Dynamic and thermodynamic behavior of supercooled Na–Ba metaphosphate liquids: Role of modifier cation ordering

Abstract

The Q-speciation and the role of modifier dynamics on network relaxation in the supercooled mixed-alkali–alkaline-earth (MAAE) Na–Ba metaphosphate liquids are investigated using a combination of ³¹P nuclear magnetic resonance (NMR) spectroscopy, calorimetric, electrical conductivity, and rheological measurements. Progressive replacement of Na with Ba in these glasses is shown to result in an increasing disproportionation of Q² species via the reaction: 2Q² = Q¹ + Q³. Unlike mixed-alkali liquids, the Na–Ba metaphosphate liquids display a monotonic variation in isothermal electrical conductivity, glass transition temperature, calorimetric and kinetic fragility, and isothermal viscosity. It is hypothesized that this monotonic variation arises from the lack of elastic facilitation of network relaxation via coupled hopping of Na–Ba pairs as these modifier cations are prohibited from mixing randomly due to the differences between their size, mass, charge, and mobility. Isobaric heat capacity measurements provide supporting evidence in favor of a such a nonrandom mixing between the modifier cations in Na–Ba metaphosphate glasses and liquids.