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Local environments of erbium and lutetium in sodium silicate glasses
Journal of Alloys and Compounds 250 (1997) 536–540
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Local environments of erbium and lutetium in sodium silicate glasses Mark R. Antonio, L. Soderholm, A.J.G. Ellison Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 -4831, USA
Abstract The coordination environments of Er and Lu in sodium silicate glasses are probed through a unique analysis of L 3 X-ray absorption near edge structure (XANES) combined with extended X-ray absorption fine structure (EXAFS). By using the observed splitting of the empty 5d manifold of the lanthanides in combination with the EXAFS data, we demonstrate that Er and Lu are 6-coordinate in well-defined sites with relatively high symmetry. Keywords: Local environments; Erbium; Lutetium
1. Introduction
2. Experimental
The optical responses of 4f single-ion states have been extensively studied and are currently exploited in a number of devices, such as lasers, telecommunications fibers, fluorescing screens and amplifiers. However, a dearth of information about local coordination of 4f ions in glasses has slowed the expansion of this technology into areas relevant to fiber optic and semiconductor applications. Information about systematics in 4f-ion coordination is crucial to a predictive understanding of the optical properties of 4f-ions in glasses. Limited energy-level information can be obtained from inelastic neutron scattering, where the neutron’s magnetic moment interacts with the unpaired spins on the f-ion to provide insight about the local environment [1]. Extended X-ray absorption fine structure (EXAFS) has also been used to probe the 4f environments in glasses [2,3]. We note the previous work on the local structure of Er 31 in silica and sodium silicate glass [4]. In this paper, we have expanded these approaches to include XANES, X-ray absorption near edge structure, as well as EXAFS, on two different glasses containing erbium and lutetium. We demonstrate that an approximate value of the 5d band splitting, reported as the crystal field strength parameter D, can be determined from the peak splitting of the second differential XANES at the Er and Lu L 3 -edges. Such splitting is commonly observed in the L 3 -edge XANES of the 4d and 5d transition-metal ions [5–7], and herein we report the observation of 5d band splitting in the 4f ions Er 31 and Lu 31 .
Polycrystalline specimens of LnPO 4 (Ln;Er, Lu) were prepared according to published procedures [8]. Together with Reacton grade Ln 2 O 3 , obtained from Aesar, these were used as reference materials for the XAFS analyses. The glasses were prepared by pour quenching at 1450 8C, the details of which are provided elsewhere [1]. Diffraction data show that both 3Na 2 O?Er 2 O 3 ?6SiO 2 and 3Na 2 O?Lu 2 O 3 ?6SiO 2 are amorphous glasses. Erbium and lutetium L 3 -edge (8358 and 9244 eV, respectively) and L 1 -edge (9751 and 10 870 eV, respectively) X-ray absorption data were collected at ambient temperature on beam line X-23A2 at the National Synchrotron Light Source with a Sik311l double crystal monochromator. At 9000 eV with a 1 mm vertical entrance slit, the total instrumental energy resolution was estimated to be 2.0 eV, which is 2.33 smaller than the natural line widths (ca. 4.5 eV [9]) of the Er and Lu L 3 levels. Hence, the XAFS shown here are somewhat broadened (¯9%) by instrumental effects. In order to avoid sample thickness artifacts and to obtain accurate edge resonance intensities and positions, the XANES was detected using the electronyield technique [10]. The L-edge regions were scanned independently under the same experimental conditions with a step size of 0.25 eV/ pt (L 3 edges) and 0.4 eV/ pt (L 1 edges). The scan-to-scan energy calibration was maintained to 60.2 eV. All XAFS analyses were performed with EXAFSPAK [11] using FEFF 6.01a [12]. The fitting procedure and all reported parameters have their standard meaning [13].
