Stereochemical Order around Sodium in Amorphous Silica - The

Mar 28, 2000 - Sodium-covered silica films formed on silicon substrates are examined by X-ray photoemission spectroscopy (XPS) and X-ray absorption fi...
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J. Phys. Chem. B 2000, 104, 3438-3445

Stereochemical Order around Sodium in Amorphous Silica C. Mazzara,† J. Jupille,† A.-M. Flank,‡ and P. Lagarde*,‡ Laboratoire CNRS/Saint-Gobain “ Surface du Verre et Interfaces ”, BP 135, F-93303 AuberVilliers, France, and LURE, Baˆ t 209D, B.P. 34, F-91898 Orsay Cedex, France ReceiVed: July 16, 1999; In Final Form: NoVember 22, 1999

Sodium-covered silica films formed on silicon substrates are examined by X-ray photoemission spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (EXAFS) in ultrahigh vacuum conditions. The presence of sodium, which diffuses within the silica matrix at 300 K results in a shift of the O 1s core level toward lower binding energy with respect to the bulk silica close to the energy separation between the O 1s level associated with bridging and nonbridging oxygen in soda-silica glass. The EXAFS at the sodium K edge shows that a defined stereochemichal order, similar to that which is observed in glass, arises in the silica matrix around this element since sodium atoms are surrounded by oxygen atoms at an average distance of 2.3 Å and by a second shell which is assigned to silicon atoms located at 3.8 Å.

1. Introduction In the absence of translational periodicity, the structural characterization of glasses relies on both the representation of randomness which corresponds to the long-range picture of these materials and the description of the local order which, in most cases, extends at least over the scale of the interatomic length.1 The modeling of glasses and amorphous materials reflects these two aspects. In particular, all structural approaches of glasses, random network models,2-4 crystalline models,5-7 and simulation,8 do account for some local order. The Zachariasen’s random network model2 and its application to silica by Warren9 gave rise to the classical two-dimensional representation of glasses that can be found in many textbooks. In the view introduced by these authors, the silica glass network ideally consists of SiO4 tetrahedra sharing each of their four oxygen with four other such tetrahedra. The oxygen atoms which are all linked to two silicon atoms are labeled bridging oxygen (BO). In the real material, some oxygen atoms happen to be bonded to only one silicon atom. These are called nonbridging oxygen (NBO) atoms. The Si-O bond distances and the O-Si-O angles lie in narrow ranges around the mean values 1.61 Å and 109.7°,10 so that the SiO4 tetrahedron undergoes only mild distortions with respect to the highly symmetric tetrahedral shape. This explains why this entity is often taken as a basis to built models of amorphous silica.2,4,8 The absence of long-range order manifests itself by a broad unimodal distribution in the Si-O-Si angles from 120 to 180°.11,12 Crystalline models5 of silica suggest more sophisticated representations of local order which are based on either bonding topologies of crystalline polymorphs of the material under study6 or other specific tridimensional topologies.7 In simulations, as in the abovementioned random network models,2-4 the local geometric constraints are simply acccounted for by the tetrahedral shape of the oxygen environment of silicon. The addition of alkali metal oxides to the pure silica strongly reduces its melting temperature and thus facilitates its manufacture.13 Indeed, the understanding of the complex environment * Corresponding author; e-mail: [email protected] † Laboratoire CNRS/Saint-Gobain. ‡ LURE.

