Probing the homogeneous distribution of sodium atoms in silicate glasses

Abstract

Sodium, silicon, and oxygen—the major constituents of magmatic silicate melts in the Earth's crust and mantle—are also common ingredients in commercial flexible and scratch-proof glasses for advanced optoelectronic devices. The distribution of alkali metal cations in these amorphous networks is known to control their overall strength, flexibility, and chemical resistance. While the exact nature of their distribution remains a longstanding problem in understanding the glass structures, metal cations have often been expected to form heterogeneous nano-scale percolation channels where they have exclusive proximity towards nonbridging oxygens (NBOs; Na–O–Si species). Contrary to this picture, we show that nuclear magnetic resonance correlation spectroscopy between sodium and oxygen atoms demonstrates the presence of bridging oxygens (BOs; Si–O–Si species), as well as NBOs, in the immediate vicinity of sodium atoms. Our results indicate a metal distribution which is more homogeneous than the previously envisaged Modified Random Network (MRN) model, which represents the extreme end of heterogeneity with metal percolation channels encapsulated exclusively by nonbridging oxygens, by placing qualitative limits on the extent of segregation of alkali channels from silica-enriched regions as suggested by the MRN model. A refined structural model of oxide glass is proposed in terms of a “perturbed” metal ion distribution, which features a rather homogeneous metal dispersion with differences in Na affinity towards the bridging and non-bridging oxygen species. Such perturbed alkali distribution may be used to account for the anomalous viscosity of magmatic melts, as well as revealing the origins of pronounced toughness in complex oxide glasses.