THERMAL-STABILITY AND OPTICAL-PROPERTIES OF K2O-NB2O5-TEO2 GLASSES

The thermal stability and optical properties of xKNbO(3). (100-x)TeO2 (x=5 similar to 32) and (30-x)K2O. xNb(2)O(5).70TeO(2) (x=5 similar to 20) glasses whose some glass ceramics show a second harmonic generati on have been studied The glasses consisting of about 80 mol% TeO2 show high termal stabilities with around Delta T=T-x-T-g=150 degrees C aga inst crystallisation where T-g and T-x are the glass transition and cr ystallisation onset temperatures, respectively. A good correlation bet ween Delta T and the degree of undercooling is found implying a low fr equency of homogeneous nucleation in the thermally, stable glasses. Th e two third rule, T-g/T-m=2/3 holds well for the TeO2 based glasses wi th a fragile character, where T-m is the melting temperature A linear correlation between refractive index (1.83-2.13) and density (4.12-5.1 8gcm(-3)) is observed. Nb2O5 acts to maintain relatively large values of refractive indices, molar refraction and polarisability of the glas ses, but K2O breaks Te-O-Te bonds leading to low glass transition and low melting temperatures.