of alkali silicate glasses is an important issue regarding the industrial importance of these materials. The basic chemistry underlying this drastic change in behavior is the breaking of the siloxane bonds by alkali ions and the concomitant appearance of the so-called nonbridging oxygen which are linked only on one side to silicon and on the other side to the alkali ion. The composition of commercial silica glasses is based on the same chemical principles. It involves network-forming elements whose role is to build chains linked by strong chemical bonds, mostly covalent, and network-modifying elements which reduces the connectivity of the glass former network.1 The chemical environment of the silicon in silicate glasses is often described by the binary model14 which states that there is a one-to-one correspondence between alkali metal ions and nonbridging oxygen. The chemical environment around silicon atoms being described by means of the notation Qn, in which the superscript n stands for the number of bridging oxygen lying within the first shell around silicon atoms, with n ranging from 4 to 2; the binary model means that Qn+1 must be completely transformed into Qn before Qn-1 could appear. This model is often considered as leading to a realistic picture of the chemical environment of the silicon atoms in glass, although some discrepancies between predictions and measurements are evidenced by careful nuclear magnetic resonance.14,15 The neighborhood of alkali ions in silica glasses is more puzzling than that of silicon. In the Zachariasen-Warren view,2,9 the alkali metal ions are only stressed to reduce the connectivity of the silica network without bringing about any new constraints. In particular, at variance with the silicon atom surrounded by oxygen tetrahedra, alkali ions are not supposed to meet any particular coordination shell, but to simply occupy the available sites of the silica network. More recent studies have provided evidence for some ordering in the chemical environment of the network-modifying ions.16 On the basis of extended X-ray absorption fine structure (EXAFS) analyses at the sodium edge of silica glasses, Greaves et al.3,4 have developed a “modified random network model”, in which alkali ions and NBO to which alkali atoms are mostly linked concentrate into channels surrounding zones dominated by pure silica. There is also indication of an ordering within the modifiers network. By

10.1021/jp9924474 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000

Stereochemical Order around Sodium in Amorphous Silica means of neutron diffraction, Gaskell et al. have pointed to a medium-range ordering of the calcium ions, over about 10 Å, in the calcium silicate glass.16 From simple chemical considerations supported by the abovementioned binary model,14,15 each alkali ion of a silica glass is expected to induce the formation of one NBO atom. Indeed, estimates derived from the quantitative analysis of the O1s spectrum by X-ray photoemission spectroscopy (XPS) confirm this expectation.17 In silica glass, alkali ions are surrounded by 4 to 5 oxygen atoms, depending on the nature of the ion.3,18 Such a number of oxygen neighbors leads to the assumption that the chemical environment of alkali ions also involves bridging oxygen even if, as pictured by the “modified random network”, a given nonbridging oxygen can be in the vicinity of several alkali ions. Quantum calculations support the presence of bonding and nonbonding oxygen at the vicinity of the alkali ions and suggest that the NBO-alkali ion distance should be shorter by about a quarter of angstrom than the BO-alkali distance.19 However, analyses of the sites of the alkali ions in glasses show only a unique distance to oxygen, which ranges from 2.3 to 2.4 Å.3,4,19 Indeed, a limitation to distinct experimental observations of BO and NBO in the chemical environment of the alkali ions comes from the fact that the concentration ratio alkali ion/NBO is about constant (which, due to the chemistry of the glass (see above) is close to unity) whatever the molar concentration of alkali oxide may be. In a previous work, sodium deposited on amorphous silica at 300 K in a vacuum has been shown to diffuse within the bulk of the silica network.20 Since pure silica is believed to mostly involve bridging oxygen atoms,14 such samples offer the opportunity to examine the behavior of sodium atoms inserted in an environment which consists almost exclusively of bridging oxygen atoms. To obtain sodium-silica samples at equilibrium from the point of view of the diffusion, silica films formed on silicon substrates have been preferred to bulk silica samples. Analyses have been focused on the description of the chemistry of the sodium dissolved in the silica film via examinations of both the charge transfer from the sodium to the silica network by XPS and the chemical environment by EXAFS. The former and the latter method are of obvious relevance herein. XPS has been widely used on glass surfaces to characterize the electron transfer from glass modifying elements, such as alkali and alkali-earth metals, toward nonbridging oxygen atoms. In most cases, the XPS O1s core level of nonbridging oxygen is separated by an energy shift close to 2 eV from the corresponding level of bridging oxygen. EXAFS is a common method to describe the chemical environment in noncrystalline materials, since it has the ability to determine both bond distances and coordination numbers around given types of atoms by selecting the relevant absorption edges. 2. Experimental Section Photoemission experiments have been performed in an ultrahigh vacuum system21 equipped with an X-ray photoemission spectrometer (XPS) VG CLAM II. The XPS data have been collected by using the Mg KR line. Data manipulations have been performed by running a VGX900 software. Photoemission peaks have been fitted using Voigt functions accounting for heterogeneity in surface potential. X-ray absorption data on the sodium K edge have been recorded on the SA32 soft X-ray beam line of Super-ACO equipped with a two-crystal monochromator and using beryl (1010) crystals. At the sodium K edge energy the resolution is

