B K-Edge XANES of Superstructural Units in Borate Glasses

O. Sipr*, A. Simunek* and F. Rocca1

*Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-162 53 Prague, Czech Republic

^IFN-CNR, Istituto di Fotonica e Nanotecnologie del Consiglio Nazionale delle Ricerche, Sezione "CeFSA " di Trento, Via Sommarive 18,1-38050 Povo (Trento), Italy

Abstract. The potential of x-ray absorption near-edge structure (XANES) spectroscopy for studying medium range order in borate glasses is assessed by theoretical modelling of the spectra. B K edge XANES is calculated in case that B atoms are located in isolated B03 and B04 units and in case that B atom are located in superstructural units of 9-15 atoms. It is found that boroxol ring and diborate and ditriborate superstructural units give rise to spectra which differ from spectra obtained by a mere superposition of spectra of isolated B03 and B04 units. On the other hand, spectra of pentaborate and triborate units do not differ significantly from spectra of isolated B03 and B04 .

Keywords: borate glasses,structure,XANES PACS: 61.43.Fs,61.10.Ht,81.05.Kf,66.30.Hs

INTRODUCTION

It is generally believed that borate glasses are formed by interpenetrating networks of basic superstructural units of B and O atoms (such as boroxol rings), with metal ions located in the voids [1]. The superstructural units consist of about 10-15 atoms and contain B atoms both in three­fold and in four-fold coordinations. Knowing the distri­bution of B atoms among various sites would be very helpful in understanding the variety of properties of bo­rate glasses. While nuclear magnetic resonance (NMR) is quite reliable in quantifying the ratio of B atoms in three-fold and four-fold coordinated sites, it is much less able to distinguish between various superstructural units [1, 2, 3, 4]. However, such information could be in prin­ciple attainable by means of x-ray absorption near-edge structure (XANES) analysis. Moreover, x-ray absorption techniques may be applied to a wide set of samples, in­cluding thin films, surfaces, diluted systems etc., where NMR cannot be used. Therefore, it would be interesting to know how sensitive B K-edge XANES is to the short-range and medium-range order in these materials.

Recently, several measurements of B K-edge XANES of borate glasses were made by different groups [5, 6, 7, 8]. Theoretical [9] as well as experimental [10, 11, 12] analysis of spectra of B-containing minerals demon­strated that B K edge XANES can distinguish whether B atoms are located at three-fold coordinated sites ^ B or whether they are located at four-fold coordinated sites I41B. For further progress, it would be highly desirable to know to what extent the XANES spectroscopy can be used for studying the distribution of B atoms not just among ^ B and ^ B sites but also among superstructural units. In order to achieve this, we calculate XANES spec­

tra of several superstructural units and search for signifi­cant features characteristic for their geometry.

COMPUTATIONAL METHOD

Of the many superstructural units which can be identi­fied in borate crystals, we concentrated on those which are displayed schematically in Figure 1. When occur­ring in various borate crystals, these units are usually slightly distorted. Here we assume them in an idealized geometry as if assembled from rigid B 0 3 triangles and

B 0 4 tetrahedra, with B - 0 distances R P]B-O"

=1.36 A and

R[4]B_0=1.47 A. The B K edge XANES spectra of these systems were calculated ab initio via a real-space full-multiple-scattering technique [13]. The muffin-tin shape of the potential was assumed.

Construction of the Potential

Our focus is on superstructural units in glasses. There­fore, we have to employ a scattering potential which — apart from reflecting the influence of the B and O atoms of the superstructural unit itself — takes into account also the influence of the other atoms of the solid. This is not a straightforward task in a glass because the posi­tions of more distant atoms are not fixed and generally not even known. An earlier study found, however, that B K edge XANES of disordered systems is dominated by the geometry around the B atoms and not by the potential [9]. Therefore, it will be sufficient to deal with the more distant atoms just in an approximative way.

CP882, X-ray Absorption Fine Structure—XAFS13 edited by B. Hedman and P. Pianetta

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boroxol ring ditriborate

FIGURE 1. Structural diagrams of superstructural units in­vestigated in this work.

In this work, the scattering potentials are constructed by averaging over muffin-tin potentials generated for several borate crystals which contain the superstruc­tural unit in question; such a procedure is analogous to the virtual crystal approximation (VCA). In particu­lar, potentials of crystalline B203 , calciborite, ludwigite and vonsenite were used for obtaining the potential of isolated B03 , potentials of high-pressure phase B 20 3 , axinite, danburite, datolite and sinhalite were used for obtaining the potential of isolated B0 4 , potentials of Cs20-9B203 and K 2OB 20 3 were used for obtaining the potential of boroxol ring, potentials of aNa20-3B203 , K20-2B203,Li20-2B203 andZnO2B203 were used for obtaining the potential of the diborate unit, potentials of Ag2OB203 , BaO-2B203 and K20-2B203 were used for obtaining the potential of the ditriborate unit, po­tentials of aNa20-3B203, K20-5B203, 5K2019B203

and Na 2 04B 2 0 3 were used for obtaining the potential of the pentaborate unit and potentials of Ag 2 04B 2 0 3 , BaO-2B203, /3Na20-3B203, CaOB203 , Cs20-3B203

and Li20-3B203 were used for obtaining the potential of the triborate unit.

The potentials of individual crystals (prior to averag­ing) were generated according to the Mattheiss prescrip­tion. The Is core hole was taken into account by relying on the final-state rule ("relaxed and screened" approxi­mation). We verified that our conclusions do not depend

on the particular choice of crystals from which the aver­age potential is formed.

