Disorder, configurational relaxations and heat capacity of glasses

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. These are observed by dielectric and mechanical relaxation spectroscopy at temperatures well 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 also shows a substantial non-vibrational contribution from the availability of configurational states in localized regions embedded in a rigid matrix. In this essay, the kinetic and the rmodynamic aspects of such states, their temperature, density and time dependence and their possible connection with the low-temperature tunneling states are considered. It seems clear 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.