J. Phys. Chem. B, Vol. 104, No. 15, 2000 3439 close to 0.5 eV. The data have been collected in the total electron yield (TEY) mode from the sample, while the incoming flux was monitored by the TEY from 500 Å of titanium supported by a 2-micron polyethylene foil. Thanks to an upstream toroidal mirror, the size of the photon beam onto the sample is 200 µm × 400 µm. To determine phase shifts for the analysis of the EXAFS spectra, model compounds have also been examined, namely Na2O and Na2SiO3 powders (Strem Chemicals). Much care has been taken to avoid hydration of these samples before their transfer to the experimental chamber. Moreover, we have run the sodium K edge of two glasses with compositions 10% Na2O, 90% SiO2 and 25% Na2O, 75% SiO2, respectively (Y. Vails, CRMHT, Orleans). Native silica films have been used in the present work for XPS and EXAFS experiments. In the latter case, measurements have also been performed on a thermal silica film. Native silica is the thin layer of silicon oxide which grows onto polished (100) wafers of pure silicon whose thickness has been evaluated, in the present case, to 20 Å by XPS20 and to 32 Å by microprobe analysis. An average value of 26 ( 6 Å has been retained. The thermal silica film under study has been obtained by heating a (100) silicon wafer at 870 K under oxygen for 2 days, to achieve an oxide thickness of 50 Å.22 The microprobe analysis leading to an estimate of 56 Å, the thickness has been taken equal to 53 ( 3 Å. In both vacuum chambers, silicon crystals are mounted on tantalum plates so that they are heated by electron bombardment of the plates. Prior to the introduction of the samples in the experimental chamber, they have been cleaned with pure acetone and then rinsed with pure water. Once in a vacuum, samples are outgassed at 870 K for several hours in a vacuum and then, while kept at the same temperature, they are exposed to 10-6 mbar of oxygen for 30 min to remove the carbon contamination. Sodium has been evaporated on the silica films at room temperature from a carefully outgassed SAES getter. The stability of the evaporation rate from the sodium source has been checked by several experiments on a test sample. Fluxes have been calibrated by means of the examination of sodiumcovered samples by microprobe analysis. It has also been verified that the bare samples were sodium free. Figure 1 shows the near edge spectra for the two deposits of sodium on a native silica samples which are examined in the present work, i.e., silica samples S1 and S2. They correspond to average concentrations of 11 ( 2.5 and 20 ( 5 atomic percent (at. %) of sodium, for samples S1 and S2, respectively. The accuracy is estimated by considering the error bar on the determination of the silica thickness and a systematic error of 5% arising from the measurement of the sodium concentration. The jump at the edge grows roughly by a factor of two, in line with a stable evaporation rate. Therefore, X-ray absorption shows that the amount of sodium which can be deposited is proportional to the incoming sodium flux, a behavior definitely different from that observed upon the exposure to sodium of a clean silicon surface at room temperature,23 in which case the amount of sodium deposited is within the monolayer range. Other features from this figure will be discussed below. The average sodium concentration in the thermal silica film of sample S3 has been estimated to 6.5 ( 0.7 at. %. 3. Results Upon deposition of sodium on a native silica layer at room temperature, it has been previously demonstrated that sodium diffuses into the silica matrix with a slight enrichment at both the silica surface and the interface of the silica film with the

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Figure 2. Photoemission spectra of the O1s region of a native silica layer supported on a Si(100) crystal: (a) clean surface (solid line); (b) sodium-covered surface; (c) soda-lime glass surface; (d) sample (b) after annealing at 870 K (dotted line). The decomposition of spectra (b) and (c) into two components is shown as well as the residues of the decomposition.

