Temperature effects on the network structure of oxide melts and their consequences for configurational heat capacity

Oxide melts generally have important configurational components to their thermodynamic and transport properties, which imply energetically significant changes in structure with temperature. Spectroscopic and scattering studies have begun to reveal the microscopic nature of these changes. Both in-situ methods, and studies of glasses with varying fictive temperature, have proven useful. Here recent findings in three relatively well-studied types of systems are reviewed, and the implications of known temperature effects on particular aspects of the short-range structure discussed: Q(n) species in alkali silicates, Si-Al network disorder and "defect" species in aluminosilicates, and boron coordination in borates and borosilicates. To estimate the importance of these changes for the bulk properties, a simple ideal solution approximation is applied to calculate contributions to the configurational heat capacity; this is extended as well to test possible effects of non-ideality. In some cases (e.g. aluminosilicates and borates), known structural changes seem to be able to account for significant fractions of measured configurational properties, in others (e.g. alkali silicates), less clearly identified changes, perhaps involving intermediate-range structure, may play key roles. The calculated contribution to the heat capacity for a given structural reaction often has a positive slope at low temperature, reaches a maximum, then decreases, suggesting that in many systems, successive aspects of temperature-induced disorder may come into play with increasing temperature. (c) 2008 Elsevier B.V. All rights reserved.