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Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

Alkali lime silicate glasses containing 5 wt% of CoO were investigated by Co K-edge XANES and EXAFS and optical absorption spectroscopy. Our results reveal the presence of tetrahedral Co2+ connected with the glass network, with a IVCo–O–Si angle of 134°. Changing the alkali from K+ to Na+ induces an increase of the local disorder around Co2+, as shown by a decrease of the contribution from the second neighbors in the EXAFS signal. We propose two models for interpreting the structure of the second shell of neighbors. Our results provide a structural basis for rationalizing the optical properties of Co2+ species in glasses.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

Structure-property relationships in glasses help predict some important physical and chemical properties of glasses and their changes as a function of temperature or glass composition. The chemical dependence of glass coloration is a perfect illustration of these relationships.[1] Co2+ is one of the most effective pigments to color glasses. The optical absorption spectra of these glasses are interpreted as arising from Co2+ in tetrahedral sites (IVCo).[2] However, the structural role of IVCo is unclear. IVCo has been interpreted as a “quasi-molecular complex” independent of the glassy network.[3] In K-bearing silicate glasses and melts, several divalent cations such as Fe2+, Ni2+, and Zn2+ in tetrahedral sites exhibit a rigid linkage with the glass network, as shown using EXAFS.[4-6] These studies demonstrate that this topology corresponds to tetrahedral transition metal ions (TM) in a network forming position, with K+ cations acting as charge compensators.[7] Such structural data are not available on Co-bearing glasses. Previous EXAFS[8] and neutron diffraction[9] studies on sodium disilicate glasses confirmed the presence of IVCo in sodium silicate glasses but did not indicate any linkage of Co-sites to the glassy network. In addition, the influence of alkaline-earth cations on Co-speciation is unknown.

In this communication, we report spectroscopic evidence for the local ordering around four-fold coordinated Co in alkali-lime silicate glasses, as deduced from Co K-edge EXAFS and XANES data. Two models are proposed, which demonstrate the presence of structural order around IVCo tetrahedra. In light of these structural data, we show how the nature of the alkali and the presence of alkaline-earth influence the optical absorption of Co-bearing silicate glasses.

2 Experimental Procedure

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

Glasses with the composition R2O–CaO–4SiO2 (R = K, Na) containing 5 wt% CoO, hereafter referred to as RCa, were synthesized from high-grade R2CO3, CaCO3, SiO2, and CoO. The starting materials were decarbonated overnight at 800°C, then melted in a Pt/Rh crucible for 3 h at 1200°C. After quench, they were crushed and melted again for 3 h at 1200°C. The melts were quenched by pouring the glass between two copper plates and annealed overnight at 400°C. Optical absorption spectra of polished slabs of glass were recorded using a Perkin Elmer Lambda 1050 spectrophotometer in the range 2500–400 nm. X-ray absorption spectroscopy measurements were performed on beamline 13-BM-D at the Advanced Photon Source (Chicago, IL). The storage ring conditions were 7 GeV and 100 mA positron current. A Si (111) double crystal monochromator was used and the intensity of the transmitted beam was measured using an argon-field ionization chamber. Co K-edge (7709 eV) was calibrated using a Co metallic foil. XANES and EXAFS spectra were recorded in transmission mode using variable energy steps (0.25 eV/step from −15 eV to +25 eV from the edge; 3 eV/step from +25 eV to +800 eV above the edge) and an accumulation time of 2 s/step. The spectra were normalized and EXAFS analyzed using the “ATHENA” and “ARTEMIS” softwares.[10]

3 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

The Co K-edge XANES spectra of KCa and NaCa are compared to the crystalline reference IVCoZnSiO4 (Fig. 1). The pre-edge (feature A) intensity in KCa is as intense as in CoZnSiO4 indicating that Co2+ is in tetrahedral sites. The pre-edge intensity decreases by 5% in NaCa. The main-edge exhibits similar shapes for both glasses with two features B and C (Fig. 1) separated by ~12 eV as in CoZnSiO4. This confirms the tetrahedral coordination of Co2+ in this glass. As observed in the XANES and first derivative XANES spectra (not shown), feature B broadens and shifts towards lower energy from KCa to NaCa, as feature C vanishes, which suggests a more distributed environment for Co2+ in the latter, with a lower contribution of IVCo2+.

image

Figure 1. Co K-edge normalized XANES spectra of KCa (middle) and NaCa (top) glasses compared with crystalline CoZnSiO4 (bottom), a reference for IVCo2+ structurally similar to Zn2SiO4 willemite. Spectra are displayed vertically for clarity. Inset: zoom on the pre-edge region (feature A).

