Bismuth fluoroborate glasses with compositions xBi 2 O 3 · (40 − x)LiF · 60 B 2 O 3 (x = 0, 5, 10, 15, and 20) are synthesized by meltquench method. Optical characterization was carried out to examine variation of optical band gap energy (E g ) and Urbach energy (E U ) with respect to the concentration. It reflects the effect of stepwise replacement of non-oxide and less polarizable LiF by an oxide and more polarizable (Bi 2 O 3 ) group on the optical properties of the samples. The value of E g decreases with increase in concentration of Bi 2 O 3 . The samples were subjected to annealing at different temperatures (300• C, 350• C, and 400• C), and the effect of annealing on the optical properties of various samples was again studied. Annealing affects remarkably the values of E g and E U in the samples with x = 0.
The Melt-quench method is used to synthesize zinc fluoroborate glasses with compositions xZnO 5, 10, 15, and 20). Optical characterization was carried out to examine the variation of optical bandgap energy (E g ) and urbach energy (E U ) with respect to the concentration for the samples annealed at different temperatures (300• C, and 400• C). Annealing shows its effect on the samples with the variations in the values of E g and E U . These variations are explained on the basis of formation of different molecular species like BO 3 − units, boroxol rings, and the change in the number of nonbridging oxygen atoms.
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