Solid StateCommunications,Vol. 31,pp. 359—364.PergamonPressLtd. 1979.Printedin GreatBritain.

ANOMALIES IN THE X-RAY DIFFUSESCATTERING FROM Cr-DOPEDV2O3 AT THE

HIGH TEMPERATUREMETAL—INSULATOR TRANSITION

F. Sinclair~andR. Colella

Departmentof Physics,PurdueUniversity,West Lafayette,IN 47907,U.S.A.

(Received23 February 1979, in revisedform 23 March 1979byE.F. Bertaut)

Severalanomaliesin theX-ray diffuse scatteringof Cr-dopedV203 havebeenobservedin proximity of thehightemperaturemetal—insulatortransition.It is speculatedthatchargedensitywavesmight be involved.Theoriginal interpretationin termsof aMott transitionisbroughtupagainfor discussion.

1. INTRODUCTION acceptedby themajority of scientistsactivein thefield. An attractivehypothesishasbeenput forwardby

WE WANT TO REPORTin this papersomeanomalies Overhauser[5], who suggeststhe possibilitythat someobservedin the X-ray diffuse scatteringpatternsof of the unusualpropertiesobservedin connectionwithCr-dopedV2O3 in theneighborhoodof themetal— this transitionmaybe linked to thepresenceof achargeinsulator (MI) transition(at around100°C).It is known densitywave (CDW)in thesystem.In sucha situationthatwhenV203 is dopedwith chromiumin sucha way electronbandgaps(in k-space)would openup atthat about 1% of theV atomsare replacedwith Cr positionsdefinedby the CDW wavevectors,therebyatoms,a largeincreasein resistivity is observed(2 orders affectingdrasticallythenumberof carriersabletoof magnitude)as thetemperatureisincreasedabove contributeto theelectricalconductivityof the crystal.room temperature[1, 2], with small shiftsin lattice Diffuse scatteringstudiesshouldbe ableto probethisparametersandatomicpositions,butwithout change hypothesis.in latticesymmetry.l’his transitionhasbeeninter-pretedby McWhaneta!. asa “Mott transition” [1, 2],

2. EXPERIMENTALbutthereis somecontroversyon thecorrectnessofthis view 13, 4]. Wewill notreview in detail the various Several0—20scanswereperformedalong theargumentsagainstor in favor.The mostpuzzling feature c*.axis andotherdirectionson a varietyof 0.8%Cr-of the transition,clearly observedby Kuwamotoeta!. dopedV2O3 singlecrystals[6], grown from the melt

[41,is a conspicuoushysteresisof about50°~ by thetn-arc [7] technique.Eachsamplewascut withthe resistivityvstemperatureplot. A morecareful a diamondbladeparallelto the(hkl)planeschosenforanalysisof thedatashowsthat the portionsof the thescan(in mostcasesthesewereplanesperpendiculargraphwithhighestslopeexhibitsmall butwell discern- to thec-axis)andcarefullyetchedwithCP4in orderible steps,which correspondcloselyto sharppeaks to removeanypowderleft ftom the cutting.In thisobservedin differentialspecificheatruns,executed way it waspossibleto stronglyreducethe powderlinesconcurrentlywith theresistivity runs [41.These in thediffuse scatteringpatterns.All crystalsusedinfeaturesclearly indicatethat the transitionproceeds theexperimentsdescribedIn this paperprovidedgoodby steps,probablythrougha domainmechanism.The Lauebackreflection pictureswith sharpspotsdespiteexistenceof alargehysteresisandstepsin thetransition thepresenceof surfacecracksand imperfections.Thecannotbe easilyreconciledwith thepictureof aMott crystalsalwaysdeteriorated,especiallyin their super-transition,in whichall propertiesdependon the degree ficial appearance,asa resultof cycling throughtheof electroniclocalization,andthereforeshouldnot be transition.Typical valuesfor full width at half maxi-asluggishfunctionof temperature.Severalmodels mumof Braggpeaksin 0—20 scanswerein thehavebeenproposed[4], noneof which aregenerally neighborhoodof 0.6°.Cu—Karadiation(X = 1.54A)

wasmonochromatizedby a curvedquartzmono-* Worksupportedby theNationalScienceFoundation! chromator.Thebeamsizeat thesamplepositionwas

MRL ProgrameDMR 77-23798. about0.2 x 3 mm2.Typicalworking conditionswere

t Presentaddress:BrandeisUniversity,PhysicsDepart- 50kVand20mA. Thecountingtimesin Figs. 1 and2

ment,Waltham,MA 02154,U.S.A. arein general200 secperpoint.