0925-8388 / 97 / $17.00 1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02736-3
M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
3. Results and discussion The normalized L 3 -edge XANES of LnPO 4 , Ln 2 O 3 and glasses 3Na 2 O?Ln 2 O 3 ?6SiO 2 for Ln5Er, Lu are displayed in Fig. 1a. The intense and symmetrical single resonances in the L 3 -edge XANES of LnPO 4 is typical of those seen for other trivalent lanthanides [14]. These resonances are due to dipole-allowed (Dl51) electronic transitions from the Ln 2p 3 / 2 initial state to the empty 5d-orbital manifold. The fullwidth at half-maxima (FWHM) of the Lorentzianshaped edge resonances, shown in Fig. 1a, are a convolution of the initial state (2p 3 / 2 core hole) and the final state (5d 1 ) widths. The final state width is the 5d bandwidth, which is subject to a number of effects, most notably splitting due to the crystal-field environment, in addition to orbital hybridization / mixing due to bonding [15]. As shown in Fig. 1a, the FWHM of the resonant-white lines obtained from our samples vary considerably, and sometimes exhibit marked asymmetry. Fitted values of the resonant line widths are listed in Table 1. It has been previously demonstrated that the L 3 -edge
Fig. 1. Lutetium (solid lines) and erbium (dashed lines) L 3 -edge XANES for Ln 2 O 3 , LnPO 4 , and Ln glasses for Ln;Lu, Er obtained through electron-yield at ambient temperature. (b) Second differential L 3 -edge XANES.
537
Table 1 Widths (FWHM, eV) of the 2p 3 / 2 →5d resonance in the normalized, electron-yield Er and Lu L 3 -edge XANES as obtained by curve fitting with Lorentz and arctangent functions, convolved with a 2 eV Gaussian broadening function a
LnPO 4 Ln 2 O 3 Ln-Glass
Er (FWHM)
D (eV)
Lu (FWHM)
D (eV)
5.1 7.2 6.9
b
5.1 7.3 7.3
b
3.4 2.8
3.4 2.8
D, the separation of the two peaks in the double differential of the edge resonance is also listed. Fitting range: 220 to 125 eV. a See F.W. Lytle, R.B. Greegor and E.C. Marques in M.J. Phillips and M. Ternan (eds.), Proc. 9 th Int. Cong. Catalysis, Calgary, Vol. 5, The Chemical Institute of Canada, Ottawa, 1988, p. 54, and; G. Krill, J. Phys., 47 Colloque C8 (1986) C8-907. b When D is smaller than the spectral resolution of 2 eV, the crystal field splitting of the 5d orbitals cannot be determined.
resonance observed in the XANES of 4d and 5d transition elements show two (or more) well-resolved peaks that have been attributed to transitions between crystal-field states within a split d manifold [6,7,16]. The overall d-orbital splitting is expected to be as large as 4 eV for the 4d band and 5–6 eV for the 5d band [17]. Thus, it can be argued that the XANES peak splitting provides an approximate value of the crystal-field strength parameter, D, whose magnitude can be diagnostic of the crystal-field symmetry about the metal ion or, in other words, its coordination environment. Recently, the simple procedure of taking the second differential of L 3 -edge XANES was shown to make possible the resolution of separate crystal-field-split transitions that are not resolved in the primary XANES [5]. This d-band splitting is demonstrated in Fig. 2b for the W L 3 -edge XANES of WO 3 . The crystal field splitting of 5d orbitals can be observed in the second differential L 3 -edge XANES of 5d transition metals, e.g., W and Ir, with a coordination environment that produces a large d-orbital splitting. In general, D decreases for three common coordination geometries as planar.octahedral.tetrahedral. The generalizations and simplifications of crystal field theory [17,18] are nicely borne out in the comparison of, for example, the shape and width of W L 3 -edge resonances near 10 206 eV for oxides with tetrahedral and octahedral coordination. As we show in Fig. 2, for W(VI) with a tetrahedral oxygen environment (Na 2 WO 4 ?2H 2 O) the resonance is sharp, whereas for an octahedral oxygen environment (WO 3 ), the resonance is broad. As has been noted elsewhere [19], the FWHM of the resonance for WO 3 (8.0 eV) is significantly broader than that for Na 2 WO 4 ?2H 2 O (5.3 eV). Moreover, the second differential W L 3 -edge XANES for WO 3 , shown in Fig. 2b, has two valleys with a separation of 3.9 eV, whereas the corresponding data for Na 2 WO 4 ?2H 2 O does not. When D is smaller than the spectral resolution of ca. 2.0 eV, the crystal field splitting of the 5d orbitals cannot be observed even in the second differential XANES. This is the case for [WO 4 ] 22 , which
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M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
Fig. 2. Tungsten L 3 -edge XANES (a) and second differential XANES (b) for WO 3 and Na 2 WO 4 ?2H 2 O.
