1 INTRODUCTION

Upon first consideration, the term 'structures of glasses' appears to be an oxymoron. How can materials defined by a lack of long range, periodic structure have structures which are characteristic of specific compositions? On the other hand, we know that the properties of three different samples of glasses of the same nominal composition, produced independently in three different laboratories, and annealed in the same manner, will be identical within reasonable limits. Our basic understanding of the solid state then indicates that these glasses have, if not identical, at least very similar structures. It follows that a lack of long range, periodic structure does not, in and of itself, preclude the existence of structure at a level that will control the properties of material.

Early discussions of glass structures centered on silicate glasses, especially vitreous silica and alkali silicate glasses. The first models for glass structures were based on the structures of silicate crystals. The microcrystal approach suggested that glasses are simply masses of very small, or micro, crystals. The small size of these crystals can be used to explain the lack of structure in the x-ray diffraction pattern. Lebedev and others in Russia favored a somewhat different version of the microcrystalline model, which they termed the crystallite model. Their crystallites differ from microcrystals in that the structures are deformed versions, i.e., not perfect lattices, of those of the crystals and are not merely smaller sized versions of normal crystals. A glass then was assumed to...