Figure 1. Sodium near edge spectra of (a) two glasses with compositions 10% Na2O, 90% SiO2 (dots), and 25% Na2O, 75% SiO2 (solid line); (b) sodium-covered native silica for 5 mn (dots, sample S1 ) 11 at. % of Na) and 10 mn (solid line, sample S2 ) 20 at. % of Na) evaporations; (c) reference compounds: Na2SiO3 (dots) and Na2O (solid line).

silicon substrate.20 At variance, when the deposit is performed at 200 K, a sodium film is formed on the silica surface,24 which compares to the observations of Madey et al.25 3.1. The Photoemission O 1s Chemical Shift. When sodium diffuses into the silica, an electron transfer takes place between the sodium atoms and the oxygen atoms belonging to the silica, which manifests itself in an O1s core level shift toward lower binding energy. Photoelectron spectra of the O1s level from native silica films are shown in Figure 2. Figure 2b has been recorded after dissolution of sodium via deposition at room temperature, up to a content of 18 at. %.20 As expected, due to the presence of sodium, a strong shoulder, which will be herein labeled ONa1s, appears at -2 eV on the lower binding energy side of the core level spectrum. Such a binding energy shift closely compares to that observed between bridging and nonbridging oxygen in the case of silica glass.17,26 For com-

parison, the spectrum 2c has been collected on a commercial soda-lime sample which has been exposed for hours in a vacuum to the X-ray source of our photoemission system so as to enhance27 the surface sodium concentration. There is a general consensus that the high and low energy O1s components recorded on soda-lime glass surfaces come from bridging and nonbridging oxygen, respectively. The shift between these two species depends on the composition of the glass. In commercial glass, the BO-NBO shift is lowered by the presence of alumina since the O1s binding energy associated with Al-O-Si bridges is lower than that corresponding to the Si-O-Si siloxane bridges.26 Nevertheless, common glass compositions are characterized by shifts lying around 2 eV, which is the value observed in the case of the soda-lime glass which is analyzed herein (Figure 2c). Notably, on Na2O(SiO2)3 glass,26 whose sodium atomic concentration, close to 17%, is very similar to that of sample 2b, the BO-NBO shift amounts to 2.2 eV, which is exactly equal to the shift measured in spectrum 2b. The sodium dissolved in the bulk of the silica film can be removed by annealing at 870 K in a vacuum. Within the detection limit of the XPS analyzer, the Na1s peak is no longer detected since the XPS O1s spectrum recorded after the removal (Figure 2d), with the same setup of the photoelectron analyzer, shows no visible change either in shape or in intensity with respect to the fact that it was before the deposit of sodium. O1s spectrum 2d can be almost perfectly superimposed onto that from the clean surface (Figure 2a). In addition, it has been reproducibly observed that successive deposits and removals did not produce any change in the XPS area ratios characteristic of the composition and thickness of the silica layer. This is indicative both of the absence of alkali-induced oxidation of the substrate and of the desorption of the sodium which has been deposited in the metallic form. Finally, in the case of the native silica films which is the thinner layer which has been used in the present work, the photoemission spectra of the Si 2p level (not shown here) exhibit only the two peaks corresponding to Si0 and Si4+ which are separated by 4 eV28 with no signature of any intermediate oxidation state. Therefore, within the limit of that observation, there is no evidence for silicon suboxides in the films, either before or after sodium deposition.

Stereochemical Order around Sodium in Amorphous Silica

Figure 3. Real and imaginary parts of the Fourier transforms of the experimental data (solid line) and of the ab initio FEFF6 calculation (dots) for Na2O (top) and for Na2SiO3 (bottom).

3.2. EXAFS Data. The absorption edges corresponding to the two sodium-covered native silica samples with 11 (sample S1) and 20 at. % (sample S2), respectively, are shown in Figure 1a. The second sodium deposition has been performed after desorption of the sodium deposited during the first exposure by a 1200 s annealing in a vacuum at 870 K. After heating, no sodium edge could be detected which, consistent with XPS, demonstrates that the sodium dissolved in silica can be completely removed by an appropriate high-temperature treatment. Also shown, on a different y-scale, are the near-edge structures of crystalline Na2SiO3 and of the oxide Na2O. The former well compares to the spectra of samples S1 and S2. The similar overall spectral shapes, with a double peak within a 15 eV range above the edge followed by a smooth resonance around 1110 eV, provides with a first indication that near environments of sodium are similar in all three samples, with a more detailed structure for Na2SiO3 due to its crystalline character. The nearedge structures of the two glasses with different sodium contents present the same features although the intensity ratio between the two first peaks beyond the edge is slightly different (Figure 1c). Therefore, the local structure around the sodium atom in both the silica and the glass samples appears close to that of Na2SiO3, but affected by a topological disorder. Such behavior of the near-edge structure is often encountered when comparing an amorphous system with the corresponding crystalline compound. A closely related example concerns quartz and amorphous silica as seen from the silicon K edge.29 EXAFS data have been analyzed in a now-classic way with a normalization procedure which uses the Heitler approximation

