An x-ray photoelectron spectroscopic study of the chemical states of fluorine atoms in calcium silicate glasses
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An x-ray photoelectron spectroscopic study of the chemical states of fluorine atoms in calcium silicate glasses Satoshi Hayakawa, Akira Nakao, Chikara Ohtsutki, and Akiyoshi Osaka Department of Bioscience and Biotechnology, Faculty of Engineering, Okayama University, Tsushima, Okayama-shi 700, Japan
Shuji Matsumoto and Yoshinari Miura Department of Environmental Chemistry and Materials, Faculty of Environmental Science and Technology, Okayama University, Tsushima, Okayama-shi 700, Japan (Received 16 December 1996; accepted 16 April 1997)
X-ray photoelectron spectroscopy has been used to examine the chemical states of fluorine in the glasses of composition xCaF2 s50 2 xdCaO ? 50SiO2 (x 5, 10, 15, 20, and 25 mol %) and xCaF2 s50 2 xy2dCaOs50 2 xy2dSiO2 (x 5, 10, 15, and 20 mol %). The analysis of the F1s spectra indicated that Ca21 and F 2 ions introduced as CaF2 are favorably located among the Si–O skeleton forming Ca–F clusters. The fraction of the bridging and nonbridging oxygen atoms was derived from the O1s spectra, and the network of the fluorine-containing glasses was concluded to depend only on the ratio CaOySiO2 .
I. INTRODUCTION
Incorporation of fluorine to multicomponent oxide glasses can modify their chemical and physical properties. Fluorine can lower the melting and glass transition temperatures and control refractive index, or provide fluorine ion conductivity. Variation of those properties is closely dependent both on the fluorine content and on the chemical bonding states of fluorine atoms.1–3 Especially, the latter may vary in a complex manner in consequence of the change in glass composition as has been reported in the literature.4–18 Infrared (IR) spectroscopic studies on alkaline earth oxyfluorosilicate melts suggested that the fluorine atoms were in the form of Si–F bonds when the melts were acidic,13,14 whereas other IR and Raman spectroscopic studies suggested that no Si–F bonds were present in calcium oxyfluorosilicate glasses.15,16 On the basis of x-ray photoelectron spectroscopy (XPS) and other techniques, Iwamoto et al. reported that the chemical states of fluorine were covalent Si–F bonds when CaOySiO2 , 1 and the F 2 ions were surrounded by Ca21 ions when CaOySiO2 . 1.17,18 However, their conclusions contradict with those in other spectroscopic studies.13–16 Therefore, the chemical states of fluorine atoms in glass and their compositional dependence have not fully been understood yet. In the present XPS study we have definitely distinguished the fluorine atoms in two chemical states by employing fresh surfaces of glass specimen fractured in ultrahigh vacuum (,7 3 1028 Pa) and by optimum charge neutralization with applying a Ni mesh screen together with flooding low energy (5–7 eV) electrons.19 The present paper will report the x-ray photoelectron spectra of F1s and O1s core levels for calcium oxyfluorosilicate glasses. The chemical states J. Mater. Res., Vol. 13, No. 3, Mar 1998
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