American Mineralogist, Volume 79, pages 622-632, 1994

X-ray absorption spectroscopy of silicon dioxide (SiOr) polymorphs:The structural characterization of opal

DrrN Lr, G. M. Bmcnorr, M. K,lsRArDepartment of Chemistry, University of Western Ontario, London, Ontario N6A 587, Canada

M. E. Flrrr, R. A. SnccoDepartment of Earth Sciences, University of Western Ontario, I-ondon, Ontario N6A 587, Canada

X. H. FrNc. K. H. TlN. B. X. Y.lnc*Canadian Synchrotron Radiation Facility, Synchrotron Radiation Center,

Universitv of Wisconsin. Madison. Wisconsin 53589. U.S.A.

Ansrntcr

SiK- and Sil-edge X-ray absorption spectra obtained using synchrotron radiation arereported for 6:3-coordinated stishovite and 4:2-coordinated a quartz, a cristobalite, coe-site, amorphous silica (a-SiOr), and opal. The SiK and SiI near-edge features are inter-preted on the basis of a qualitative MO model of SiOX and SCF-Xa calculation of modelmolecules. Some edge features are attributed to the multiple scattering effect of the moredistant shell atoms in the crystal structure. The K- and l-edge features reflect the maxi-mum densities of unoccupied Si 3s, 3p, and 3d states in the conduction band and arequalitatively in agreement with calculated densities of states. Comparison of SiK- and SiL-edge XANES demonstrates the bond mixing of Si 3p and 3s orbitals and of Si 3p and 3dorbitals. Also, for 4:2-coordinated silica, the transition of Si 2p electrons to the t2 state ofhigh Si 3p character becomes dipole allowed. For stishovite and coesite, states dominatedby Si 3s apparently have a large amount of Si 3p orbital character, probably because ofpressure-induced mixing of Si 3s and 3p orbitals. The SiK- and Sil-edge shifts are system-atically related to the coordination number of Si atoms, Si-O bond length, Si-Si distance,Si-O-Si angle, Si-O bond valence, and Si NMR chemical shift of SiO, polymorphs. TheSiK- and SiZ-edge XANES indicate that the local structure of two opals investigated is amixture of a-SiO, and a cristobalite structural units, and the relative proportions of thetwo structural components are semiquantitatively determined. EXAFS structure param-eters (bond distances, coordination number, and Debye-Waller factor) of quartz and sti-shovite are obtained and shown to be in good agreement with the X-ray structures. Si insixfold and fourfold coordination can be distinguished unambiguously from SiK- and Sil-edge XANES features and SiK-edge EXAFS analysis. These results are very useful forcharacterizing the structure and bonding of the mantle silicates and silicate glasses.

Ixrnoouc'rroxExtensive studies on phase transformations of Mg- and

Fe-bearing silicates at high temperature and high pressurehave been conducted to understand the mineralogical andpetrological relations and seismological discontinuities ofthe Earth's mantle. The phase transformations in MgSiO.and MgrSiOo, two of the most important silicate com-positions, have been established as follows (Liu, 1975):with increasing pressure, the sequence of transformationsis enstatite (MgSiO.) to B spinel and stishovite, to z spineland stishovite, to ilmenite, and finally to perovskitephases, whereas forsterite (MgrSiOo) transforms to 0 spi-nel, to 7 spinel, and finally to perovskite and periclase.

* Present address: Advanced Photon Sources, Argonne Na-tional l-aboratory, Argonne, Illinois 60439-4814, U.S.A.

0003-004x/94l0708-0622$02.00

The densification of silicates at high pressure is causedby more efficient packing of anions or an increase in thecoordination number of cations (or both).

There are many polymorphic modifications of silicondioxide (SiOr) at high temperature and high pressure, in-cluding 4 :2-coordinated structures (u qtartz, cristobalite,tridymite, and coesite) and a 6:3-coordinated structure(stishovite). Stishovite was initially synthesized at highpressure above 100 kbar (Stishov and Popova, 1961) andsubsequently discovered in an impact breccia from Me-teorite Crater, Arizona (Chao et al., 1962). Stishovite hasa rutile-type structure (Sinclair and Ringwood, 1978). Thedetailed crystal chemistry of stishovite was described byHill et al. (1983) and Ross et al. (1990). The electronicstructures of a quartz, B cristobalite, and stishovite havebeen calculated using various approaches. The calculatedvalence band structures were used to interpret reasonably

622

LI ET AL.: XAS STUDY OF SiO" POLYMORPHS 623

the experimental photoelectron spectra (XPS and UPS)and X-ray emission spectra (XES). The recent referenceson these calculations have been listed in Li et al. (1993).Wiech and Kurmaev (1985) also studied the electronicstructure of crystalline and vitreous SiO, using SiK XES.

The valence band (VB) structure and bonding of aquartz, B cristobalite, and stishovite are well understoodfrom experimental XPS and XES, as well as from theo-retical calculations. X-ray absorption near-edge structure(XANES) can provide important information on the un-occupied electronic states of solids. The Silr..-edgeXANES of a qttartz was first reported by Brown et al.(1977). The SiK- and Sil,r.r-edge XES and X-ray absorp-tion spectra (XAS) of stishovite and a quartz have beenstudied comparatively (Iguchi, 1977; Brytov et al., 1979).Recently, Davoli et al. (1992) also studied the Si-rK-edgeand OK-edge XANES of densified a-SiO, at high pres-sure. However, the assignments for the XANES spectraof both a qluar1':z and stishovite were in conflict, and theconduction band (CB) structure and unoccupied densitiesof states of SiO, are not fully understood, mainly becauseof theoretical and experimental limitations. We recentlyreported the high-resolution SiK- and SiZ-edge XANESof a quarlz and 6:3-coordinated stishovite using synchro-tron radiation (Li et al.,1993).

