CFSE Cr mullite

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American Mineralogist, Volume77, pages251-257, 1992

Crystal-field spectroscopic tudy of Cr-dopedmullite

Ko IxpoaDepartmentof Advanced Materials Scienceand Engineering,YamaguchiUniversity, 755 Ube, Japan

flanrrrur ScnNnronnDLR Institute for Materials Research.D-5000 K<iln 90. Germany

MlslHror ArlslKA

Department of Geology, ShimaneUniversity, 690 Matsue,Japan

Hnr,vrur Rlcn'nDepartment of Geosciences, niversity of Marburg, D-3550 Marburg, Germany

Ansrru.cr

Unpolarized crystal-field spectrawere measured n the wavelength angeof 340 to 1540nm by reflection from mullite powders.The mullite sample contained 8. wo/oCrrO, andwas synthesizedby reaction sintering from AlrOr, SiOr, and CrrO, powders.The spectrawere converted to wavenumber scale and were interpreted by curve fittings, applying

Gaussian functions. The results yielded two pairs of absorption peaks. One of them isattributed to Cr3+ ons replacingoctahedralAl3+ ions. The other pair ofabsorption peaksis assigned o Cr3* ions occurringat interstitial octahedral sites.

INtnooucrroN

Mullite is considered o be a candidate for advancedceramics or use at high temperaturebecausemullite ma-trix materials exhibit low thermal expansion, very lowthermal conductivity, and excellent creep resistance.Mullite has the general ormula Al4+2,Si2 "O,0 , (0.2 s

x < 0.5) (Durovid, 1969; Cameron, 19771, chneider,1986) with the most frequently occurring compositionsat aboutx : 0.25(l mullite: 3Al,O3'2SiO,)andx : 0.40(l mullite: 2Al,O3.SiO,). Study of phaseequilibria yield-

ed a solid-solution field of mullite in the systemAlrO3-SiO, at high temperature Aramaki and Roy, 1962; Pask,1990; KIug et a l., 1990).Recently t was reported hatmullite can contain up to 84 wt0/oAlrOr, nearly corre-sponding o I mullite (Klug et al., 1990). Solid solution

between sillimanite (x : 0) and 3l mullite (x : 0.25),which was studied by Hariya et al. (1969),seems o bepossibleat elevatedpressureonly.

The averagemullite structure has spacegrotp Pbamand is characterizedby chainsofAl-O octahedra desig-

nated as Ml) and double chains of Al,Si-O tetrahedraextendingparallel to the c axis. The Ml octahedra arerelatively regular n shapewith a mean M-O distance of

l.9ll A. The tetrahedraare classifiednto twogroups:

one of them contains Sio* and Al3+ ions (designatedasM2). The other contains only Al3+ ons and is associatedwith O(C) O vacancies designatedas Al*). The Al* tet-rahedrahave somewhat onger mean M-O distances han

the M2 tetrahedra 1.803A insteadof 1J02 A, Saalfeldand Guse, 98l; Burnham,1963;SadanagataI.,1962).Ml octahedra and M2 and Al* tetrahedra have point

symmetries Crn(2/m), C,(m) ar;'d C"(n), respectively.Within the polyhedralchains O(C) O vacancies ccur that

ooo3404x 9210304-02 $02.00

balance he chargedeficiencydue to varying Al,Si ratios,

as seen n Figure l. These O(C) O vacanciesoccur on the

averageafter everyfifth O(C) atom for'zl mullite and after

every eighth for I mullite in the c direction. Becauseof

the presenceof O(C) vacancies the channels are dis-

turbed, leaving a vacancy.

Saalfeld 1979),McConnell and Heine (1985),Angel

and Prewitt (1987),and Morimoto (1990)demonstrated

that an incommensurate structurewith modulation of two

sets of ordering patterns with different symmetries oc-

curs. Both setshave chains of Ml octahedraparallel toc. In one case, he chains ofoctahedra are connectedby

tetrahedral double chainswith all tetrahedrabeing cross

linked by O(C) bridging atoms, as in sillimanite. In the

other case,each second etrahedrapair points away from

the double chain, producing Al* tetrahedra.This struc-

tural arrangement esembles hat of r alumina.

