Thermal stability and optical properties of K2O-Nb2O5-TeO2 glasses

The thermal stability and optical properties of xKNbO 3 .(100-x)TeO 2 (x=5∼32) and (30-x )K 2 O. xNb 2 O 5 . 70TeO 2 (x=5∼20 ) glasses whose some glass ceramics show a second harmonic generation have been studied. The glasses consisting of about 80 mol% TeO 2 show high termal stabilities with around ΔT=T X -T g =150°C against crystallisation where T g and T X are the glass transition and crystallisation onset temperatures, respectively, A good correlation between ΔT and the degree of under-cooling is found implying a low frequency of homogeneous nucleation in the thermally stable glasses. The two third rule, T g /T m =2/3 holds wellfor the TeO 2 based glasses with a fragile character, where T m is the melting temperature A linear correlation between refractive index (1.83-2.13) and density (4.12-5.18 gcm -3 ) is observed. Nb 2 O 5 acts to maintain relatively large values of refractive indices, molar refraction and polarisability of the glasses, but K 2 O breaks Te-O-Te bonds leading to low glass transition and low melting temperatures.