In this paper, we present high-resolution SiK- and SiI-edge XANES and SiK-edge EXAFS of 6:3-coordinated sti-shovite and 4:2-coordinated a quafiz, a cristobalite, coesite,amorphous SiOr, and opals and use the spectra to infer theunoccupied electronic states of these forms of silica (SiOr).In addition, we explore the relation of the XANES spectrato the crystal structures of SiO, polymorphs and discussimportant new applications of XANES and EXAFS to studyamorphous silicate materials.

ExpnnrunxrAr- METHoDS

Natural and synthetic stishovite samples were provid-ed by Lin-gun Liu, Australian National University. Thenatural stishovite was separated from sandstone of theMeteorite Crater, Arizona, by German researchers (Liu,1993 personal communication). The synthetic stishovitewas obtained by allowing coesite and graphite to react at120 kbar. Natural single-crystal ot quartz and the two opalsamples were from the mineral collection in the Depart-ment of Earth Sciences, University of Western Ontario(UWO). Two cristobalite samples were synthesized atroom pressure and 1600 "C, and the coesite sample wassynthesized at 1000 "C and 50 kbar using the 1000-t cu-bic-anvil press at UWO. All samples were checked byX-ray powder diffraction (XRD) before X-ray absorptionspectra (XAS) measurements were obtained. Both sam-ples of opal were highly disordered and nearly amor-phous. Opal no. 1232was a precious opal from Queens-land, Australia, and has an XRD pattern similar to sampleH (Jones and Segnit, l97l). Opal no. 800 was from Vir-gin, Nevada, and had an XRD pattern intermediate be-tween that of samples G and F (Jones and Segnit, l97l).

The K-edge XAS were measured using a double-crystal

monochromator (DCM) with synchrotron radiation at thechamber pressure of l0-6 torr. The DCM employs InSb(l I l) as the monochromator crystals. The spectra of thefine powder samples were recorded using both total elec-tron yield (TEY) and fluorescence modes. The energy res-olution for the DCM is about 0.8 eV at 1840 eV so thatthe line width for the SiK-edge of a quartz is as small as1.7 eV. The K-edge XANES spectra were obtained at aninterval of 0.2 eV, with 2 s for each data point; the EX-AFS data were collected at an interval of 0.3-4 eV, with2-5 s per data point from 1900 to 2800 eV, so that thedata have the same interval in k space after Fourier trans-form. The L-edge XANES were collected at the chamberpressure of l0-8 torr with a Grasshopper monochromatorby TEY, which has an energy resolution of about 0. I eVat 100 eV. The DCM and Grasshopper beam lines areaffiliated with the Canadian Synchrotron Radiation Fa-cility (CSRF) and accommodated on the Aladdin storagering operating at either 800 MeV, with current from 50to 180 mA, or I GeV, with current from 40 to 80 mA,in the Synchrotron Radiation Center (SRC), Universityof Wisconsin. The designs and performances were de-scribed for the DCM by Yang et al. (1992) and for theGrasshopper beam line by Bancroft (1992).

Rrsur.rs AND DrscussroN

Interpretation of XANES spectra

Figure I shows the SiK- and SiZ,.r-edge XANES of aquartz and stishovite, along with the calculated unoccu-pied density of state (Nucho and Madhukar, 1980; Li andChing, I 98 5). There are no significant differences betweenour TEY and fluorescence spectra, but all present obser-vations and interpretation are based on the TEY spectra.The lr.-edge spectra are aligned using the XPS Si 2pbinding energy (BE) of d q\artz aI 103.4 eV (Wagner etal., 1982), and the SiK-edge spectra are correlated withthe Sil,r.r-edge spectra by taking the SiKa, X-ray emis-sion line of a quartz at 1740.0 eV. The peak positionsare summarized in Table 1, in which AE is the differencebetween the XANES resonance and the corresponding Si1s or 2p BE ofa quartz. The AE value for the correspond-ing peaks A-G in the l- and K-edge spectra of both min-erals are very similar, although the relative intensitiesdiffer considerably. The slight AE differences ofthe cor-responding L- and K-edge spectra features are probablyattributable to differences in Z and K core-hole relaxa-tion. The calculated density of state features also corre-spond reasonably well in most cases with the Z- andK-edge features (Fig. 1), although, again, relative inten-sities differ considerably.

XANES features involve complicated processes and arenot fully understood theoretically. For molecules, the in-ner-potential barrier model (Dehmer, 19721' Hudson etal., 1993) and multiple scattering Xa calculation (Tse etal., 1989) have been used successfully to interpret themajority of features in the XANES spectra of many mol-ecules. For semiconductors and insulators, calculated

624 LI ET AL.: XAS STUDY OF SiO, POLYMORPHS

an+J

tr

Frtrdt{

P

!ri

oP

l.oo3

- 1 0 0 1 0 2 0Energy (eV)

Fig. l. High-resolution Si1(- and SiZ,,.,-edge XANES of 4:2-coordinated d quartz and 6:3-coordinated stishovite. The cal-culated unoccupied densities of states are cited from Nucho andMadhukar (1980) for d quartz and from Li and Ching (1985) forstishovite. The Si 2p XPS BE of a quartz is 103.4 eV (Wagneret al., 1982), and the Si ls BE is calculated by the sum ofthe Si2p XPS BE and Sir(a, XES at l74O.O eV. The SiK- and SiI,.,-edge XANES spectra are aligned to zero using the Si ls and 2pBE, respectively.