359

360 ANOMALIES IN THE X-RAY DIFFUSESCATFERING FROM Cr-DOPESV203 Vol.31, No.5

cts/sec I.R.T. T~I42°C R.T.

(a) (b) • (c)

_______________________ _______________________ 28—degrees ~So’ lo’ 1 i I I I~’ I I J

55 60 56 60 55 60

Fig. 1. The arrowsmarkthe positionof a peakpartially hiddenbehindthe(009) Umweganregungpeakat 59.7°.Notehow peakpositionandintensitychangethroughoutthe temperaturecycle.Thepeakat 54.2 isthe(116)powderline of V203.

Samplesurfaceswere typically in the rangeof manycouplesof active reflectionsfor whichthe dif-10—15mm

2. Thecrystalswerealwayskept in a cylin- ferencein Miller indicesgive 009. Thisis theconditiondricalenclosuresurroundedby a Mylar shroud,in for theexistenceof multiple reflections [8]. It waswhich thetemperaturecouldbe raisedup to 200°C subsequentlyfoundthat thepeakat 59.7°disappearedandmore. The enclosurewascontinuouslyflushed whenthecrystalwasrotatedaroundthescatteringwith highly purified He gas,getteredat 850°Cin vector,which confirmsthis analysis.Thepeakattitanium. Thisprecautionwastakenin orderto avoid 20 = 54.2°is the (116)powderline of V

2O3 thatsurfacecontamination,and,particularly,formationof couldnot be removedby etchingin this particulardifferentoxidesduring temperaturecycles.Thisalso sample,indicatingthe presenceof a smallamountofreducedconsiderablythebackgrounddueto scattering randomlyorientedV2 03 asan inclusion. An estimatefrom thegas surroundingthe sample.Thefinal peakto of the amountof such inclusionscanbe derivedfrombackgroundratio for theBraggpeakswas 1~ the intensityof the (006)Braggpeakwhich was

Figure 1 showsthe resultsof a scanalongthe —‘ 45,000cps,comparedwith 4 cpsfor the powderc*.axis.Thepeakat 20 = 59.7°correspondsto (009) line.in hexagonalindices.This is a forbiddenreflection,but It is apparentin Fig. 1(a) that a peakis presentatmay benon-zerodueto Umweganregung(multiple 20 = 57.7°asa bulge on theleft shoulderof the(009)reflection)effects [8]. It is in facteasyto verify that peak(seearrow). Figure 1(a)correspondsto a roomfor V203, whosespacegroupisR3c (n. 167),thereare temperaturescan.As the temperatureis increased,the

Vol.31, No.5 ANOMALIES IN THE X-RAY DIFFUSESCATFER1NGFROM Cr-DOPEDV2O3 361

(c) (e)

8-(a) ~6-

• 28:49.4

8- ~4;o • 2 ~ ~ ~ >a) ~- • •

i .e •*s••. •••• ••~4 . 40 5O~ * *

......—..... •..‘.....•...‘ . 28-degrees2-~ •. .~. , i

,

I I I 24 25

(b) (d) (f)

