbyplacesinKarnataka,India,wherethelocaldaily temperaturesdifferbyabout2^◦C.

2. Materials and methods

Two different representative soilswere collected from two differentregions(HospeteandSandur)inKarnataka,India.

These cities are situated atabout 350km toward north of Bangalore,India.Theyarenotmorethan∼30kmapartfrom eachother.Allovertheyear,Hospeteexperiencesabout2^◦C higher temperature than Sandur. This temperature differ- encecouldbeduetothefactthatHospetehasmuchmore dry-landthanSandur,whichincludeforestregions.Thevir- ginsoilsamplescollectedfromHospeteandSandurregions werelabeledasHSandSS,respectively.Toassessthecom- position and properties of the samples a host ofcomplex

characterizationswere performed.Initially, the as-collected samplesweregroundtofinepowders.Theelementalcomposi- tionandmorphologicalcharacterizationsofthesampleswere performedusingthetechniqueslikeEnergyDispersiveAnal- ysisofX-rays(EDAX)(Zeiss,Germany),X-raydiffraction(XRD) (PANAlytical,UK),ScanningElectronMicroscope(SEM)(Zeiss, Germany)andVibratingSampleMagnetometry(VSM)(Quan- tum Design, USA). Thesamples were foundto bestrongly magnetic,astheyshowstrongattractiontoamagnetduring preliminary assessment. Afterinitial characterizations, the magneticparticles wereseparatedfromtheoriginalsoilvia apermanentmagnetofmaximumfieldstrengthof∼1T.The fractionofthe magneticparticles ofthesamplewas about 64% and 68% forHS and SSsamples, respectively(i.e., the non-magnetic part was about 36% and 32% for HS and SS samples,respectively).Thenon-magneticsamplesobtained afterthemagneticseparationwere labeledasHS1andSS1 forHospeteandSandursoils,respectively,whereasthemag- neticsamplesobtainedaftertheseparationwerelabeledas HS2andSS2,respectively.Thepresenceofdifferentcrystalline phasesinallthesampleswasidentifiedusingXRD.Thestruc- turalandmagneticpropertiesofHS2andSS2sampleswere studiedusingXRD,⁵⁷Feinternalfieldnuclearmagneticres- onance(IFNMR) spectroscopy,⁵⁷FeMössbauerspectroscopy, X-ray photoelectron spectroscopy (XPS) (AXIS ULTRA, UK) andVSM.

XRDpatternsofthesampleswerecollectedusingPANalyt- icalX’PertProdiffractometer(Cu-K_␣radiation).Thecrystalline phasespresentinthesoilsampleswereidentifiedbyreferring totheBraggpositionsandtheirrelativeintensities.Rietveld refinements[17]ofthestructuralparameterswerecarriedout usingthecomputerprogramFullProf[18].Thecrystallographic informationfortherefinementwasobtainedfromstandard crystallographic informationfiles (CIF) from ICSDdatabase.

57Fe-Mössbauerspectroscopywascarriedoutintransmission modeatRT(usingSEECo.USAinstrument).Thespectrawere recordedusingagas-filledproportionalcounter.Thevelocity ofthedrivewascalibratedusing␣-FefoilatRT.Theobtained Mössbauerspectrawereleastsquarefitusing“NORMOS”pro- gram[19,20].TheM-Hloopsofthesampleswererecordedat RTusinga“PPMS9T”magnetometer.

Ahome-madepulsedIFNMRspinechospectrometerwith two“equalpulsesequences”[21]wasusedforthemeasure- mentofthesamplesatRTandat77K.Adetaileddescription andworkingprincipleoftheIFNMRinstrument isprovided elsewhere [22]. In a typical room temperature experimen- tal setup, a LC parallel probe was excited with a pulse sequenceof‘/2––/2’(twoequalpulsessequence).Forthe twoequalpulses,anoptimizedpulsewidthof1␮swasused asthe/2pulse,forwhichtheechoamplitudewasthehigh- est.The /2 pulsewas followedbya delay time of20␮s beforearrivalofthesecond/2pulse(1␮s).Forlowtempera- ture(77K)experiment,weusedaspeciallydesignedcoaxial probe. Thepulse-widthsofthetwo equalpulseswere also keptconstantevenat77K.However,thedelaybetweenthe two pulses was increased to 30␮s, in order to accommo- date longer spin–spin relaxationtime atlow temperature.

