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Pb2+ environment in lead silicate glasses probed by Pb-LIII edge XAFS and 207Pb NMR
Journal of Non-Crystalline Solids 243 (1999) 39±44
Pb2 environment in lead silicate glasses probed by Pb-LIII edge XAFS and 207Pb NMR F. Fayon a, C. Landron a
a,b,*
, K. Sakurai c, C. Bessada a, D. Massiot
a
Centre de Recherches sur les Mat eriaux a Haute Temp erature, 1D Ave de la Recherche Scienti®que, 45071 Orl eans cedex 2, France b Laboratoire pour l'Utilisation du Rayonnement Electromagn etique, 91405, Orsay cedex, France c National Research Institute for Metals, Sengen, Tsukuba, Ibaraki 305, Japan Received 16 November 1997; received in revised form 18 September 1998
Abstract The local structural environment of lead (Pb) atoms in lead silicate glasses has been studied by two complementary spectroscopic techniques: Pb-LIII edge X-ray absorption ®ne structure (XAFS) and 207 Pb solid state nuclear magnetic resonance (NMR). XAFS results have been obtained with two kinds of experimental devices: a synchrotron radiation source and a laboratory spectrometer. The weak photon ¯ux is then compensated by a higher stability of the source and a longer acquisition time. The experiments were carried out on selected compositions of lead silicate glasses in which we observed PbO3 and PbO4 pyramidal units with short Pb±O bond lengths typical of a covalent bonding state. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In binary lead silicate glasses, lead atoms are often described as glass former at high lead content and as network modi®er at low lead content [1±7]. Such dierences in the structural role of Pb2 would imply signi®cant modi®cations of its local environment and of its coordination. Both solid state nuclear magnetic resonance (NMR) spectroscopy [8,9] and X-ray absorption ®ne structure (XAFS) spectroscopy [10±12] have proven to be powerful tools in elucidating the local environments of atoms in disordered systems like glasses. XAFS is based on the interpretation of the interference between outgoing and backscattered
* Corresponding author. Tel.: +33-2 38 51 55 31; fax: +33-2 38 63 81 03; e-mail: [email protected].
photoelectrons emitted during an X-ray absorption process. Thus, XAFS describes the arrangement of atoms through atomic radial distribution functions. Structural information related to the symmetry of the coordination shell can be deduced from the NMR parameters: isotropic chemical shift and chemical shift anisotropy. In a previous study we have investigated a wide range of compositions (from 30 to 70 mol% PbO) in the PbO±SiO2 system using 29 Si and 207 Pb high resolution solid state NMR and observed that the glassy network was made of SiO4 tetrahedra and covalent PbOn pyramids [13]. In order to improve the understanding of the lead silicate glass structure, we have combined, in the present work, data obtained by 207 Pb NMR and XAFS on lead silicate glasses to characterize the local environments of lead atoms and their distributions over a large range of compositions.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 0 9 - 6
40
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
2. Experimental 2.1. Sample preparation Three lead silicate glasses containing 31, 50.5 and 66 mol% of PbO (named A, B and C, respectively) were prepared from reagent grade PbO and SiO2 . The glass samples (3 g) were melted in Pt crucibles in an electric furnace under air, and quenched by partially immersing the crucibles in water. Melting temperatures are reported in Table 1. Melting times were minimized in order to reduce the volatilization of PbO. An additional 0.02 mol% Fe2 O3 was included in the batch to reduce the 207 Pb relaxation times. Compositions were checked by electron microprobe and are reliable to within 1 mol%. For XAFS experiments all powdered samples were conventionally mounted on thin sticky tape. 2.2. XAFS experiments X-ray absorption experiments require an intense source of continuous radiation in the X-ray range. The Pb LIII edge energy is 13.035 keV. A high ¯ux of tunable monochromatic X-rays can be obtained from a powerful laboratory generator or from a synchrotron radiation source. Both systems were used in this study and their eciencies are compared in terms of their ability to provide reliable characterization of disordered solids.