The mullite structure is able to incorporate consider-

able amounts of transition metal ions. The most impor-

tant species re Ti3+, Ti4+, V3+, V4+, Cr3+,Mn3*, and

Fer+ ons (Schneider,1990). n a recent study, solubility

of up to 12wto/oCrrO, hasbeen ound (Rageret al., 1990).

According to EPR spectroscopic tudies,Rageret al. sug-gested hat Cr3+ entersMl octahedraand octahedral n-

terstices n the channelsor O vacanciesoccurring in themullite structure.Crystal-fteldspectroscopic tudieswere

carried out in order to provide further information on the

distribution of Cr in the mullite structure.

ExpxnrNtnNr.lr-

Mullite sampleswere synthesizedby the reaction sin-

tering technique. Chemically pure a-AlrO, (54.5 wto/o),

25r



252

l q

Fig. . Schematicrawingof the crystal tructure f mullitein a viewdown he c axis.

a-SiO, (38.0 wt0/0), nd Cr2O3 7.5 wto/o)powders were

used for the syntheses.After grinding the starting pow-

ders with ethyl alcohol in an agatemortar, the mixture

wasannealedn a PtRh crucibleat 1650'C for l0 d. Thegrain boundary glasspresentafter the reaction sinteringprocesswas eachedwith an HF/HCI acid solution. Since

the microprobe studiesyielded no evidencefor the oc-

currenceof any glasswithin the mullite crystals, he ma-

terial can be considered o be single-phasemullite after

acid treatment. Electron microprobe analysisof the sam-ple

showed hat the mullitewas approximately of

%-typecomposition, containing 8.1 wto/oCrrO, and 0.1 wt0/oFe

as Fe3*(Table ).The acid-treatedmullite powdersare of greencolor and

were checkedprior to spectroscopicmeasurementswith

the polarizing microscope and by X-ray diffraction. No

impurity minerals or glasswere obseryed n the samples

so analyzed.Unpolarized crystal-fieldspectrawere measured n the

wavelength ange rom 340 nm (29400"--t)

to 1540nm

(6500 cm-') using a Hitachi 3235 automatic recording

spectrophotometer.A white plateof barium sulphatewas

usedas reference.Applying the Kubelka-Munk formula,

the reflectancespectra obtained were converted to ab-

sorption spectra on a wavenumber scalewith 100 cm'

intervals. Curve fittings were performed on a personal

computer, applying Gaussian functions. Spin-forbidden

energy evels were neglectedbecauseno tracesof these

levelswere observed n the spectrum.These evelsgen-

erally show line spectraof weak intensity for I6lCr3+ ndprobably do not appreciably nfluence he results.

IKEDA ET AL.: SPECTROSCOPYOF Cr-DOPED MULLITE

TABLE . Chemical omposition nd cell dimensions f mullite

Oxide comDonent AlrO3 Cr"O. FerO" SiO,

Composition wto/4 63.7 8. 1 0. 1 28.1

Molar ratioChemical ormula([email protected],.XAl,*,Si'-JzOsa"cel l dimensionsA) a:7.559(2) b:7.702(2\ c: 2.896(1)

* The x: O.231iCt and Fe are assumedas octahedraland trivalent.

centers.Although Cr may occur in severalvalencestates(seeRager et al., 1990), only the trivalent statewas con-

sideredbecause he spectragaveno indication ofCr ions

different rom Cr3+.