The echoamplitude (NMR signal) wasrecorded asa func- tionoffrequencyvaryingfrom65MHzto80MHzinstepsof 0.1MHz.

Fig.1–(i)Thephotographsoftheas-collectedsoilsamplescollectedfromSandur(SS)andHospete(HS).(ii)SEMimagesof theHSandSSsamples.(iii)EDAXspectraoftheHSandSSsamples.

Table1–ElementalcompositionobtainedfromEDAX experiment.

SoilSample Elements Atomic%

HS O 57.97

Al 3.60

Si 3.50

Fe 34.93

SS O 79.01

Al 2.18

Si 3.65

Fe 15.16

3. Results and discussion

3.1. Samplesbeforeseparationofthemagneticphases

Thephotographsoftheas-collectedHSandSSsamplesare shown inFig. 1(a).TheHospete soil(HS) isdark brown in colorwhereastheSandursoil(SS)isbrownish.Thissuggests thatboththesoilscontainhighamountofironoxidephases.

Fig.1(b)showstheSEMmicrographsofHSandSSsamples, respectively.Theirregularstructuralmorphologyoftheparti- clesinthesoilsisevidentfromtheSEMmicrographsofboth thesamples.Fig.1(c)showstheEDAXspectraforHSandSS samples,respectively.BoththesamplescontainFe,Si,AlandO asmajorelements.Theelementalcompositionofboththesoil samples(HSandSS)obtainedfromEDAXislistedinTable1.

Astheamountoftheseelementslargelydependsonsam- plingareaofthesample,wecannotconcludeontheaccuracy ofelementalcomposition.However,itseemsthatHScontains higherpercentage(34.93at%)ofFethanthatofSS(15.16at%).

However,boththesamplescontainoxygenasthemostabun- dantelement(57.96at%forHSand79.01at%forSS).Presence ofthese twomajor elements indicates thatboth the sam- ples contain high amount of iron oxide phases (␣-Fe2O3, Fe3O4and␥-Fe2O3)assupportedbytheXRDresultsdiscussed below.Assigningappropriate(stoichiometric)amountofoxy- gentothealuminosilicate(4–5mol%,Al2SiO5)andlow-quartz

(2–3mol%, SiO2)phases,a simplecalculationsuggests that thereareoxygendeficientregions(oxygenvacancies)present inthesamples.ItcanberationalizedfromTable1thatHSsam- ple contains moreoxygen deficientiron containingphases than the SS sample.TheXRD patterns ofHS and SSsam- plesare shown inFig. 2(a).TheXRD patternsindicate that samplescontainironoxides(␣-Fe2O3,Fe3O4and␥-Fe2O3)as majorphasesalongwithsmallamountsofaluminosilicates and quartz.TheM–Hloops forthe HSand SS samplesare showninFig.2(b).ItisclearthatHSsamplehassaturation magnetization(MS)of∼8emu/gwhereasSSsampleshowsan MSvalueof∼19emu/g.ThisconfirmsthatSSsampleisslightly moremagneticascomparedtoHSsample.

3.2. Samplesafterremovalofthenon-magnetic phases

Aftergrindingtheascollectedsamplestofinepowders,the magneticpartofboththesampleswereseparatedfromthe non-magnetic/weaklymagneticparts.However,astheparti- clessizeswerestillbigger,presenceofsmallmagneticgrains inthenon-magneticmatrixandviceversa,cannotberuled out.Thenon-magneticpartofthesamplesobtainedafterthe magneticseparation were labeledasHS1 and SS1forHos- pete and Sandur soils,respectively.Similarly, themagnetic partsofthesampleswerelabeledasHS2andSS2.Afterthe magneticseparationwehavefurthercollectedtheXRDpat- ternsofboththemagneticandnon-magneticpartsofboth thesamples.Fig.3(a)showstheXRDpatternsofallthesam- plesmeasuredafterthemagneticseparation.Itcanbeseen that non-magnetic phaseslike aluminosilicatesand quartz areprominentoverthemagneticphases.Differentcrystalline formsofaluminosilicatespresentinthesamplesareidentified asKyanite(Space-group:P−1)andSillimanite(Space-group:

Pnma),asgiveninFig.3(a).Thesoilscontainmostlyalumi- nosilicatesandquartzasthenon-magneticphases.

