Solid State Communications,Vol. 5,pp. 267-270, 1967. PergamonPressLtd. Printed in GreatBritain

MOSSBAUER STUDIESOF Fe2~IN PARAMAGNETIC FAYALITE (Fe3SiO4)~

M. Eibschiitz andU. Ganielt

The WeizmannInstitute of Science,Rehovoth, Israel

(Received13 February1967 by E. Burstein)

Mössbauerabsorptionspectrain Fe3SiO4weretakenover the temperaturerange80 - 1000°K. Thequadru-pole splitting of theabsorptionline asafunction oftemperaturewasmeasured. The results show, thatthe electric field gradienttensorat the sites of the Fe

2~ions hasdifferentvalues, correspondingto thetwo sitesof different point symmetrywhich areknown from x-raydata.

AS PART of a series of investigationson divalent of gravity of the correspondingpeaks. Someiron compounds,beingcarried out in our labor- typical unretouchedSpectraarereproducedinatones, we studied Fayalite(Fe

2SiO4)by means Fig. 1.of theMössbauereffect.

Insteadof thetwo peaksof equalintensity,Fe3SiO4waspreparedby mixing Fe203, whichareusually observedwhenthe ferrous ion

Si02 and Fe in stoichiometric proportions. The occupiesasite of distortedoctahedralsymmetry,mixturewas sealedin an evacuatedsilica tube we found in the presentcasethat oneof the linesandplacedin a 1000°C furnacefor a periodof hadsplit into two, this secondarybecomingmore24 hr. The resultingcompoundwascheckedby pronouncedat high temperatures. This ratherx-ray photographsto checkthat it wasa single interestingfeatureis easily explainedby thephasematerial, crystallographicdata. Fe2SIO4belongsto the

spacegroup ~nma (D ~~6h), having four moleculesThe MO~ssbauerspectrawere obtainedby perunit cell. ~ The ferrous ions occupytwo

meansof a constantvelocity automaticrecord- non-equivalentpositions, of which therearefouring spectrometer.1 The absorberwascontained of eachtype perunit cell. EachFe

2’~ion isin an improvedversion of a cryofurnacedescrib- surroundedby adistortedoctahedronof oxygened by SharonandTreves2, andspectrawere ions. The point symmetry at thesite of the Fe~takencovering atemperaturerangebetween80 ion hasonly a centerof symmetry (T) at oneandl000°K. The sourcewas ~Co In Pd. ~ The position anda mirror plane(m) at the other. 8

spectrawere recordeddirectly on an x-y Thedifferent point symmetriesat the two sitesrecorderandthe quadrupolesplittings were give rise to different valuesfor the componentsdeterminedfrom thedistancebetweenthe centers of theelectric field gradient(EFG) at thesesites

_________________________________________ andhence- to different quadrupolesplittings* The researchreportedin this documenthas (Q.S) of the upperM~óssbauerlevel of the Fe

beensponsoredin part by theAir ForceMater- nucleus. This in turn explainsthesecondaryials LaboratoryResearchand‘ieclinology splitting of theMc~issbauerpeak. The resultsofDivision AFSCthroughtheEuropeanOffice of the quadrupolesplitting at both sites asa functionAerospaceResearch,United StatesAir Force of temperatureareshownin Fig. 2.ContractF 61052-67-C-0040.

t Part of a Ph.D. thesisto be submittedby U. At low temperatureswe found (to withinGaniel to the FeinbergGraduateSchool of the the experimentalresolution)the samevalue forWeizmannInstitute of Science, the Q.5. at thetwo sites, namely (3.05 ±0.03)

267

268 MOSSBAUERSTUDIES OF Fe2~’ Vol.5, No. 4

Sourc* Co57 in Pd Absorber ~ Si 04 the spin orbit interactioncan leadto evenfurther splitting. For eachsuchatomic level,

the EFG tensorcan be calculatedI! theeigen-. ~ / functionsareknown. Thequadrupolesplitting,

T= 296 ~ in the caseof Fe (I = 3/2) is givenby

~ e2 q Q ( ~ + ~ 2 / ~ ) %

T= 387~K whereq = V,,, Q is the quadrupolemomentoftheI = 3/2 nuclearlevel and

/ 5’ /. ,.._‘. ~ v—V \I V\ xx yyJI zzT~639°K •, : “ /

Sincethe electronicrelaxationtimes areexpectedto be muchsmaller than J h / ~EQ

• (~“3.10_Bsec), theactualvalue of ~EQ which-s the nucleus“feels” becomeathermal average

of the valuesof ~EQ in thedifferent states. At* low temperaturesonly the lowestatomic levels

contributeto this averageso that ~EQ normallyT= 759°K : approachessome“saturation” value, which, for

• : Fe2~compounds,is typically around3 mm/sec.7• It is only at highertemperatures,that the higher

• levelsbecomemore important andthus the

detailsof the crystal field influenceon theQ. S.of theMössbauerlines becomeimportant. Inthecaseof Fe

2SIO4,the existenceof two differ-

I I ent typesof lattice sites for theFe2~ions means

-4 -2 0 2 4 that therearedifferent crystal fields at those

Veloct 1mm.’ e sites, a situationwhich manifestsitself moreY i S pronouncedlyat hightemperatures.