carrying out measurements with a conventional generator have been continuously developed and improved during the past years. A lot of work has been devoted to developing a spectrometer associated with a generator producing monochromatic X-rays eciently from a continuum spectrum generated in a tube. A major improvement has come with the development of a special X-ray generator that is suitable for XAFS experiments, since most conventional X-ray sources for industrial use have so far been designed for either diffraction or ¯uorescence experiments. The NRIM XAFS spectrometer, developed at Tsukuba (Japan) [14±16], provides a maximum tube-current of 1.1 A at low tube-voltage (14.5±18.0 kV) with a narrow line focus (0.1 mm ´ 10 mm at 6° take o). We have used a bent crystal (Ge (2, 2, 0), as monochromator with 1° divergence and 0.1 mm receiving slits, producing intense monochromatic X-ray ¯ux on the sample. We observed 5 ´ 106 counts/(s mm2 ) around the Cu K edge (8.99 keV). This laboratory X-ray source gives a smooth continuum spectrum in the energy region between 6.5 and 30 keV except for the anode (molybdenum) characteristic lines (Ka1 : 17.4793 keV, Ka2 : 17.3743 keV, Kb1 : 19.6083 keV). The measuring time required to obtain a similar signal/noise ratio compared to synchrotron radiation experiments, for the lead silicate samples, with a thickness x given by: lx 2, is estimated around 2 h (l is the absorption coecient).
2.3. Laboratory spectrometer (NRIM)
2.4. Synchrotron radiation (station D44 of the DCI)
While the availability of synchrotron sources has facilitated the use of XAFS, instruments for
The advantages of using synchrotron radiation as an X-ray source for absorption experiments are
Table 1 Structural parameters obtained by least squares ®tting the XAFS spectra for the oxygen coordination shell around a lead atom in the vitreous samples A, B, and C. A double shell is assumed. Bond lengths: d(Pb±O), coordination numbers N(O), and Debye±Waller factors r Sample
Melting temperature (°C)
d(Pb±O) (nm)
N(O)
r (nm) 0.003
A
1200
B
900
C
900
0.222 0.240 0.222 0.241 0.222 0.242
2 1.7 2 1.95 2 1.8
0.0045 0.0051 0.0045 0.0055 0.0045 0.0065
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
now well recognized [17]. They result from the high intensity, the tunability and the continuous spectrum of beams produced by the bending magnets of storage rings [18]. The photon ¯ux is more than two orders of magnitude higher, in the case of the D44 station, than the laboratory system. The counting rate is about 109 counts/s mm2 at the sample position. Some lead LIII EXAFS spectra were compared with data recorded at the D44 station of the 1.85 GeV, 300 mA DCI storage ring at the synchrotron radiation facilities of LURE (Orsay, France). The X-ray beam is made monochromatic by a double Bragg re¯ection (3 1 1) of two parallel Si crystals. Absorption measurements on glass powders have been performed in transmission mode with ionization chambers as the detection system. Absorption spectra of the lead silicate glasses were recorded in 7 min. Fig. 1 compares the XAFS oscillations
function recorded above the Pb LIII -edge on sample B containing 50.5 mol% of PbO. We observe that the low value of the photon ¯ux in laboratory XAFS is compensated by a longer recording time for the spectra recorded using the highly stable photon source at the NRIM laboratory. 2.5. Data processing Data processing uses the general procedure [19,20] of pre-edge, background removal, normalization and extraction of the v(k) XAFS oscillations as recommended by the report of the international workshop on standards and criteria in XAFS [21]. The pseudo radial distribution function is obtained by a Fourier transform of the v(k) function. Quantitative information related to the oxygen coordination shell is obtained by isolating the ®rst peak in the radial distribution function and taking into account amplitude and phase shift. The ®ltered contribution of the peak related to oxygen is the XAFS signal is extracted by an inverse Fourier transform. The k-space range used for the Fourier transform was limited by kmin 35 nmÿ1 and kmax 130 nmÿ1 . The structural parameters associated with the ®rst neighboring oxygen atoms are calculated by a curve-®tting method based on the fundamental expression [22] of the XAFS oscillations. Accuracy is of the order of 10% for the coordination num for the shell radius. bers and 0.03 A 2.6.
Fig. 1. Comparison of the v(k) oscillation function recorded above the Pb LIII -edge on the same vitreous sample with (A) the conventional XAFS spectrometer of the station D44 of the DCI storage ring (Orsay, France), (B) the laboratory spectrometer developed at NRIM (Tsukuba, Japan).
