STRUCTURAL CHARACTERISATION OF BISMUTH-SILICATES 

M. Todea and S. Simon 

Babes-Bolyai University, Faculty of Physics, Cluj-Napoca, Romania todeam@phys.ubbcluj.ro

Bismuth-silicate glasses acquired special applications as low loss optical fibres, infrared transmitting materials or as active medium of Raman-active fibre optical amplifiers and oscillators. 
However, the structural role played by Bi2O3 in glasses without or with low content of conventional glass formers is complicated and poorly understood. The problem is complex because crystalline Bi2O3 can form several polymorphs with highly distorted corner-, and edge-sharing polyhedra and consequently the structural model for bismuth-silicate glasses is still under discussion.
Structural changes induced by temperature rising in binary silico-bismuthate glasses of xSiO2·(100-x)Bi2O3 composition, 20 ≤ x ≤ 40  mol %, are investigated by differential thermal analysis. Several exothermic peaks are recorded for all samples. They are attributed to structural relaxation processes taking place by temperature rising. The shift of the first crystallisation peak toward higher temperatures by increasing of silicon dioxide content indicates that the number and size of nucleation centres is diminished by SiO2 substitution for Bi2O3. The temperature at which the melting process starts pronouncedly diminishes as SiO2 content exceeds 30 mol % and the samples gain higher glass stability. 
The vitreous samples were partially crystallised by heat treatment. The iron doped samples were investigated by electron paramagnetic resonance.