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Figure 4. Real and imaginary parts of the Fourier transforms of (a) 11 at. % (dots) and 20 at. % (solid line) of a sodium evaporation on native silica and 6.5 at. % (dashes) on thermal silica, (b) glasses with 10% (dots) and 25% (solid line) Na2O.

and then a k-weighted Fourier transform with a Hanning apodization window spanning from 2.35 to 8.65 Å-1. The model compound data have also been processed in that way and have been modeled using the FEFF6 code30 whose only input is the atomic positions of the atoms. A fitting procedure has then been applied where the only adjustable parameters are the DebyeWaller factors of the different shells of atoms. As seen in Figure 3, there is a good agreement with the experimental data. The quality of the fit appears less correct in the case of Na2O, probably because this sample is highly hygroscopic, but also because, in both simulations, only the single scattering events have been taken into account. Multiple scattering could play a significant role in the case of a more symmetric structure like that of sodium oxide. Nevertheless, the main result is that we can be confident using the FEFF6 code in terms of backscattering amplitudes and phase shifts and that we are allowed to use it for the understanding of the sodium site in the silica layer. The FEFF6 code has also been used to build a model for the site of sodium dissolved in silica in the last part of the present EXAFS section. Figure 4 compares the modulus and imaginary parts of the Fourier transforms for sodium-covered samples S1, S2, and S3 (top), and it also gives (bottom) the same functions for the two soda-silica glass compositions. The similarity between all spectra is consistent with the previously demonstrated diffusion of sodium into the silica network and supports the suggestion which has been made by examining the profiles of the sodium edges that sodium occupies sites which are close to those encountered in a Na2O-SiO2 glass obtained from a melt in which, as it is expected from the high electropositive character

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Figure 5. Plots of the quantity Log(X1/X2) for different filtered first shell EXAFS data. In all cases, X2 refers to the bulk glass with 10% Na2O (sample G1): G2/G1 ) dots, sample S3/G1 ) solid line, sample S2/G1 ) dashed line, sample S1/G1 ) dot-dashed line.

of sodium, the first shell involves only oxygen atoms. To check whether sodium could diffuse in silicon, the corresponding EXAFS signature has been modeled by taking a sodium-silicon distance of 2.7 Å, a value close to both the experimental31 and the calculated32 interatomic distance of sodium when deposited onto a clean silicon surface. The discrepancy with the spectra shown in Figure 4 was judged strong enough to discard any sodium diffusion into the silicon network. The local environment of the sodium inserted within the bulk of the silica is then a shell of oxygen atoms very similar to that found in the glasses but also, as proved by the near-edge structure (Figure 1), to that found in Na2SiO3. A second interatomic distance (at about

Mazzara et al. 3.2 Å, uncorrected from phase shift) is also present, which will be discussed below. The local environment of the sodium in the bulk glasses under examination is almost independent of the sodium concentration. At variance, the amplitudes of the Fourier transforms associated to the sodium-covered silica strongly depend on the sodium content. In EXAFS, the amplitude scales with the coordination number N of the central atom and with the so-called Debye-Waller factor σ which describe the degree of disorder of the shell of atoms under consideration. In an amorphous solid, this parameter accounts for both the crystallographic disorder and the thermal motion of the atoms. Coordination numbers and Debye-Waller factors correlate in the fitting process so that the respective variations of these parameters lead to uncertainties about the description of the system by compensating each other. Figure 4 shows that the average interatomic first distance between sodium and oxygen remains almost constant for all three sodium-silica samples. It is also very close to that found in the bulk glasses with some differences in the low distance side of the main peak. For atoms of similar nature whose chemical environments differ only by the values taken by coordination numbers and Debye-Waller factors, as in the case of sodium in sodium-covered silica and silicate, a way to decipher the origin of the EXAFS amplitude is to plot the logarithm of the amplitude ratio for two samples as a function of k2. Such a plot is expected to give rise to a straight line whose slope and intercept with the y-axis correspond to the difference in DebyeWaller factor and to the logarithm of the coordination number ratio, respectively. These plots are shown in Figure 5 for the first shell. Within the uncertainties due to the limited energy domain of the EXAFS analysis, the coordination numbers and the degree