Electron configurationand energydiagrarn

As can be seen n Figure 2,three types of electron con-

figurations are associatedwith octahedrallycoordinated

and tetrahedrally coordinated trivalent Cr. Octahedralcoordination has only one type since here is no distinc-

tion betweenthe high spin stateand the low spin state,

whereas etrahedral coordination may have the high spinstateor the low spin state.The tetrahedrallycoordinated

high spin state is less stable becauseof smaller CFSE(crystal-field stabilization energy).The tetrahedrally co-

ordinated ow spin state s stableat low temperaturesand

becomesunstableat high temperaturesas a result of site

expansion (Ikeda and Yagi, 1982).The higher the tem-peraturethe more likely the I4rCr3+ ill be found in the

high spin state. However, this state is unstable and the

Cr3+ eventually becomeshighly stabilized n octahedral

sitesat high temperatures.Consequently, he spectramay

be analyzedassumingoctahedralcoordination.For octahedrally coordinated trivalent Cr there are three

spin-allowed transitions as shown in Figure 3. Usually

only the two lower onesare observed,whereas he thirdtransition often overlaps with charge ransfer bands lo-

cated n the ultraviolet region, as statedelsewhere.Some-

times, spin-forbidden transitions of very weak intensity

are observed.

The crystal-field spectrum of the Cr-doped mullite is

shown in Figure 4. Generally, octahedrallycoordinated

Cr3+ ons give rise to two main absorption bandsbelong-

ing to spin-allowed transitions, oTr" * aAr, and oT,, -aAr"of the F term. The experimental crystal-field spec-

t 2

2. 9 2. 0

tzg

t2

Rnsur,rs Oct.highspin Tet. ighspin Tet. ow spinCFSE' 2Ao 0 . 8A t l . 8A i

The assignments fabsorption bandshave beencarried

out assuming cubic site symmetry Or(m3m) for all Cr Fig.2. Electron onfigurationsf trivalentCr.

e9



IKEDA ET AL.: SPECTROSCOPYOF Cr-DOPED MULLITE 253

oTtn P)

4Trg

aTzg

o(,co€ o.goo.o

ot0.2

LE 0.1

o.1

8 to 12 14 t6 18 20 22 24 26 28Wovenumber,l000cm-l

Fig. 4. Crystal-fieldpectrum f chromiummullite and heresults fcurve fittings.The l-3 pair s assignedo channel c-tahedra,he2-4pair o M1 octahedra.he sumof theGaussianfunctions s shownalong he solid curve as dots.The broad

absorption f low-intensity entering t 10500 m-r is due o

thepresencef traces f octahedrallyoordinatede2+.

Curve fittings and assignments

Results of curve fittings by Gaussian unctions and as-

signmentsare shown in Figure 4 and Table 2. The first

IR overtone of absorbedHrO of low intensity observed

around 6900 cm-' was correctedby smoothing.Accord-

ing to Bealesand Day (1980), he first overtoneof OH in

glasss ocatedat 7300cm-r. However, he nfraredspec-

tra recorded or Cr-dopedmullite yielded no evidence or

the presence f OH bandsat 3650 cm-'.

Tailing effectson the base ine from the ultraviolet and

infrared regionswere approximatedby locating Gaussian

functions a|32900 and 3400 cm '. The correction from

the infrared regionwas negligible.Absorption due to im-

purity ferrous ions was fitted by one Gaussiancompo-

nent, though strictly speaking t should be fitted by two

Gaussiancomponents,especially or large mpurity levels

(Ikeda,unpublisheddata).However, a one-component it

is sufficient or the very small Fe content of mullite. The

mullite may have both ferric and ferrous ions, but the

peak for ferric ions that is expected o appearat approx-

Tmr-e2. Assignments f spectrum eaks o transitions nd

crystal-fieldarametersor chromiummullite

rransitiono'

tl;lrt"channel€ctahedra

25300

18850

22600

1s800

15800699

18960

2Trg

tEn

tAzg

Fig. 3. Schematicenergydiagram ofoctahedrally coordinat-

ed trivalent Cr.

trum consistsof three main bands, ncluding the middle

steplike shoulder of lower intensity, as seen n Figure 4.