41
207
Pb NMR experiments
The 207 Pb NMR experiments were carried out on a Bruker DSX 300 spectrometer (7.0 T), using a standard static NMR probe. The 207 Pb spectra of the glass samples are so wide that Magic Angle Spinning (MAS), even at high speed, does not yield any resolution enhancement. The very broad 207 Pb resonance exceeds the excitation bandwidth and the spectrum has to be acquired piece-wise according to the previously described VOCS protocol Variable Oset Cumulative Spectrum (VOCS) [23,24,10]. The 207 Pb static spectra were acquired as sums of individual spectra taken at dierent osets. The oset step of 50 kHz is chosen such that the reconstructed irradiation pro®le
42
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
covers the whole spectrum. For each oset, static 207 Pb NMR spectrum is collected using a Hahn echo sequence with a recycle delay varying from 3 to 9 s. All experiments were carried out at room temperature. 207 Pb chemical shifts were referenced relatively to Pb(CH3 )4 at 0 ppm. 3. Results Both XAFS and NMR spectroscopies are atom selective and sensitive to their local order. Nevertheless they relate to dierent parameters. The interpretation of XAFS spectra gives radial distributions that are interpreted in terms of average pair distances and coordination numbers, while the NMR parameters: isotropic chemical shift, chemical shift anisotropy and their distributions are directly measuring the shielding by electrons of the principal magnetic ®eld (chemical shift interaction) that can be correlated to coordination numbers and local geometry. Whereas XAFS only gives an average signature of the lead environment, NMR also provides some kind of anisotropic characterization through the chemical shift anisotropy which re¯ects the local geometry. We shall ®rst present the results obtained from the two techniques. 3.1. XAFS Analysis of the XAFS experimental spectra above the LIII edge yields structural information about the oxygen coordination shell around lead atoms in silicate glasses with low and high Pb2 concentrations. The results related to Samples A, B and C, recorded on the NRIM laboratory system, are illustrated in Fig. 2. The main peak of the radial distribution function resulting from the Fourier transform of the XAFS oscillation is correlated to the cation±ligand bonds. In these leadglass samples, in spite of the high value of the atomic number Z, the radial distribution function is characterized by the absence of the peak associated with the Pb2 cation±cation bonds. The structural parameters presented in Table 1, are obtained by a least squares ®tting procedure. We have constrained the optimization ®tting proce-
Fig. 2. Comparison of the modulus of the Fourier transform FT(R) of the v(k) function recorded at the Pb LIII -edge on dried powders produced from glasses (A) 31 mol% of PbO, (B) 50.5 and (C) 66 mol% of PbO. FT(R) is uncorrected for the phase shift of the Pb±O pair.
dure for the glass spectra analogous to the structure of the corresponding crystal. A double coordination shell is proposed and the number of oxygens in the ®rst subshell is ®xed to a value of two. Results are in good agreement with previous studies by X-ray diraction analysis [25]. Analysis of the Pb LIII edge reveals no drastic modi®cation of the nearest sub-shell oxygen structure and a monotonic increase in the average Pb±O bond length as a function of lead concentration in the second sub-shell. The short Pb±O bond lengths associated with a low coordination number are typical of a covalent bonding state. However, this technique is not sensitive to the local symmetry and cannot propose either a tetrahedral or pyramidal arrangement for the Pb units. Taking the spectrometer resolution into account, no chemical shift has been observed in the spectra as was reported by Wang et al. [7] from XPS experiments (1 eV). In addition to the lengthening of the Pb±O bond in the second subshell with increasing lead concentration, we note an increase in the Debye± Waller factor indicating a higher disorder in these glasses. 3.2.
207
Pb NMR
In a previous paper, we established an empirical correlation between 207 Pb chemical shift and
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
structural parameters in solids, based on the 207 Pb MAS NMR study of lead-containing crystalline reference compounds including silicates and oxides. We showed that it is possible to dierentiate lead involved in covalent or ionic bonding states from 207 Pb isotropic shift position and 207 Pb chemical shift anisotropy (CSA) [26]. The 207 Pb spectra of the three glasses studied are reported in Fig. 3. These spectra are characterized by very wide lines with 207 Pb positive isotropic chemical shift positions and large chemical shift anisotropy. These spectra thus unambiguously characterize covalent PbO3 or PbO4 pyramidal units [26,10]. The additional broadening is ascribed to the disorder in the second coordination sphere induced by cation substitution (2Pb/Si) as well as bond length and bond angle distributions. Both the linewidth (3700 ppm) and the averaged position (d 520 ppm) of the spectrum observed for the glass having the highest lead content (67 mol% PbO) are dierent from those of the two other samples (3400 ppm, d 50 ppm). This behavior is in agreement with previous results obtained in continuous wave [2] and pulsed [10] NMR studies
43
that showed a clear increase in chemical shift with lead content above about 60 mol% PbO. This chemical shift variation is interpreted as the eect of an increase in the number of covalent Pb±O±Pb bonds and thus a more pronounced connectivity between the PbOn units (n 3,4) involved in the formation of a lead oxide network. This interpretation is supported by the very large distribution of 207 Pb isotropic shifts that has been estimated to be close to 1200 ppm: it covers most of the observed range for PbOn pyramids in crystalline silicates. 4. Discussion As expected, the two dierent spectroscopic approaches provide dierent clues about the local environment of lead in these glasses. The very anisotropic nature of the lead local structure is evidenced by the large chemical shift anisotropies observed in the NMR spectra. The short Pb±O bond lengths extracted from the XAFS spectra, are associated with a low coordination number. Furthermore, the empirical correlation established between 207 Pb chemical shift and the mean Pb±O bond length [26] can be used to estimate the mean Pb±O distances from NMR measurements. Even if it is possible, from NMR measurement, to evaluate an average PbO bond distance varying from 0.234 to 0.239 nm for the three glasses, only XAFS is able to describe the evolution of this parameter with composition. Due to the very wide range of coordination numbers (3±12) on which the NMR correlation was established, an attempt to deduce exact bond length would lead to a misinterpretation. As previously noted, in covalent compounds, 207 Pb chemical shifts are mainly driven by the bond angle and the nature of the second neighbors [26]. 5. Conclusion
Fig. 3. (a) 207 Pb VOCS spectrum obtained as a sum of the six single full echo spectra at variable oset, (b) 207 Pb VOCS spectra of Samples A, B and C.
