Composition dependence of the short range order structures in 0.2Na2O + 0.8[xBO3/2 + (1-x)GeO2] mixed glass formers

The atomic level structures of ternary 0.2Na₂O + 0.8[xBO_3/2 + (1-x)GeO₂] glasses have been carefully examined for the first time using a combination of Raman, infrared (IR) and ¹¹B magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. It was found that, like in other mixed glass former (MGF) systems, there is an unequal sharing of the sodium oxide between the two glass formers, B and Ge, across this series 0 < x < 1. It is found that for all x > 0, the Na₂O preferentially reacts with boron to create a larger number of tetrahedral boron, BO_4/2^-1 ≡ B⁴, units than would be found for a binary sodium borate glass of the same mole fraction, 0.20, of Na₂O. In addition to this higher than expected fraction of B⁴ units as determined by the ¹¹B MAS-NMR measurements, the Raman spectra further suggest the presence of high concentrations of octahedrally coordinated GeO_6/2^-2, Ge⁶, units in the GeO₂ rich end of this compositional series, x ≤ 0.2. Across this series, it is found that there are at most five short range order (SRO) structural units present in these glasses, BO_3/2 (B³), B⁴, GeO_6/2^-2 (Ge⁶), GeO_3/2O^-1 (Ge³), and GeO_4/2 (Ge⁴), where the superscript gives the number of bridging oxygens (BOs) bonded to the central atom. The composition, x, dependence of all of these SRO structural units were determined by combining the results all of these spectral techniques and requiring that the total sum of all the negatively charged SRO units balance the positive (+0.4) charges from the Na⁺ ions and that the total amount of B balance to 0.8×, the total amount of Ge balance to 0.8*(1-x), and the total amount of oxygen balance to1.8-0.4× as required by the stoichiometry of this compositional series.