If it is supposed hat the bands at approximately 16000

cm-' and 23500 cm-' belong to the two spin-allowed

transitionsof a single Cr3+center n mullite, the band at

approximately 19000 cm-r cannot be assigrred o that

center,even if it is a spin-forbidden band. The band at

approximately23500 cm ' is a double band, udgingfrom

the flatness and broadening of the peak. Peak splitting

and broadening caused by site distortions from cubic

symmetry are generallyconsidered o be approximately

1000cm-' , and even or Jahn-Teller pecies,or instance

octahedralCu2* ons, they are 2000 cm ' at most (Figgis,

1969).OctahedralCr3+ ons are not Jahn-Tellerspecies

in theground

state(Earnshaw,1970).Excited statescor-

respond to the Jahn-Teller efect (Kettle, 1972; Burns,

1972), but large peak splittings for Cr3+ ions resulting

from this effecthave never been ound; this is unlike Fe2*

ions, for which approximatesplitting of 5500 cm-r was

observed for a very distorted M4 site of amphibole(Goldman and Rossman, 1977).The observed splitting

magnitude of approximately 3000 cm-' thereforecannot

be due to a single site. We believe instead that two Cr3+

siteswith slightly different distortionsmust be taken into

account.

aTrq-

Tzs-1T."*

zJre-

,Q - oA,n

1oDqBc

CFSE

188506 1 8

22620



2s4 IKEDA ET AL.: SPECTROSCOPYOF Cr-DOPED MULLITE

TaBLE . Octahedral mean M-O distances of Cr-doped mullite

in comparisonwith those of Crjree mullite

Spectroscopicmean M-Odistance of Cr4oped mullite

Dro(3m) and C2(2), espectively.The assignment of theoTr"t oAr"transition of Cr3+ ons in kosmochlor given

by Ikeda and Ohashi (1974) s incorrect. Revised crystal-fieldparameters re 15640,645,and2630cm-'for l0Dq,B, and C, respectively Ikeda, unpublished data).

With respect o A and R values of the referencemate-

rials, the Cr3+ ons are most probably incorporated into

the Ml octahedra,replacing Al3+ ions (Table 2). Thisassignment s reasonableaccordingto the preferenceofCr3+ ons for octahedralcoordination.The Cr-containingMl octahedraare relatively regular n shape,havingpoint

symmetry Crn(2/m), though a slight distortion from cubic

symmetry On(m3m) especially n the dipyramidal base s

observed.The Ml-O distancescalculated rom spectro-scopicdata on the basisof data for spinel and kosmochlor

both fit well with observeddistances,with fitness ratios

of 100.2and 100.9010,espectively. hesevalues ndicate

a sligfit site expansionwith substitution, with slightly bet-ter agreementon the basisof data for spinel (Table 3).

Geometry of channelsRageret al. (1990)suggestedhat someof the Cr3* ons

in mullite are incorporated interstitially, possibly in the

structural channelsparallel to c. The frequent occurrence

of O(C)* atoms insteadof O(C) atoms may locally alter

the straight structural channels.The normal crosssection

ofuninterrupted channels s drawn in Figure 5a and des-ignated as N, whereas Figures 5b and 5c give possible

normal crosssectionsof channelsat the alteredpositions.

Obviously two kinds of crosssectionsoccur as a result of

the occurrenceof O(C)* atoms, larger cross sections D)

and smaller ones(S).The S cross sectionsare associatedwith Al* tetrahedra.

The structural channelscan also be viewed as bundles

of chains of tetrahedraparallel to c, as shown in Figure

5. The correspondingchains ofoctahedra are designatedas A (Figs. 5a-5c) and B (Figs. 5b and 5c) chains.There

are six kinds of channel octahedra, hree each of the A

and B type:l. Normal sizedoctahedrahaving displacementsof O

atoms in upper and lower vertices relative to ideal posi-

tions, designatedas A(N) and B(N) octahedra, orming

uninterrupted A and B chains, respectively, as seen nFigures5a and 6.