Tounderstandtheeffectofdailyenvironmentaltemper- ature onthe phase compositionand phase transformation

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Fig.2–(a)XRDpatternsforthesoilsamplesHSandSS.(b)M–Hloopsofthestudiedsoilsamples:HS(black)andSS(Red), measuredatRT.

Fig.3–TheXRDpatternsofthesoilsamples:(a)HS1andSS1,(b)HS2andSS2.Foridentificationpurposesstandard(ICSD) XRDpatternsoftheobservedphasesarealsoshown.(c)TheRietveldrefinedXRDpatternsforthesoilsamples.Thedata pointsinredcircles,blacklinesandbluelines(belowXRDpattern)representtheexperimentaldata,fittotheexperimental dataandthedifferencepattern(mismatchbetweenexperimentaldataandrefineddata),respectively.Theverticalbars belowtheXRDpatterns(ingreencolor)representBraggpositions.

Table2–TheRietveldrefinedXRDparameters.Theabbreviations‘a’and‘V’standfortheunitcellparameterandcell volume,respectively.Theerrorinestimationof‘a’isinthe4thdecimal.

Sample Phase(wt%)spacegroup Cellparameters(Å),volume(ų)

HS2 Fe3O4/␥-Fe2O3(5.3±0.5)[Fd-3m] a=8.4077,V=594.3

␣-Fe2O3(81.5±1.3)[R-3c] a=5.0364,c=13.7541,V=302.13 Lowquartz(13.2±1.0)[P3221] a=4.8365,c=5.5604,V=112.64

SS2 Fe3O4/␥-Fe2O3(20.9±1.0)[Fd-3m] a=8.3775,V=587.95

␣-Fe2O3(71.1±1.2)[R-3c] a=5.0329,c=13.7451,V=301.52 Lowquartz(8.0±0.6)[P3221] a=4.8186,c=5.5682,V=111.97

behavior, the magnetic part of the samples were studied extensively.TheXRDpatternsofthemagneticpartofthesoil samples(HS2andSS2)showpresenceofFe3O4/␥-Fe2O3(Fd- 3m),␣-Fe2O3(R-3c)andlowquartz(P3221)phases(Fig.3(b)).The RietveldrefinedXRDpattersareshowninFig.3(c).Themajor BraggreflectionsareindexedinFig.3(c)andtheindicesofindi- vidualphasesare color-coded forclarity. From theRietveld refinementresults(Table2),weobservethatboththesam- ples(after separationbyamagnet) stillcontain␣-Fe2O3 as themajorphase,i.e.,∼81%forHS2and∼71%forSS2sam- ples.However,theamountofmagneticphase(Fe3O4/␥-Fe2O3) isestimatedtobe∼5.3±0.5wt%forHS2and∼20.9±1.0for SS2samples,i.e.,theSS2samplecontainsmoreamountof magneticphasethanHS2sample.Also,thetotalamountof

ironoxidephases(Fe3O4/␥-Fe2O3+␣-Fe2O3)ishigherinSS2 thanHS2sample(Table2).Therearesmallquantitiesofthe non-magnetic low-quartz (SiO2) phasestill present inboth thesamples(∼13wt%inHS2soiland8wt%inSS2soil).Note that,althoughtheBraggpositionscorrespondtoFd-3mstruc- tureofFe₃O₄phase,␥-Fe₂O₃canalsoassumeFd-3mstructure [23]. Hence, quantificationofthe magneticpart into Fe3O4

and␥-Fe2O3phases,byXRDalone,isnotpossible,unless␥- Fe2O3possessesP4332structure[24,25].However,itisknown that␥-Fe2O3ordefected-Fe3O4(Fe²⁺partiallyoxidizedtoFe³⁺) usually has lower lattice parameter than the Fe₃O₄ phase [23,25,26].Interestingly,incaseofHS2,thecellparameter(a)is 8.4077 ˚A,whilethatforSS2is8.3775 ˚A.ThissuggeststhatSS2 mightcontaindefected-Fe3O4asprominentphase.Incaseof