20- • 29:49.4°

8- /

a)” . . o

cn ~ - 28~49.4 .~ t~w0.3 ~ è.~ 4 - • . -~/ \E—U. ~ •. t ... •1

2 -~-‘:.~ ~ ~ .~ ~ ~—. /C I

I I I I tO ~ i ~ i I I .~ *40 50 22 24 26 28 .° • • *

~ •**S •• •.**S

28 — degrees w — degreesI I I I i i i i I ,

24 25w— degrees

Fig. 2. 0—20scansalongthece-axisat differenttemperatures.(a) T= 95°C(increasing).(b) T= 126°C(insulatorstate).(0c) T= 21°C(endof temperaturecycle). In (a) thepeakat 48.4°is barelyvisible. In (b) it is fully developedat 49.4 anddisappearsat roomtemperaturein part(c). The peakat44.4°is the (002)powderline of VO. In (d)ano.-scanat 20 = 49.4°showsthat, afterthethird temperaturecycle,theextrapeakvisible in (b) at 20 = 49.4°isfoundagainslightly off thec*~axisat ~ = 24.0°.In (f) a slow~-scanof thepeakfoundin(b) at T 134°C(insulatorstate)is shown.Thecountingtimewasincreasedfor betterstatistics.Thepeakwidth &i., amountsto0.3°.(e) showsthesameu-scanat theendof thethird temperaturecycle.

peakat 57.7°graduallydisappears(or, perhaps,is results.The crystalwassubjectedtothreetemperaturehiddenbehindthe(009)peak),butreappearsagainon cyclesthroughtheM—I transition,betweenroom tern-theright shoulderof (009),at 20 = 62.0°,after the peratureand134°C.Threepeaksalongthec*~axisM—I transitionhastakenplace,at T= 142°C.As the alternatelyappearedanddisappearedat different tern-temperatureis decreased,thepeakdisappearsagainbut peratures,locatedat threewell definedangularreappearsat room temperatureat 20 = 57.7°[Fig. positions(20 = 49.2°,70.6°,and 124.8°,respectively).1(c)]. Notethe reductionof thebackgroundandthe The distinctivefeatureof all thesethreepeakswas their(116)powderline in part(c) of Fig. I. No strong extremesharpness,of the orderof 0.3°(in bothvariations(within ~ 20%)were observedin theBragg c.~.,and20 scans),which isless thanthe instrumentalpeaksduring this temperaturecycle. Whenthe crystal resolution(of theorderof ‘0.5°in ~ at the abovewascooledto — 63°Cfor a few hours,thepeakat mentioned20 values).Figure2 showstheevolutionof57.7°Cdisappeared,andthe backgroundandthe (116) oneof thesepeaksat 20 = 49.2°,absentat roomtern-line went up to the original values.No appreciable peratureat thebeginningof the temperaturecycle, aschangeswere foundafter subsequentheatingto room thecrystalgoesinto the insulatorstateandcomesbacktemperature. to themetallic state.Thepeakintensityof the (0,0, 12)

Anotherseriesof scanswere performedalongthe reflectionon thesameplot would be about25,000cps.c*.axis on a different crystal,with ratherintriguing Notethat no appreciablechangecanbe observedin the

362 ANOMALIES IN THE X-RAY DIFFUSESCATFERINGFROM Cr-DOPEDV2O3 Vol.31, No.5

38-

/

MS I I

50 (00 (50

Co

Fig. 3. The changein 20* angleat which thediffuse backgroundishalf of its final valueis plottedasa functionof7’. Thehysteresis1oopcouldnotbe completedbecausethecrystalshatteredby thermalstressesat pointS(T= 100°C).

peakto theleft, at 20 = 44.4°,dueto the(002)powder detail,entirelydifferent resultswere obtained.Noline of VO [9]. Theidentificationof powderlineswas extrapeakswere observed,but thebackgroundlevelconfirmedby checkingthat theirintensitiesweremore wasvery different from thatshownin Fig. 2. Its valueor less constantin w scans,whereastheextrapeakswe at 20 10°wasabout 1.5 ctssec’,thenincreasedaretalkingaboutwereextremelysharpin w scans. graduallyandlevelledoff at about20 = 60°,at aFigure2(f) showsoneof thesew scans.The peakat valueof 20ctssec’ or more. As the crystalwas20 = 49.2°showedup again,ratherweakly, during the broughtthroughtheM—I transition,theangle,20*, atsecondtemperaturecycle, butdid not appearduring which halfof thefmal constantbackgroundlevel wasthe thirdcycle. It was foundagain,however,at theend obtained,decreaseddrasticallybeforethetransitionof the thirdcycle at RT, slightly off thec*~axis,as (in otherwords,the slopeof the risingportionof theshown in Fig. 2(d). Whenthe peakwasfound absentin diffusescatteringintensityvs20 increaseconsiderably),a20-scanat RT afteroneof the previoustemperature then camebackto moreor lessthe initial value asthecycles,an~ scanconfirmedthat indeedthepeakhad temperaturewasdecreased.A markedhysteresiswaspractically disappeared(Parte).Thepeaksat observedin the temperaturedependenceof 20 * (Fig. 3),20 = 70.6°and124.8°appearedonly during the first verysimilar to thehysteresisloopobservedin resis-partof thefirst temperaturecycle, when7’ was tivity measurements[4], which indicatesthat theincreasing,andthendisappearedandwereneverfound observedbackgroundvariationsareundoubtedlyagain,exceptfor thepeakat 20 = 70.6°which relatedto the M—I transition.This hugeconstantback-reappearedin the insulatorphaseduring the2ndand groundis thesignatureof quasi-randomatomic dis-3rd temperaturecycle, andin theendwasfoundat RT placements,a sortof a liquid-like state,for whichtheslightly off thec*~axis. angle20* characterizesthemaximumvalueof the

