- Email: [email protected]
Potassium surface enrichment in mixed alkali glass irradiated with electrons
LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 337 (2004) 268–271 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Potassium surface enrichment in mixed alkali glass irradiated with electrons Josef Zemek a, Ondrej Gedeon
b,*
a b
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic Received 16 February 2004; received in revised form 29 March 2004
Abstract Sodium-potassium-silicate glass was irradiated with electrons of energy of 1600 eV. The changes in the surface composition were analyzed by means of the angular-resolved X-ray induced photoelectron spectroscopy (ARXPS). Low electron dose irradiation enriched the uppermost glass surface with alkali ions, considerably more with potassium than with sodium ions. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.80.-x
1. Introduction A solid irradiated with a high-energy electron beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. This irradiation introduces disorder in the original structure, e.g. causing amorphization of crystalline samples accompanied with volume changes [2]. A study of the influence of electron bombardment on the alkali glass helps us to understand the elemental processes induced in the irradiated glass. Interaction of electrons with glass is also an important point in the radioactive waste deposition, in which electrons produced with various energies lead eventually to the irreversible changes in glass. One of the most interesting and widely studied phenomena in alkali-oxide glass irradiated with electron beams is the alkali ion migration. The phenomenon can be observed even for the very low-energy beams. The changes induced in glass have been related with alternations in alkali spectrum-line found in electron probe microanalysis (EPMA) [3–5] or in Auger electron spectroscopy (AES) [6–9]. *
Corresponding author. Tel.: +420-2 2431 1918; fax: +420-2 2431 3200. E-mail address: [email protected] (O. Gedeon). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.04.025
While one can observe alkali migration to the bulk of the glass by EPMA, usage of the surface sensitive methods, on the other side, yields the alkali surface enrichment. A formation of neutral alkali atoms was suggested [7,10] as a consequence of the electron irradiation. Later, Brow [11] has shown that metallic sodium can be found on the surface of irradiated binary sodium-silicate glass. Application of the top-surface sensitive ion scattering spectroscopy (ISS) enabled Then and Pantano [12] to observe the alkali signal increase (alkali surface concentration) as a direct consequence of the low-dose electron irradiation. It was another confirmation of alkali migration towards the surface under a low-dose electron bombardment. Miotello et al. [13] suggested Gibbsian segregation of alkali metal species to be a driving force for the out-migration. They were able to explain the intensity changes of the alkali signal on base of the continuity equation, in which gradients of the electrochemical potential and Gibbsian segregation stand as formal forces. The equation was completed with an ‘evaporation law’ at the surface. However, this formal set of macroscopic equations says little about the real mechanism of the alkali migration. On the other hand, the surface potassium enrichment was recently demonstrated by XPS for potassium-lime-silicate glass irradiated with a low electron dose [14]. Here, the increase of the potassium signal (and simultaneous
LETTER TO THE EDITOR
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
decrease of calcium signal) with a low-dose electron irradiation was suggested to be caused only by the surface relaxation, accompanied by the potassium topmost surface layer enrichment. The mixed alkali effect (MAE), referring to the nonlinear change of many physical properties, most notably the mobility, as a result of the replacement of the alkali ion by other alkali element, is well known among glass scientists more than a century. Although both the sodium and potassium are monovalent, and therefore both elements play very similar role in the chemistry of glass, different structural environments around the ions are expected due to their size. The mean distance of the for sodium and alkali ion-oxygen is around 2.3 A around 2.6 A for potassium [15]. The larger ion size for potassium also results in the higher average number of nearest neighbors in glass bulk and also in a slower diffusion. The different structural arrangements around different alkali elements serve then as a basic stones for an explanation of MAE. The dynamic structure model (DSM) [16] distinguishes optimized sites for an alkali ion, where the site meets chemical requirements of the cation, ‘possible’ (convertible) sites, which need to be converted to optimized ones, and finally sites optimized to the other alkali cation. MAE is then explained as a result of seeking proper sites in the environment where the competing cations form sites optimized for them. The resulting migration pathways are then strongly correlated with a distribution of the other alkali cation. However, it has to be stressed in connection with the presented paper that all results reporting MAE are based on bulk experience and cannot be directly transferred to the situation, where migration is strongly influenced with a real-surface-vacuum interface. Moreover, the differences of roles played by sodium and potassium cations in surface processes are only poorly known. On the other hand, the high-energy (50 keV) electron irradiation of sodium-potassium-silicate glass confirmed MAE under electron irradiation [17]. In this paper, the changes in composition of sodiumpotassium-silicate glass surface irradiated with low-energy electrons are studied by means of angular-resolved XPS. An attention is given to differences between sodium and potassium ions in their mobility, and their role in the surface relaxation.