Fig.4–TheXPSspectraof(a)HS2and(b)SS2samplesoverthewholemeasuredenergyrange.TheFe2pXPSspectraof(c) HS2and(d)SS2samples.

boththe samples,aslightmisfitatafewdiffractionangles (andanunaddressedpeakat∼27.5^◦(markedasasterisk)for HS2sample) correspond tothe aluminosilicatesinthe soil samples.However,theabundanceofthisphaseis<2wt%in thewholesample.

Fig.4(a,b)showstheXPSspectraofthetwosoilsamples (HS2andSS2).TheFe2pandO1speaksarewellpronounced alongwiththe C1speak (duetothe carbontapeuseddur- ingthemeasurement).TheFe2pXPSspectraoftheHS2and SS2sampleswerefurthermeasuredwithbetterenergyreso- lutionandareshowninFig.4(c,d).Itisclearthat,thebinding energyfortheironliesbetween700and735eV.Theintense peak,whichappearsaround710eV,confirmsthepresenceof Fe²⁺(Fe2P_3/2)inboththesamples.Thepeakat∼713eVcor- respondstoFe³⁺(Fe2P3/2).Thesatellitepeakscorresponding toFe²⁺andFe³⁺appearintherange∼715–722eV.Thepeaks seenat∼724eVand∼728eVcorrespondtotheFe²⁺andFe³⁺

ions(Fe2P_1/2),respectively.Thepeaksobservedintherange

∼730–736eVcorrespondtothesatellitelinesofFe²⁺andFe³⁺

ions(Fe2P_1/2).TheXPSresultscorroboratetheXRDresultsthat bothFe²⁺andFe³⁺ionsarepresentinthesample,i.e.,␣-Fe2O3, Fe3O4and␥-Fe2O3phasesarepresentinboththesamples.The peakobservedat∼708eVgivestheimpressionthatmetallic iron(Fe⁰)mightbepresentinboththesamples.However,we havenotobservedanytraceofmetallicFephaseinanyofthe characterizationmethodsreportedhere.Earlier,asimilarpeak wasobservedforlowspinFe²⁺state[27–29].Theoriginoflow spinFe²⁺statecanbeduetothepresenceofhighamountof defectsinthesamplesasindicatedfromtheEDAXmeasure- ments(Table1).Hence,thispeakmaycorrespondtothesame lowspin2+oxidationstateofFe.

TheMössbauerspectraofboththesamplesmeasuredat RTaregiveninFig.5.KeepingtheXRDresultsinmind,that boththesamplescontainFe₃O4,␣-Fe₂O₃and␥-Fe₂O₃phases, and eachspectrumwas fitwithfoursub-spectra shownas foursextetsinFig.5(a).AlltheMössbauerparametersobtained fromtheleast-squaresfittingarelistedinTable3.Alltheiso- mershift(IS)valuesaregivenwithrespecttothe⁵⁷Cosource (Rh-matrix)atRT.Thedominantsextet(S1)seeninbothsam- ples(darkgreencolor,B_hf=51.5T)isassignedtothe␣-Fe₂O₃ phase[25].Thesextet(S2)showningreencolor(B_hf=49.7T)is assignedtothe␥-Fe2O3phase.Theothertwoadditionalsex- tetshavingB_hf=48.9T(S3)and45.9T(S4)areassignedtothe AandBsitesofFe3O4phase,respectively.Asinglet(P)corre- spondingtoparamagneticphaseisalsoobservedinboththe samples,whichmightbeduetothepresenceofsomeFeatoms inthenon-magneticphase[9].Clearly,theHS2samplecon- tainsmore␣-Fe2O3phase.Acomparisonbetweenthearea%

obtainedfor␣-Fe2O3andfortheferrimagneticphases(␥-Fe2O3

andFe₃O4)suggeststhatthefractionof␣-Fe₂O₃contentinHS2 ismuchhigherthanSS2sample.