With the third sample,which we wish to discussin correlationlength.

Vol.31, No.5 ANOMALIES IN THE X-RAY DIFFUSESCATFERINGFROM Cr-DOPEDV2O3 363

In onecaseascanalongthe (110)directionwas system.The photographicimagedisappearedcompletelyperformed,butno significantanomalieswere observed whenthecrystalwasrotatedoff thec~.-valueat whichalongthis direction. thepeakhadbeenobserved.Weverifiedthat thebeam

Severalotherscanswere performedon different producingtheseimageswas actuallycomingfrom thesamples.In mostcasesextrapeaksof thekind described crystal.earlierhavebeenobserved,alwayslocalizedalongthe The fact that reproducibility fromsampletoc8-axis,thatcould notbeidentified aspowderpeaksof sampleispoor maybe relatedtoirregularity in Cr

anyvanadiumoxide.The appearanceanddisappearance concentration.A microprobetestrun on oneof ourof thesepeaksat variousstagesof the thermalcycle were samplesdetectedCr concentrationsrangingfrom 0.5toclearly relatedto theM—I transition.In two or three 0.8%. It is knownthatCr plays a dominantrole incasesno anomalywhatsoeverwasobserved;in other makingthelatticeunstableenoughto trigger a M—Icasesit wasfoundthat the samecrystalwould give transitionat temperaturesslightly aboveRT [1, 2,4].different diffraction patterns(with or without extra Wementionedin theintroductorypartof this paperpeaksorhigh background)whentakenseveraldays that theM—I transitionin Cr-dopedV

2O3 maypossiblyapart,evenwithout anythermalcyclingbetweenthem. be linked to thepresenceof CDW’s. In suchacase,a

In summingup, thereis no doubtthatextrapeaks CDW statewill mostlikely to be set up in a domainwith incommensurateQ vectorsalongthec*~axishave structure,similar to whathappensin chromiumforbeenobservedin Cr-dopedV203.The Q valuesare spindensitywaves.Many unusualpropertiesof pot-fixed for a givensample,but cannotbe reproducedfrom assium,suchasthe torqueanisotropy,haverequiredsampleto sample.In somesamplesnoeffect of anysort the postulateof domain-structuredCDW’s [10]. If ais observed;in othersamples,in which extrapeaksare similarsituationexistsin V203,it is only accidentalnotobserved,the diffusebackgroundis anomalously that thewhole crystal,or mostof it, will developahigh andits 20 dependenceis definitely relatedto the singledomainwith a CDW satellite locatedon theM—I transition. c*~axis.ThattheM—I transitionin Cr-dopedV203

proceedsby domainshasbeenshownby Kuwamoto

3. DISCUSSIONAND CONCLUSIONS etal. [4].Wehaveno clear-cutexplanationfor the anomalies It hasbeensuggested[5] that peakssuchasthose

