Residual and configurational entropy: Quantitative checks through applications of Adam–Gibbs theory to the viscosity of silicate melts

In this paper the traditional view that glasses possess residual entropy, which can be determined by calorimetric means, is quantitatively supported by applications of Adam and Gibbs configurational entropy theory to the temperature, composition and pressure dependences of the viscosity of silicate melts. This theory is also in harmony with the mechanisms of viscous flow, as understood from NMR experiments, according to which viscosity is controlled by the rate of bond rearrangements between network-forming cations and oxygens. As a matter of fact, Adam and Gibbs basic expression relating structural relaxation times to the reciprocal of the product of temperature and configurational entropy can be derived from a phenomenological analysis of the temperature dependence of the activation energy for viscous flow. Adam–Gibbs theory thus works well for silicate melts because network-modifying cations also play a role in bond rearrangements such that, as a bulk property, configurational entropy is actually relevant to structural relaxation and flow.