A Focus on Glasses
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new theoretical and experimental techniques. In the remainder of this article, I will list some of the theoretical and experimental advances which I believe are needed for an increased scientific u n d e r s t a n d i n g of glassy and other amorphous materials. Opportunities for Theory The core of the present scientific problem is the m a t h e m a t i c a l t r e a t m e n t of strongly interacting atoms whose equilibrium positions show strong spatial disorder. This first requires efficient specification of atomic p o s i t i o n s , followed by sufficiently accurate calculation of physical properties. Simple extensions from the theory of liquids or crystalline solids have not yielded a convincing or powerful theory of glassy materials. There is a large and rich body of experimental information already available, so it appears that the greatest need and opportunity is for novel theoretical approaches, tailored to the central fact that glasses do not have the point group symmetries of molecules nor the space group symmetries of crystalline solids. Well-defined idealized structural models are needed for these materials, or large classes of them, against which to treat reality as a perturbation. This is at least as important for conceptual purposes as it is for c o m p u t a t i o n . T h u s , t h e r e is still argument 1 4 1 5 w h e t h e r the ZachariasenWarren continuous random network 1618 is a correct idealized model for vitreous silica. We need quantitative treatment of the order and disorder in glasses. What kind of statistical averaging is appropriate for predicting particular properties? How much structure must be specified for each property? Can we introduce probability distributions at a very early stage of the formalism? What form do selection rules take in systems that have no rotational or translational symmetry beyond regions which are a few angstroms in dimension? The short-range order (SRO) or nearest neighbor atomic arrangements are fairly easily determined by diffraction experiments, but the resultant radial distribution (or pair correlation) function is not enough even to adequately predict the mass density of the glassy material. It is therefore essential to learn more about structure on the next scale of distance; but we don't know how best to describe this intermediate range order (IRO). Should we seek higher order correlation functions, ring statistics, dihedral angle distributions or s o m e t h i n g else? Virtually n o t h i n g is
known directly about the IRO in any glass, yet this is the range of structure where a glass typically begins to differentiate itself from a crystal of the same material. It is n e c e s s a r y to u n d e r s t a n d m o r e clearly the nature of defects in amorphous materials, to determine defect structures experimentally and be able to interpret and p r e d i c t the v a r i o u s m a n i f e s t a t i o n s of defects as dopants, traps, color centers, etc. 7 ' 910 Are defects in glasses intrinsically related to their glass-forming ability? Glasses are meta
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