Raman scattering and Medium Range Structure of (BaO)$_{\mathrm{x}}$(B$_{\mathrm{2}}$O$_{\mathrm{3}})_{\mathrm{100-x}}$-glasses.

In Raman scattering of titled glasses we observe a triad of modes (770 cm$^{\mathrm{-1}}$, 750 cm$^{\mathrm{-1}}$, 705 cm$^{\mathrm{-1}})$ on the low frequency side of the famous Boroxyl-Ring (BR) mode (808 cm$^{\mathrm{-1}})$ as the BaO content x increases in the 0 \textless x \textless 40{\%} range. Raman polarization experiments reveal that the triad of modes show a rather low depolarization ratio ($\rho $ \textless 0.25) suggesting that each mode results from a symmetric stretch of O atoms in mixed-3-member-rings, analogous to the BR-mode. In the 0 \textless x \textless 15{\%} range, the conspicuous absence of these mixed-ring modes is consistent with the composition range representing the immiscibility range. At x \textgreater 15{\%}, the 770 cm-1 mode scattering strength grows precipitously to show a maximum near x$_{\mathrm{c}} \quad =$20{\%}, consistent with presence of Ba-tetraborate structural grouping (SG). At x \textgreater 15{\%}, we also observe scattering strength of the 750 cm$^{\mathrm{-1}}$ mode to increase with x and show a maximum near x$_{\mathrm{c}} \quad =$ 33{\%}, consistent with formation of Ba-diborate SG. Finally, a mode near 705 cm$^{\mathrm{-1}}$, whose scattering strength increases linearly with x in the 15{\%} \textless x \textless 40{\%} range, we identify with Ba-metaborate SG. FTIR measurements permit a measurement of the B$_{\mathrm{4}}$/B$_{\mathrm{3}}$ fraction and also the LO-TO splittings.