reveals a single valley. Conversely, when D is larger than the spectral resolution, then crystal field splitting of the 5d orbitals can be observed. This was demonstrated for WO 3 . In both of the W compounds reported here, tungsten is hexavalent, and therefore it has the same formal electronic configuration as Lu 31 (4f 10 5d 0 ). The second differential L 3 -edge XANES of the Ln data, shown Fig. 1b, are quite revealing. For LnPO 4 , there is just one deep valley, whereas for Ln 2 O 3 and the Ln glasses there are two well-resolved valleys with a separation of 3.4 and 2.8 eV, respectively. The similarity of the resonant-line splitting, together with the similarity in electronic configurations of the ions under study, leads us to conclude that the splitting observed in our Er and Lu L 3 -edge data is the result of a crystal field splitting of the empty, lanthanide 5d manifold. Whereas crystal field splitting effects such as those described herein are commonly observed in the L 3 XANES of transition metal ions, this is not the case to date for the 4f ions. For the crystalline Ln 2 O 3 and glass 3Na 2 O? Ln 2 O 3 ?6SiO 2 samples we observe using L3 XANES crystal field splitting of the 5 d orbitals of Er 31 and Lu 31 . These observations are consistent with the 6-fold coordination of the Ln 31 in Ln 2 O 3 and the glass samples. The Ln 31 ions in LnPO 4 have 8-fold, dodecahedral coordination which,
gives rise to a splitting that is too small to detect with this technique. This small splitting is consistent with predictions based on the rare-earth site coordination [18]. The Ln L 1 -edge XANES of LnPO 4 , Ln 2 O 3 and glasses 3Na 2 O?Ln 2 O 3 .6SiO 2 for Ln5Er, Lu are shown in Fig. 3. Unlike the corresponding L 3 -edge XANES of Fig. 1, there are no intense peaks that can be directly assigned to specific electronic transitions. For L 1 XANES, the initial state is the Ln 2s orbital. Dipole-allowed transitions from the 2s to the empty 6p manifold have yet to be unambiguously identified in L 1 -edge XANES data. Nevertheless, quadrupole-like transitions from the 2s to the empty 5d manifold have been observed under certain conditions, particularly for W 61 [20]. The peaks beyond 0 eV in the data of Fig. 3 are now generally recognized as scattering resonances [21]. From the base of the rising edge, there appears to be a slight shoulder extending from about 1 / 3 to 1 / 2 of the edge step intensity. Among other things, this unresolved feature may be a weak s→d electronic transition. Strong electronic resonances near the base of the rising L 1 -edge are often observed for absorbing atoms with particular site symmetry. These so-called pre-edge peaks are especially intense for absorbing atoms in a site without inversion symmetry, such as for a tetrahedral ligand field [20]. In this case, the peak is due to a transition from the 2s initial state to a final state of mixed d and p orbital character. In contrast, if the absorbing atom site has inversion symmetry, such as for an octahedral ligand field, pre-edge features are not observed. The absence of any pre-edge peaks in all of the L 1 -edge XANES of Fig. 3 argues against significant p-d mixing in any of these materials. The k 3 x(k) EXAFS and the corresponding Fourier transform (FT) data are shown in Fig. 4a and Fig. 4b, respectively, for the erbium and lutetium phosphates, oxides and glasses. The first and most intense peak at ca.
Fig. 3. Lutetium (solid lines) and erbium (dashed lines) L 1 -edge XANES for Ln 2 O 3 , LnPO 4 and Ln glasses for Ln;Lu, Er obtained through electron-yield at ambient temperature.
M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
539
Fig. 4. (a) Lutetium (solid lines) and erbium (dashed lines) L 3 -edge k 3 x(k) EXAFS and (b) the corresponding Fourier transform (FT) magnitude of the k 3 x(k) EXAFS of LnPO 4 , Ln 2 O 3 and Ln glasses.
