NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE

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NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL

GEOMETRY AND CATION TYPE

G. Brown, G. Waychunas, J. Stohr, F. Sette

To cite this version:

G. Brown, G. Waychunas, J. Stohr, F. Sette. NEAR-EDGE STRUCTURE OF OXYGEN IN INOR- GANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-685-C8-689. �10.1051/jphyscol:19868129�. �jpa-00226028�

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JOURNAL D E PHYSIQUE

Colloque C8, supplement au n o 12, Tome 47, decembre 1986

NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE

G . E . BROWN J r , G . A . WAYCHUNAS, J. STOHR* and F. SETTE**

Scho@l of Earth Sciences and Center for Materials Research.

Stanford University, Stanford, CA 94305, U.S.A.

" I B M Almaden Research Center, San Jose, CA 95120-6099. U.S.A.

* * A T & T Bell Laboratories, Murray Hill, N J 07974, U.S.A.

ABSTRACT NEXAFS measurements a t the oxygen K-edge have been carried out on a variety of crystalline oxide samples t o examine the effects of differences in local coordination environment of oxygen on near-edge structure. All spectra display an intense white line a t about 543 eV and a strong shape resonance about 20 eV above the edge. The white line shifts to higher energies with increasing oxygen coordination number, although differences in nearest-neighbor type can affect this correlation. In addition, the Ca and transition-metal containing oxides display features 6-10 eV below and 6-9 eV above the white line. The intensities of these features vary inversely with the number of 3d-electrons but their energies are independent of cation oxidation state. Large changes in coordination number and geometry affect edge structure less than variations in types of nearestneighbor cations. The inverse distance-energy relationship is poorly obeyed in these compounds when oxygen- cation bond lengths are compared with the position of the strongest shape resonance for a given structure type.

The near-edge structure is qualitatively interpreted using the results of recent X a multiple scattered-wave calculations.

INTRODUCTION

Recent attention has been focused on near-edge x-ray absorption fine structure spectroscopy as a structural tool because of the success of multiple -scattering theory in explaining edge features (1,2) and the development of empirical models that relate bond lengths about an absorber to the position of u-resonances, particularly for gas phase molecules and chemisorbed species on solids (3,4). Although many NEXAFS studies of cation ab- sorbers have been carried out and show a rich diversity of near-edge fine structure with variations in absorber environment, little attention has been focused on anions such as oxygen, the most important anion in many solids and on solid surfaces and the most abundant element in the earth. This investigation was undertaken to examine the effects of structure type and types of nearest-neighber cations on oxygen K-iTXAF%S. Well- characterized insulator crystalline oxides with severai different structure types having distinctly different, oxygen environments were utilized.

EXPERIMENTAL METHODS

The samples studied included the following (structure types and oxygen coordination numbers given in parentheses): GeO, (wquartz OIII] and rutile O[IJI]); SiO, (a-quartz, cristobalite, and coesite, all O[III]); TiO, (rutile O[IlI]); A1208, Ti,08, Fe,08 (corundum O[IV]); Mg2Si0,,Ca$i0,, Fe,SiO,, Ni$iO, (olivine O[IV]); CaO, FeO, NiO (NaCI - B1, Om). All samples were powdered t o < 5 p m diameter particles, using a SPEX BN mortar and pestle, and were sedimented using ethanol on Cu-metal plates attached t o a non-magnetic stainless steel sample holder. The sample holder was attached to a vacuum mmipulator and placed in a VG photoemis- sion instrument. All data were collected on the 4 " branch line of beam line I a t the Stanford Synchrotron Radi- ation Laboratory. During data collection, the SPEAR ring was operated a t 3.0 GeV and up to 80 mA. A 1200 line/mm holographic grating was installed in the grasshopper monochromator for these measurements. At the energy of the oxygen K-edge (-543 eV) this grating permitted an energy resolution of 3-4 eV. The base pres- sure of the ultrahigh vacuum chamber was less than 3 x 10-lo torr during data collection. All measurements were made in electron yield using a spiraltron electron multiplier floated to a battery box and connected to a current amplifier. The yield spectra were normalized by dividing the signal from the sample by that from a Ge-coated

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868129

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NORMALIZED ABSORPTION (ELECTRON YIELD) Ul NORMALIZED ABSORPTION (ELECTPON YIELD) A "7 rn : 0" C m - 73 0 z 0 0 0 0 F

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reference grid in the incident beam. Edge spectra were obtained by step scanning from 500 to 600 eV in 0.3 eV steps with a 2 second count time a t each step. Sample charging occurred in some cases, increasing the signal- to-noise level and reducing resolution, but was generally minimal. The angle of the incident beam on the sample surface was typically 2 3 0 " to increase the signal from the bulk sample relative to the surface. A more com- plete description of the data collection procedure used is given in [5].

