Journal of the Ceramic Society of Japan 119 [10] 720-725 2011 Special Article The 65th CerSJ Awards for Academic Achievement in Ceramic Science and Technology: Review

# Characterization of glasses based on basicity

Tokuro NANBA

Graduate School of Environmental Science, Okayama University,

3–1–1 Tsushima-naka, Kita-ku, Okayama 700–8530

Basicity is defined as an electron donating ability of a material, and hence, an oxide material containing oxide ions with high electron density is regarded to be in high basicity. It has been demonstrated that basicity can be evaluated by binding energy of core level electrons, which is measured by X-ray photoelectron spectroscopy. Based on the experimental results obtained from a large number of oxide glasses, correlations among basicity, O1s binding energy, structure and properties of glasses have been investigated. In the present paper, some of the research outcomes are introduced.

Key-words: Glass, Characterization, Structure, Properties, Material design, Basicity

[Received July 20, 2011; Accepted August 24, 2011]

### 1. Introduction

It has been understood that glass is a salt, which is formed by an acid-base reaction, where $ SiO_{2} $ and $ B_{2}O_{3} $ are the examples of acids, and $ Na_{2}O $ and CaO are classified as bases. According to an acid-base concept proposed by Lewis, $ ^{1)} $ acid is defined as an acceptor of electrons, and base is as electron donor. Under the definition, basicity is interpreted as a measure of electron donating ability. In oxide glasses, electrons donated are excess electrons possessed by oxide ions. The excess electrons are present in outer shell levels at valence band, and hence glasses containing much oxide ions with high electron density are regarded as those in high basicity. Electron density of outer shell levels is related to various properties of materials, such as chemical reactivity and stability, optical absorption and emission, electronic and magnetic functions, and so on. Besides properties, structure of materials is also dominated by basicity. $ ^{2)} $ Therefore, basicity is a predominant factor for both properties and structure of glasses, and it may cover exhaustively the relation among composition, structure and properties.

Basicity can be determined by various experiments, that is, measuring activity of oxide ions, optical absorption, and electron spin resonance spectra. An optical basicity concept proposed by Duffy and Ingram $ ^{3)} $ is based on the optical absorption of probe ions such as $ Tl^{+} $ , $ Pb^{2+} $ , and $ Bi^{3+} $ with an $ ns^{2} $ electron configuration. The absorption peaks of $ {}^{1}S_{0} $ $ \rightarrow{}^{3}P_{1} $ transition are observed at ultra-violet region, and the peak position is sensitive to the glass composition. The peak shift is responsible for the change in electron density of outer shell levels of probe ions, which is attributable to the change in number of electrons donated from the matrix glass, that is, the change in basicity of the matrix glass. In $ TeO_{2} $ and $ Bi_{2}O_{3} $ glasses, however, the absorption edge is present near visible light region, and hence the absorption peak of $ Pb^{2+} $ ions, for example, will be hidden in the absorption of these glasses. Thus, optical basicity is applicable to the most of conventional glasses which are transparent to UV light.

Change in the electronic states of outer shell levels, that is, change in the chemical bonding character, leads to a change in electrostatic potential inside the outer shell, resulting in the change in energy of core orbital electrons. Energy of electrons in core orbitals as well as outer shell levels is determined by X-ray photoelectron spectroscopy (XPS). It is therefore expected that XPS is used as an experimental method to evaluate basicity of a material. Then, the author's research group has measured XPS spectra for various glasses and examined the applicability of binding energy of O1s core orbital as a measure of basicity. In the present paper, the achievements in a series of investigations on basicity are introduced, where structure and properties of glasses are characterized on the basis of basicity.

### 2. Evaluation of basicity by XPS measurement

In theory, energy of core level electrons is measurable with XPS, and in practical, however, the measurement is quite difficult, because most of