Crystallisation, dielectric, polar and optical characteristics of $Li_2B_4O_7-SrO-Bi_2O_3-Nb_2O_5$ glasses and glass nanocomposites

Glasses in the composition (100-x)$Li_2B_4O_7.x(SrO-Bi_2O_3-Nb_2O_5)$ ($10 \leq x \leq 60$, in molar ratio) were prepared. The thermal stability factor $(K_{gl})$ was found to increase with increase in SBN composition. The activation energies involved for the crystal growth in the present study were calculated from basic Kissinger plots. The Avrami parameter associated with the crystallisation events was found to be $\approx 3$. The formation of nanocrystalline layered perovskite $SrBi_2Nb_2O_9$ (SBN) phase via an intermediate fluorite phase was confirmed. The optical band gap and Urbach energy of the transparent glasses and glass nanocomposites (heat treated at 500°C) were found to be composition dependent. The samples that were heat treated at 550°C exhibited broad dielectric anomalies in the vicinity of ferroelectric to paraelectric transition temperature of the parent SBN ceramics. A shift in the phase transition temperature monitored via dielectric and pyroelectric measurements was observed with variation in SBN content. The incidence of pyroelectric and ferroelectric effects in the present composites confirmed their polar nature