2. Larger octahedrahaving displacementsof O atomsin upper and lower vertices,designatedas A(D) and B(D)

octahedra, orming intemrpted A and B chainswith thenormal sized octahedra,respectively, as seen n Figures5c and 6.

3. Smaller octahedrahaving displacementsof O atomsof upper and lower vertices,designatedas A(S) and B(S)

octahedra, orming intem.rptedA and B chainswith thenormal sized octahedra,respectively, as seen n Figures5b and 6.

A and B octahedrahave significantly ditrerent shapesas shown in Figure 6.

Spectroscopically etermined mean M-O distancesof

1.982(Based n

spinel)

1.997A(Basedon

kosmochlor)Mean M-O

distanceofCr-free mullitefrom structure

refinement

Fitness atio in percentage

(Basedonspinel)

(Basedonkosmochlor)

A chain B chain(A) (A)

Octa-hedron

rypeA chain B chain A chain B chain

("/"1 (w (%) (%)

5

J

N

DD'M1

2.003 2.OO72.035 2.0722.061 2.1362j04 22112.143 2.286

1 . 9 1 1 4

99.0 98.897.4 95.796.2 92.894.2 89.692.5 86.7

100.2%

99.7 99.598.1 96.496.9 93.594.9 90.393.2 874

100.9%

A/otejSpectroscopicmean M-O distances were obtainedafter the fifthpower rule, Equation 1 in the text. Each fitness ratio was obtained bydividing he spectroscopicmean M-O distanceby the value rom the struc-ture refinement.Crystallographicmean M-O distances are based on dataof Saalfeld and Guse (1981).Centralpositionswere approximated n the

followingway for S and D octahedra: Midpointsof the baseplanediagonalswere connected. Then, the midpointsof this line and the top-bottom di-agonal were connected n order to locate the approximatecenter as themidpointof this final line(seealso Fig. 6).

imately 22000 cm I (Ikeda, unpublisheddata) s not de-tectable because fthe very low concentrationoftotal Feand superpositionof sucha peakwith the observedmajorabsorptionpeak.The standard deviation between he in-tegrated fitting curves and the observed spectrum was0.7o/o.

The Tanabe-Sugano ecular equations for 3d3 config-

uration wereapplied for energycalculations TanabeandSugano,1954a,1954b;Bates,1962).The results onfirmthe presenceoftwo sets ofbands, one having higher en-ergy han the other (18850cm-' insteadof 15800cm '

in lODq values).

DrscussroN

Coordination polyhedraof Cr

Mean M-O distances around octahedral trivalent Crions can be estimated applying the following formula(White et al., 197l):

NA ' : (R'Zn;' (l )

whereA and R denote l0Dq and the mean M-O distancefor a standard specimenand A' and R' denote those foran unknown specimen. Values for standardswere R :

1.926A (Smythand Bish, 1988) ndA : 18250A lSvir-idov et al., 1973) for Cr-doped spinel (MgAlrOo)and R: 2.001A (Cameron t al.. 1973)an d A : 15640 m- r(Ikeda and Ohashi, 1974) for kosmochlor (NaCrSirOu).They wereconsidered o be representativeofregular andirregular octahedra.The octahedral site symmetries are



IKEDA ET AL.: SPECTROSCOPYOF CT-DOPEDMULLITE 255

( a ) ( b ) ( c )

Fig.5. Cross ections f structural hannelsn a viewdownthe c axis. a)N cross ectionN: normal), b) S cross ection(S intemrptionof the structural hannels ue o thepresenceof O(C)* ormingAl* tetrahedron),c)D cross ectionD : localalteration f structural hannels ue o thepresencef O(C)*].

