The influence of B2O3 on structure and ionic conductivity of lithium phosphate-niobate glasses

Glasses in the series (100-x)[0.4Li₂O-0.2Nb₂O₅-0.4P₂O₅]-xB₂O₃ were prepared with B₂O₃ content ranging from 0 to 48 mol%. The glass transition temperature (T_g) increases with up to 8 mol% B₂O₃, peaking at 512 °C, then decreases to 490 °C at 40 mol% B₂O₃. Raman spectra at low B₂O₃ content indicate the presence of NbO₆ octahedra. XRD patterns of crystallized samples reveal NbOPO₄ formation across the entire composition range. Confrontation of the Raman spectra of glasses and crystalized glasses resulted in the assignment of the broad Raman band at 777–804 cm⁻¹ to Nb−O−Nb vibrations in the NbO₆ octahedra. ¹¹B MAS NMR shows a transition from BO₄ to BO₃ units, while ³¹P MAS NMR suggests mixed borate-phosphate structures. Using 2D NMR techniques, mixed species were identified in the glass network. Ionic conductivity remained stable up to 16 mol% B₂O₃ but decreased significantly at higher concentrations due to reduced Li⁺ ion mobility in the mixed phosphate-borate network.