densities of states within the energy band theory (Brownet al., 1977), the multiple scattering approach (Natoli andBenfatto, 1986), the one-electron approximation molec-ular orbital diagram (Iguchi, 1977; Brytov et al., 1979;Kisiel et al., 1989), and a quasi-molecular method (Fi-latova et al., 1985) have been proposed to interpret thenear-edge features. Lately, we have successfully inter-preted qualitatively the SiK- and Sil-edge features of4:2-coordinated a quartz on the basis of the SiK- andSiZ-edge XANES, as well as SCF-Xa calculations of mo-lecular model compounds (Sutherland et al., 1993).Therefore, for semiconducting and insulating solids, thestrong near-edge features, like the strong line structuresin the XAS of molecules, correspond to the transitions ofinner-shell electrons to unoccupied states in the CB. Ifone assumes Z-S coupling in absorber atoms, these tran-sitions are also governed by the dipole selection rules, Ar: + 1 . A s : 0 . a n d A J : + 1 .

The SiZ-edge XANES (Iguchi, 1977) and, both SiK-and SiZ-edge XANES (Brytov et al., 1979) of ot quartzand stishovite were reported and interpreted on the basisof the MO calculations of SiOl and SiO! clusters. Re-cently, Azizan et al. (1987) used the MO calculation ofthe SioX-cluster to interpret the direct and inverse pho-toemission spectra of crystalline and amorphous SiO, andto study the electronic structures of these materials. Inthe ground state of the SiOl- moleculelike cluster havingTo point symmetry, the s orbitals of the Si atom trans-form to an ar representation, the p orbitals transform toa t2 representation, and the d orbitals split into triplydegenerate t, and doubly degenerate e representations.The transitions of Si ls electrons to these unoccupiedstates are allowed by electric dipole selections (Iguchi,1977; Hansen et al., 1992\.

For 4:2-coordinated d qvartz, the Si XANES spectraare characteristic ofa tetrahedral SiOo cluster. Indeed, themain peaks, A, C, E, and G, in both K- and L-edgeXANES are aligned well and can be assigned generallyfrom the gas phase spectra of the tetrahedral analoguesS(OCH3)" (Sutherland et al., 1993), SiHo friedrich et al.,1979), and SiF" (Ferrett et al., 1988). Based on the mo-lecular orbital approach mentioned above (Tossell, 1975aIguchi, 1977; Azizan et al., 1987), peak A in the Z-edgeXANES corresponds to a transition of Si 2p electrons tothe antibonding a, state (Si 3s-like state). The splitting ofthis peak by about 0.6 eV is attributed to the spin-orbitalinteraction of Si 2p orbitals. O'Brien et al. (1991) deter-mined the CB minimum of d quartz at 106.73 eV on thebasis of the SiZr 3 XES at 97 .3 eV and the known opticalband gap of 9.1 eV (Ravindra and Narayan, 1987).Therefore, peak A is also called a core exciton below theCB minimum of a quartz (Bianconi, 1979). Peak A inthe K-edge XANES is weak because the transition of Sils - Si 3s-like state is dipole forbidden. Peak C in theSiK-edge spectrum is attributed to the dipole-allowedtransition ofSi ls electrons to the antibonding t, state ofmajority Si 3p character, and peak C in the l-edge spec-tra to the transition ofSi 2p electrons to the t2 state. Thehigh intensity of peak C in the l-edge spectmm showsthat even though the peak corresponds to a Si 2p - pJiketransition, ? t, - t, transition is, in fact, dipole allowed(Iguchi, 1977; Hansen etal.,1992). This is also consistentwith the acentric tetrahedral symmetry and the stronghybridization ofthe Si 3s, 3d, and 3p orbitals in a quartz.Peaks E and G in the l-edge XANES are assigned to theempty Si 3d states (Liu et al., 1992), also called shaperesonances (Bianconi, 1979). ln the To crystal field, the3d orbitals are split into e and t2 states, and the e statesare favored in energy over t2. Peak E is assigned to tran-sitions to the e states, and peak G to the t, states. The Si3d states in a quartz also have contributions from Si pelectron states, which are responsible for peaks E and Gin the SiK-edge XANES of a quartz.

Peaks D and F in both K- and Z-edge spectra of aquartz correspond with peaks in the density of state.However, these two peaks are not present in both SiK-and Sil-edge XANES of a-SiO,, the theoretical calcula-

LI ET AL.: XAS STUDY OF SiO, POLYMORPHS

TaBLE 1. SiK- and Sit..-edge XANES of SiO, polymorphs

625

K-eclge (eV) ,z.3-edge (eV)

Assignments" Assignments-'

A

cDEFG

A

cDEFG

A

cDEFu

A

EG

A

cEG

1844.9

1846.81850.71854.51857.41864.3

1844.9

1846.741849.91852.61856.51863.6

1843.6

1846.8

1851 .11855.01858.41864.0

1844.9

1846.91853.41863.8

1844.9

1846.81852.71863.6

1 . 5

3.47.3

12.114.020.9

1 . 5

3.36.59.2

13.120.2

0.2

3.4

7.71 1 . 615.020.6

1 . 5

3.510.020.4

5.67.9

10.016.823.0

1 . 5

3.49.3

20.2

a qualtz

Si 1s + a, (Si 3s3p)

Si 1s - t, (Si 3p3s)multiple scatteringS i l s - e ( S i 3 d - 3 p )multiple scatteringSi ls + t, (Si 3d3p)

Cristobalite

Si ls + a, (Si 3s-3p)

Si 1s + t, (Si 3tr3s)multiple scatteringSi 1s + e (Si 3d3p)multiple scatteringSi 1s - t, (Si 3d-3p)