RESULTS

With the exception of the boroxol ring, all the superstruc­tural units in Figure 1 contain two inequivalent B atoms. Therefore, the spectrum of the unit is a superposition of partial spectra generated at PlB and ^ B sites. Our aim is to find out whether XANES can distinguish between systems in which B atoms are located in isolated B0 3

and B04 units on the one side and systems in which B atoms are located in superstructural units on the other side. Therefore, the spectrum of each superstructural unit is compared with an appropriately weighted superposi­tion of spectra of isolated B0 3 and B04 units. The re­sults are summarized in Figure 2. Note that spectra gen­erated at the ^ B sites are characterized by a pre-peak at ~10 eV and a main peak at 19-20 eV and spectra gen­erated at the ^ B sites are characterized by a single main peak at 15 eV (see our earlier work [9] on the differences between XANES generated at three-fold and four-fold coordinated B atoms).

It follows from Figure 2 that in a system with only three-fold coordinated B atoms (such as B 2 0 3 glass [14]), one could be able to identify the signal coming from the boroxol ring from the small peak around 25 eV because it appears in a region where no other peaks oc­cur. XANES spectroscopy could thus contribute to the complex and important task of quantifying the ratio of boroxol rings in vitreous B 2 0 3 [2, 3, 15, 16, 17, 18].

Possibly, one might also be able to deduce the pres­ence of diborate and ditriborate units from the small shoulders occurring at the high-energy side of the main PlB peak (around 21 eV). However, this feature could be too faint to be identifiable.

Spectra of pentaborate and triborate units do not sig­nificantly differ from a superposition of spectra of iso­lated B03 and B04 units, meaning that these units cannot be identified via XANES.

SUMMARY

Theoretical B K edge XANES spectra of borate super-structural units are quite robust with respect to the choice of the scattering potential, implying that it is determined first of all by the geometry. The presence of boroxol rings and possibly also of diborate and ditriborate units in bo­rate glasses could be inferred features in their from B K edge XANES. On the other hand, spectra of pentaborate and triborate units are very similar to spectra of isolated B03 and B04 units.

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10 15 20 25 energy (arb. origin) [eV]

FIGURE 2. Thin lines represent individual spectra gener­ated at K B and ^ B sites in isolated B0 3 and B04 (full grey lines) and in superstructural units (dashed black lines). Spectra of whole superstructural units were obtained as appropriately weighted averages of individual spectra generated at ^B and ^ B sites either in isolated B0 3 and B04 (thick full grey lines) or in superstructural units (thick dashed black lines).

ACKNOWLEDGMENTS

This work was supported by the CNR-AV CR Common Research Project "Local order in nanosize and disordered systems" and by the GA AV project IAA100100514. The research in the Institute of Physics AS CR was supported by the project AV0Z-10100521 of AV CR.

REFERENCES

1. P. J. Bray, "NMR and NQR studies of borates and borides," in Borate glasses, crystals and melts, edited by A. C. Wright, S. A. Feller, and A. C. Hannon, The Society of Glass Technology, Sheffield, 1997, p. 1.

2. J. Tossell, Journal Of Non-Crystalline Solids 183, 307 (1995).

3. P. Umari, and A. Pasquarello, Phys. Rev. Lett. 95, 137401 (2005).

4. J. Zwanziger, Solid State Nuclear Magnetic Resonance 27, 5 (2005).

5. R. Carboni, G. Pacchioni, M. Fanciulli, A. Giglia, N. Mahne, M. Pedio, S. Nannarone, and F. Boscherini, Appl. Physics Lett. 83, 4312 (2003).

6. K. Handa, J. Ide, T. Koganezawa, K. Ozutsumi, G. Dalba, O. Norikazu, and N. Umesaki, European Journal of Glass Science & Technology Part B to be published (2006).

7. G. Yang, G. Moebus, and R. J. Hand, European Journal of Glass Science & Technology Part B to be published (2006).

8. F. Rocca, C. Armellini, F. Boscherini, R. Carboni, M. Malvestuto, and O. Sipr, European Journal of Glass Science & Technology Part B to be published (2006).

9. O. Sipr, A. Simunek, J. Vackar, G. Dalba, and F. Rocca, European Journal of Glass Science & Technology Part B to be published (2006).

10. H. Sauer, R. Brydson, P. Rowley, W. Engel, and J. Thomas, Ultramicroscopy 49, 198 (1993).

11. L. Garvie, H. Hubert, W. Petuskey, P. Mcmillan, and P. Buseck, Journal Of Solid State Chemistry 133, 365 (1997).

12. M. Fleet, and S. Muthupari, Journal Of Non-Crystalline Solids 255,233(1999).

13. D. D. Vvedensky, "Theory of X-ray Absorption Fine Structure," in Unoccupied Electronic States, edited by J. C. Fuggle, and J. E. Inglesfield, Springer, Berlin, 1992, p. 139.

14. A. H. Silver, and P. J. Bray, /. Chem. Phys. 29, 984 (1958). 15. A. Hannon, D. Grimley, R. Hulme, A. Wright, and

R. Sinclair, Journal Of Non-Crystalline Solids 111, 299 (1994).

16. J. Swenson, and L. Borjesson, Physical Review B 55, 11138(1997).

17. C. Joo, U. Werner-Zwanziger, and J. Zwanziger, Physics And Chemistry Of Glasses 41, 317 (2000).

18. J. Sakowski, and G. Herms, Journal Of Non-Crystalline Solids 292,, 304 (2001).

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Melville, New York, 2007AIP CONFERENCE PROCEEDINGS VOLUME 882

X-RAY ABSORPTION

FINE STRUCTURE—XAFS13

13th International ConferenceStanford, California, U.S.A. 9 - 14 July 2006

EDITORSBritt Hedman

Piero PianettaStanford Synchrotron Radiation Laboratory

Stanford, California, U.S.A.

SPONSORING ORGANIZATIONInternational XAFS Society