Daltonides, berthollides and inorganic glasses

Abstract

With the aim of restoring glasses to their place within the general framework of inorganic chemistry, the author begins by recalling that their deliberate isolation, like that of certain alloys, dates back to Proust's law of constant proportions (1797) and the Dalton atomic theory (1808). The concept of stoichiometry was used to cover up the most glaring exceptions, and, for more than a century, substances that could not be made to fit these new rules were ignored. The first reaction against the rigidity of this attitude, and the use of the term “berthollide”, came from Kurnakov (1914). Shortly afterwards, Schottky and Wagner, using the statistical thermodynamics of ordered solid phases, revealed the border-line character of “daltonides” (1930). Electronic compounds, oxide bronzes, interstitial compounds and chalcogenides provide the author with typical examples of crystallized berthollides. Although a special role can be attributed to certain privileged compositions for each of them, the classic concept of stoichiometry cannot satisfactorily be applied to them. The distinction between glasses and crystallized berthollides lies in the difference in the origin of local variations in composition, and in the greater discrepancy in glass between overall and localized compositions. The short-range order takes the form of structure units, the articulation of which, in chains within a three-dimensional network of bonds, plays a rôle similar to that of the crystallographic structure of crystals. The author shows that privileged overall compositions can also be distinguished, on the basis of phase separation phenomena, the singular points of certain physical properties and, even more directly, the co-ordination numbers of the atoms. In ternary glasses, these concepts lead to a definition of privileged zones, comparable to substitutional solid solutions in the case of crystallized compounds. Their boundaries account fairly well for the vitreous regions revealed by experiment. The case of the systems GeAsSe and AsTeI is touched on briefly.