The local environment of Pb2 has been probed in lead silicate glasses by combining XAFS and 207 Pb NMR informations. Lead is three or fourfold coordinated by oxygen atoms in a pyramidal geometry with short Pb±O bond lengths and we
44
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
observe no modi®cation of the lead coordination number either by XAFS nor by NMR over the whole composition range. As the lead content increases, the connectivity between the PbOn pyramids increases to form a lead oxide based network in the high lead content glasses. References [1] E.M. Rabinovich, J. Mater. Sci. 11 (1976) 925. [2] M. Leventhal, P.J. Bray, Phys. Chem. Glasses 6 (1965) 113. [3] R. Dupree, N. Ford, D. Holland, Phys. Chem. Glasses 28 (1987) 78. [4] A.-M. Zahra, C.Y. Zahra, B. Piriou, J. Non-Cryst. Solids 155 (1993) 45. [5] L. Liu, Z. Phys. B 90 (1993) 393. [6] V.O. Kabanov, T.M. Podolskaya, O.V. Yanush, Glass Phys. Chem. 22 (1996). [7] P.W. Wang, L. Zhang, J. Non-Cryst. Solids 194 (1996) 129. [8] H. Eckert, Prog. Nucl. Magn. Reson. Spectrosc. 24 (1992) 159. [9] R. Dupree, D. Holland, in: M.H. Lewis (Ed.), Glasses and Glass Ceramics, Chapman & Hall, London, 1989. [10] D.A. Keown, G.A. Waychunas, G.E. Brown, J. NonCryst. Solids 74 (1985) 325.
[11] M.R. Antonio, L. Soderholm, A.J.G. Ellison, J. Alloys Comp. 250 (1997) 536. [12] C.W. Ponader, H. Boek, J.E. Dickinson, J. Non-Cryst. Solids 201 (1996) 81. [13] F. Fayon, C. Bessada, D. Massiot, I. Farnan, J.P. Coutures, J. Non-Cryst. Solids 232±234 (1998) 403. [14] K. Sakurai, Rev. Sci. Instrum. 64 (1993) 2702. [15] K. Sakurai, Rev. Sci. Instrum. 64 (1993) 267. [16] K.Sakurai, Jpn. J. Appl. Phys. Suppl. 32-2 (1993) 261. [17] D.E. Sayer, F.W. Lytle, E.A. Stern, Adv. X-Ray Anal. 13 (1970) 248. [18] D.C. Koningsberger, R. Prins (Eds.), X-Ray Absorption, Wiley, New York, 1988. [19] F.W. Lytle, D.E. Sayers, E.A. Stern, Phys. Rev. B 11 (1975) 4825. [20] B.K. Teo, EXAFS: basic principles and data analysis, Inorg. Chem. Conc., vol. 9, Springer, Berlin, 1986. [21] Report of international workshops on standards and criteria in XAFS, Physica B 158 (1989) 701. [22] A. Filipponi, J. Phys.: Condens. Matter 6 (1994) 8415. [23] D. Massiot, I. Farnan, N. Gautier, D. Trumeau, A. Trokiner, J.P. Coutures, Solid State NMR 4 (1995) 241. [24] Y.Y. Tong, J. Magn. Res. A 119 (1996) 22. [25] H. Morikawa, Y. Takagi, H. Ohno, J. Non-Cryst. Solids 53 (1982) 173. [26] F. Fayon, I. Farnan, C. Bessada, J. Couture, D. Massiot, J.P. Coutures, J. Am. Chem. Soc. 119 (29) (1997) 6837.
