General Approaches in the Description of the Structure of Glasses

Abstract

Taking into account the great variety of substances with extremely different compositions, known to exist in the vitreous state, it seems impossible at a first glance even to formulate such a problem like the general description of the structure of glasses. Moreover, even from a given melt with a definite composition different glasses can be prepared by varying the cooling rates. In the discussion of the structure of glasses in most textbooks devoted to the vitreous state only the structure of oxide and, in particular, of silicate glasses is analyzed. This is due, on one hand, to historic reasons, connected with the early widespread application of such glasses and, on the other hand, because oxide glasses represent, in fact, a striking example for a large variety of possible structures. Thus, an extended discussion is required to describe the structural properties of oxide glasses in a more or less comprehensive way. Other glass-forming systems, for which thorough structural investigations have been performed , are metallic glass-forming alloys and organic polymer glass-forming systems. A number of cases of structure determinations for glasses with more exotic compositions can also be mentioned. In line with the general approach of the present book we will not attempt to give a precise description of the structure of particular glasses but would like to summarize in the following sections structural hypotheses and results of structural investigations which allow one to draw more or less general conclusions concerning the structural principles of glasses, valid for any type or, at least, for large classes of glass-forming systems. From a historical point of view the first such a general principle was developed by the Norwegian crystallo-chemist G. V. Goldschmidt. This is the reason why we start the discussioll with his ideas. 4.2 Goldschmidt's Rule Goldschmidt noted in 1926 that for binary ionic glass-formers (oxides, halides, chalcogenides) there exists a distinct correlation beteen the ability of a substance to form a glass and the ratio of its anionic (Ra) and cationic (Rc) radii. According to Goldschmidt's ratio criterion for typical glass-formers the inequality Rc 0.2 < Ra < 0.4 t4.1) has to be fulfilled (Goldschmidt (1926) [255]).