R E S U L T S A N D D I S C U S S I O N GeO, and SiO, Polymorphs

Figure 1 shows oxygen K-NEXAFS spectra for crystalline GeO, in the rutile and a-quartz structure types. In GeO, (rutile), each oxygen is coordinated by three Ge atoms a t 1.88 A average distance [d(Ge-Ge) = 2.86 A, Ge-O-Ge = 99.8 "1 (6), the Ge atoms are octahedrally coordinated by oxygens, and the GeO, octahedra form edge-shared chains along c. In GeO, (quartz), each oxygen is coordinated by two Ge atoms a t 1.739 A average distance [d(Ge-Ge) = 3.150 A, Ge-O-Ge = 129.8 "1 (7), the Ge atoms are tetrahedrally coordinated by oxygens, and the structure is a framework of.corner-shared GeO, tetrahedra. The key difference between the two spectra involves the white lines: GeO, (rutile) exhibits two strong features separated by 6 eV whereas GeO, (quartz) exhibits only one. Differences are also observed in the shape resonances. These differences are presumably due to the geometric differences between the two GeO, polymorphs,

The 0 K - N E W S (this study) and Ge K - N E W S (8) spectra of GeO, (quartz) can be compared by choosing the zero of energy for each a t the pre-edge inflection (535.5 eV for the 0 K-edge and 11,104 eV for the Ge K- edge). The O K-NEXAFS spectrum consists of a main white line 5.0 eV above the pre-edge inflection and shape resonances a t 12.7 and 26.5 eV above the inflection. The corresponding values observed a t the Ge edge are 2.5 eV (white line) and 10.0, 12.8, and 21.5 eV (shape resonances). The general shape of both edges is similar to the metal K-edges observed for third-row tetrahedral oxyanions (Q), tetrahedral GeC1, (lo), and tetrahedral Si, (X

= F, C1, CH,) (11). The bound- and continuum-state multiple scattered wave X a calculations for SiF, and SiCl, (12), and Si(OH), (9) indicate that the main white line is due to a I s to t, a-antibonding, bound-state transition and that the features a t about 12 and 20 eV above the edges are due to u and t2 resonant eigenchannel scattering, respectively. X a calculations for tetrahedral GeCI, (10) yield similar features, although detailed assignments were not made. The features of the 0 and Ge K - N E W S spectra of GeO, (quartz) should be due to similar bound-state and continuum transitions. We are not aware of any X a calculations for GeO, or GeO, clusters.

Comparison of the 0 and Ge K-NEXAFS spectra of GeO, (rutile) does not show as many similarities as the spectra of GeO, (quartz). The Ge K - N E W S spectrum (8) consists of a strong white line with three resolvable features (at 2.5, 4.0, and 7.2 eV above the pre-edge inflection) and two relatively weak shape resonances (at 16.5 and 26.0 eV). The 0 K-NEXAFS spectrum (Fig.1) shows two features (at 3.0 and 9.0 eV above the pre-edge inflection) and three shape resonances (at 21.5, 28.5, and 47.5 eV above the inflection). The low resolution level of our oxygen spectrum limits further comparison of white line features. Lacking Xcu calculations for edge- shared GeO, clusters representative of the rutile structure type, bound-state X a calculations for TiO, and FeO, (13) (assuming 0, point symmetry) provide a qualitative guide for assignment of the 0 K-edge feature a t 3.0 eV above the inflection and the Ge K-edge feature a t 2.5 eV above the inflection to a 1s to t,, bound-state transition; the lower energy a,, molecular orbital of GeO, should be filled and this is not a likely final state.