Pb2 environment in lead silicate glasses probed by Pb-LIII edge XAFS and 207Pb NMR F. Fayon a, C. Landron a
a,b,*
, K. Sakurai c, C. Bessada a, D. Massiot
a
Centre de Recherches sur les Mat eriaux a Haute Temp erature, 1D Ave de la Recherche Scienti®que, 45071 Orl eans cedex 2, France b Laboratoire pour l'Utilisation du Rayonnement Electromagn etique, 91405, Orsay cedex, France c National Research Institute for Metals, Sengen, Tsukuba, Ibaraki 305, Japan Received 16 November 1997; received in revised form 18 September 1998
Abstract The local structural environment of lead (Pb) atoms in lead silicate glasses has been studied by two complementary spectroscopic techniques: Pb-LIII edge X-ray absorption ®ne structure (XAFS) and 207 Pb solid state nuclear magnetic resonance (NMR). XAFS results have been obtained with two kinds of experimental devices: a synchrotron radiation source and a laboratory spectrometer. The weak photon ¯ux is then compensated by a higher stability of the source and a longer acquisition time. The experiments were carried out on selected compositions of lead silicate glasses in which we observed PbO3 and PbO4 pyramidal units with short Pb±O bond lengths typical of a covalent bonding state. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In binary lead silicate glasses, lead atoms are often described as glass former at high lead content and as network modi®er at low lead content [1±7]. Such dierences in the structural role of Pb2 would imply signi®cant modi®cations of its local environment and of its coordination. Both solid state nuclear magnetic resonance (NMR) spectroscopy [8,9] and X-ray absorption ®ne structure (XAFS) spectroscopy [10±12] have proven to be powerful tools in elucidating the local environments of atoms in disordered systems like glasses. XAFS is based on the interpretation of the interference between outgoing and backscattered
* Corresponding author. Tel.: +33-2 38 51 55 31; fax: +33-2 38 63 81 03; e-mail: [email protected].
photoelectrons emitted during an X-ray absorption process. Thus, XAFS describes the arrangement of atoms through atomic radial distribution functions. Structural information related to the symmetry of the coordination shell can be deduced from the NMR parameters: isotropic chemical shift and chemical shift anisotropy. In a previous study we have investigated a wide range of compositions (from 30 to 70 mol% PbO) in the PbO±SiO2 system using 29 Si and 207 Pb high resolution solid state NMR and observed that the glassy network was made of SiO4 tetrahedra and covalent PbOn pyramids [13]. In order to improve the understanding of the lead silicate glass structure, we have combined, in the present work, data obtained by 207 Pb NMR and XAFS on lead silicate glasses to characterize the local environments of lead atoms and their distributions over a large range of compositions.
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 0 9 - 6
40
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
2. Experimental 2.1. Sample preparation Three lead silicate glasses containing 31, 50.5 and 66 mol% of PbO (named A, B and C, respectively) were prepared from reagent grade PbO and SiO2 . The glass samples (3 g) were melted in Pt crucibles in an electric furnace under air, and quenched by partially immersing the crucibles in water. Melting temperatures are reported in Table 1. Melting times were minimized in order to reduce the volatilization of PbO. An additional 0.02 mol% Fe2 O3 was included in the batch to reduce the 207 Pb relaxation times. Compositions were checked by electron microprobe and are reliable to within 1 mol%. For XAFS experiments all powdered samples were conventionally mounted on thin sticky tape. 2.2. XAFS experiments X-ray absorption experiments require an intense source of continuous radiation in the X-ray range. The Pb LIII edge energy is 13.035 keV. A high ¯ux of tunable monochromatic X-rays can be obtained from a powerful laboratory generator or from a synchrotron radiation source. Both systems were used in this study and their eciencies are compared in terms of their ability to provide reliable characterization of disordered solids.
carrying out measurements with a conventional generator have been continuously developed and improved during the past years. A lot of work has been devoted to developing a spectrometer associated with a generator producing monochromatic X-rays eciently from a continuum spectrum generated in a tube. A major improvement has come with the development of a special X-ray generator that is suitable for XAFS experiments, since most conventional X-ray sources for industrial use have so far been designed for either diffraction or ¯uorescence experiments. The NRIM XAFS spectrometer, developed at Tsukuba (Japan) [14±16], provides a maximum tube-current of 1.1 A at low tube-voltage (14.5±18.0 kV) with a narrow line focus (0.1 mm ´ 10 mm at 6° take o). We have used a bent crystal (Ge (2, 2, 0), as monochromator with 1° divergence and 0.1 mm receiving slits, producing intense monochromatic X-ray ¯ux on the sample. We observed 5 ´ 106 counts/(s mm2 ) around the Cu K edge (8.99 keV). This laboratory X-ray source gives a smooth continuum spectrum in the energy region between 6.5 and 30 keV except for the anode (molybdenum) characteristic lines (Ka1 : 17.4793 keV, Ka2 : 17.3743 keV, Kb1 : 19.6083 keV). The measuring time required to obtain a similar signal/noise ratio compared to synchrotron radiation experiments, for the lead silicate samples, with a thickness x given by: lx 2, is estimated around 2 h (l is the absorption coecient).