˚ (before phase shift correction) in all the FT data of 1.9 A Fig. 4b is due to scattering by oxygen atoms. Beyond ˚ in the FT data, there is significant structure for about 2 A LnPO 4 and Ln 2 O 3 which has been assigned to scattering by phosphorous and Ln atoms, see Fig. 4b. In contrast, ˚ in the FT data for the erbium and lutetium beyond 2 A glasses, there are no structurally significant peaks. The absence of any structurally significant peaks beyond this distance provides clear evidence that the glasses do not possess long range order. This is consistent with the X-ray diffraction data for these same glasses. The short-range Ln 31 –O order manifests itself in the k 3 x(k) EXAFS of Fig. 4a. The EXAFS for the glasses looks like a damped, single-frequency sinusoid, whereas for LnPO 4 and Ln 2 O 3 the EXAFS appears to be a combination of multiple sinusoids with different frequencies. Curve fitting analysis of the k 3 x(k) EXAFS for the glasses reveals 6661 O ˚ The metrical results obtained from the atoms at 2.2(1) A. EXAFS analysis of crystalline LnPO 4 and Ln 2 O 3 are in agreement with the reported crystal structures [22]. Using the data interpretation discussed above, we can make several remarks about the rare earth environment in the glass samples. First, because of the larger FWHM, and the splitting observed from the second-differential-edge spectra shown in Fig. 1b, we can conclude that both Er and Lu are 6-coordinate. This is corroborated by fitting to the EXAFS data, which shows 6 oxygen near neighbors for both glass samples. This is also consistent with neutron work on similar glasses [1,23] and with other EXAFS studies of the heavier lanthanides in glasses [2,4]. For comparison, a variety of other work suggests a coordination of 8 for the heavier lanthanides [24,25] in solution. The determination from the L 1 XANES data of a Ln coordination site with inversion symmetry argues against coordinations of 5 or 7 in our glass samples. The first coordination sphere in our glass samples is also well defined, and of relatively high symmetry. The edge shows
a well defined d-state splitting, indicative of well separated states such as those found in octahedral symmetry. Lowering of the symmetry would further split the d states, and whereas the peak width would perhaps remain the same, the observed splitting would disappear. Again the interpretation of the L 1 data that the site possesses a center of inversion supports this conclusion. Finally, the EXAFS data from the glass samples show a well defined first coordination sphere for the Ln ion in the glasses, whereas the more distant coordinations are not well defined. It should be noted that the 4f-state splittings are largely determined by the first cordination sphere, and therefore the optical properties of the Er glass should be fairly well resolved.
Acknowledgments The authors thank Farrel Lytle for the tungsten data shown in Fig. 2, and for insightful discussion. The NSLS is supported by the U.S. Department Of Energy, Division of Material Sciences and Division of Chemical Sciences. This work was supported by the U.S. DOE, Basic Energy Sciences-Chemical Sciences, under contract No. W-3l-109ENG-38.
References [1] A.J.G. Ellison, C.K. Loong and J. Wagner, J. Alloys Comps., 207 / 08 (1994) 170. [2] C.W. Ponader and G.E. Brown Jr., Geochim. Cosmochim. Acta, 53 (1989) 2893. [3] E.M. Larson, A.J.G. Ellison, F.W. Lytle, A. Navrotsky, R.B. Greegor and J. Wong, J. Non-Cryst. Solids, 130 (1991) 260. [4] M.A. Marcus and A. Polman, J. Non-Cryst. Solids, 136 (1991) 260–265.
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[5] J.-H. Choy, K.-K. Kim, G. Demazeau and D.-Y. Jung, J. Phys. Chem., 98 (1994) 6258. [6] S.R. Bare, G.E. Mitchell, J.J. Maj, G.E. Vrieland and J.L. Gland, J. Phys. Chem., 97 (1993) 6048. [7] G.N. George, W.E. Cleland Jr., J.H. Enemark, B.E. Smith, C.A. Kipke, S.A. Roberts and S.P. Cramer, J. Am. Chem. Soc., 112 (1990) 2541. [8] M.M. Abraham, L.A. Boatner, T.C. Quinby, D.K. Thomas and M. Rappaz, Radioactive Waste Manage., 1 (1981) 181. [9] M.O. Krause and J.H. Oliver, J. Phys. Chem. Ref. Data, 8(2) (1979) 329. [10] F.W. Lytle, in H. Winick et al. (eds.), Applications of Synchrotron Radiation, Gordon and Breach, New York, 1989, p. 135. [11] P.A. O’Day, J.J. Rehr, S.I. Zabinsky and G.E. Brown Jr., J. Am. Chem. Soc., 116 (1994) 2938. [12] J. Mustre de Leon, J.J. Rehr, S.I. Zabinsky and R.C. Albers, Phys. Rev. B, 44 (1991) 4146. [13] B.K. Teo, EXAFS: Basic Principles and Data Analysis, SpringerVerlag, Berlin, 1986, 349 pp. [14] J. Rohler, in K.A. Gschneidner Jr., L. Eyring and S. Hufner (eds.), Handbook Phys. Chem. Rare Earths, North Holland, Amsterdam, 1987, pp. 453–545.
