Bull. Minéral. (1983), 106, 57-75

Raman spectroscopic studies of silicate and related glass structure -a review

Etudes structurales des verres silicatés par spectroscopic Raman-analyse critique.

Introduction

Silicate liquids are important in many geologic pro¬ cesses, and much effort has been devoted to under¬ standing their structure in relation to their bulk proper¬ ties. The methods used to probe this structuçe have included a range of spectroscopic and diffraction tech¬ niques, and modelling based on measurements of ma¬ croscopic properties. The refractory nature of silicate systems has often posed serious experimental pro¬ blems for direct study of their high-temperature li¬ quids, and a number of studies have carried out mea¬ surements on glasses quenched from these melts and extrapolated the results and conclusions to the liquids. The validity of this approach has been questioned, and will be discussed below. One of the more successful spectroscopic techniques for the study of silicate glass and melt structure has been vibrational spectroscopy, especially Raman scattering. Raman spectroscopy has been used with great success in the study of vibratio¬ nal properties of crystals, and has more recently been applied to the vibrational structure of disordered mate¬ rials. Although the nature of vibrations in such amor¬ phous systems is not yet well understood, suitable interpretation of the observed spectra may lead to mo¬ dels of the molecular structure which can be compared with models derived from other types of study. The present article reviews the application of Raman spec¬ troscopy to the structure of silicate and aluminosilicate glasses and melts.

These studies are considered in increasing order of complexity. The vibrational and molecular structure of vitreous silica has been studied extensively, and al¬ though its vibrational spectrum is not fully under¬ stood, some general conclusions may be drawn. In the following discussion, the term molecular structure is used to denote the average or static geometrical arran¬ gement of bound atoms, or to describe the geometry of a particular unit within the glass structure. This is distinguished from the vibrational or dynamic struc¬ ture of the glass, where vibrating units are defined by the atomic displacements associated with a particular mode of vibration. In the binary alkali and alkaline-earth silicates, a large number of experimental studies have investigated the variation of Raman spectra with composition, and some general agreement has been reached which allows assignment of band groups to structural units. The case for aluminosilicatés is more complex and structural interpretations are highly controversial. Finally a little work has been carried out on aluminosilicate glasses containing titanium, iron or phosphorus, but more experimental data are needed before their structural role may be fully understood.

Glass vs. melt structure

Glasses are known to differ substantially from melts in their macroscopic properties, and the extrapolation