Fig. 5(b) shows the M versus H loops for HS2 and SS2 samples.Thesamples(themagneticparts)showsaturation magnetization(MS)of∼16emu/gand∼22emu/gforHS2and SS2,respectively.Asseenearlier,theMSvalueforHSandSS sampleswere 8emu/gand 19emu/g,respectively(Fig.2(b)).

Hence,thesaturationmagnetizationvaluesforthemagnetic partofthesamplesincreasedonlyslightlyforSS2sample,but considerablyforHS2sample.However,thesaturationmagne- tizationvaluesarenotquitesimilartotheMSvaluesofFe3O4

and␥-Fe₂O₃phase[29–33].Thissuggeststhattheseparation ofmagneticandnon-magneticpartsisnotideal.

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Fig.5–(a)The⁵⁷FeMössbauerspectraofboththesamples(HS2andSS2)measuredatRT.(b)MagnetizationloopsforHS2 andSS2samples,measuredatRT.

Table3–MössbauerspectralparametersobtainedfromtheleastsquarefittingoftheMössbauerspectraoftheHS2and SS2samples.IS,isomershift(withrespectto⁵⁷Co-source(Rh-matrix));2␧,quadrupolenuclearlevelshift;B_hf,magnetic hyperfinefield;Area%,percentareaunderthesub-spectrum.Theestimatederrorinthevalueofeachquantityisalso given.

Sample Phase Sub-spectra IS(mm/s)±0.005 2␧(mm/s)±0.005 B_hf(T)±0.5 Area%±2%

HS2 ␣-Fe2O3 S1 0.027 −0.179 51.5 83.88

␥-Fe2O3 S2 0.020 0.002 49.7 2.63

Fe3O4(Asite) (B-site) S3 S4 0.200 0.560 −0.020 0.05 48.9 45.9 4.387.59

Paramagnetic P 1.52

SS2 ␣-Fe2O3 S1 0.027 −0.167 51.5 66.20

␥-Fe2O3 S2 0.020 0.002 49.7 5.83

Fe3O4(Asite) (B-site) S3 S4 0.200 0.560 −0.020 0.050 48.9 45.9 5.5716.28

Paramagnetic P 6.12

TheIFNMRisatechniquetoassessthemagneticproper- tiesofferromagneticandferrimagneticmaterials[21,34-36].

InatypicalIFNMRexperimentthetransitionofnuclearspins between m_I=+

½

and m_I=−

½

state is governed by an RF pulse,whichcreatesaresonancecondition.Theconditionfor resonanceisgivenbyω=H_int,whereωistheprecessionfre- quencyofthenuclearspinsandalsothematchingfrequency oftheRF pulse, isthegyromagneticratioof⁵⁷Fenucleus (=1.382MHz/T),H_intissameasthemagnetichyperfinefield (B_hf)experiencedbytheNMRactivenucleus.Fromtheelemen- talanalysisandtheMössbauerstudiesitisclearthatbothof oursamples(HS2andSS2)contain␥-Fe2O3andFe3O4asmajor ferrimagneticphasesinitscomposition.Asdiscussedinthe Mössbauerspectroscopyresults,themagnetichyperfinefield ofthesetwophases(␥-Fe2O3andFe3O4)variesbetween48and 52TatRTandbetween50and55Tat77K,whichmatcheswith manyotherpreviousreports[29,37–41].TheseB_hfvaluescorre- spondtoanNMRfrequencybetween60and72MHzatRTand between65and76MHzat77K.Asexpected,␣-Fe₂O₃phase beingantiferromagnetic,nosignalwasobservedatbothmea- surementtemperatures(RT and77K).TheobtainedIFNMR resultsforboththestudiedsamples(HS2andSS2)aredis- cussedbelow.