[15] U. Staub, M.R. Antonio, L. Soderholm, M. Guillaume, W. Henggeler and A. Furrer, Phys. Rev. B, 50(10) (1994) 7085. [16] M.R. Antonio, U. Staub, J.S. Xue and L. Soderholm, Chem. Mat., (1996) in press. [17] F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Interscience, New York, 4th ed., 1980, 1367 pp. [18] B.N. Figgis, Introduction to Ligand Fields, Wiley-Interscience, New York, 1966, Chapter 12. ¨ ¨ [19] F. Hilbrig, H.E. Gobel, H. Knozinger, H. Schmelz and B. Lengeler, J. Phys. Chem., 95 (1991) 6973. [20] J.A. Horsley, I.E. Wachs, J.M. Brown, G.H. Via and F.D. Hardcastle, J. Phys. Chem., 91 (1987) 4014. [21] F.W. Lytle and R.B. Greegor, Appl. Phys. Lett., 56(2) (1990) 192. [22] A.F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 5th Ed., 1986, 1382 pp. [23] A.J.G. Ellison, C.K. Loong and J. Wagner, J. Appl. Phys., 75 (1994) 6825. [24] E.N. Rizkalla and G.R. Choppin, in K.A. Gschneidner Jr. and L. Eyring (eds.), Hndbk. on the Physics and Chm. of Rare Earths, Elsevier Science, Amsterdam, 1991, pp. 393–442. [25] S.F. Lincoln, in Advances in Inorangic and Bioinorganic Mechanisms, Academic Press, London, 1986, 217 pp.
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Local environments of erbium and lutetium in sodium silicate glasses Mark R. Antonio, L. Soderholm, A.J.G. Ellison Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 -4831, USA
Abstract The coordination environments of Er and Lu in sodium silicate glasses are probed through a unique analysis of L 3 X-ray absorption near edge structure (XANES) combined with extended X-ray absorption fine structure (EXAFS). By using the observed splitting of the empty 5d manifold of the lanthanides in combination with the EXAFS data, we demonstrate that Er and Lu are 6-coordinate in well-defined sites with relatively high symmetry. Keywords: Local environments; Erbium; Lutetium
1. Introduction
2. Experimental
The optical responses of 4f single-ion states have been extensively studied and are currently exploited in a number of devices, such as lasers, telecommunications fibers, fluorescing screens and amplifiers. However, a dearth of information about local coordination of 4f ions in glasses has slowed the expansion of this technology into areas relevant to fiber optic and semiconductor applications. Information about systematics in 4f-ion coordination is crucial to a predictive understanding of the optical properties of 4f-ions in glasses. Limited energy-level information can be obtained from inelastic neutron scattering, where the neutron’s magnetic moment interacts with the unpaired spins on the f-ion to provide insight about the local environment [1]. Extended X-ray absorption fine structure (EXAFS) has also been used to probe the 4f environments in glasses [2,3]. We note the previous work on the local structure of Er 31 in silica and sodium silicate glass [4]. In this paper, we have expanded these approaches to include XANES, X-ray absorption near edge structure, as well as EXAFS, on two different glasses containing erbium and lutetium. We demonstrate that an approximate value of the 5d band splitting, reported as the crystal field strength parameter D, can be determined from the peak splitting of the second differential XANES at the Er and Lu L 3 -edges. Such splitting is commonly observed in the L 3 -edge XANES of the 4d and 5d transition-metal ions [5–7], and herein we report the observation of 5d band splitting in the 4f ions Er 31 and Lu 31 .