2. Experimental Glass was melted using a high purity batch in a Pt crucible and cast on a metal plate to prevent crystallization. The plate of glass was then annealed for 16 h. The glass was transparent and contained no bubbles or cords. The composition of glass was: 85 mol% SiO2 , 5.0 mol% Na2 O, and 10.0 mol% K2 O. Before irradiation with electrons, a clean glass surface was prepared by
269
fracturing of the glass rod under UHV condition using a standard cleaving device.The clean pristine glass surface was step-by-step irradiated by electron beam of energy of 1600 eV, 0.1–0.2 A/m2 the beam current density, and scanned uniformly over fractured glass surface. The impact angle of the electron beam was perpendicular to the sample surface. The composition of the freshly fractured and electron irradiated glass surfaces was studied by an ADES 400 angular-resolved photoelectron spectrometer (VG Scientific, UK) equipped with a twin anode X-ray source, standard Al/Mg anodes, and a hemispherical analyzer. Spectra were recorded using Mg Ka source, operated at a power of 110 W and at constant pass energy of 100 eV. The detection angles were 0° and 60° counted from the surface normal. Although some changes introduced by X-rays in glass are reported [18,19], our exposure conditions were very mild: non-focused beam, low source power, and low-photon density were used to eliminate the influence of X-ray irradiation during spectra recording. It needs to be underlined that no electron flooding was used during spectra recordings. As a result, the surface was slightly positively charged. However, the charging continuously decreased during experiment from the initial value of 3.5 eV to the final value of 2.3 eV. In other words, the surface has been becoming more and more conductive with increasing electron dose. Peak areas were determined following the Shirley’s inelastic background subtraction method [20]. Atomic concentrations of elements found in the analyzed thickness were determined from the O 1s, Si 2p, Na 1s, and K 2p lines assuming a simple model of a homogeneous semi-infinite solid [21]. The relative sensitivity factor approach [22] was applied to calculate atomic concentrations. The peak areas were corrected for ii(i) the measured transmission function of the spectrometer, which comprises all instrumental factors influencing the measured quantity [23], i(ii) the photoelectric cross-sections [24], and (iii) the inelastic mean free paths [25] of photoelectrons in question, determining their motion in SiO2 . Experimental uncertainties accompanied with XPS quantitative analysis are estimated to be 7%. The value covers overall uncertainties of the method that are mostly introduced by the background subtraction and the correction procedure used for the calculation of concentrations from intensities of spectral lines.
