Thermal, Structural, Optical, and Dielectric Properties of $(100-x)Li_2B_4O_7 - x(BaO-Bi_2O_3-Nb_2O_5)$ Glasses and Glass-Nanocrystal Composites

Transparent glasses in the system $(100 - x)Li_2B_4O_7 - x(BaO-Bi_2O_3-Nb_2O_5)$ (x = 10, 20, and 30) were fabricated via the conventional melt-quenching technique. The amorphous and glassy characteristics of the as-quenched samples were established by the differential thermal analyses (DTA) and X-ray powder diffraction (XRD) studies. Glass-nanocrystal composites (GNCs) i.e., the glasses embedded with $BaBi_2Nb_2O_9$ (BBN) nanocrystals (10-50 nm) were produced by heat-treating the as-quenched glasses at temperatures higher than $500^0 C$. Perovskite BBN phase formation through an intermediate fluorite-like phase in the glass matrix was confirmed via XRD and transmission electron microscopic (TEM) studies. The optical transmission properties of these GNCs were found to have a strong compositional (BBN content) dependence. The refractive index (n = 1.90) and optical polarizability $(\alpha_0 = 15.3 × 10^{-24} cm^3)$ of the GNC (x = 30) were larger than those of as-quenched glasses. The temperature dependent dielectric constant $(\epsilon_r)$ and loss factor (D) for the glasses and GNCs were determined in the 100-40 MHz frequency range. Theεr was found to increase with increase in heat-treatment temperatures, while the loss of the glass-nanocomposites was less than that of as-quenched glasses. The sample heat-treated at $620^0 C/1 h (x = 30)$ exhibited relaxor behavior associated with a dielectric anomaly in the $150-250 ^0 C$ temperature range. The frequency dependence of the dielectric maximum temperature was found to obey the Vogel-Fulcher relation $(E_a = 0.32 eV and T_f = 201 K)$