channel octahedra as obtained applying formula I are

listed in Table 3. Mean M-O distances calculatedfrom

data for the structureof Cr-freemullite (Saalfeld nd Guse,1981)are also isted. Comparisonbetween he spectro-

scopically determined mean M-O distances and thosefrom the structurerefinementyields various fitness atiosasgiven in Table 3 in percentages.Obviously, the resultsfor B(S) and especially of A(S) octahedra it best. How-ever, a considerablediscrepancy rom l00o/ostill exists.An improvement can be achievedby the presence f twoor more successiveO(C)* atoms in the mullite structure,

resulting n smaller octahedra A(S') and B(S') octahedra]and largeroctahedra A(D') and B(D') octahedra],all hav-ing rectangularbaseplanes (Fig. 6). It should be noted

that these octahedra do not exist in the normal mullitestructure, but the structure is flexible enough to accom-

modate heir topology. Spectroscopically alculatedmean

M-O distancesare in good accordancewith mean values

for A(S') and B(S') octahedraas determined from struc-ture refinement(Table 3). Slightly smaller fitness valuesindicate a small contraction of the channel siteswhen Cr

is incorporated. Kosmochlor-baseddata give better re-

sults, although the difference between those of kosmo-

chlor and spinel is slight.

Probablesite occupancies nd CFSE

Site occupancy distributions of Cr between Ml octa-hedra and channel octahedrahave been estimated fromintegrated ntensitiesof the two main bands, oTr,* oAr*)

* (oT,* * oAr"),assuming that transition probabilities

between the two sites are identical. Simple comparisonyields 40 and 600/o ccupancyof Ml octahedraand chan-nel octahedra, respectively. Ground-state trivalent Cr ionsare not Jahn-Tellerspecies, nd largesite distortions thataffect the transition probabilities significantly cannot beconsidered or the Cr positions in the mullite structure.That is, the predicted distortions in structure provide areasonableenvironment for Cr ions. As a consequence,the inferred relative occupanciesmay be accurate.

A largedifferenceof CFSEwas observedat room tem-perature or Cr3+occurring n Ml octahedraand channel

A-chqin A(N)

ffixffi-xB-chqin B(N) B(s) B(s') B(o) B(0,)

Fig.6. Symmetriesf channel ctahedra.(N) andB(N)oc-tahedra n the eft and he otheroctahedran the middle and

on theright.S' andD' octahedraequire ouplingwith S andDoctahedran order o form A andB chains.

octahedra Table 2), showinga CFSEgap of 3660 cm-t

for the two sites. f Cr3+ ons enter either A(S') octahedra

or B(S') octahedra, he observedCFSEpreferentialenergy

of Ml octahedracannotbe explained. t may be conclud-

ed that Cr3+ ons enter A(S') and B(S') octahedra.This

means hat the mullite structurecontains hreenonequiv-

alent octahedralCr3* sites, .e., Ml octahedra,A(S') oc-

tahedra,and B(S') octahedra. t is estimated hat there s

at most only a CFSEgapof 480 cm-' for the two non-

equivalent channel octahedra. Therefore, nearly equal

distribution of Cr3+ ons may occur between he two kindsof channel octahedra. f Cr ions are equally distributed

betweenA(S') octahedraand B(S') octahedra, he relative

occupancies fthe threenonequivalentoctahedra ate 40,

30, and 300/0,espectively.This is in accordancewith the

order of CFSEpreference.

In a study of Cr-bearing diopside of the CaMgSi,O.-

CaCrCrSiOu and CaMgSirOu-CaCrAlSiOu eries (Ikeda

and Yagi, 1977,1982),an averageCFSEgapofonly 700

cm-r was found for trivalent octahedralhigh spin Cr and

trivalent tetrahedral ow spin Cr. Accordingly, the 3660-

cm ' gap observed or the Ml octahedraand the channel

octahedra n Cr-bearingmullite is not too small. The oc-

tahedral CFSE preferenceenergyof 4340 cm-' lies be-

tween hoseof t6lcu2+ nd [alcu2+n CuAlrOospinel.Reed(1973')considered hat Madelung and short-rangeener-

gies reduce the magnitude of the octahedral preference

energy.The sameconsiderationmay be applicable o Cr-

doped mullite.Recently, Wojtowicz et al. (1988), Wojtowicz and

Lempicki (1988), Knutson et al. (1989),and Liu et al.(1990)carriedout spectroscopic tudieson Cr-containing(=0.05 wto/oCr) mullite glassceramics.The mullite glass