3. Results and discussion The photoelectron spectra were recorded under two distinctly different detection angles. Ignoring effects of electron elastic scattering, the mean escape depth [26] of
LETTER TO THE EDITOR
270
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
photoelectrons is described by k cos a, where k is the inelastic mean free path (IMFP) [25] and a is the detection angle. For the detection angle of 60°, the corresponding mean escape depth is one half of that analyzed with the normal detection angle. Therefore, quantitative results obtained at the detection angle of 60° can be understood as more surface sensitive and can be considered as average concentrations within the top surface layer. Typically, the IMFP values are about 2 nm for Si 2p, O 1s and K 2p photoelectrons in SiO2 excited by Mg Ka radiation (1254 eV) [25]. It is worth mentioning, that the IMFP of the Na 1s electrons is as low as 0.6 nm due to their low kinetic energy. The freshly fractured glass surface was enriched by alkali ions with respect to the nominal bulk glass composition as the result of the surface relaxation processes. Similar results have been published recently for potassium-lime-silicate glass [14]. The sodium enrichment was significantly higher (2.1 times) than the potassium one (1.7 times). In addition, the topmost layer was even more enriched by sodium (2.8 times). On the other hand, the topmost layer was depleted by potassium in comparison to the bulk. The sodium surface enrichment can be rationalized by its higher mobility. The mobility of the ion restricted to the short distances (neighbor sites) is determined by the bond strength, availability of the proper site (convertible or optimized, in terms of DSM), and geometry of the site in regard of its size. In case of a large cation, the hop to the available site can be succeeded only after some structural rearrangement, which opens the doorway for the hop [27]. Mobility to longer distances is correlated with connectivity of the proper sites, or, in other word, with an existence of diffusion pathways. The diffusion coefficients for the composition of the used glass should be close to the ‘mobility crossover’ [28], where the mobility of both alkali species is the same. Although some experimental results indicate that the diffusion coefficient of sodium is still higher for
our glass [29], it is questionable if such information can be simple transferred to the mobility of ions near glass fresh-surface. A large number of charge defects need to be compensated promptly. Hence, the time scale of mobility plays an important role. The sodium ions near the surface can compensate surface defects more quickly unlike potassium ions, which need some time for opening their doorways in the structure. After low dose electron irradiation further surface alkali enrichment proceeds. Similar surface enrichment by alkali atoms as potassium, rubidium and caesium studied by ISS was observed for electron irradiated glass surfaces at the similar electron dose [12]. Fig. 1 well demonstrates the remarkable topmost enrichment with potassium ions accompanied with a mild decrease of potassium in the thicker layer. The maximum enrichment is reached for the relatively low dose of 25 C/m2 . The glass with covalently bonded silica sub-network loosens with electron irradiation and positively charged point defects can be quickly compensated in the environment rich to electrons. As a result the irradiated region is negatively charged with a number of negatively charged point defects. It is unclear, however, if the enhanced mobility of alkali species under electron irradiation is a result of a formation of new point defects (new migration paths) predicted by point defect model (PDM) of ion diffusion in glass [30]. The loosened structure also makes potassium more mobile, as its mobility is more restricted by glass structure than sodium ions. As a result, potassium ions can compete with sodium ions in a more effective compensation of surface defects. It is suggested, the potassium surface enrichment is caused by the energetically favorable replacement of sodium ions by potassium ions. In other words, the potassium is driven towards surface by thermodynamics of the surface relaxation, which overrides the kinetics dominant in relaxation processes directly after the glass fracture.
7
Relative concentrations
6 5 4 3 2 1 0 0
200
400
600
800
1000
1200
1400
1600
1800
2
Dose [C/m ]
Fig. 1. Concentrations of potassium (diamonds) and sodium (squares) relative to their values after the glass fracture. Open symbols correspond to the relative concentrations obtained under low-angle (the topmost surface layer) while the closed symbols mark results yield from the thicker surface layer. The lines connected symbols serve only as a guide for an eye.
LETTER TO THE EDITOR
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
The potassium concentration in the thicker layer decreases with dose. The irradiated depth approximately corresponds to the information depth of potassium and therefore potassium topmost surface enrichment supplied from the thicker layer cannot be fully compensated by potassium ions from deeper regions (not irradiated) due to the low potassium mobility. In addition, the transport of potassium cations to the topmost layer may cause the structure rearrangement, effectively resulting in the macroscopic compressive stress [31]. It further decreases the mobility, again more of potassium than sodium. Contrary to the potassium behavior, sodium concentration in the topmost layer follows the concentration changes in the thicker layer; again due to the relatively high, structurally more weakly dependent, sodium mobility.
4. Conclusion The mixed sodium-potassium-silicate glass was irradiated with low-energy electrons. After the low-dose irradiation the significantly high topmost surface enrichment by potassium is observed. It is attributed to the real-surface relaxation energetically favored by potassium ions.
Acknowledgements This work was financially supported by Grant Agency of the Czech Republic through the grant no. 104/03/0976. It was also part of the research project CEZ: MSM 223100002 Preparation and properties of advanced materials – modelling, characterization, and technology.
