Dielectric polarizability of SiO2 in niobiosilicate glasses

Understanding the mechanisms contributing to dielectric properties of glasses is critical for designing new compositions for microwave frequency applications. In this work, dielectric permittivity was measured using a cavity perturbation technique at 10 GHz for a series of niobiosilicate glasses with the compositions (100-2x)SiO₂- xNb₂O₅- xLi₂O where x = 32.5, 30, 25, and 15 mol%. Permittivity measurements and glass compositions were used to calculate the polarizability of each cation-anion unit in the glass network using the Clausius-Mossotti equation. The SiO₂ polarizability in niobiosilicates was calculated to be 6.16 ų, which is much higher than the SiO₂ polarizability in fused silica glass (5.25 ų), alkali modified silicates (5.37 ų), and aluminosilicates (5.89 ų). The increasing trend in SiO₂ polarizability is attributed to the disruption in the connectivity of the SiO₄ tetrahedral network as it accommodates different network formers. The high SiO₂ polarizability of 6.16 ų accurately predicts measured dielectric permittivity when Nb₂O₅ = 25, 30, and 32.5 mol%, but overpredicts measured permittivity when Nb₂O₅ ≤ 15 mol%, which is attributed to a decrease in SiO₂ polarizability as the percentage of corner sharing SiO₄ tetrahedra with NbO₆ octahedra goes down. This work demonstrates that SiO₂ polarizability depends on chemistry and connectivity of the glass, which has important implications in designing glass compositions for microwave frequency applications.