Figure 2 compares 0 K - N E W S spectra of three polymorphs of crystalline SiO, and of GeO, (quartz). Except for the broader and slightly lower energy white line of GeO,, these spectra show minor differences. The same as- signments as those made above for the near-edge features of GeO, (quartz) apply to the 0 K-edge spectra of the SiO, polymorphs cristobalite, and coesite - all with oxygen coordinated by two Si cations a t distances of 1.61 A, Si tetrahedrally coordinated by oxygen, and the corners of all SiO, tetrahedra shared). The major structural difference among these polymorphs involves Si-O-Si angles: a-quartz [I44 " 1 (14), cristobalite [146']

(15), and coesite [I37 - 180 " 1 (16); GeO, (quartz) has a smaller angle (130 " ) and a longer tetrahedral distance (1.74 A). The lack of significant variations in the 0 K - N E W S spectra of the SiO, polymorphs is consistent with recent X a calculations on three-atom Si-O-Si clusters with different Si-O-Si angles (17). These predict only minor variations in oxygen K-edge spectra except a t Si-O-Si angles of near 1 8 0 " . Unfortunately, the calculated spectra bear little resemblance to those observed for the SiO, polymorphs, suggesting that the three-atom cluster is not an adequate model of the oxygen environment in SiO,.

TiO, and GeO,: R u t i l e S t r u c t u r e T y p e [V'M"'O,]

Figure 3 compares 0 K - N E W S spectra of GeO, and TiO,, both in the rutile structure type described above (6). The differences between these spectra are related to the fact that the metal d-orbitals are unfilled in TiO, and are filled in GeO,. The higher resolution 0 K-EEL spectrum of TiO, (rutile) observed by Grunes (18) shows

+ Figs.1-6. Oxygen K-NEXAFS of c r y s t a l l i n e oxides of various s t r u c t u r e types. Abcissa i s energy s c a l e i n e l e c t r o n v o l t s . The numbers shown on s p e c t r a a r e t h e peakpositions minus 500 eV. See t e x t f o r d e t a i l s .

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C8-688 JOURNAL DE PHYSIQUE

that the first peak (at 534.5 eV) in our 0 K-edge spectrum of TiO, actually consists of two peaks separated by 2.5 eV. Natoli's (3) inverse distance-energy relationship is not followed for the most intense shape resonances of GeO, [d(Ge-0) = 1.88 A; E(sr) = 564.0 eV] and TiO, [d(Ti-0) = 1.959 A; E(sr) = 571.5 eV].

The assignments made below are based on Grunes' (18) interpretation of Ti K-edge absorption spectra, Ti L,- EEL and 0 K-EEL spectra of TiO, (rutile). The final states for the two features near 534.5 eV are t,, (dr*) and e, (a*) antibonding orbitals containing about 70% Ti 3d character and about 20 - ^30%0 2p character (13). The greater intensity of these transitions in the 0 K-edge relative to the Ti K-edge is due to the greater presence of 0 2p character near oxygen than near Ti. Based on the reasoning above and the XLY calculations for TiO, (13), the first feature in the 0 K-edge of GeO, (rutile) does not involve t,, or e, final states since these MO's are filled.

If the intense feature is due t o a bound-state transition, the final state is probably a t,, antibonding orbital involving 0 2p character and very little pure metal character. The intense feature at 546.0 eV in the TiO, spectrum has been assigned to final states of a,, and t,, character (13,18); as argued above, it is likely that the white line of GeO, (rutile) a t 544.5 eV also involves a t,, final state. The shape resonances a t higher energy have not been assigned due to the lack of appropriate continuum calculations.

Corundum Structure Type ~ M , N O s ]

In the approximately hexagonal closest-packed corundum structure type, each oxygen is coordinated by four trivalent cations and each cation is six coordinated by oxygens. Figure 4 shows 0 K-NEXAFS spectra for A120,, Ti,O,, and Fe,O,. The two major observations are (1) the presence of an intense, pre-edge feature at 535.0 and 534.0 eV of the Ti,O, and Fe,O, spectra, respectively, and its absence in the 4 0 , spectrum; and (2) the higher intensity of the pre-edge feature for Ti,Oa relative to Fe,O,. As in the case of the 0 K-edge of TiO,, two pre- edge features should be resolvable in the 0 K-edge spectra of Ti,O, and Fe,O, but are not seen in our spectra.

These observations and X a calculations (13) suggest that the final states for the transitions causing the pre-edge feature of Ti,O, and Fe,O, involve substantial metal d-character of e, and t,, antibonding types. The greater intensity in the Ti,O, spectrum is due to the presence of more unfilled metal d-orbitals on Ti" (dl) than on FeS+

(d5); Its absence in the spectrum of Al,O, is caused by the fact that the empty Al 3d orbitals lie a t too high an energy for mixing with 0 2p orbitals (19). The observed 0 K-edge spectrum for A1,03 matches band theory calculations (e.g. 20) well. The position of the most intense shape resonance a t about 20 eV above the white line does not follow the inverse distance - energy relationship (3).