References [1] W.J. Weber, R.C. Ewing, C.A. Angell, G.W. Arnold, A.N. Cormack, J.M. Delaye, D.L. Griscom, L.W. Hobbs, A. Navrotsky, D.L. Price, A.M. Stoneham, M.C. Weinberg, J. Mater. Res. 12 (8) (1997) 1946.
271
[2] R.C. Ewing, W.J. Weber, V.F. Clinard Jr., Prog. Nucl. Energy 29 (2) (1995) 63. [3] A.K. Varshneya, A.R. Cooper, M. Cable, J. Appl. Phys. 37 (5) (1966) 2199. [4] L.F. Vassamillet, V.E. Caldwell, J. Appl. Phys. 40 (4) (1969) 1637. [5] O. Gedeon, V. Hulinsky, K. Jurek, Mikrochim. Acta 132 (Suppl.) (2000) 505. [6] J.P. Lacharme, P. Champion, D. Leger, Scanning Electron Microsc. (1981) 237. [7] R.J. Gossink, T.P.A. Lommen, Appl. Phys. Lett. 34 (1979) 444. [8] D.M. Usher, J. Phys. C 14 (1981) 2039. [9] A. Miotello, J. Phys. C 19 (1986) 445. [10] P.G. Whitkop, Surf. Sci. 110 (1981) 261. [11] R.K. Brow, J. Non-Cryst. Solids 175 (1994) 155. [12] A.M. Then, C.G. Pantano, J. Non-Cryst. Solids 120 (1990) 178. [13] A. Miotello, G. Cinque, P. Mazoldi, C.G. Pantano, Phys. Rev. B 43 (5) (1991) 3831. [14] O. Gedeon, J. Zemek, J. Non-Cryst. Solids 177 (2003) 178. [15] G.N. Greaves, S.J. Gurman, C.R.A. Catlow, A.V. Chadwick, S. Houde-Walter, B. Dobson, C.M.B. Henderson, Philos. Mag. A 64 (1991) 1059. [16] A. Bunde, M.D. Ingram, P. Maass, J. Non-Cryst. Solids 172 (1994) 1222. [17] O. Gedeon, M. Zimova, K. Jurek, J. Non-Cryst. Solids 279 (2001) 14. [18] J.E. Shelby, J. Appl. Phys. 51 (5) (1980) 2561. [19] A. Sharma, H. Jain, A.C. Miller, Surf. Interf. Anal. 31 (2001) 369. [20] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [21] C.S. Fadley, R.J. Baird, W. Sickhaus, T. Novakov, S.A. Bergstrom, J. Electron Spectrosc. Relat. Phenom. 93 (1974) 4. [22] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin–Elmer, Eden Prairie, MN, 1992. [23] P. Jiricek, Czech J. Phys. 44 (1994) 261. [24] I.M. Band, I.Y. Kharitonov, M.B. Trzhaskovskaya, Atomic Data Nucl. Data Tables 23 (1979) 443. [25] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interf. Anal. 20 (1993) 77. [26] J. Zemek, S. Hucek, A. Jablonski, I.S. Tilinin, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 443. [27] O. Gedeon, J. Machacek, M. Liska, Phys. Chem. Glasses 44 (2003) 331. [28] H. Jain, N.L. Peterson, H.L. Downing, J. Non-Cryst. Solids 55 (1983) 283. [29] H. Wabayashi, R. Teray, in: O.V. Mazurin, M.V. Streltsina, T.P. Svaiko-Svaikovskaya (Eds.), Handbook of Glass Data C, Elsevier, Amsterdam, 1987. [30] H. Jain, A. Sharma, Phys. Chem. Chem. Phys. 4 (2002) 3232. [31] M.D. Ingram, J.E. Davidson, A.M. Coats, E.I. Kamitsos, J.A. Kapoutsis, Glastech. Ber. Glass Sci. Technol. 73 (2000) 89.
