Secondary relaxations and the properties of glasses and liquids

Abstract

Thermally activated translational and/or rotational motions of groups of atoms or molecules occur in localized regions of the rigid matrix of glasses, glassy liquid crystals and glassy crystals. Known as secondary relaxations, these are observed by dielectric and mechanical relaxation spectroscopy at temperatures near and below T_g and show features which are remarkably similar amongst the various types of disordered solids. An analysis of the heat capacity and entropy of the three types of disordered solids also shows a substantial non-vibrational contribution from the availability of configurational states in localized regions in an internal thermodynamic equilibrium embedded in a rigid matrix. In this article, the relevance of the kinetic and thermodynamic aspects of such relaxations, their temperature, density and time dependence and their link with the low-temperature tunneling states in a glass are considered.

That the tunneling centres responsible for the low temperature thermodynamic behaviour of a glass are linked with, or identified as, those local regions where thermally excited orientational and/or translational diffusion over short distances in an otherwise rigid glassy matrix occurs, is tested by three sets of different experiments, namely, (i) physical ageing, (ii) regions created in crystals by neutron irradiation and (iii) regions created by addition of a second component to a glass. All of these seem to suggest that the concept of a disordered solid as an elastic continuum is unsatisfactory and that a description of its heat capacity should also include the energy associated with the configurational states involved in secondary relaxations.