Temperature-Driven Structural Transitions in Molten Sodium Borates Na2O–B2O3: X-ray Diffraction, Thermodynamic Modeling, and Implications for Topological Constraint Theory

Abstract

Temperature-dependent measurements of the X-ray structure factor of molten Na2B4O7 reveal a continuous structural transition. We demonstrate that the thermodynamic model of ideal associated solutions is capable of predicting this evolution of melt structure, between a low density, depolymerized melt at ≳300 K above the liquidus, toward a dense, polymerized melt close to the glass transition. This temperature-dependent nature of melt structure is predicted to be strongly composition-dependent, with the B–O coordination number depending on temperature only in the range 20–50 mol % Na2O, which appears to be manifest in the broad maximum observed in the glass-transition temperatures. We discuss the ramifications of these findings for the application of topological constraint theory, with relevance to industrial glass design and manufacture, crystal growth from melts of nonlinear optical materials, geochemistry, and the understanding of melt fragility and the glass transition.