Journal of Non-Crystalline Solids 337 (2004) 268–271 www.elsevier.com/locate/jnoncrysol
Letter to the Editor
Potassium surface enrichment in mixed alkali glass irradiated with electrons Josef Zemek a, Ondrej Gedeon
b,*
a b
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic Received 16 February 2004; received in revised form 29 March 2004
Abstract Sodium-potassium-silicate glass was irradiated with electrons of energy of 1600 eV. The changes in the surface composition were analyzed by means of the angular-resolved X-ray induced photoelectron spectroscopy (ARXPS). Low electron dose irradiation enriched the uppermost glass surface with alkali ions, considerably more with potassium than with sodium ions. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 61.80.-x
1. Introduction A solid irradiated with a high-energy electron beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. This irradiation introduces disorder in the original structure, e.g. causing amorphization of crystalline samples accompanied with volume changes [2]. A study of the influence of electron bombardment on the alkali glass helps us to understand the elemental processes induced in the irradiated glass. Interaction of electrons with glass is also an important point in the radioactive waste deposition, in which electrons produced with various energies lead eventually to the irreversible changes in glass. One of the most interesting and widely studied phenomena in alkali-oxide glass irradiated with electron beams is the alkali ion migration. The phenomenon can be observed even for the very low-energy beams. The changes induced in glass have been related with alternations in alkali spectrum-line found in electron probe microanalysis (EPMA) [3–5] or in Auger electron spectroscopy (AES) [6–9]. *
Corresponding author. Tel.: +420-2 2431 1918; fax: +420-2 2431 3200. E-mail address: [email protected] (O. Gedeon). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.04.025
While one can observe alkali migration to the bulk of the glass by EPMA, usage of the surface sensitive methods, on the other side, yields the alkali surface enrichment. A formation of neutral alkali atoms was suggested [7,10] as a consequence of the electron irradiation. Later, Brow [11] has shown that metallic sodium can be found on the surface of irradiated binary sodium-silicate glass. Application of the top-surface sensitive ion scattering spectroscopy (ISS) enabled Then and Pantano [12] to observe the alkali signal increase (alkali surface concentration) as a direct consequence of the low-dose electron irradiation. It was another confirmation of alkali migration towards the surface under a low-dose electron bombardment. Miotello et al. [13] suggested Gibbsian segregation of alkali metal species to be a driving force for the out-migration. They were able to explain the intensity changes of the alkali signal on base of the continuity equation, in which gradients of the electrochemical potential and Gibbsian segregation stand as formal forces. The equation was completed with an ‘evaporation law’ at the surface. However, this formal set of macroscopic equations says little about the real mechanism of the alkali migration. On the other hand, the surface potassium enrichment was recently demonstrated by XPS for potassium-lime-silicate glass irradiated with a low electron dose [14]. Here, the increase of the potassium signal (and simultaneous
LETTER TO THE EDITOR
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
decrease of calcium signal) with a low-dose electron irradiation was suggested to be caused only by the surface relaxation, accompanied by the potassium topmost surface layer enrichment. The mixed alkali effect (MAE), referring to the nonlinear change of many physical properties, most notably the mobility, as a result of the replacement of the alkali ion by other alkali element, is well known among glass scientists more than a century. Although both the sodium and potassium are monovalent, and therefore both elements play very similar role in the chemistry of glass, different structural environments around the ions are expected due to their size. The mean distance of the for sodium and alkali ion-oxygen is around 2.3 A around 2.6 A for potassium [15]. The larger ion size for potassium also results in the higher average number of nearest neighbors in glass bulk and also in a slower diffusion. The different structural arrangements around different alkali elements serve then as a basic stones for an explanation of MAE. The dynamic structure model (DSM) [16] distinguishes optimized sites for an alkali ion, where the site meets chemical requirements of the cation, ‘possible’ (convertible) sites, which need to be converted to optimized ones, and finally sites optimized to the other alkali cation. MAE is then explained as a result of seeking proper sites in the environment where the competing cations form sites optimized for them. The resulting migration pathways are then strongly correlated with a distribution of the other alkali cation. However, it has to be stressed in connection with the presented paper that all results reporting MAE are based on bulk experience and cannot be directly transferred to the situation, where migration is strongly influenced with a real-surface-vacuum interface. Moreover, the differences of roles played by sodium and potassium cations in surface processes are only poorly known. On the other hand, the high-energy (50 keV) electron irradiation of sodium-potassium-silicate glass confirmed MAE under electron irradiation [17]. In this paper, the changes in composition of sodiumpotassium-silicate glass surface irradiated with low-energy electrons are studied by means of angular-resolved XPS. An attention is given to differences between sodium and potassium ions in their mobility, and their role in the surface relaxation.