CoesiteSi ls + a, (Si 3s-3p)

Si 1s + t, (Si 3tr3s)

multiple scatteringSi ls + e (Si 3d-3p)multiple scatteringSi 1s + t2 (Si 3d3p)

a-SiO"

Si 1s + a1 (Si 3s-3p)

Si 1s + t, (Si 3tr3s)Si 1s + e (Si 3d€p)Si 1s - t, (Si 3d-3p)

OPal

Si ls + a, (Si 3s3p)

Si 1s - t, (Si 3p3s)Si 1s + e (Si 3d-3p)Si 1s + t2 (Si 3d-3p)

Stishovite

Si 1s * a1s (Si 3s-3p)

Si 1s - t1, (Si 3p.3s)multiple scatteringSi ls + t,e (Si 3d-3p)multiple scatteringSi 1s + ee (Si 3d3p)

105.4106.0107.81 1 1 . 3114 .5118 .5125.0

105.6106.2108.01 1 1 . 31't4.2115 .9127.8

105.9106.5107.81 1 1 . 3114 .3116 .5125.9

104.8105.4107.7114 .81 3 1 . 1

105.5106.2108.11 1 5 . 1130.4

106.0106.8109.8

113 .6120.7126.4

2.02.64.47.9

1 1 . 115.121.6

2.22.84.68.3

10.812.521.2

2.53 .14.47.9

10.913.122.5

1 .42.24.3

1 1 . 327.3

2.12.84.7

1 1 . 727.O

Si 2P* - a' (Si 3s3P)Si 2Po - a' (Si 3s3P)Si 2P - 1" (Si 3P€s)multiple scatteringSi 2P - g (Si 3d3P)multiple scatteringSi 2P - 1, (Si 3d3P)

Si 2P,. - al (Si 3s3P)Si 2Po - a1 (Si 3s€P)Si 2P - 1" 15; 3O*",multiple scatteringSi 2P - g (Si 3d3P)multiple scatteringSi 2P - 1. (Si 3dAP)

Si 2P* - a' (Si 3s3P)Si 2Pn * a' (Si 3s€P)Si 2P - 1, (Si 3'is)multiple scatteringSi 2P - s (Si 3d€P)multiple scatteringSi 2P + 1. (Si 3d3P)

Si 2P* - a' (Si 3s3P)Si 2P' - a' (Si 3s3P)Si 2P - 1, (Si 3tr3s)Si 2P - s (Si 3d3P)Si 2P - 1" (Si 3d3P)

Si 2P" - a' (Si 3s-3P)Si 2P, - al (Si 3s-3P)Si 2P + 1. (Si 3P-3s)Si 2P + s (Si 3d-3P)Si 2P - t" 15; 36-30,

A 1846.3 2.92.6 Si 2Po - alc (Si 3s-3P)3.4 Si 2Po - a,s (Si 3s-3P)6.4 Si 2P - 1'" (Si 3tr3s)c 1849.0

D 1851.3E 1853.4F 1860.2G 1866.4

Si 2P - 1- (Si 3d-3P)multiple scatteringSi 2P - ee (Si 3d€P)

. The XPS Si 2p BE of c quartz is 103.4 eV, and the SiKa, X-ray emission energy is 1740.0 eV, so that the Si 1s BE is calculated to be 1843.4 eV.The AEis the energy difference between the resonances and the Si 1s BE for the SiK€dge XANES and between the resonances and the Si 2p BEfor the Silz3-edge XANES.

t. The maiority character of the conduction band states is given first.

10.217.323.0

tions of gaseous S(OCH3)" and Si(CH3)4 (Sutherland etaI., 1993), and the SiZ-edge XANES of SiH4 friedrichet al., 1979) and SiF4 (Ferrett et al., 1988). On the otherhand, McComb et al. (1992) have done multiple-scatter-ing calculations (MS) for zircon (ZrSiO.). The results in-dicated that with increasing the size of the cluster, al-though s- and d-like final states are essentially localizedin the first coordination shell, the p-like DOS is modifiedby outer shells. When the MS calculation was made forthe SiOl-cluster, the calculated spectrum is similar tothat of a-Sior. When up to seven shells are included inthe MS calculation, the SiK-edge electron energy loss near-edge structure (ELNES) is reproduced. The peaks, be-cause of the MS effect, which is absent in the SiOf- spec-trum, correspond well to peaks D and F in our Si_iK-edge

XANES spectrum of a quartz. From this argument, peaksD and F are probably related to the extended energy bandstructure or the MS of the more distant shell atoms inthe crystal structure. Davoli et al. (1992) reported theSiK-edge spectra of densified vitreous silica; however, theirspectra had lower signal to noise ratios, and their MScalculations are not in their experimental results or theirconclusions. More recently, Bart et al. (1993) interpretedpeaks D, E, F, and G in the SiK-edge spectrum of a quartzas the MS from the more distant atoms using Natoli'smodel (Natoli, 1984), but they did not even include thefirst Si-O shell in the AE vs. l,/R2 correlation. Actually,we tried to use this model, but it does not work for ourspectra of d qtrartz and many other silicate minerals.