FIG. 1In caseswherethe point symmetryat the

Mössbauerspectraof Fe2Si04 at nuclearsite is higherthan in Fe2SiO4, the con-.

different temperatures. The source siderationsoutlinedabovepermit actualcalcul-wasCo

5~in Pd. Countingtime was ationsof crystal field parametersfrom the30 sec. per channelfor thefirst two temperaturedependenceof the Q. s, ~~ In thespectraand100 secfor the othertwo, presentcasehowever, thepoint symmetryat

both sites is so low, that any calculationof thistype would require a largenumberof parameters.

mm/secat 80°K. This is aboutthe character- To attemptsuchacalculation, it is necessaryistic value for the Q. S. in compoundsof Fe2+ ~ that theexperimentalresultsbe of high accuracy,at low temperatures,in adistorted octahedral in order to getunambiguousresults, On theenvironment. At highertemperatures,on the otherhand, thesecondarysplitting describedotherhand, the secondarysplitting becamemore aboveis relatively small, andthus the resolut-pronounced,giving at 1000°K thevalues: ion is rather poor. This reducestheaccuracy(1.56 ± 0. 06) mm/secfor onesite and considerably. Hence, determinationof the(1.40 ±0. 06) mm/secfor the other. These crystalfield parametersvia somecurvefittingfactscanbe understoodif we considertheEFG proceduredoesnot seemmeaningful.at the site of the Fe2~ion. The most importantcontribution to theEFG comesfrom the elect- In anattempt to identify eachof the second-ronic d-shell, which hasthe configurationd6. ary peakswith oneof the two sites, we tried theIn an octahedralfield, this leadsto a lower following procedure. We preparedthecompoundtriplet T

21, andan upperorbital doublet, E,. CaFeSiO4,for which it hasbeenshown ‘° ~ ~ theDeviationsfrom octahedralsymmetry canthen Ca

2~ Ion substitutesfor the Fe2 ion in the sitesplit the orbitally degeneratelevels further, and with the mirror plane(m). TheMóssbauer

Vol.5, No.4 MOSSBAUER STUDIES OF Fe2~’ 269

I I I I I

301—

. S0

: 25- 0’.5.

E 0 1E 0 •%

— 020- ‘~, .0 1

Oo •.Absorber Fe2 Si 0~

~ 1.5- 00~ •Oo

09. 1.0~0

0

0.5

00 I I I I

• 200 400 600 800 000

Temperature [KJ

FIG. 2

Quadrupolesplitting asafunction of temper-aturefor Fe

2SiO4- Full circle andopencircles give the experimentalvaluesfor thetwo different iron sites.

spectrumof this compoundconsistedof two lines SinceFe2SiO4 undergoesatransition andwithout the secondarysplitting observedin becomesantiferromagneticbelow 65°K 10, it isFe2SiO4, which meansthat all theferrous Ions possiblethat further investigationsat loweroccupyequivalentsites. The Q. S., however, temperaturescould helpto clarify the situationturned out to be considerablysmaller thanin further. ~Fe2SiO4, namely: (2. 18 ±0. 03) mm/secatroomtemperatureand(1.21 ±0. 03) mm/secat We would like to thankProf. S. Shtrikman880°K. Thus theEFG in CaFeSiO4 is not the for suggestingthis studyandfor his constantsameas in eitherof the two sitesof Fe

2~in interest.Fe2SiO

4, andhencethis methoddid not give usthe identificationwe hadhopedfor.

References

1. M5ssbauerEffect Spectrometer,producedby “Elron”, Electronic Industries,Haifa, Israel.

2. SHARON B. andTREVESD., Rev. Sci. Instr. 37, 1252 (1966).

3. Purchasedfrom New EnglandNuclearCorporation, U.S.A.

4. HANKE K., NeuesJb. Miner. Mh. 8, 192 (1963).

5. InternationalTablesfor x-ray Crystallography,Vol.!, Kynoch Press,Birmingham (1952).

6. Seee.g. INGALLSR., Phys. Rev. 133, A787 (1964).

270 MOSSBAUER STUDIESOF Fe2 Vol.5, No.4

7. Strictly speaking,spin orbit effectsandmixing of higher statesinto the groundstateinfluencethe value of the EPG at low temperatures,but in most casesthegeneralargumentsdescribedabovearestill valid. -

8. EIBSCHUTZ M., GANIEL U. andSHTRIKMAN S., Phys. Rev. 151, 245 (1966).

9. EIBSCHTZ M,, GANIEL U. andSHTRIKMAN S., Phys. Rev., (in press).

10. SAUTOROR.P., NEWNHAM R.E. andNOMURA S., J. Phys. Chem. Solids, 2’?, 655 (1966).

11. NEWNUAM R.E., CARON L. G. andSAUTOROR. P., J. Amer. Cer. Soc. 49, 284 (1966).

12. KUNIG W., LINDQUIST R.H. andCONSTAVARIS G., reportedin the 12th AnnualConferenceon MagnetIsmandMagneticMaterials (1966).

Des spectresd’absorbtionMdssbaueren Fe2SiO4ont

dtéfaites dansunerangêede temperaturedu 80 - I OO°K.On amesur4le couplagequadrupolairede la raie d’absorbtlonfonction de la temperature. Les rdsultatsmontrentque le tenseurgradientdu champdlectriquedansles positionsdesions de Fe

2~a desvaleursdifferentes, correspondantaux symmetriesdepointsdiffdrents connusdesdonnéescrystallographiques.