AMORPHOUS SOLID

any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel. Solids and liquids are both forms of condensed matter; both are composed of atoms in close proximity to each other. But their properties are, of course, enormously different. While a solid material has both a well-defined volume and a well-defined shape, a liquid has a well-defined volume but a shape that depends on the shape of the container. Stated differently, a solid exhibits resistance to shear stress while a liquid does not. Externally applied forces can twist or bend or distort a solid's shape, but (provided the forces have not exceeded the solid's elastic limit) it springs back to its original shape when the forces are removed. A liquid flows under the action of an external force; it does not hold its shape. These macroscopic characteristics constitute the essential distinctions: a liquid flows, lacks a definite shape (though its volume is definite), and cannot withstand a shear stress; a solid does not flow, has a definite shape, and exhibits elastic stiffness against shear stress. Figure 1: The state of atomic motion. On an atomic level, these macroscopic distinctions arise from a basic difference in the nature of the atomic motion. Figure 1 contains schematic representations of atomic movements in a liquid and a solid. Atoms in a solid are not mobile. Each atom stays close to one point in space, although the atom is not stationary but instead oscillates rapidly about this fixed point (the higher the temperature, the faster it oscillates). The fixed point can be viewed as a time-averaged centre of gravity of the rapidly jiggling atom. The spatial arrangement of these fixed points constitutes the solid's durable atomic-scale structure. In contrast, a liquid possesses no enduring arrangement of atoms. Atoms in a liquid are mobile and continually wander throughout the material. Additional reading Works on solids in general include Lawrence H. Van Vlack, Elements of Materials Science and Engineering, 6th ed. (1989), an elementary textbook; Charles A. Wert and Robb M. Thomson, Physics of Solids, 2nd ed. (1970), an intermediate-level text; Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986), the standard college textbook; Neil W. Ashcroft and N. David Mermin, Solid State Physics (1976), an advanced textbook; George E. Bacon, The Architecture of Solids (1981), an introduction to bonding and structure; and Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed. (1960, reissued 1989), the classic reference work on chemical bonding. Gerald D. MahanOn amorphous solids in particular, a lucid introductory text accessible to a nontechnical reader is Richard Zallen, The Physics of Amorphous Solids (1983), with coverage of structural models for the various classes of amorphous solids as well as percolation theory, a modern paradigm for disordered systems. A classic advanced work is N.F. Mott and E.A. Davis, Electronic Processes in Non-crystalline Materials, 2nd ed. (1979), which features many of the theoretical contributions of Nobel Laureate coauthor Mott. A text providing a thorough treatment of oxide glasses is J. Zarzycki, Glasses and the Vitreous State (1991; originally published in French, 1982). A reference work with wide coverage of recent research topics, including detailed treatment of chalcogenide glasses, is S.R. Elliott, Physics of Amorphous Materials, 2nd ed. (1990). A comprehensive collection of detailed reviews is contained in R.W. Cahn, P. Haassen, and E.J. Kramer (eds.), Materials Science and Technology, vol. 6, Glasses and Amorphous Materials, ed. by J. Zarzycki (1991), including coverage of glass technology, formation, and structure, oxide glasses, chalcogenide glasses, metallic glasses, polymeric glasses, and the optical, electric, and mechanical properties of glasses. Amorphous silicon is treated in detail in another work, R.A. Street, Hydrogenated Amorphous Silicon (1991). Richard Zallen