Polycrystalline specimens of LnPO 4 (Ln;Er, Lu) were prepared according to published procedures [8]. Together with Reacton grade Ln 2 O 3 , obtained from Aesar, these were used as reference materials for the XAFS analyses. The glasses were prepared by pour quenching at 1450 8C, the details of which are provided elsewhere [1]. Diffraction data show that both 3Na 2 O?Er 2 O 3 ?6SiO 2 and 3Na 2 O?Lu 2 O 3 ?6SiO 2 are amorphous glasses. Erbium and lutetium L 3 -edge (8358 and 9244 eV, respectively) and L 1 -edge (9751 and 10 870 eV, respectively) X-ray absorption data were collected at ambient temperature on beam line X-23A2 at the National Synchrotron Light Source with a Sik311l double crystal monochromator. At 9000 eV with a 1 mm vertical entrance slit, the total instrumental energy resolution was estimated to be 2.0 eV, which is 2.33 smaller than the natural line widths (ca. 4.5 eV [9]) of the Er and Lu L 3 levels. Hence, the XAFS shown here are somewhat broadened (¯9%) by instrumental effects. In order to avoid sample thickness artifacts and to obtain accurate edge resonance intensities and positions, the XANES was detected using the electronyield technique [10]. The L-edge regions were scanned independently under the same experimental conditions with a step size of 0.25 eV/ pt (L 3 edges) and 0.4 eV/ pt (L 1 edges). The scan-to-scan energy calibration was maintained to 60.2 eV. All XAFS analyses were performed with EXAFSPAK [11] using FEFF 6.01a [12]. The fitting procedure and all reported parameters have their standard meaning [13].
0925-8388 / 97 / $17.00 1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02736-3
M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
3. Results and discussion The normalized L 3 -edge XANES of LnPO 4 , Ln 2 O 3 and glasses 3Na 2 O?Ln 2 O 3 ?6SiO 2 for Ln5Er, Lu are displayed in Fig. 1a. The intense and symmetrical single resonances in the L 3 -edge XANES of LnPO 4 is typical of those seen for other trivalent lanthanides [14]. These resonances are due to dipole-allowed (Dl51) electronic transitions from the Ln 2p 3 / 2 initial state to the empty 5d-orbital manifold. The fullwidth at half-maxima (FWHM) of the Lorentzianshaped edge resonances, shown in Fig. 1a, are a convolution of the initial state (2p 3 / 2 core hole) and the final state (5d 1 ) widths. The final state width is the 5d bandwidth, which is subject to a number of effects, most notably splitting due to the crystal-field environment, in addition to orbital hybridization / mixing due to bonding [15]. As shown in Fig. 1a, the FWHM of the resonant-white lines obtained from our samples vary considerably, and sometimes exhibit marked asymmetry. Fitted values of the resonant line widths are listed in Table 1. It has been previously demonstrated that the L 3 -edge
Fig. 1. Lutetium (solid lines) and erbium (dashed lines) L 3 -edge XANES for Ln 2 O 3 , LnPO 4 , and Ln glasses for Ln;Lu, Er obtained through electron-yield at ambient temperature. (b) Second differential L 3 -edge XANES.
537
Table 1 Widths (FWHM, eV) of the 2p 3 / 2 →5d resonance in the normalized, electron-yield Er and Lu L 3 -edge XANES as obtained by curve fitting with Lorentz and arctangent functions, convolved with a 2 eV Gaussian broadening function a
LnPO 4 Ln 2 O 3 Ln-Glass
Er (FWHM)
D (eV)
Lu (FWHM)
D (eV)
5.1 7.2 6.9
b
5.1 7.3 7.3
b
3.4 2.8
3.4 2.8
D, the separation of the two peaks in the double differential of the edge resonance is also listed. Fitting range: 220 to 125 eV. a See F.W. Lytle, R.B. Greegor and E.C. Marques in M.J. Phillips and M. Ternan (eds.), Proc. 9 th Int. Cong. Catalysis, Calgary, Vol. 5, The Chemical Institute of Canada, Ottawa, 1988, p. 54, and; G. Krill, J. Phys., 47 Colloque C8 (1986) C8-907. b When D is smaller than the spectral resolution of 2 eV, the crystal field splitting of the 5d orbitals cannot be determined.