The very different spectra for 6:3-coordinated stisho-

Si 1s BE

A C D

f--"\

f

626 LI ET AL.: XAS STUDY OF SiO. POLYMORPHS

m+)

p

htrdtr

+)

ti

H

+)

trom

1E40 1850 1860 1A70

Energy (eV)

Fig. 2. SiK-edge XANES spectra of 6:3-coordinated stisho-vite and 4:2-coordinated a quartz, a cristobalite, coesite, amor-phous SiOr, and opal. The Si ls BE is indicated by an arrow,and the peaks are labeled as in Fig. l. The features above theedge (peak C) have been enhanced.

vite can be interpreted qualitatively with the help of theSiK- and SiZ-edge spectra of SiF6 (Ferrett et al., 1986;Hudson et al., 1993) and the MO calculation of theSiO[- cluster (Tossell, 1975b1, Iguchi, 1977). In octahe-dral symmetry, the s state transforms to an ars represen-tation, the p state transforms to a tr, representation, andthe d state is split into tr, and e, orbitals. Therefore, peakA in the L-edge spectrum is due to the transition of Si2p electrons to the antibonding a,, orbital (Si 3sJike state).The splitting of peak A is apparently larger than the Si2p spin-orbit splitting of 0.6 eV, but this splitting mustbe mainly due to the Si 2p spin-orbit interaction. Peak Ain the K-edge spectrum is relatively weak because thetransition of Si ls - 3s is not dipole allowed. Peak C isattributed to the transition ofSi ls electrons to the anti-bonding tr" orbital (Si 3plike state). This transition is notformally allowed in the l-edge spectrum, and so peak Cis very weak in the l,-edge spectrum, as in the corre-sponding spectrum of SFu (Hudson et al., 1993). In ad-dition, the hybridization of Si 3s, 3d, and Si 3p orbitalsin stishovite is much weaker than that in a quartz, be-

cause of the octahedral crystal field in stishovite. PeaksE and G in the Z-edge spectrum can be assigned readilyto the d-like shape resonance of tr" and es character, re-spectively. The energy separation ofthe two Si 3d bandsin stishovite is larger than in a quartz, as required for thepresent assignments, but it does not obey the rule A,., :-o/nL"., (Liu et al., 1992). The empty Si 3d bands are alsomixed with some Si p-like states, which are responsiblefor the weak peaks E and G in the K-edge spectrum ofstishovite. The origins of peak D in the K-edge XANESand peak F in the K- and L-edge spectrum of stishovitemay be related also to the extended energy band structureor MS effects, as discussed for a quartz.

The qualitative assignments of the near SiK- and SiL-edge spectra of a qlartz and stishovite are summarizedin Table l. These assignments are reasonably consistentand also in reasonable agreement with calculated unoc-cupied density of state (see Fig. l). Unfortunately, thedensity of state calculations gave neither the s, p, or dcharacter of these unoccupied states nor the probabilityof transition to these states (Nucho and Madhukar, 1980;Li and Ching, 1985), and it is not possible to relate theexperimental and theoretical intensities even semiquali-tatively. We can, however, give a qualitative and reason-able assignment that gives satisfactory relative intensities.

XANES spectra and crystal chemistry

Figure 2 shows the SiK-edge XANES spectra of cris-tobalite, coesite, a-SiOr, and opals, as well as ot qvartzand stishovite, in which the Si I s BE for a quartz is in-dicated by an arrow at the top left. The SiK-edge spec-trum of a-SiO, is in agreement with the results of Davoliet al. (1992). The peak positions and the assignments arealso summarized in Table l. It is immediately apparentthat the XANES spectrum of the 6:3-coordinated sti-shovite is very different from those of4:2-coordinated aquartz, cristobalite, and coesite. In 4:2-coordinated SiOr,the strongest peak, C, at about 1846.8 eV, characterizesthe t4lsi with O in the SiK-edge XANES, and peak A, dueto the dipole-forbidden transition ofSi ls - Si 3s, is veryweak. However, peak C in the stishovite K-edge spectrumis shifted by about 2.2 eY to higher energy compared withpeak C in 4:2-coordinated SiOr, and peak A is signifi-cantly intensified, even though the transition ofSi ls -Si 3s is also dipole forbidden in the octahedral field. PeaksF and G also shift to higher energy (Fig. 2). Therefore,on a simple fingerprint basis, it is apparent that SiK-edgeXANES spectra should be very useful for distinguishingIarSi and t6lSi in silicate minerals and glasses.

For 4:2-coordinated cristobalite and coesite, the gen-eral spectral profiles are similar to that of a quartz. Thefour main peaks, A, C, E, and G, are assigrred to thetransitions ofSi ls electrons to a, (Si 3slike state), t, (Si3p-like state), e, and t, (Si 3dlike state) states in tetra-hedral symmetry. However, we also can observe someapparent changes in the SiK-edge spectra of these 4:2-coordinated SiOr. First, peak A, because of a dipole-for-

LI ET AL.: XAS STUDY OF SiO, POLYMORPHS 627

bidden Si ls * 3s-like transition, is expected to be weakin the K-edge spectra, as observed for most 4:2-coordi-nated SiO, materials. However, for coesite, peak A issignificantly more intense and shifts toward lower energy.The energy separation between peaks A and C for coesiteis about 3.2 eY, similar to that for stishovite. The relativeintensity of peak A for coesite is much greater than thatfor a quartz and cristobalite but smaller than that forstishovite. This indicates that the Si 3s state below theCB minimum is mixed with more of the Si 3p state inthe high-pressure structures. Second, peaks E and G inthe cristobalite spectrum shift to lower energy, but theenergy separation for these two peaks remains similar tothat for d quarlz and coesite. Peaks D and F, because ofthe MS effect, also shift to lower energy and become weakcompared with the corresponding peaks for a quartz andcoesite. These apparent differences in the postedge regionof a cristobalite from those of a quartz and coesite mustbe related to the crystal structures.

