Mixed modifier effect in Na2O–BaO–B2O3 glasses: Structure-forming role of alkali-alkaline earth combinations

This study investigates the mixed modifier effect on structure in Na₂O–BaO–B₂O₃ glasses, driven by variations in the modifier ratio (Na₂O/BaO) across different glass-forming regions (B₂O₃ concentrations). Vibrational spectroscopy methods (IR and Raman) were used to analyze the short-range order (expressed as the fraction of tetrahedral boron, N₄) and the intermediate-range order (represented by superstructural units) structure.

The results reveal a near-linear decrease in N₄ and a higher concentration of asymmetric (metaborate) triangles in more alkaline glasses which is particularly pronounced in the most modified series (33.3 mol.% modifiers). In glasses with low modifier content (≤20 mol.%), the fraction of superstructural units with one tetrahedron (T₁) exhibits nonlinear behavior, whereas in the high-modified series (33.3 mol.%), this dependence follows a linear trend, showing a steady increase in T₁ units’ fraction with increasing Na₂O content.

Structural analysis of Na₂O–BaO–B₂O₃ glasses revealed that the structural effects observed during BaO/Na₂O substitution in three glass series are primarily governed by two key factors: the preferential formation of mixed boron-oxygen network anions (stoichiometric analogues of multicomponent crystalline compounds) and the phase diversity of crystalline compounds in the binary Na₂O–B₂O₃ and BaO–B₂O₃ systems. These findings challenge the conventional approach of considering multicomponent borate glass structures as simple combinations of binary subnetworks.

The structural calculations were supported by density and molar refractivity modelling for both low- and high-modified glass series. The calculated density values agreed well with experimental data, showing deviations comparable to those obtained in high-accuracy thermodynamic calculations.