resonance observed in the XANES of 4d and 5d transition elements show two (or more) well-resolved peaks that have been attributed to transitions between crystal-field states within a split d manifold [6,7,16]. The overall d-orbital splitting is expected to be as large as 4 eV for the 4d band and 5–6 eV for the 5d band [17]. Thus, it can be argued that the XANES peak splitting provides an approximate value of the crystal-field strength parameter, D, whose magnitude can be diagnostic of the crystal-field symmetry about the metal ion or, in other words, its coordination environment. Recently, the simple procedure of taking the second differential of L 3 -edge XANES was shown to make possible the resolution of separate crystal-field-split transitions that are not resolved in the primary XANES [5]. This d-band splitting is demonstrated in Fig. 2b for the W L 3 -edge XANES of WO 3 . The crystal field splitting of 5d orbitals can be observed in the second differential L 3 -edge XANES of 5d transition metals, e.g., W and Ir, with a coordination environment that produces a large d-orbital splitting. In general, D decreases for three common coordination geometries as planar.octahedral.tetrahedral. The generalizations and simplifications of crystal field theory [17,18] are nicely borne out in the comparison of, for example, the shape and width of W L 3 -edge resonances near 10 206 eV for oxides with tetrahedral and octahedral coordination. As we show in Fig. 2, for W(VI) with a tetrahedral oxygen environment (Na 2 WO 4 ?2H 2 O) the resonance is sharp, whereas for an octahedral oxygen environment (WO 3 ), the resonance is broad. As has been noted elsewhere [19], the FWHM of the resonance for WO 3 (8.0 eV) is significantly broader than that for Na 2 WO 4 ?2H 2 O (5.3 eV). Moreover, the second differential W L 3 -edge XANES for WO 3 , shown in Fig. 2b, has two valleys with a separation of 3.9 eV, whereas the corresponding data for Na 2 WO 4 ?2H 2 O does not. When D is smaller than the spectral resolution of ca. 2.0 eV, the crystal field splitting of the 5d orbitals cannot be observed even in the second differential XANES. This is the case for [WO 4 ] 22 , which
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M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
Fig. 2. Tungsten L 3 -edge XANES (a) and second differential XANES (b) for WO 3 and Na 2 WO 4 ?2H 2 O.
reveals a single valley. Conversely, when D is larger than the spectral resolution, then crystal field splitting of the 5d orbitals can be observed. This was demonstrated for WO 3 . In both of the W compounds reported here, tungsten is hexavalent, and therefore it has the same formal electronic configuration as Lu 31 (4f 10 5d 0 ). The second differential L 3 -edge XANES of the Ln data, shown Fig. 1b, are quite revealing. For LnPO 4 , there is just one deep valley, whereas for Ln 2 O 3 and the Ln glasses there are two well-resolved valleys with a separation of 3.4 and 2.8 eV, respectively. The similarity of the resonant-line splitting, together with the similarity in electronic configurations of the ions under study, leads us to conclude that the splitting observed in our Er and Lu L 3 -edge data is the result of a crystal field splitting of the empty, lanthanide 5d manifold. Whereas crystal field splitting effects such as those described herein are commonly observed in the L 3 XANES of transition metal ions, this is not the case to date for the 4f ions. For the crystalline Ln 2 O 3 and glass 3Na 2 O? Ln 2 O 3 ?6SiO 2 samples we observe using L3 XANES crystal field splitting of the 5 d orbitals of Er 31 and Lu 31 . These observations are consistent with the 6-fold coordination of the Ln 31 in Ln 2 O 3 and the glass samples. The Ln 31 ions in LnPO 4 have 8-fold, dodecahedral coordination which,
gives rise to a splitting that is too small to detect with this technique. This small splitting is consistent with predictions based on the rare-earth site coordination [18]. The Ln L 1 -edge XANES of LnPO 4 , Ln 2 O 3 and glasses 3Na 2 O?Ln 2 O 3 .6SiO 2 for Ln5Er, Lu are shown in Fig. 3. Unlike the corresponding L 3 -edge XANES of Fig. 1, there are no intense peaks that can be directly assigned to specific electronic transitions. For L 1 XANES, the initial state is the Ln 2s orbital. Dipole-allowed transitions from the 2s to the empty 6p manifold have yet to be unambiguously identified in L 1 -edge XANES data. Nevertheless, quadrupole-like transitions from the 2s to the empty 5d manifold have been observed under certain conditions, particularly for W 61 [20]. The peaks beyond 0 eV in the data of Fig. 3 are now generally recognized as scattering resonances [21]. From the base of the rising edge, there appears to be a slight shoulder extending from about 1 / 3 to 1 / 2 of the edge step intensity. Among other things, this unresolved feature may be a weak s→d electronic transition. Strong electronic resonances near the base of the rising L 1 -edge are often observed for absorbing atoms with particular site symmetry. These so-called pre-edge peaks are especially intense for absorbing atoms in a site without inversion symmetry, such as for a tetrahedral ligand field [20]. In this case, the peak is due to a transition from the 2s initial state to a final state of mixed d and p orbital character. In contrast, if the absorbing atom site has inversion symmetry, such as for an octahedral ligand field, pre-edge features are not observed. The absence of any pre-edge peaks in all of the L 1 -edge XANES of Fig. 3 argues against significant p-d mixing in any of these materials. The k 3 x(k) EXAFS and the corresponding Fourier transform (FT) data are shown in Fig. 4a and Fig. 4b, respectively, for the erbium and lutetium phosphates, oxides and glasses. The first and most intense peak at ca.