Figure 3 shows the SiZ-edge XANES of SiO, poly-morphs, a-SiO, and opals, and the Si 2p BE for a quartzis also indicated by an arrow. The peaks are labeled as inFigure 1. The basic spectral features for a quartz, cristo-balite, and coesite are qualitatively similar to each otherbut differ from those of stishovite. The Sil-edge (peakA) for stishovite also shifts to higher energy by about IeV compared with the torSiO, materials. The Sit-edgespectra can also be used as a structural fingerprint to dis-tinguish tarSi and r6tSi in silicate minerals and glasses. Forthe 4:2-coordinated SiO, materials, the strong peak, C, at107 .9 + 0. I eV, characterizes the t4rsi atoms in the SiZ-edge XANES. However, peak A, an exciton below the CBminimum, appears to shift toward higher energy in thesequence from a quartz to cristobalite to coesite. Thisslight shift for peak A is related to the local structure ofthe Si atoms and is also qualitatively in agreement withvarious crystal chemical parameters, e.g., Si-O bondlength, Si-Si distance, Si-O bond valence, and MAS NMRchemical shifts (also see Table 2). As we noted above,comparison with the Sil-edge spectrum of gaseousS(OCH3)" (Sutherland et al., 1993), SiHo Griedrich et al.,1979), and SiFo (Fenert et al., 1988) suggests that rhe SiZ-edge absorption spectra of various SiO, materials are

1 0 0 1 1 0 1 2 0 1 3 0 1 4 0

Energy (eV)

Fig. 3. SiZ-edge XANES spectra of 6:3-coordinated stishov-ite and 4:2-coordinated d qrtartz, a cristobalite, coesite, amor-phous SiO., and opal. The Si 2b BE is indicated by an arrow,and the peaks are labeled as in Fig. l.

dominated by the tatSiO! cluster. Hence, the presently re-solved differences among the spectra of a quartz, a cris-tobalite, and coesite, and also among the spectra betweencrystalline and amorphous SiO. materials, likely repre-sent contributions from beyond the first shell of Si; inother words, the extra features reflect the medium-rangestructure of a quartz, a cristobalite, and coesite.

a+)

XFldh

+)

tr

.1

+)

Fr

o

Tlale 2. SiK- and Sit,"-edge XANES and the crystal chemistry of SiO, polymorphs

c quanz a cristobalite Stishovite

AEfor peak C in K-edge- (eV)AEfor peak A in t23-edge'(eV)Si:O coordination numbersi-o (A)

si-si (A)sLo-si f)Si-O bond valence (s)MAS NMR 6 (ppm)

3.06144.O

4.23- 107.1

3.07146.8

4.29- 108.5

3.42.84:21 .61*

3.09'.150.8'-

4.22-110.0*

5.63.06:31.76 ( x 2)1 .81 (x 4 )3.24

130.73.82

- 1 9 1 . 1

3.32.54:21.605

3.42.34 :21 .61

. The AE is the difference between the energy for peak C and Si 1s BE for SiK-edge XANES and between the energy for peak A and Si 2p BE forSiI23-edge XANES.

" There are two Si sites in coesite, but the XANES has no capability to distinguish them. These data are the average of two different Si sites.

628 LI ET AL.: XAS STUDY OF SiO, POLYMORPHS

a+). d

4H

xtr(6ti

. H

.oFr

co

+)gtiov)

Siff-edge

D

E

A

I

A

I

Itl.I

I

I

'rI

F

20%

istobalite

20% a-60%

40% a-SiO.

ao%60% a-SiO,

8O% a-SiO,

a-SiO.

18J0 1840 1850 1860 1870

Energy (eV)

Fig. 4. SiK-edge XANES spectra of cristobalite and a-SiO,,together with the composite spectra for different proportions ofcristobalite and a-SiOr. The spectra of cristobalite and a-SiO,were collected at similar experimental conditions and normal-ized by 1/10. A linear background has been removed, and peakC is normalized to the same height in all the spectra. Peak Ebecomes more intense, as marked by a solid triangle, with anincrease in the proportion ofcristobalite.

For a-SiOr, the four main peaks, A, C, E, and F, char-acterizingtorSi, are also present in both SiK- and Sil-edgeXANES; even peak E in the l-edge spectmm becomesbroader and weaker. This confirms that the local struc-ture of a-SiO, has tetrahedral symmetry, as in the crys-talline 4:2-coordinated modifications. This is, of course,well known from the early diffraction study of a-SiO, byWong and Angell (1976). On the other hand, peaks Dand F are absent from both SiK- and L-edge XANES.This confirms the assignments of peaks D and F to theMS effect related to intermediate range structure becausethe MS effect from the more distant shell atoms arestrongly quenched by the structural disorder.

The energy position (A-E) for peak C in the K-edgeXANES and peak A in the L-edge XANES and somecrystal chemical parameters of the 4:2-coordinated SiO,materials and 6:3-coordinated stishovite are summarizedin Table 2. In this table, AE is defined as in Table l, andthe bond valence, s, is calculated according to the formulas: exp[-(R - R)/Bl, where R is the bond length, andRo and B are fitted constants for Si-O bonds (Brown,l98l). The shift in the SiK-edge toward higher energy instishovite is caused by the different effective charge onthe Si atoms, which is related to a change of coordinationnumber of Si, from 4 in all 4:2-coordinated SiO, mate-rials to 6 in stishovite. This is also in agreement with theSi Ka X-ray emission shift and MO calculation (Okuraet al., 1990). However, the shift in the SiK-edge XANESis much more significant than that in SiKc X-ray emis-sion. Therefore, the SiK-edge XANES is a more sensitivefingerprint for determining the coordination of Si in sil-icate minerals and glasses. The SiK-edge shift towardhigher energy is also related to variations in crystal chem-ical parameters of stishovite and the other SiO, poly-morphs (see Table 2): with an increase in Si-O bond length,Si-Si bond distance, and negative Si MAS NMR chemicalshift (Smith and Blackwell, 1983), a decrease in the Si-O-Si bond angle, and a reduction in Si-O bond valencefrom the 4:2-coordinated SiO, polymorphs to stishovite,the SiK-edge shifts toward higher energy.