2.3. Laboratory spectrometer (NRIM)
2.4. Synchrotron radiation (station D44 of the DCI)
While the availability of synchrotron sources has facilitated the use of XAFS, instruments for
The advantages of using synchrotron radiation as an X-ray source for absorption experiments are
Table 1 Structural parameters obtained by least squares ®tting the XAFS spectra for the oxygen coordination shell around a lead atom in the vitreous samples A, B, and C. A double shell is assumed. Bond lengths: d(Pb±O), coordination numbers N(O), and Debye±Waller factors r Sample
Melting temperature (°C)
d(Pb±O) (nm)
N(O)
r (nm) 0.003
A
1200
B
900
C
900
0.222 0.240 0.222 0.241 0.222 0.242
2 1.7 2 1.95 2 1.8
0.0045 0.0051 0.0045 0.0055 0.0045 0.0065
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
now well recognized [17]. They result from the high intensity, the tunability and the continuous spectrum of beams produced by the bending magnets of storage rings [18]. The photon ¯ux is more than two orders of magnitude higher, in the case of the D44 station, than the laboratory system. The counting rate is about 109 counts/s mm2 at the sample position. Some lead LIII EXAFS spectra were compared with data recorded at the D44 station of the 1.85 GeV, 300 mA DCI storage ring at the synchrotron radiation facilities of LURE (Orsay, France). The X-ray beam is made monochromatic by a double Bragg re¯ection (3 1 1) of two parallel Si crystals. Absorption measurements on glass powders have been performed in transmission mode with ionization chambers as the detection system. Absorption spectra of the lead silicate glasses were recorded in 7 min. Fig. 1 compares the XAFS oscillations
function recorded above the Pb LIII -edge on sample B containing 50.5 mol% of PbO. We observe that the low value of the photon ¯ux in laboratory XAFS is compensated by a longer recording time for the spectra recorded using the highly stable photon source at the NRIM laboratory. 2.5. Data processing Data processing uses the general procedure [19,20] of pre-edge, background removal, normalization and extraction of the v(k) XAFS oscillations as recommended by the report of the international workshop on standards and criteria in XAFS [21]. The pseudo radial distribution function is obtained by a Fourier transform of the v(k) function. Quantitative information related to the oxygen coordination shell is obtained by isolating the ®rst peak in the radial distribution function and taking into account amplitude and phase shift. The ®ltered contribution of the peak related to oxygen is the XAFS signal is extracted by an inverse Fourier transform. The k-space range used for the Fourier transform was limited by kmin 35 nmÿ1 and kmax 130 nmÿ1 . The structural parameters associated with the ®rst neighboring oxygen atoms are calculated by a curve-®tting method based on the fundamental expression [22] of the XAFS oscillations. Accuracy is of the order of 10% for the coordination num for the shell radius. bers and 0.03 A 2.6.
Fig. 1. Comparison of the v(k) oscillation function recorded above the Pb LIII -edge on the same vitreous sample with (A) the conventional XAFS spectrometer of the station D44 of the DCI storage ring (Orsay, France), (B) the laboratory spectrometer developed at NRIM (Tsukuba, Japan).
