New Interpretation of Na⁺‑Ion Conduction in and the Structures and Properties of Sodium Borosilicate Mixed Glass Former Glasses

A new study has been made of the well-known sodium borosilicate glass system to improve the overall and detailed understanding of both the atomic level structures in these glasses and the correlation of these structures to their physical properties. The specific intent is to examine the isocompositional, Na2O content, variation of the Na+-ion conductivity with the mixing ratio x of the amounts of B2O3 and 2SiO2 (Si2O4, which keeps the number of glass former cations constant across this series) in these glasses. This study deepens our ongoing examination of the mixed glass former effect (MGFE) on the Na+ ion conductivity in these glasses. In doing so, we also report and examine the MGFE on the density, the mechanical moduli, and the glass-transition temperature, Tg. The most significant structural change that occurs in these glasses is the formation of large fractions of tetrahedral borons, B4, that leads to densification and strengthening of the glass structure and, as a result, causes what appears to the very first negative MGFE in the alkali-ion conductivity in an oxide glass. Until this study, all studies of the MGFE in oxide glasses have shown that the alkali-ion conductivity is a positive function of the mixing ratio of the two glass formers. The weak negative effect reported here appears to be a direct result of the positive MGFEs observed in the density, the Tg, and all of the mechanical moduli of these glasses. Weak negative MGFE in the Na+-ion conductivity appears to be consistent with an increasing strain energy to Na+ conduction caused by the densification of the structure, leading to the increased mechanical energy necessary to force the dilation of the volume necessary to accommodate the Na+-ion motion between sites. This negative MGFE in the volumetric strain energy appears therefore to overcome a slight reduction in the coulombic binding energy between the Na+ ion and its counter anions caused by the formation of weakly basic B4 anion sites. An improved model for ion conduction in solid electrolyte glasses is developed as a result of modeling the composition dependence of the Na+-ion conductivity in these glasses