Structural characterization of opal

Opal is a compact form of natural hydous silica (SiOr.nHrO) and has been classified by Jones and Segrrit accord-ing to the nature of its XRD pattem (Jones and Segnit,l97l). Amorphous opal has been further classified as glass-like network structure opal and gel-like structure opal byLanger and Fldrke (1974) on the basis of XRD, chemical,thermoanalytic, and IR spectroscopic studies. Adams et al.(1991) reported that chemical shifts from MAS NMR spec-tra do not distinguish various types ofopal, but the "Si linewidths decrease with an increase in structural order.

The SiK- and SiZ-edge XANES spectra of two opalsamples, nos. 1232 and 800, are compared with the spec-tra of the other crystalline SiO, polymorphs in Figures 2and 3, respectively. The SiK-edge spectra ofthe opal sam-ples are similar to that of a-SiOr, but we can see someweak features due to long-range ordered structure, partic-ularly for opal no. 800. For example, peak E in the K-edgespectrum of opal no. 800 shifts toward lower energy, asin the spectrum of cristobalite rather than that of a quartz.The SiZ-edge spectra of the opals demonstrate that thetwo opals are essentially similar to a-SiO, but also havesome spectral features of crystalline cristobalite. For in-stance, peak A is split, and peak D becomes more signif-icant, as in the cristobalite spectrum. In summary, thetwo opals essentially have the structural character ofa-SiO, but also contain a small proportion of critsobalitestructural units; features of a quartz are not present ineither SiK- or SiZ-edge spectra of theses two opals. Opalno. 800 has a greater component of cristobalite structural

LI ET AL.: XAS STUDY OF SiO, POLYMORPHS 629

@

!dh

lr

4

h

F

k

hoa

IA+)

H

hH

dtr

Pptr

H

+)

t{ou

99 102 105 108 1 1 1 114

Energy (eV)

Fig. 5. SiZ-edge XANES spectra of cristobalite and a-SiO',together with the composite spectra for various proportions ofcristobalite and a-SiOr. The spectra of cristobalite and a-SiO'were collected at similar experimental conditions and normal-izedby I/Io. Alinear background has been removed, and peakC is normalized to the same height in all these spectra. Twoapparent changes, marked by solid triangles, can be observedwith an increase in the proportion ofcristobalite. First, peak Abecomes more intense and more significantly split. Second, peakD becomes more significant.

units than the precious opal, no. 1232. These results arein good qualitative ag,reement with the X-ray diffractionpatterns ofthe opals.

To determine the compositional and structural featuresof the two opals semiquantitatively, we have calculatedcomposite SiK- and SiZ-edge XANES spectra for variousproportions of cristobalite and a-SiOr, as shown in Fig-ures 4 and 5, respectively. The SiK-edge spectra (Fig. a)of cristobalite and a-SiO, were collected by TEY at sim-ilar conditions and normalized by I/Io, where 1o is theintensity of photon flux and 1 is the intensity of TEYsignal. The linear background was removed, and peak Cwas normalized to the same height in all spectra. Themost apparent feature of the composite spectra is thatpeak E, marked by a solid triangle in Figure 4, becomes

1E3o '*';:::" l:il "

"nergy (ev)

Fig. 6. (kft) The Si-rK-edge XANES spectra of two opal sam-ples, nos. 1232 ar'd 800, are shown (solid lines) and comparedwith the composite spectra (solid and dotted lines) of 100/o cris-tobalite + 90olo a-SiO' and 300/o cristobalite f 700/o a-SiOr, re-spectively. (Right) The SiZ-edge spectra of these two opals areshown (solid lines) and compared with the composite spectra of400/o cristobalite * 600/o a-SiO, (solid and dotted lines).

more intense, and also peaks D and F become more sig-nificant, with an increase in the proportion ofcristobalite.

Similarly, the Sil-edge spectra of cristobalite and a-SiO,(Fig. 5) were also collected by TEY at similar conditionsand normalizedby I/Io. The linear background was sub-tracted, and peak C was normalized to the same heightin all spectra. In the composite spectra, two apparentchanges, marked by solid triangles in Figure 5, occur withan increase in the proportion of cristobalite. First, peakA becomes more intense and is split more significantlybecause of the spin-orbit interaction of Si 2p orbitals.Second, peak D, attributable to the MS effect of the ex-tended structure, becomes more significant.

The SiK-edge spectra ofopal nos. I 232 and 800, shownas solid lines in the left part of Figure 6, were also col-lected at similar experimental conditions, and the datawere reduced in a similar way, as for cristobalite anda-SiOr. Composite spectra of l0o/o cristobalite * 900/oa-SiO, and 300/o cristobalite * 70o/o a-SiO' simulate vi-sually the spectra ofopal nos. 1232 and 800, respectively,quite successfully. This analysis indicates that the twoopal samples are essentially characterized by a-SiO,structural units but also contain small proportions of thestructural units of crystalline cristobalite. The preciousopal, no. 1232, contains about l0o/o cristobalite structuralunits, and opal no. 800 contains about 300/o cristobalitestructural units. These estimates are in qualitative agree-ment with XRD, which indicates that there is some crys-talline cristobalite present. However, the XANES meth-od can also provide information on the local structureand can semiquantitatively determine the proportions ofvarious structural units in the opals.