41
207
Pb NMR experiments
The 207 Pb NMR experiments were carried out on a Bruker DSX 300 spectrometer (7.0 T), using a standard static NMR probe. The 207 Pb spectra of the glass samples are so wide that Magic Angle Spinning (MAS), even at high speed, does not yield any resolution enhancement. The very broad 207 Pb resonance exceeds the excitation bandwidth and the spectrum has to be acquired piece-wise according to the previously described VOCS protocol Variable Oset Cumulative Spectrum (VOCS) [23,24,10]. The 207 Pb static spectra were acquired as sums of individual spectra taken at dierent osets. The oset step of 50 kHz is chosen such that the reconstructed irradiation pro®le
42
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
covers the whole spectrum. For each oset, static 207 Pb NMR spectrum is collected using a Hahn echo sequence with a recycle delay varying from 3 to 9 s. All experiments were carried out at room temperature. 207 Pb chemical shifts were referenced relatively to Pb(CH3 )4 at 0 ppm. 3. Results Both XAFS and NMR spectroscopies are atom selective and sensitive to their local order. Nevertheless they relate to dierent parameters. The interpretation of XAFS spectra gives radial distributions that are interpreted in terms of average pair distances and coordination numbers, while the NMR parameters: isotropic chemical shift, chemical shift anisotropy and their distributions are directly measuring the shielding by electrons of the principal magnetic ®eld (chemical shift interaction) that can be correlated to coordination numbers and local geometry. Whereas XAFS only gives an average signature of the lead environment, NMR also provides some kind of anisotropic characterization through the chemical shift anisotropy which re¯ects the local geometry. We shall ®rst present the results obtained from the two techniques. 3.1. XAFS Analysis of the XAFS experimental spectra above the LIII edge yields structural information about the oxygen coordination shell around lead atoms in silicate glasses with low and high Pb2 concentrations. The results related to Samples A, B and C, recorded on the NRIM laboratory system, are illustrated in Fig. 2. The main peak of the radial distribution function resulting from the Fourier transform of the XAFS oscillation is correlated to the cation±ligand bonds. In these leadglass samples, in spite of the high value of the atomic number Z, the radial distribution function is characterized by the absence of the peak associated with the Pb2 cation±cation bonds. The structural parameters presented in Table 1, are obtained by a least squares ®tting procedure. We have constrained the optimization ®tting proce-
Fig. 2. Comparison of the modulus of the Fourier transform FT(R) of the v(k) function recorded at the Pb LIII -edge on dried powders produced from glasses (A) 31 mol% of PbO, (B) 50.5 and (C) 66 mol% of PbO. FT(R) is uncorrected for the phase shift of the Pb±O pair.
dure for the glass spectra analogous to the structure of the corresponding crystal. A double coordination shell is proposed and the number of oxygens in the ®rst subshell is ®xed to a value of two. Results are in good agreement with previous studies by X-ray diraction analysis [25]. Analysis of the Pb LIII edge reveals no drastic modi®cation of the nearest sub-shell oxygen structure and a monotonic increase in the average Pb±O bond length as a function of lead concentration in the second sub-shell. The short Pb±O bond lengths associated with a low coordination number are typical of a covalent bonding state. However, this technique is not sensitive to the local symmetry and cannot propose either a tetrahedral or pyramidal arrangement for the Pb units. Taking the spectrometer resolution into account, no chemical shift has been observed in the spectra as was reported by Wang et al. [7] from XPS experiments (1 eV). In addition to the lengthening of the Pb±O bond in the second subshell with increasing lead concentration, we note an increase in the Debye± Waller factor indicating a higher disorder in these glasses. 3.2.
207
Pb NMR
In a previous paper, we established an empirical correlation between 207 Pb chemical shift and
F. Fayon et al. / Journal of Non-Crystalline Solids 243 (1999) 39±44
structural parameters in solids, based on the 207 Pb MAS NMR study of lead-containing crystalline reference compounds including silicates and oxides. We showed that it is possible to dierentiate lead involved in covalent or ionic bonding states from 207 Pb isotropic shift position and 207 Pb chemical shift anisotropy (CSA) [26]. The 207 Pb spectra of the three glasses studied are reported in Fig. 3. These spectra are characterized by very wide lines with 207 Pb positive isotropic chemical shift positions and large chemical shift anisotropy. These spectra thus unambiguously characterize covalent PbO3 or PbO4 pyramidal units [26,10]. The additional broadening is ascribed to the disorder in the second coordination sphere induced by cation substitution (2Pb/Si) as well as bond length and bond angle distributions. Both the linewidth (3700 ppm) and the averaged position (d 520 ppm) of the spectrum observed for the glass having the highest lead content (67 mol% PbO) are dierent from those of the two other samples (3400 ppm, d 50 ppm). This behavior is in agreement with previous results obtained in continuous wave [2] and pulsed [10] NMR studies
43
that showed a clear increase in chemical shift with lead content above about 60 mol% PbO. This chemical shift variation is interpreted as the eect of an increase in the number of covalent Pb±O±Pb bonds and thus a more pronounced connectivity between the PbOn units (n 3,4) involved in the formation of a lead oxide network. This interpretation is supported by the very large distribution of 207 Pb isotropic shifts that has been estimated to be close to 1200 ppm: it covers most of the observed range for PbOn pyramids in crystalline silicates. 4. Discussion As expected, the two dierent spectroscopic approaches provide dierent clues about the local environment of lead in these glasses. The very anisotropic nature of the lead local structure is evidenced by the large chemical shift anisotropies observed in the NMR spectra. The short Pb±O bond lengths extracted from the XAFS spectra, are associated with a low coordination number. Furthermore, the empirical correlation established between 207 Pb chemical shift and the mean Pb±O bond length [26] can be used to estimate the mean Pb±O distances from NMR measurements. Even if it is possible, from NMR measurement, to evaluate an average PbO bond distance varying from 0.234 to 0.239 nm for the three glasses, only XAFS is able to describe the evolution of this parameter with composition. Due to the very wide range of coordination numbers (3±12) on which the NMR correlation was established, an attempt to deduce exact bond length would lead to a misinterpretation. As previously noted, in covalent compounds, 207 Pb chemical shifts are mainly driven by the bond angle and the nature of the second neighbors [26]. 5. Conclusion
Fig. 3. (a) 207 Pb VOCS spectrum obtained as a sum of the six single full echo spectra at variable oset, (b) 207 Pb VOCS spectra of Samples A, B and C.
