The Structure of Industrial Glasses Probed by Mössbauer Spectroscopy

Glassy materials, whether man-made or naturally occurring, have been recognised for centuries though attempts to understand their nature are relatively recent.1 They are produced by rapidly cooling the material after melting and are non-crystalline. Only certain materials may readily form glasses. Unlike crystalline materials, where there is a periodic structure which may be determined by diffraction techniques, there is no direct way of determining the arrangement of atoms in glasses. Their diffraction patterns consist of diffuse rings from which the radial distribution function of atoms may be deduced. Indeed unlike crystals, which have a unique three-dimensional structure which depends on their composition, temperature, and pressure, a glass does not have a unique structure but it depends upon the conditions under which it was formed, principally the rate at which it was cooled through the glass transition point. The free energy of the material is higher in the glassy, disordered, state than in the crystalline, ordered, state. Hence thermodynamically the crystal is the stable equilibrium state. However, a glass has strong directional covalent bonds which must be broken in order to be converted to a crystal which is more ionic, hence the time taken for a glass to reach its equilibrium crystalline state may be infinite for all practical purposes.