TheIFNMRspectraforboththesamples(HS2andSS2)mea- suredatRTisshowninFig.6(a)and(b),respectively.Forboth

thesamplesthreeisolatedpeaksareobserved.Thepeakswere assignedinaccordancewiththepreviousdiscussionandare consistentwithpreviousreports[36,38,39].Thepeakatabout 63.5,67.5and69.5MHzareassignedtotheFeatomspresentat theB-siteofFe3O4,theA-siteofFe3O4and(boththesitesof)

␥-Fe2O3phase,respectively.ItisclearfromtheIFNMRspec- troscopythatboththeFe3O4and␥-Fe2O3phasesarepresent inthesamples.Wehavequantifiedtheamountofeachphase ineachsamplebyintegratingtheareaundereachcurvein Fig.6.TheresultsaretabulatedinTable4.FromTable4itcan berationalizedthatthesiteoccupancyofFecationsinAandB sitesofFe3O4is∼2:1.Furthermore,thesumoftheintegrated areaunderallthepeaksis0.62fortheHS2sample(Fig.6(a)) and0.90fortheSS2sample(Fig.6(b)).Thisclearlysuggests thattheamountofmagneticphases(Fe3O4+␥-Fe2O3)inHS2 sampleislowerthan thatofSS2sample.These resultsare furtherverifiedbytheIFNMRmeasurementsat77K,whichis discussedbelow.

The internalfield of the ferrimagneticmaterials largely dependsonthetemperature;atlowertemperaturetheinter- nalfieldishigher.TheIFNMRspectrameasuredat77Kare giveninFig.6(c,d).SimilartotheIFNMRspectrameasuredat RT,wehaveobservedthreepeakseach,forboththesamples (HS2andSS2)at77K.However,thelinewidthofthepeaksis highandthepeaksareoverlapping.Thelowfrequencypeaks

Fig.6–The⁵⁷FeIFNMRspectra(echoamplitudeversusfrequency)of(a)HS2(b)SS2samplesmeasuredatRTusinga two-equal-pulsessequence(1␮s–25␮s–1␮s).Thecoloredlinesrepresentthefittedcurvestothemeasureddatapoints.The

57FeIFNMRspectraof(c)HS2and(d)SS2samples,measuredat77Kusingatwo-equal-pulsessequence(1␮s–30␮s–1␮s).

TheblueandpinklinescorrespondtoBandAsitesofFe3O4,respectively,andthegreenlinesrepresent␥-Fe2O3.Thered linerepresentsthefittedcumulativecurvetothemeasureddatapoints.

Table4–NMRparametersobtainedfromtheexperimentattwodifferenttemperatures(RTand77K).Phasecomposition indicatesthemagneticphase;NMRfrequencyisthecenterfrequency,internalfieldobtainedfromthecorresponding centralNMRfrequency.AreaisobtainedafterintegratingtheNMRfrequencycurves.

Soilsample Temperature(K) Phasecomposition Correspondingfrequency(MHz) Internalfield(T) Areain%

HS2 RT Fe3O4B-site 63.48 45.93 41

A-site 67.59 48.90 20

␥-Fe2O3 69.53 50.30 39

77 Fe3O4B-site 70.16 50.62 57

A-site 71.94 52.12 26

␥-Fe2O3 73.05 52.80 17

SS2 RT Fe3O4B-site 63.55 45.98 40

A-site 67.48 48.82 22

␥-Fe2O3 69.55 50.32 38

77 Fe3O4B-site 69.96 50.62 54

A-site 72.11 52.17 27

␥-Fe2O3 73.02 52.83 19

at∼ 70and∼72MHzare assignedtotheB-siteand A-site ofFe3O4 phaseand theotherpeakat∼ 73MHzisassigned to(bothA-andB-sitesof) the␥-Fe₂O₃ phase.Theorigin of the overlapping ofthe twopeaks corresponding to A-and B-sites ofFe3O4 iswell known inthe literatureas‘Verwey transition’[40–45]. This correspondsto the transitionfrom metallictoinsulatingbehaviorofFe3O4belowthetempera- tureTV≈122K.Thischangeisalsoassociatedwithastructural transformationfromcubicinversespineltomonoclinicstruc- ture.Thisleadstoachangeinmagneticpropertiesandboth the sitesacquiring similarinternal field/hyperfinefield. As aresult,boththepeaks,correspondingtoA-andB-sitesof Fe3O4,appearveryclosetoeachotherasseeninFig.6(c,d).