observed.Themoststriking featureson whichwe want reportedin Fig. 2 are secondordersatellitesofto concentratearethesharppeaks,anexampleof which (4, 4, q) CDW’s. At low temperature,below 170K,is presentedin Fig. 2. Thesepeaksarenotdue to oxide V203 transformsto anantiferromagneticinsulatorformationon thesurfaceduringour experimentbecause: phase,with a spindensitywavecharacterizedby a(i) theyappearedanddisappearedduring thetemperature (4~4.0)wavevector [111.It isconceivablethat aCDWcycle;(ii) theV203 crystalswere kept in a highly systemcould beset up with aQ vectorcantedat somepurifiedoxygen-freeenvironment;(iii) the observed anglewith respectto ahighsymmetryaxissuchaspeakscouldnotbe consistentlyidentifiedwith anyof (110),analogousto whathappensin KCP andin layeredthe powderlinesof all knownvanadiumoxides(includ- compoundssuchasTaS2.ing the Magnéliphases)listedin the powderdiffraction Eventhoughwe arenot in aposition to draw firmfile (J.C.P.D.S.1977). conclusionsonthe interpretationof the observed

If thesepeaksare thegenuinesignatureof periodic anomaliesin the diffuseX-ray scatteringpatternoflattice displacements,severalquestionsmerit consider- Cr-dopedV2 03, theappearanceanddisappearanceofation: Why aretheysosharp,sharperthan any other unidentifiablepeaksaswell asthe highbackgroundordinaryBraggpeak?Why areall thesephenomenaso phenomenadescribedin Fig. 3 constituteanunmistak-poorly reproduciblefrom sampleto sample(in contrast ableproofthat subtlestructuralperturbancesincom-to theresistivitymeasurementswhich showa mensuratewith theunit cell accompanytheM—Iremarkableconsistency)? transitionin this crystal,which castsfurtherdoubton

At the presenttime, we cannotoffer anexplan- the original interpretation[1, 2] of apurely electronication for theextremesharpnessof the extrapeaks Mott transitionin the absenceoflong rangestructural

effects.observed.It couldbe an unusualcombinationof peakshapeandresolutionfunction.Photographicimagestakenwith films placedin front of the countershowedtwo verysharpverticallines(Ka1 andKa2) corre- Acknowledgements— Theauthorsgreatlybenefitedspondingto imagesof theX-ray source,with the from manydiscussionswith A.W. Overhauserandexpectedgeometricalshapeas definedby the collimating J.M. Honig.

364 ANOMALIES IN THE X-RAY DIFFUSESCATTERINGFROM Cr-DOPEDV203 Vol.31, No.5

REFERENCES found to takeplacebetween95°and126°Cwhenthe temperaturewasincreased,andbetween721. p.B. McWhan,TM. Rice & J.P.Remeika,Phys. and55°Cwhenthe temperaturewas decreased.

Rev.Lett. 23, 1384 (1969). 7. T.B. Reed& E.R.Pollard,J. CrystalGrowth 2,2. D.B. McWhan& J.P.Remeika,Phys.Rev.B2, 243 (1968).

3734(1970). 8. H. Cole, F.W. Chambers& H.M. Dunn,Acta3. J.M. Honig, G.V. Chandrasekhar& A.P.B.Sinha, Cryst. 15, 138 (1962).

Phys~Rev.Lett. 32, 13 (1974). 9. ThisVO peakcannotbe ascribedto reductionof4. H. Kuwamoto,H.V. Keer,J.E.Keem, V203. If this wasthe case,its intensitywould

S.A.Shivashankar,L.L. VanZandt& J.M. Honig, haveincreasedduring the durationof the exper-J. Phys. 37,C435(1976). iment (of the order of 3—4 weeks).Furthermore,

5. A.W. Overhauser(private communication). considerationsbasedon thermochemicaldataon6. This valuefor theCr concentrationwas adopted V2O3,confirmedby our experimentalwork at

for all thesamplesusedin this work becauseit Purdue,suggestthatV203 cannotbe reducedatwasfound [41 that theM—I transitionis sharper T~ 150°C.Small inclusionsof VO weremostandbetterdefinedfor this materialthanfor 1% likely formedduring thegrowthprocessby theCr-dopedV2 03. In manycasestheM—I transition tn-arc technique.wasmonitoredby resistivitymeasurementsin SitU 10. A.W. Overhauser,Adv.Phys.27, 343 (1978).(on theX-ray diffractometer)andit wasinvariably ii. R.M. Moon,Phys. Rev.Lett. 25, 527 (1970).