Figure 6. Experimental EXAFS k‚X(k) data (upper part) and the corresponding modulus and imaginary parts of the Fourier transform (lower part) for the glass with 10% Na2O (left) and for the sample S3 (6.5 at. % Na on thermal silica, right). The experimental data are shown with the solid line and the FEFF6 calculations are in dots. Numerical values for the fits are from Table 1.

Stereochemical Order around Sodium in Amorphous Silica of disorder in the two soda-silica glasses G1 and G2 appear to be independent of the sodium concentration. In Figure 5, the y-axis intercept of the dotted curve, which is the logarithm of the ratio between the two coordination numbers, is very close to zero and its slope (the difference between the two DebyeWaller factors) is very small. Then, from Figure 5, and assuming an oxygen coordination of 5 for the bulk glasses G1 and G2, the sodium in sample S1 (11% in native silica) shows a coordination of about 2, and 3.5 in samples S2 and S3 (20 and 6.5%, in native silica and thermal silica, respectively). Differences also appear in the Debye-Waller factors which can be deduced from the slopes of the curves. To achieve a more quantitative description of the sodiumcovered silica films, we have built a cluster by taking the 5-fold oxygen local environment of sodium in Na2SiO3 as a basis. Assuming that these oxygen atoms of the amorphous silica network are linked to silicon, the second shell should be silicon and the distance of this second neighbors shell can be calculated given some inputs: (1) The interatomic distances Si-O and Na-O must be respectively close to 1.61 and 2.4 Å. The value of 1.61 Å has been systematically measured, within 0.03 Å, for glassy or crystalline silicon oxides and compounds,4,10,11 and this Si-O distance is known to be very stable for different silicon oxide compounds. An interatomic distance of 2.3-2.4 Å for Na-O has also been measured in glasses4 and corresponds to the average value in sodium oxide and Na2SiO3. (2) The valence bond angle of oxygen must be close to its average value of 150° found for bridging oxygens in silicon oxide.33 This results in a Na-Si distance of about 3.8 Å, and then the above cluster of oxygen has been surrounded by a second shell of silicon at 3.8 Å. A FEFF6 calculation has then been performed with the following constraints. First, an amplitude scaling factor which takes into account both the so-called “reduction factor” of the EXAFS analysis and the absolute value of the Debye-Waller factor for sample G1 has been determined. Then the various coordination numbers and the variations of the disorder parameters have been taken as a first guess from Figure 5. We then have allowed the sodium-oxygen interatomic distance to vary slightly. Multiple scattering terms have not been considered since their effect is expected to be negligible because of the structural disorder. The results are shown in Figures 6 and 7 and the numerical values are gathered in Table 1. In the above model, the nature of the second neighbor, which has been assumed to be silicon, could be questionned. Alternative solutions where the second shell is made of either oxygen or sodium atoms have been built in order to compare the present data with the results obtained in silicate glasses using the same technique.34,35 In the fitting procedure, the parameters of the first shell have been fixed to the values listed in Table 1. It appears that no valid structural solution can be obtained which could explain the second peak found in the Fourier transforms, in particular for the low sodium concentration sample (S3). The goodness of the fit appears to decrease at the higher concentrations (samples S1 and S2). This could indicate that this simplified model does not fully describe the actual local order around sodium when, at high concentrations, there is a definite probability for one sodium to be in the second neighbor shell of one equivalent atom. This will be discussed in a forthcoming paper. Exact values of the coordination numbers may be, in some extent, model dependent since the calculation assumes that all of the sodium atoms are incorporated in the volume of the silica layer, while it can be stressed that sodium atoms which segregate either at the surface of the oxide or at its interface with silicon20

