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 - xBi(2)WO(6) (0 less than or equal to x less than or equal to 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 diffracti on (XRD) studies. The glassy nature of the as-quenched glasses was establis hed by differential thermal analyses (DTA). Glass-ceramics comprising Bi2WO 6 nanocrystallites with high optical transparency were obtained by controll ed heat-treatment of the glasses at 723 K for 6 h. High resolution transmis sion electron microscopy (HRTEM) of the glass-ceramics demonstrate the pres ence of nearly spherical nanocrystallites (2-33 nm range) of Bi2WO6 embedde d in Li2B4O7 glass matrix. The dielectric constants of both the as-quenched glasses and glass-ceramics decrease with increase in frequency (40 Hz to 1 00 kHz) at 300 K. Interestingly, the dielectric constant of the as-quenched glass undergoes a maximum around the crystallization temperature of the ho st glass reaching an anamalously high value (approximate to 10(7)) at 800 K . Different dielectric mixture formulae were employed to analyze the dielec tric 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 composition al and crystallite size dependent. (C) 2001 Elsevier Science B.V. All right s reserved.