Fig. 3. Lutetium (solid lines) and erbium (dashed lines) L 1 -edge XANES for Ln 2 O 3 , LnPO 4 and Ln glasses for Ln;Lu, Er obtained through electron-yield at ambient temperature.
M.R. Antonio et al. / Journal of Alloys and Compounds 250 (1997) 536 – 540
539
Fig. 4. (a) Lutetium (solid lines) and erbium (dashed lines) L 3 -edge k 3 x(k) EXAFS and (b) the corresponding Fourier transform (FT) magnitude of the k 3 x(k) EXAFS of LnPO 4 , Ln 2 O 3 and Ln glasses.
˚ (before phase shift correction) in all the FT data of 1.9 A Fig. 4b is due to scattering by oxygen atoms. Beyond ˚ in the FT data, there is significant structure for about 2 A LnPO 4 and Ln 2 O 3 which has been assigned to scattering by phosphorous and Ln atoms, see Fig. 4b. In contrast, ˚ in the FT data for the erbium and lutetium beyond 2 A glasses, there are no structurally significant peaks. The absence of any structurally significant peaks beyond this distance provides clear evidence that the glasses do not possess long range order. This is consistent with the X-ray diffraction data for these same glasses. The short-range Ln 31 –O order manifests itself in the k 3 x(k) EXAFS of Fig. 4a. The EXAFS for the glasses looks like a damped, single-frequency sinusoid, whereas for LnPO 4 and Ln 2 O 3 the EXAFS appears to be a combination of multiple sinusoids with different frequencies. Curve fitting analysis of the k 3 x(k) EXAFS for the glasses reveals 6661 O ˚ The metrical results obtained from the atoms at 2.2(1) A. EXAFS analysis of crystalline LnPO 4 and Ln 2 O 3 are in agreement with the reported crystal structures [22]. Using the data interpretation discussed above, we can make several remarks about the rare earth environment in the glass samples. First, because of the larger FWHM, and the splitting observed from the second-differential-edge spectra shown in Fig. 1b, we can conclude that both Er and Lu are 6-coordinate. This is corroborated by fitting to the EXAFS data, which shows 6 oxygen near neighbors for both glass samples. This is also consistent with neutron work on similar glasses [1,23] and with other EXAFS studies of the heavier lanthanides in glasses [2,4]. For comparison, a variety of other work suggests a coordination of 8 for the heavier lanthanides [24,25] in solution. The determination from the L 1 XANES data of a Ln coordination site with inversion symmetry argues against coordinations of 5 or 7 in our glass samples. The first coordination sphere in our glass samples is also well defined, and of relatively high symmetry. The edge shows
a well defined d-state splitting, indicative of well separated states such as those found in octahedral symmetry. Lowering of the symmetry would further split the d states, and whereas the peak width would perhaps remain the same, the observed splitting would disappear. Again the interpretation of the L 1 data that the site possesses a center of inversion supports this conclusion. Finally, the EXAFS data from the glass samples show a well defined first coordination sphere for the Ln ion in the glasses, whereas the more distant coordinations are not well defined. It should be noted that the 4f-state splittings are largely determined by the first cordination sphere, and therefore the optical properties of the Er glass should be fairly well resolved.
Acknowledgments The authors thank Farrel Lytle for the tungsten data shown in Fig. 2, and for insightful discussion. The NSLS is supported by the U.S. Department Of Energy, Division of Material Sciences and Division of Chemical Sciences. This work was supported by the U.S. DOE, Basic Energy Sciences-Chemical Sciences, under contract No. W-3l-109ENG-38.
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