Theeffectofthelocaldailytemperatureonthecomposi- tionofthesoilsisdescribedbelow.MostpartoftheHospete is dry-land whereas large part ofSandur includes aforest area.Sandurcityisonlyabout30kmfromHospete.Although, the solar radiation receivedbyboth the citiesis verysim- ilar, the local temperature experienced daily is about 2^◦C higher atHospete than Sandur. Hence, the environmental parameters, e.g.,humidity, atboththe placesare different.

Both the cities are known fortheir iron ore deposits. The soil alsocontains a high amount ofiron-oxide phases. As weseefromtheaboveresults,thesoilscollectedfromboth the placescontainaveryhigh amountofnon-magnetic ␣- Fe2O3(hematite)alongwithferrimagneticFe3O4(magnetite)

j mater res technol.2019;8(2):2192–2200

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and␥-Fe2O3(maghemite)phases.However,the␣-Fe2O3con- tentinHospetesoilisrelativelymuchhigherthanthatofthe Sandursoil.

According to earlier investigations, the ferrimagnetic iron-oxide phases (Fe3O4 and ␥-Fe2O3) transform to sta- ble ␣-Fe2O3 phase between 200 and 350^◦C [29]. Fe3O4 first transforms to ␥-Fe2O3 phase between 200 and 250^◦C, the

␥-Fe2O3 phase then transforms to ␣-Fe2O3 between 300 and 350^◦C. These transformations are often observed to occurinnatureevenatlowertemperatures,butveryslowly depending on the temperature and other environmental conditions.

According to Mössbauer spectroscopy (Fig. 5(a)), VSM (Fig. 5(b)) andIFNMRspectroscopy(Fig.6)results,themag- neticpartoftheSS2sampleshowedrelativelyhigheramount ofmagneticphases(Fe3O4and␥-Fe2O3)thanHS2sample.Note thattheparticleshavesizeshigherthan1␮m(Fig.1(i)),hence, theseparationofthemagneticandnon-magneticpartcan- notbedoneperfectlybyapermanentmagnet,because,even thefinely-groundparticlescancontainboththephasesinthe sameparticle.ThisisevidentfromtheXRDpattern(Fig.2(a)) ofthenon-magneticpartsofthesamples(HS1andSS1).How- ever,thisprocessseparatesonlythemagneticparticlesthat areattractedbythemagnetoffieldstrength1T.Hence,with- outlosinganygenerality,wemaysaythatthemagneticparts ofthe samplescontainproportionallyhigher ferrimagnetic phases.Alltheresultstakentogether,weobservedthatthe as-collectedHS(Hospete)samplehasloweramountofmag- neticphases(Fe3O4and␥-Fe2O3)thanthatoftheas-collected SS(Sandur)sample.Consideringthatequalamountofiron- oxide phases were created atthe beginning (as thesetwo cities,HospeteandSandur,areclosetoeachother),therate oftransformationofFe3O4and␥-Fe2O3to␣-Fe2O3ishigher inHospetesoilthaninSandursoil,becausetheSandursoil containshigheramountofferrimagneticiron-oxidephases.

Hence,thephasecompositionofthesoilisintimatelyrelated tothelocaldailytemperature.

4. Conclusion

Thestructure,elementalcomposition,phasecompositionand magneticpropertiesoftworepresentativeironorescollected fromtwodifferentregions(HospeteandSandur)ofKarnataka, India, were investigated. Both the samples predominantly contain ceramic phases with Fe and O as the major ele- ments along witha small amount ofAland Si. We found that ␣-Fe2O3 is the dominantphase (∼80wt%) inboth the samples.Otheriron-oxidephasessuchas␥-Fe2O3andFe3O4

alsoconstitutethesamplesalongwithtracesofaluminosil- icate(Al₂SiO5)andlow-quartz(SiO₂).Presenceofalltheiron oxidephaseswasconfirmedbycomplementarycharacterizing methodssuchasXPS,Mössbauerspectroscopyandinternal- fieldNMRspectroscopy.Weobservedthatthesoilcollected fromlowtemperatureregion(Sandur)containsmoreamounts offerrimagneticoxide(Fe3O4,␥-Fe2O3)phases,whereasthat collectedfromthehightemperatureregion(Hospete)contains more␣-Fe2O3phaseinthesoil.

Conflicts of interest

Theauthorsdeclarenoconflictsofinterest.

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