2. Experimental Glass was melted using a high purity batch in a Pt crucible and cast on a metal plate to prevent crystallization. The plate of glass was then annealed for 16 h. The glass was transparent and contained no bubbles or cords. The composition of glass was: 85 mol% SiO2 , 5.0 mol% Na2 O, and 10.0 mol% K2 O. Before irradiation with electrons, a clean glass surface was prepared by
269
fracturing of the glass rod under UHV condition using a standard cleaving device.The clean pristine glass surface was step-by-step irradiated by electron beam of energy of 1600 eV, 0.1–0.2 A/m2 the beam current density, and scanned uniformly over fractured glass surface. The impact angle of the electron beam was perpendicular to the sample surface. The composition of the freshly fractured and electron irradiated glass surfaces was studied by an ADES 400 angular-resolved photoelectron spectrometer (VG Scientific, UK) equipped with a twin anode X-ray source, standard Al/Mg anodes, and a hemispherical analyzer. Spectra were recorded using Mg Ka source, operated at a power of 110 W and at constant pass energy of 100 eV. The detection angles were 0° and 60° counted from the surface normal. Although some changes introduced by X-rays in glass are reported [18,19], our exposure conditions were very mild: non-focused beam, low source power, and low-photon density were used to eliminate the influence of X-ray irradiation during spectra recording. It needs to be underlined that no electron flooding was used during spectra recordings. As a result, the surface was slightly positively charged. However, the charging continuously decreased during experiment from the initial value of 3.5 eV to the final value of 2.3 eV. In other words, the surface has been becoming more and more conductive with increasing electron dose. Peak areas were determined following the Shirley’s inelastic background subtraction method [20]. Atomic concentrations of elements found in the analyzed thickness were determined from the O 1s, Si 2p, Na 1s, and K 2p lines assuming a simple model of a homogeneous semi-infinite solid [21]. The relative sensitivity factor approach [22] was applied to calculate atomic concentrations. The peak areas were corrected for ii(i) the measured transmission function of the spectrometer, which comprises all instrumental factors influencing the measured quantity [23], i(ii) the photoelectric cross-sections [24], and (iii) the inelastic mean free paths [25] of photoelectrons in question, determining their motion in SiO2 . Experimental uncertainties accompanied with XPS quantitative analysis are estimated to be 7%. The value covers overall uncertainties of the method that are mostly introduced by the background subtraction and the correction procedure used for the calculation of concentrations from intensities of spectral lines.
3. Results and discussion The photoelectron spectra were recorded under two distinctly different detection angles. Ignoring effects of electron elastic scattering, the mean escape depth [26] of
LETTER TO THE EDITOR
270
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
photoelectrons is described by k cos a, where k is the inelastic mean free path (IMFP) [25] and a is the detection angle. For the detection angle of 60°, the corresponding mean escape depth is one half of that analyzed with the normal detection angle. Therefore, quantitative results obtained at the detection angle of 60° can be understood as more surface sensitive and can be considered as average concentrations within the top surface layer. Typically, the IMFP values are about 2 nm for Si 2p, O 1s and K 2p photoelectrons in SiO2 excited by Mg Ka radiation (1254 eV) [25]. It is worth mentioning, that the IMFP of the Na 1s electrons is as low as 0.6 nm due to their low kinetic energy. The freshly fractured glass surface was enriched by alkali ions with respect to the nominal bulk glass composition as the result of the surface relaxation processes. Similar results have been published recently for potassium-lime-silicate glass [14]. The sodium enrichment was significantly higher (2.1 times) than the potassium one (1.7 times). In addition, the topmost layer was even more enriched by sodium (2.8 times). On the other hand, the topmost layer was depleted by potassium in comparison to the bulk. The sodium surface enrichment can be rationalized by its higher mobility. The mobility of the ion restricted to the short distances (neighbor sites) is determined by the bond strength, availability of the proper site (convertible or optimized, in terms of DSM), and geometry of the site in regard of its size. In case of a large cation, the hop to the available site can be succeeded only after some structural rearrangement, which opens the doorway for the hop [27]. Mobility to longer distances is correlated with connectivity of the proper sites, or, in other word, with an existence of diffusion pathways. The diffusion coefficients for the composition of the used glass should be close to the ‘mobility crossover’ [28], where the mobility of both alkali species is the same. Although some experimental results indicate that the diffusion coefficient of sodium is still higher for
