On the dual role of halogen in magnesium oxyhalide bismuth borate glasses: Insight through optical absorption

The present research communication deals with some significant outcomes related to the compositional dependence of optical absorption in magnesium oxyhalide bismuth borate glasses. These vitreous materials have been synthesized over very wide composition range of xMgX2·(30-x)MgO·20Bi2O3·50B2O3 (X = Cl, F, Br and x = 0, 2, 5, 7, 10 mol%) via convenient melt-quench route. The analysis of fundamental absorption edge (from UV-Vis spectroscopy data) has been carried out in order to obtain optical band gap (Eopt) and band tailing parameter (B) corresponding to indirect allowed and forbidden Mott's transitions. The non-linear compositional variations of Eopt have predicted a dual role (network former and modifier) of halogen ions in the magnesium bismuth borate matrix. Moreover, evaluation of Urbach's energy has been done and order of its values indicates the presence of phonon-assisted indirect transitions in each synthesized composition.