The Sil-edge spectra of the two opals are shown also as

630 LI ET AL.: XAS STUDY OF SiO, POLYMORPHS

Tlele 3. EXAFS analysis of d quartz, stishovite, and amorphous SiO,

EXAFS analyses

Rehr et al. (1991) McKale et al. (1988) Curve fitting X-ray difhaction

CNBDCN

d quartz*Natural stishovite'.

Synthetic stishovite

a-SiO,t

4.64.22.23.51 . 81 . 4

1.62 2.2'1.77 4.5

1.80 3.7

1.s9 2.2

1.62 3.31.83 6.8

1.86 6.6

1 .61 3 .4

0.00080.0040

0.0026

0.0025

0.00030.0035

0.0007

0.0011

1 .611.821 .771.821 .771.58

0.0020.0070.0070.0050.0050.0026

1 .61 41 .81 41 .76 21 .81 41 .76 2

Note.'BD : bond distance (A); Ctt : coordination number; 02 : Debye-Waller factor.'X-ray diffraction data are cited from LePage et al. (1980)." X-ray diffraction data are cited from Sinclair and Ringwood (1978).t EXAFS structure parameters are derived using phase shifts and amplitudes from a quartz EXAFS analysis.

solid lines in the right part of Figure 6. Peak A is marginallystronger, and peak D is a little more significant in opal no.800 than in opal no. 1232, qualitatively indicating that opalno. 800 has more features of crystalline cristobalite. How-ever, these two spectra are in general very similar. The com-posite spectrum of 400/o cristobalite + 600/o a-SiO, fits thespectra of both opals reasonably well. Therefore, the SiI-edge spectra demonstrate that both opals contain about 400/ocristobalite structural units and are thus in qrralilaliys, fu1not quantitative, agreement with the SiK-edge spectra andXRD. However, as a semiquantitative method, SiK- andSil-edge XANES shows potential applications for the char-acterization of the structure of opal and other partly orderedsilicate materials. A more extensive range of opal samplesis being investigated to determine their structure and com-positions using this method. This new application of XANESto studies of amorphous and very disordered structures willbe further developed in this future study.

F qudtl (")

2100 2400 2700Energy (eV)

6 9 . 1 2r (A-',)

EXAFS analysis

The application of synchrotron radiation and short-range single-electron scattering theory has made EXAFSspectroscopy a powerful technique for studying the struc-ture of minerals and glasses (Waychunas et al., 1986).Figure 7 shows the calibrated and normalized averageSiK-edge EXAFS spectra and EXAFS data reductions ofd quartz. The EXAFS analyses were performed with theprogram BAN (Tyliszczak, 1992). The experimental av-erage spectrum was normalized using 1/1o (see Fig. 7a).Following background subtraction, the spectrum wasconverted from energy space into k space, and the weight-ed x(k).k is shown as the solid line in Figure 7b. TheFourier transform is shown in Figure 7c. Figure 7d showsthe back Fourier transform (solid line) for windowing thefirst shell of Si in quartz, and the dashed lines are thefitting curves constructed on the basis of the above for-mula using theoretical amplitudes and phase shifts, aswell as assumed structure parameters, bond distances, co-ordination numbers, and Debye-Waller factors (or). Alsousing Rehr and McKale phase shifts and amplitude cor-rection (Rehr et al., l99l; McKale et al., 1988), the av-erage bond distance and coordination number for eachshell were obtained. The EXAFS parameters for a quartz,stishovite, and a-SiO, are given in Table 3, and the bonddistances and coordination numbers for a quartz and sti-shovite are in good agreement with the data for X-raystructures (LePage et al., 1980; Sinclair and Ringwood,1978). The t5rsi and lorSi can be determined clearly fromthe EXAFS analysis. In general, the bond distances de-rived from EXAFS are accurate to +0.02 A, and the co-ordination numbers are accurate to +20o/o for the firstshell.

As shown in Table 3, although the Debye-Waller fac-tors derived using Rehr and McKale theoretical phaseshifts and amplitudes and from curve fitting are differentin value, the qualitative results have significance. TheDebye-Waller factor, which directly measures the localvibrations of Si-O bonds, is significantly larger for sti-shovite than for d quartz. This is also in agreement withtheir structure and bonding features. For 6:3-coordinat-ed stishovite, the Si-O bond distances are larger thanthose in 4:2-coordinated a quartz, and the Si-O bonds

1 800

I

- o2

-0.41 5

o

I o o

e

Io

* t

o 2 t < r l t

E 1 0 3 u *t(n)

12 ls

Fig. 7. The normalized Si-rK-edge EXAFS spectrum (a) andthe data reduction of a qtrartz. The weighted 1ft).k (solid lines)and the Fourier transform (dashed line) are shown in b and c,respectively. Part d shows the back Fourier transform (solid line)and the fitting curve (dashed line) for windowing the first shellof Si as in c. In part b the back Fourier transform for the firstshell is also compared with the weighted 1ft).k.

(b)

(d)

LI ET AL.: XAS STUDY OF SiO, POLYMORPHS 63r

in stishovite are of greater ionicity than they are in aquartz. Interestingly, for tarSiO+, the Debye-Waller fac-tor for a-SiO, is apparently larger than that for d qtrartz,which is consistent with the structural disorder thatcharacterizes a-SiO..

Acxxowr.rocMENTs

We thank Lin-gun Liu, Research School of Earth Sciences, Australian Na-tional University, for the provision of the natural and synthetic stishovitesamples, and we aclcrowledge the staffat the Synchrotron Radiation C-enter(SRQ, the University of Wisconsin, for thek technical assistance and theNational Science Foundation (NSF) for the support of the SRC. This work issupported by NSERC. We appreciate the very helpfirl suggestions and com-ments from J. Stebbins and two anonymous referees-

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