our glass [29], it is questionable if such information can be simple transferred to the mobility of ions near glass fresh-surface. A large number of charge defects need to be compensated promptly. Hence, the time scale of mobility plays an important role. The sodium ions near the surface can compensate surface defects more quickly unlike potassium ions, which need some time for opening their doorways in the structure. After low dose electron irradiation further surface alkali enrichment proceeds. Similar surface enrichment by alkali atoms as potassium, rubidium and caesium studied by ISS was observed for electron irradiated glass surfaces at the similar electron dose [12]. Fig. 1 well demonstrates the remarkable topmost enrichment with potassium ions accompanied with a mild decrease of potassium in the thicker layer. The maximum enrichment is reached for the relatively low dose of 25 C/m2 . The glass with covalently bonded silica sub-network loosens with electron irradiation and positively charged point defects can be quickly compensated in the environment rich to electrons. As a result the irradiated region is negatively charged with a number of negatively charged point defects. It is unclear, however, if the enhanced mobility of alkali species under electron irradiation is a result of a formation of new point defects (new migration paths) predicted by point defect model (PDM) of ion diffusion in glass [30]. The loosened structure also makes potassium more mobile, as its mobility is more restricted by glass structure than sodium ions. As a result, potassium ions can compete with sodium ions in a more effective compensation of surface defects. It is suggested, the potassium surface enrichment is caused by the energetically favorable replacement of sodium ions by potassium ions. In other words, the potassium is driven towards surface by thermodynamics of the surface relaxation, which overrides the kinetics dominant in relaxation processes directly after the glass fracture.
7
Relative concentrations
6 5 4 3 2 1 0 0
200
400
600
800
1000
1200
1400
1600
1800
2
Dose [C/m ]
Fig. 1. Concentrations of potassium (diamonds) and sodium (squares) relative to their values after the glass fracture. Open symbols correspond to the relative concentrations obtained under low-angle (the topmost surface layer) while the closed symbols mark results yield from the thicker surface layer. The lines connected symbols serve only as a guide for an eye.
LETTER TO THE EDITOR
J. Zemek, O. Gedeon / Journal of Non-Crystalline Solids 337 (2004) 268–271
The potassium concentration in the thicker layer decreases with dose. The irradiated depth approximately corresponds to the information depth of potassium and therefore potassium topmost surface enrichment supplied from the thicker layer cannot be fully compensated by potassium ions from deeper regions (not irradiated) due to the low potassium mobility. In addition, the transport of potassium cations to the topmost layer may cause the structure rearrangement, effectively resulting in the macroscopic compressive stress [31]. It further decreases the mobility, again more of potassium than sodium. Contrary to the potassium behavior, sodium concentration in the topmost layer follows the concentration changes in the thicker layer; again due to the relatively high, structurally more weakly dependent, sodium mobility.
4. Conclusion The mixed sodium-potassium-silicate glass was irradiated with low-energy electrons. After the low-dose irradiation the significantly high topmost surface enrichment by potassium is observed. It is attributed to the real-surface relaxation energetically favored by potassium ions.
Acknowledgements This work was financially supported by Grant Agency of the Czech Republic through the grant no. 104/03/0976. It was also part of the research project CEZ: MSM 223100002 Preparation and properties of advanced materials – modelling, characterization, and technology.
References [1] W.J. Weber, R.C. Ewing, C.A. Angell, G.W. Arnold, A.N. Cormack, J.M. Delaye, D.L. Griscom, L.W. Hobbs, A. Navrotsky, D.L. Price, A.M. Stoneham, M.C. Weinberg, J. Mater. Res. 12 (8) (1997) 1946.
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