Mixed glass former effect in tungsten-containing lithium borophosphate glasses: Spectroscopic structure–property correlations

The structure, thermal behaviour, and ionic conductivity of tungsten-containing lithium borophosphate glasses were investigated to study the interplay between mixed glass former effects and tungstate-modified phosphate networks. Glasses were prepared by systematically substituting P 2 O 5 with B 2 O 3 in the 40Li 2 O–20WO 3 –(40−x)P 2 O 5 –xB 2 O 3 system, while maintaining constant Li 2 O and WO 3 contents in order to isolate the effect of mixed phosphate–borate glass formers. Thermal analysis revealed a pronounced mixed glass former effect, with the glass transition temperature (T g ) reaching a maximum at intermediate B 2 O 3 content (15 mol%), followed by a gradual decrease at higher boron levels. Similar non-linear trends were observed for thermal stability (T c –T g ) and the coefficient of thermal expansion, indicating optimal network connectivity at intermediate compositions. Raman spectroscopy combined with one- and two-dimensional 11 B and 31 P MAS NMR demonstrated that these trends originate from progressive phosphate network depolymerisation, changes in boron coordination from tetrahedral 4 B to trigonal 3 B units, and the formation of mixed P–O–B linkages at low-to-moderate B 2 O 3 contents, whereas higher boron concentrations favour B–O–B connectivity and more open network motifs. Electrical conductivity measurements confirmed a positive mixed glass former effect, manifested as a non-linear enhancement of DC conductivity arising from phosphate–borate network reorganisation on a fixed tungstate-modified structural background. The DC conductivity increased by nearly two orders of magnitude, reaching a maximum of 2.49× 10 −7 Ω −1 cm −1 at 30 °C for x = 25 mol% B 2 O 3 , before slightly declining at higher boron contents. These results demonstrate that controlled introduction of a mixed glass former effect into a structurally optimised phosphate–tungstate network provides an effective strategy for further optimisation of ionic transport in lithium phosphate-based glasses. • Mixed glass former effect studied by replacing P 2 O 5 with B 2 O 3 at constant Li 2 O and WO 3 . • Advanced 1D/2D 11 B and 31 P MAS NMR revealed progressive phosphate depolymerisation. • 4 B 3 units act as network cross-linkers controlling thermal stability and network rigidity. • Mixed glass former effect enchances DC conductivity by ∼2 orders reaching 2.5× 10 −7 Ω −1 cm −1 at 30 °C. • Results show strategy to optimise ionic transport in TMO-modified borophosphate glasses.