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The EXAFS signals of KCa and NaCa show spectacular differences but nonetheless exhibit well-defined oscillations of similar phase up to 15 Å−1 [Fig. 2(a)]. For KCa, we observe additional contributions at 3.9, 4.8, 6.1, 8.3, 10.3, 12.8, and 13.7 Å−1. These contributions arise from the interference of the low-frequency signal of the first shell with a high-frequency signal, arising from a second coordination shell. This results in a second peak near 2.9 Å in the magnitude of the Fourier transform (FT) of the EXAFS spectra [Fig. 2(b)]. In NaCa, these interferences fade out resulting in the decrease of the second shell contribution in the FT. The EXAFS-derived coordination numbers N, distances d and Debye-Waller factors σ for the first O shell are given in Table 1. These values are in agreement with tetrahedral coordination of Co2+ and are consistent with previous EXAFS and neutron diffraction data.[8, 9] The increase of the Debye–Waller factor from KCa to NaCa confirms the increase of the radial disorder around Co and is also consistent with a smaller contribution of the second shell in the EXAFS signal. Fitting this second shell may not be realized with a single Co-Si distance. Two models may be used to fit the second shell contribution:

  1. Model 1: a possible solution is obtained using two Co–Si distances (Table 1). Considering a dCo-Si mean value of 3.27 Å (and an average dSi-O = 1.63 Å) gives a IVCo–O–Si angle of 134° which agrees fairly well with the TM–O–Si angles reported previously: for Ni2+ in K2NiSi3O8 (129°)[5] and Zn2+ in cordierite glass (127°).[1] This indicates corner-sharing tetrahedra and is consistent with a network forming position for Co2+ (as for Ni2+ and Zn2+) where alkali and alkaline-earth cations play a charge-compensating role. These IVTM–O–Si angles are smaller than the Si–O–(Al,Si) angles in NaAlSi3O8 and KAlSi3O8 glasses: 143° and 146° respectively.[11] This indicates that the lower valence of the TM cations causes lower repulsion between IVSi and TM ions.
  2. Model 2: using one Co–Si and one Co–Co distances also provides satisfactory fits (Table 1). These data suggest a willemite-like structure where two Co-sites are linked to one SiO4 via a threefold coordinated O. A similar model was proposed for Fe2+ in molten Fe2SiO4.[4] For NaCa, the Co–Si distance is subject to a large uncertainty because of its weak contribution to EXAFS. In this model, no additional charge compensation is required.
Table 1. EXAFS-Derived Parameters Obtained for the First and Second Shells
 KCaNaCa
First shell
N O 3.7 ± 0.34.0 ± 0.6
dCo-O (Å)1.94 ± 0.011.95 ± 0.01
σ2O (10−3 Å2)3.4 ± 0.95.0 ± 1.9
Second shell
Model 1
  N Si 1 1.5 ± 0.50.4 ± 0.2
  dCo-Si 1 (Å)3.10 ± 0.033.10 ± 0.04
  N Si 2 2.3 ± 0.71.1 ± 0.3
  dCo-Si 2 (Å)3.44 ± 0.023.50 ± 0.02
  σ2Si (10−3 Å2)3.0 ± 2.03.0 ± 2.9
Model 2  
  NCo-Si1.3 ± 0.30.2 ± 0.3
  dCo-Si (Å)3.24 ± 0.022.86 ± 0.02
  σ2Si (10−3 Å2)4.5 ± 2.52.9 ± 2.9
  N Co-Co 2.2 ± 0.30.9 ± 0.3
  dCo-Co (Å)3.28 ± 0.023.28 ± 0.02
  σ2Co (10−3 Å2)4.1 ± 3.74.7 ± 2.0
image

Figure 2. (a) Co K-edge k3χ(k) EXAFS signal of KCa (bottom) and NaCa (top) showing interferences between 6 and 14 Å−1 (indicated by arrows). (b) Un-shifted magnitude (thick line) and imaginary part (thin line) of the Fourier transforms of the EXAFS signal for KCa and NaCa, showing the contribution of the second shell at 2.9 Å.