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Figure 7. Experimental modulus and imaginary parts of the Fourier transform (solid lines) compared with the FEFF6 calculations (dots) for the sample S1 (11 at. % Na, native silica, top) and for the sample S2 (20 at. % Na, native silica, bottom). Numerical values for the fits are from Table 1.

show coordination numbers lower than those in bulk. Because the silica layers are very thin (their thickness corresponds to 10 to 20 SiO4 units), the average values extracted from the EXAFS analysis likely underestimate the bulk coordination numbers. Moreover, because of the very strong influence of the static disorder in EXAFS, only the very well defined interatomic interactions are evidenced by this technique: more loosely bounded atoms could be present around the sodium, especially in the case of sample S1, which could not be detected by EXAFS. 4. Discussion The present data confirm that sodium diffuses within the bulk of the silica at room temperature, a behavior which has been predicted by molecular dynamic calculations for lithium and, in smaller extent, for potassium.33,36,37 The EXAFS convincingly demonstrates that, once inserted in the silica network, sodium atoms become surrounded by a first shell of oxygen atoms. In the material obtained by dissolving sodium in an amorphous silica network via

SiO2 + yNa f Nay(SiO2) all oxygen atoms are, at least formally, expected to be bridging oxygen since the proportion of nonbridging oxygen is only marginal in pure silica.14 In glass obtained from melts, the presence of sodium oxide is supposed to systematically lead to the formation of nonbridging oxygen by breaking siloxane bridges in a way that can be schematized as follows

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TABLE 1: Local Environment of the Sodium Atoms for the Different Samples sodium - oxygen

sodium - silicon σ2 (Å2)

R (Å)

5 ((0.5)

0.015 ((0.003)

3.80 ((0.05)

5 ((0.5)

0.007 ((0.005)

2.30 ((0.04)

3.7 ((0.5)

0.006 ((0.003)

3.80 ((0.05)

3.7 ((0.5)

0.007 ((0.005)

2.30 ((0.04)

3.7 ((0.5)

0.009 ((0.003)

3.80 ((0.05)

3.7 ((0.5)

0.015 ((0.005)

2.30 ((0.04)

1.85 ((0.5)

0.0045 ((0.003)

3.80 ((0.05)

2 ((0.5)

0.007 ((0.005)

sample

R (Å)

G1 10% Na2O, 90% SiO2 S3 (6.5 at. %) S2 (20 at. %) S1 (11 at. %)

2.32 ((0.04)

N

+ Na2O f Si-O-Na + Na-O-Si Si-O-Si bridging oxygen nonbridging oxygens Therefore, in glass, sodium is supposed to be mostly bonded to nonbridging oxygen, even if, as already mentioned, sodiumBO bonds should also be formed. Surprisingly, EXAFS leads to very similar sodium-oxygen distances for sodium-covered silica and glass (Table 1). The presence of a second shell of atoms reveals that the local order around sodium extends up to the neighboring silica tetrahedra. The EXAFS thus demonstrates the existence of a defined stereochemical order around sodium diffusing in an amorphous silica network. This means that in this thermodynamically favorable configuration, sodium creates its own site via a relaxation of the surrounding arrangement of silica tetrahedra. Nevertheless there are a few pending questions. The experimentally observed narrow distribution of sodium-oxygen distances can be achieved only via relaxations of the silica network around the sodium atoms. Under the assumption that, as predicted by theory for silicate glass,19 the sodium-BO distance should be higher by a quarter of an angstrom than the sodium-NBO distance, which is far beyond the error attached to the determination of distances by means of the EXAFS calculation, the above result indicates that sodium diffusing in silica becomes surrounded by nonbridging oxygen atoms. Consistently, the -2 eV XPS O 1s core level shift associated to the ONa1s component which compares to the BO-NBO shift observed in soda-silica glass also favors the formation of nonbridging oxygen. Since pure silica dominantly involves bridging oxygen, this means that Si-O distances might also relax around dissolved sodium atoms. However, to account for the almost perfect recovery of the initial XPS O1s spectrum, including line shape and intensity, arising after the removal of the sodium dissolved in the silica film by annealing at 870 K (Figure 2), the silica network should undergo only local relaxations. Although the present work is dealing with sodium-silica films which are chemically different from soda-silicate glasses, the sodium site defined here can be compared to the models describing the local order in a glass. The existence of a stereochemical order around sodium atoms dissolved in silica is at variance with the “continuous random network” model in which, beyond their direct action in breaking the bridging bonds, network modifiers are supposed to occupy only available sites of the glass former network.2,9 On the contrary, the present data prove that the chemical environment of sodium involves a somewhat ordered contribution from silica tetrahedra. Therefore, the sodium-silica material which has been considered herein likely involves chains of locally ordered moieties, a description which is still compatible with the “modified random network” which postulates a local order around sodium for silica glasses and with the finding by Gaskell et al.16 of a medium-range order in the cation network of silicate glass.