Olivine Structure Type [V'M2'VSi'v0,]

In the olivine structure type, each oxygen is bonded to one Si and three divalent cations (M) in roughly tetrahedral geometry, each M cation is bonded to six oxygens, and each Si is tetrahedrally coordinated by oxygens (21). Figure 5 compares 0 K-NEXAFS spectra of four olivine isostructures with M = Mg, Ca, Fe, and Ni, respectively. A partially-resolved, pre-edge feature is seen in the spectra of C%SiO,, Fe,SiO,, and Ni,SiO,, de- creasing in intensity in the same order; this feature is absent 'in the spectrum of Mg,SiO,. These observations suggest that the final state associated with the pre-edge transition involves substantial metal 3d-character. The position of the major shape resonance (562 - 564.5 eV) and observed M-0 distances (21) follow the inverse distance-energy relationship (3) poorly. The pre-edge and main-edge features in each spectrum can be assigned to the same transitions discussed for the other structure types.

Sodium Chloride (Bl) Structure Type ["'MV'O]

In the cubic closest-packed NaCl or B1 structure type, each cation is bonded to six anions and each anion is bonded to six cations. Figure 6 compares the 0 K - N E W S for B1 structures CaO, FeO, and NiO. Higher resolution 0 K-EEL spectra of NiO (18,22) show that the white line centered a t 543.5 eV in our spectrum actually consists of two features separated by 3 eV. Alternatively, the difference in resolution level between our spectrum and 0 K-EEL spectra of NiO may be due to possible non-stoichiometry of the NiO sample we used (A. Bianconi, pers. comm.). The most important observations are (1) the presence of a pre-edge feature and a shoulder on the high-energy side of the white line in each spectrum, and (2) the decrease in intensity of these features in the order CaO > FeO > NiO. These observations suggest that the final state associated with the pre-edge feature involves metal 3d character (t,, and eJ. They also suggest that the high-energy shoulder is caused by a resonant eigenchannel scattermg with metal 3d character. The pre-edge and main-edge features can be assigned t o the same transitions as in the other metal oxide structure types. A high resolution 0 K-EEL spectrum of MgO in the B1 structure type (22) shows no pre-edge feature as expected; however, it does show a weak, well-resolved feature about 9 eV above the white line corresponding to the high-energy shoulder in our spectra. Thus this feature cannot depend on 3d metal character for transition intensity.

C O N C L U S I O N S

The most significant conclusion to be drawn from the oxygen K-NEXAFS spectra reported in this study is their overall similarity. Generally, they consist of a strong white line, a t an average value of 543 eV, and one strong shape resonance about 20 eV above the white line. In general, the position of this shape resonance does not verify the inverse distance-energy relationship that appears to hold for the simpler spectra of gas-phase and

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chemisorbed molecules (3,4). The position of the white line shifts to higher energy with increasing oxygen coordination, although nearestneighbor type affects this correlation. For example, the white line of GeO, (rutile), with three-coordinated oxygens - O[III], is 4 eV greater than for GeO, (quartz), with O[II]. However, the white line of CaO, with O[VI], is 1.5eV less than that of GeO, (rutile). For compounds containing transition-metal cations or Ca, a relatively strong pre-edge feature was observed at 6 to 10 eV below the white line. A high-energy shoulder was also observed in some of the same compounds 6-9 eV above the white line. The energies of these features are insensitive to cation oxidation state relative to corresponding features in cation L,-EEL spectra (23). For example, the pre-edge features of TiO, L8-EEL features for these compounds differ by 2 eV. The intensity of these features is correlated with the number of unfilled metal 3d orbitals. This is consistent with ear- lier assignments to final states involving substantial metal d-character (t,, and eJ. The white line in the transition-metal or Ca-containing oxides involves final states of a,, and t,, character based on Xcu multiple s e a t tered wave calculations. For the SiO, polymorphs and GeO, (quartz), the two major shape resonances a t 12 and 20 eV above the pre-edge inflection are assigned to e and t, resonant eigenchannel scattering, respectively. For the other compounds studied, no appropriate continuum calculations are available to explain the observed shape resonances.

ACKNOWLEDGEMENTS

This study was supported in part by NSF Grant EAR-8513488 (GEB and GAW) and in part by IBM (JS and FS). We thank F.D. Dikmen for help with data collection and C. W. Ponader for help in preparing the figures and the camera-ready text. SSRL is supported by the DOE and the NM.

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