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The optical absorption spectra show a modification of the local environment of Co2+ when changing K to Na (Fig. 3). The typical intense blue color of both samples is due to two triplet bands. They are assigned to the electronic transitions 4A1[RIGHTWARDS ARROW]4T1(F) (5700, 6640, 7660 cm−1) and 4A1[RIGHTWARDS ARROW]4T1(P) (15 400, 16 720, 18 500 cm−1) of Co2+ in tetrahedral symmetry.[12] Such assignment is in agreement with XANES and EXAFS data. The molar extinction coefficient of the most intense absorption band is higher for KCa [εmax = 152 L·(mol·cm)−1] than for NaCa [εmax = 105 L·(mol·cm)−1]. Replacing K by Na induces an increase in the width of the absorption bands, the shift of their position towards higher energy, the change of their relative intensity and a decrease of the overall absorbance by 25%.

image

Figure 3. Optical absorption spectra of KCa (top) and NaCa (bottom) revealing typical shape for IVCo2+. The nature of the alkali modifies the spectra.

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The molar extinction coefficients found in this study are smaller than the molar extinction coefficients of tetrahedral Co2+ in ZnO or complexes [εmax ≈ 500 L·(mol·cm)−1].[12, 13] The values for KCa and NaCa are also smaller than those reported for potassium and sodium silicate glasses, respectively.[14, 15] Altogether, it suggests that in glasses Co2+ converts to less absorbing Co-species when adding alkaline earth and decreasing the size of the alkali. In staurolite, distorted IVCo2+ induces a smaller molar extinction coefficient [εmax ≈ 100 L·(mol·cm)−1][16] and it results in an increase of the disorder around IVCo and in a decrease of the NCo-O values derived by EXAFS.[17] In contrast, in glasses, the increase of the disorder from KCa to NaCa is correlated to the increase of the fitted NCo-O value. This agrees with neutron diffraction[9] data, giving NCo-O = 5.0 ± 0.2 in Na–Co silicate glass. The Co coordination numbers found in this study and in literature suggest the presence of higher coordinated Co2+ species (i.e., V− and VICo2+). This is consistent with lower values of the molar extinction coefficients for V− and VICo2+ than for IVCo2+ [VICo2+: εmax < 10 L·(mol·cm)−1; VCo2+: εmax ≈ 100 L·(mol·cm)−1].[18] Because the optical absorption bands of IVCo2+ largely overlap the transitions of VCo (5000–11 000 cm−1 and 15 000–21 000 cm−1) and VICo (8000 and 19 000 cm−1),[19] the contributions of high-coordinated Co2+ are not directly apparent. The low intensity of these “spectroscopically silent” species does not help in recognizing them from optical absorption spectra, although the presence of high-coordinated Co2+ in silicate glasses had not been ruled out.[15, 20] Here, EXAFS data provide an important indication of their presence.

4 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

The determination using EXAFS of the local environment of Co2+ in alkali-lime silicate glasses reveals the speciation of Co2+ in well-defined regular tetrahedral sites. The analysis of the second shell suggests that the ordering of the local environment is influenced by the alkali. The structural disorder increases from KCa to NaCa. IVCo2+ is converted to optically less absorbing species (“optically silent” species). However, comparison with XANES, optical spectroscopy and previous neutron diffraction data reveals that EXAFS spectroscopy mainly gives information on the most ordered surrounding.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References

This work is part of the project POSTRE, supported by Agence Nationale de la Recherche within the program MatetPro2008. Portion of this work was performed at GeoSoilEnviroCARS (Sector 13), APS, Argonne National Laboratory supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy - Geosciences (DE-FG02-94ER14466). Use of the APS was supported by the U. S. Department of Energy, Basic Energy Sciences (DE-AC02-06CH11357).

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Conclusion
  7. Acknowledgments
  8. References
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