N

σ2 (Å2)

5. Conclusion When deposited on native and thermal silica films which are formed at the surface of a silicon crystal at room temperature, sodium atoms tend to diffuse toward the bulk of the oxide. In the present work, the charge transfer and the local order around sodium have been explored by X-ray photoemission spectroscopy and extended X-ray absorption fine structure studies of sodium deposition on native and thermal silica films. The silica network is shown to relax around the sodium atoms to produce a chemical environment which consists of a first shell of oxygen atoms at a distance of 2.30 Å, and a more distant shell of silicon atoms belonging to silica tetrahedra at 3.8 Å. This environment shows striking similarities with that of sodium in both crystalline silicate and soda-silica glass, while charge transfer produces a -2 eV shift of the O1s core level which is reminiscent of the shift associated with nonbridging oxygen in silica glass. At variance with the picture of network modifiers in both “continuous random network” and “modified random network” models, a defined stereochemical order is evidenced for sodium in silica, and this element plays an active role in building its own site in the silica network. In future work, the sodium concentration will be considered in more detail to derive a better understanding of the coordination number within the first shell and of the nature of the more distant shells, in particular the presence of sodium. Acknowledgment. We thank Y. Vaills (CRMHT, Orleans) for providing us with the soda-silicate samples and also Bruno Germain (Saint-Gobain Recherche) for the microprobe analysis of the concentration of sodium in the silica thin films. We also thank the team in charge of the Super-ACO machine. References and Notes (1) Gaskell, P. H. Models for the Structure of Amorphous Solids in Glasses and Amorphous Materials in Materials Science and Technology, Zarzycki, J., Ed., VCH: Weinheim, 1991; Vol. 9. (2) Zachariasen, W. H. J. Am. Chem. Soc. 1932, 54, 3841. (3) Greaves, G. N.; Fontaine, A.; Lagarde, P.; Raoux, D.; Gurman, S. J. Nature 1981, 293, 611. (4) Greaves, G. N. J. Non-Cryst. Solids 1985, 71, 203. (5) Wright, A. C.; Leadbetter, A. J. Physics Chem. Glasses 1976, 17, 122. (6) Phillips, J. C. J. Non-Cryst. Solids 1984, 63, 347. (7) Hosemann, R.; Hentschel, M. P.; Schmeisser U.; Bru¨ckner, R. J. Non-Cryst. Solids 1986, 83, 223. (8) Gladden, L. F. J. Non-Cryst. Solids 1990, 119, 318. (9) Warren, B. E. J. Am. Ceram. Soc. 1941, 24, 256. (10) Wright, A. C. J. Non-Cryst. Solids 1988, 106, 1. (11) Mozzi, R. L.; Warren, B. E. J. Appl. Crystallogr. 1969, 2, 164. (12) Beall, G. H. Silica, Physical Behavior, Geochemistry and Materials Applications in ReViews in Mineralogy; P. J. Heaney, C. T. Prewitt, G. V. Gibbs, Eds.; Minerological Society of America: Washington, DC, 1994; Vol. 25. (13) Cable M. Models for the Structure of Amorphous Solids in Glasses and Amorphous Materials in Materials Science and Technology, Zarzycki, J., Ed.; VCH: Weinheim, 1991; Vol. 9. (14) Dupree, R.; Holland, D.; Mortuza, M. G. J. Non-Cryst. Solids 1990, 116, 148.

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