The relationship between the short- and intermediate-range order in glasses and melts: a thermodynamic interpretation
Natalia Vedishcheva,
Institute of Silicate Chemistry of the Russian Academy of Sciences
With only a few exceptions, the majority of the structural models, known to the author, consider glass structure as a network of different basic structural units with varying numbers of bridging oxygen atoms, such as B(n) or Si(n), and neglect network-modifying cations, which are present in the form of metal-oxygen polyhedra and similarly characterise the short-range order. As a result, in such models, the requirements of mass and charge balance and that of the minimal Gibbs energy of a given system are not observed. Therefore, these models do not allow various well-known features in the structure of the short-range order to be explained. For example, they do not answer the question why, in binary borate glasses, the maximum fraction of 4-fold co-ordinated boron atoms, x4, never reaches 0.5, which is the maximum x4 value for borate crystals, or why, with increasing temperature, the fraction of the structural units BØ4- and BØ2O- decreases and increases, respectively, whilst the fraction of BØ3 units remains constant over an extended temperature interval. The suggested thermodynamic approach enables the above problems to be successfully solved, since it takes into account the presence of network-modifying cations in glasses and considers structural changes in terms of the Gibbs energy of a given system. This approach, based on the concept of chemical structure, considers glasses (melts), formed from components with different chemical natures, as solutions whose constituents are the products of interaction between the initial oxides. It is assumed that these products (also called chemical groupings) are stoichiometrically and structurally similar to crystalline compounds that form in the given system and can be found in its phase diagram. Due to this, the origin of the various basic structural units in a given glass (melt) and their inter-conversion, as the glass (melt) composition or temperature change, can be explained in terms of the presence in the glass or melt, at a given temperature, of chemical groupings that introduce strictly determined numbers of specific basic structural units (BØ4-, BØ3 and BØ2O- in borate glasses and Si(n) species in silicate glasses). Hence, the concept of the chemical structure not only defines the type and content of the basic structural units present but also explains why they form and transform into other units as the glass composition or temperature vary. Hence, this concept brings a strict physical meaning to the explanation of structural changes in glasses (melts) and yields an understanding of specific features of the structure, and establishes the relationship between the short- and intermediate-range order. The concept also shows which of the levels in the structure determines the properties of glasses.