256

ceramicshad glassymatrices with chemical compositionsdifferent from those of the mullite crystallites, whereas

the present mullite sample is glassfree. The chemical

composition of thesematerials was also quite different,including BrO, and other components.Crystal-field spec-

tra were similar to those of this study. Knutson et al.(1989) found high-field and low-fie ld sites or trivalent

Cr. The former may correspond o the Ml site discussedin this study. However, we do not believe hat this shouldbe attributed to the occurrenceof Cr in the glass phase

alone. The low-field site may include the channel sitesdiscussed n this study, although thermal history andchemical composition of the samplesare different from

thoseof our sample.

Colqcr,usroNs

Trivalent Cr ions are incorporatedinto Ml octahedraof mullite, substituting for AI3*. Moreover, Cr3+may oc-cur in interstitial octahedral sites n the structural chan-nelsof mullite.

Comparison of mean M-O distancescalculatedon the

basisof spectroscopicmeasurementwith those calculatedon the basis of X-ray structure refinements revealed hepresence ftwo kinds ofvacant octahedra n the channelsthat are suitable for Cr3+ incorporat ion. These are theA(S') octahedraand B(S')octahedraof the structure.For-mation of such octahedra equires he presence f two ormore successiveO(C)* atoms.

The presence f Cr3+ n the Ml octahedraand in chan-nel octahedra can be distinguished by meansof crystal-field spectracurve fittings, resulting n two setsofspectra.One set, which is assigned o channel Cr3+ octahedra, sinferred to be due to superposition of two peaks.Thesespectra cannot easily be resolved becauseof the prox-

imity of absorption bands having, at most, only a differ-ence of 400-cm I lODq. Therefore, the analysis of crys-tal-field spectrawascarried out without distinction of thesuperimposedbands.

Consideration of CFSE between Ml octahedra andchannel octahedraalso supports he three-siteoccupationby Q1:+ ons. That is, if Cr3* substitutes n Ml octahedraand only one kind of channeloctahedron, the observedCFSE preferenceenergy of Ml octahedra cannot bemaintainedas 3660 cm '. Resolutionof this problem sprovidedby Cr3+ ons n threesites,.e., n Ml octahedra,A(S') octahedra,and B(S') octahedra.

Integated absorption intensities yield estimated Cr3+site occupancies f approximately 40, 30, and 300/on M I ,A(S'), and B(S') octahedra,respectively. Basedon datafrom structure refinement, the atomic coordinates of Crare (0,0,0), 0.20,0.51,0),nd (0.10,0.26,0)or th e Ml ,A(S'), and B(S') octahedra, espectively.

AcxNowlnlcMENTS

K.I. and M.A. express heir gatitude to K. Matsubara of Yamaguchi

University and Y. Matsunagaof Hokkaido University for useof the spec-

trophotometer acilities. Thanks are also due to T. Koyanagi or t he mea-

surement ofspectra and to T. Sakao or typing the manuscript. The pre-

sentstudywas inancially supportedby grant0264063 from the Ministry

IKEDA ET AL.: SPECTROSCOPYOF Cr-DOPED MULLITE

ofEducation ofthe Japanese overnment.H.S. and H.R. thank H. Krause

for technical assistanceand the German ResearchFoundation (DFG) for

financial support.

RarnnrNcss crrED

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Merq,rcrrm RrcErvEDMnncn 21, l99l

MeNuscnrrr AccEmEDOcronen 14, 1991

IKEDA ET AL.: SPECTROSCOPYOF CT-DOPEDMULLITE

CFSE Cr mullite

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