Dielectric, linear and non-linear optical properties of lithium borate–bismuth tungstate glasses and glass-ceramics

Glasses of various compositions in the system (1−x) Li2B4O7−xBi2WO6 (0⩽x⩽0.35) were prepared via a melt-quenching technique. The amorphous nature of all the compositions of the as-quenched glasses under study was confirmed by X-ray powder diffraction (XRD) studies. The glassy nature of the as-quenched glasses was established by differential thermal analyses (DTA). Glass-ceramics comprising Bi2WO6 nanocrystallites with high optical transparency were obtained by controlled heat-treatment of the glasses at 723 K for 6 h. High resolution transmission electron microscopy (HRTEM) of the glass-ceramics demonstrate the presence of nearly spherical nanocrystallites (2–33 nm range) of Bi2WO6 embedded in Li2B4O7 glass matrix. The dielectric constants of both the as-quenched glasses and glass-ceramics decrease with increase in frequency (40 Hz to 100 kHz) at 300 K. Interestingly, the dielectric constant of the as-quenched glass undergoes a maximum around the crystallization temperature of the host glass reaching an anamalously high value (≈107) at 800 K. Different dielectric mixture formulae were employed to analyze the dielectric properties of these glasses and glass-ceramics. The optical properties viz. transmission, optical band gap energy and second harmonic generation (SHG) of these transparent glass-ceramics have been found to be compositional and crystallite size dependent.