The local environment of Pb2 has been probed in lead silicate glasses by combining XAFS and 207 Pb NMR informations. Lead is three or fourfold coordinated by oxygen atoms in a pyramidal geometry with short Pb±O bond lengths and we
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observe no modi®cation of the lead coordination number either by XAFS nor by NMR over the whole composition range. As the lead content increases, the connectivity between the PbOn pyramids increases to form a lead oxide based network in the high lead content glasses. References [1] E.M. Rabinovich, J. Mater. Sci. 11 (1976) 925. [2] M. Leventhal, P.J. Bray, Phys. Chem. Glasses 6 (1965) 113. [3] R. Dupree, N. Ford, D. Holland, Phys. Chem. Glasses 28 (1987) 78. [4] A.-M. Zahra, C.Y. Zahra, B. Piriou, J. Non-Cryst. Solids 155 (1993) 45. [5] L. Liu, Z. Phys. B 90 (1993) 393. [6] V.O. Kabanov, T.M. Podolskaya, O.V. Yanush, Glass Phys. Chem. 22 (1996). [7] P.W. Wang, L. Zhang, J. Non-Cryst. Solids 194 (1996) 129. [8] H. Eckert, Prog. Nucl. Magn. Reson. Spectrosc. 24 (1992) 159. [9] R. Dupree, D. Holland, in: M.H. Lewis (Ed.), Glasses and Glass Ceramics, Chapman & Hall, London, 1989. [10] D.A. Keown, G.A. Waychunas, G.E. Brown, J. NonCryst. Solids 74 (1985) 325.
[11] M.R. Antonio, L. Soderholm, A.J.G. Ellison, J. Alloys Comp. 250 (1997) 536. [12] C.W. Ponader, H. Boek, J.E. Dickinson, J. Non-Cryst. Solids 201 (1996) 81. [13] F. Fayon, C. Bessada, D. Massiot, I. Farnan, J.P. Coutures, J. Non-Cryst. Solids 232±234 (1998) 403. [14] K. Sakurai, Rev. Sci. Instrum. 64 (1993) 2702. [15] K. Sakurai, Rev. Sci. Instrum. 64 (1993) 267. [16] K.Sakurai, Jpn. J. Appl. Phys. Suppl. 32-2 (1993) 261. [17] D.E. Sayer, F.W. Lytle, E.A. Stern, Adv. X-Ray Anal. 13 (1970) 248. [18] D.C. Koningsberger, R. Prins (Eds.), X-Ray Absorption, Wiley, New York, 1988. [19] F.W. Lytle, D.E. Sayers, E.A. Stern, Phys. Rev. B 11 (1975) 4825. [20] B.K. Teo, EXAFS: basic principles and data analysis, Inorg. Chem. Conc., vol. 9, Springer, Berlin, 1986. [21] Report of international workshops on standards and criteria in XAFS, Physica B 158 (1989) 701. [22] A. Filipponi, J. Phys.: Condens. Matter 6 (1994) 8415. [23] D. Massiot, I. Farnan, N. Gautier, D. Trumeau, A. Trokiner, J.P. Coutures, Solid State NMR 4 (1995) 241. [24] Y.Y. Tong, J. Magn. Res. A 119 (1996) 22. [25] H. Morikawa, Y. Takagi, H. Ohno, J. Non-Cryst. Solids 53 (1982) 173. [26] F. Fayon, I. Farnan, C. Bessada, J. Couture, D. Massiot, J.P. Coutures, J. Am. Chem. Soc. 119 (29) (1997) 6837.