Abstract

Glasses have the structures of those liquids from which they formed on cooling. The structures of liquids are the key to the understanding of glass formation. Weyl and Marboe assume three extreme cases of liquid structures. (1) Sodium chloride melts under disproportionation of all binding forces, forming abruptly a very fluid melt which consists of ionic clusters separated by fissures (Frenkel-type liquid). (2) Organic compounds of high molecular weight consist of molecules with strong intramolecular forces and weaker intermolecular forces. Depending on the shape and size of these molecules, the liquids undergo orientation when stretched (orientable liquid of the Stewart type). (3) Cristobalite and many silicates (albite) melt because increased thermal motion gradually overcomes the binding forces in the crystal. Melting is a gradual process; the optical properties of the crystal are retained for long times above the melting temperature. The resulting “flawless” liquid of the Bernal type can be easily supercooled and forms a glass. Glass formation does not take place from atomic liquids (argon, most metals) nor from low molecular-weight liquids (benzene, carbon tetrachloride) but from liquids whose structures correspond to (2) and (3) of the idealized structures. The structure of liquids which can form glasses is discussed on the basis of a ternary diagram, the corners of which are representing the three idealized extreme structures. Conventional thermodynamic treatment can not distinguish between the average bond strength and a distribution of bond strengths. The average bond strength might be the same for (1), (2), and (3). Even if one could derive the distributions of all binding forces within a condensed system, one still cannot distinguish between the structures (2) and (3), because the difference lies in the lifetime of the “clusters” (short life) and the “molecules” (long life).

© 1963 Optical Society of America

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