Stereochemical Structure for Sodium in Native and Thermal Silica Layers
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ABSTRACT Sodium-covered silica films formed on silicon substrates have been examined by X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (EXAFS) in ultra-high vacuum conditions at 300K. The results show that sodium diffuses into the silica layer on a reversible manner and that it modifies the silica network in order to create its own site. Sodium atoms are surrounded by oxygen atoms at an average distance of 2.3 A and by a second shell which is assigned to silicon atoms located at 3.8 A. At high Na concentrations, sodium atoms are also present in the close environment of one sodium atom. Keywords : Soda-silicate glasses, X-ray Absorption Spectroscopy, Sodium Local Order, X-ray Photoemission INTRODUCTION The addition of alkali-metal oxides to the pure silica strongly reduces its melting temperature and thus facilitate its manufacturing. Indeed, the understanding of the complex environment of alkali silicate glasses is an important issue regarding the industrial importance of these materials. The basic chemistry of commercial silica glasses involves network-forming elements whose role is to built chains linked by strong chemical bonds, mostly covalent, and network-modifying elements which reduces the connectivity of the glass former network [1]. The neighbourhood of alkali ions in silica glasses is more puzzling than that of silicon. In the Zachariasen-Warren's view [2], alkali ions are not supposed to meet any particular coordination shell, but to simply occupy the available sites of the silica network. More recent studies have provided evidences for some ordering in the chemical environment of the networkmodifying ions [3]. On the basis of EXAFS analysis at the sodium K-edge of silica glasses, Greaves et. al. [4] have developped a'Modified Random Network' model, in which alkali ions and non-bridging oxygens (NBO) to which alkali atoms are mostly linked, concentrate into channels surrounding zones dominated by pure silica. Nevertheless, all these studies have been conducted on bulk materials manufactured with the usual methods. At the contrary, we present here results on thin silica films prepared using 'surface' techniques. Moreover, since pure silica is believed to mostly involve bridging oxygen atoms, such samples offer the opportunity to examine the behaviour of sodium atoms inserted in an environment which consists almost exclusively of bridging oxygen atoms.
299 Mat. Res. Soc. Symp. Proc. Vol. 606 ©2000 Materials Research Society
EXPERIMENT Photoemission experiments (XPS) have been performed in an ultra-high vacuum system equipped with an Mg Ka X-ray photoemission spectrometer VG CLAM IH.X-ray absorption data on the sodium K-edge have been recorded on the SA32 soft X-ray beam line of SuperACO equipped with a toroidal focusing mirror, and a two-crystal monochromator using beryl (1010) crystals. At the sodium K-edge energy the resolution is close to 0.5 eV. The data have been collected in the total electron yield mode (TEY) from the sample. Model compounds have also been examined, name
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