Vibrational excitations and elastic phases in Sodium Borate Glasses

Glass Transition temperatures (T$_g$s) and non-reversing enthalpy ($\Delta$H$_{nr}$) at T$_g$ of dry (Na$_2$O)$_x$(B$_2$O$_3$)$_{100-x}$ glasses across the 0\% $<$ x $<$ 44\% soda range are measured. Trends in $\Delta$H$_{nr}$(x) show a reversibility window in the 20\% $<$ x $<$ 40\% range, and fix the Intermediate Phase (IP). IR and Raman vibrational modes including Boson modes are also examined. At low x ($<$ 20\%), the Raman active 808 cm$^{-1}$ mode of boroxyl rings steadily lowers in scattering strength and red-shifts with increasing x, suggesting that the stressed-rigid quasi 2D network of B$_2$O$_3$ glass at x = 0, steadily softens with a characteristic optical elastic power-law (p$_1$ = 0.85(2)). In the 26\% $<$ x $<$ 40\% range, a mode near 770 cm$^{-1}$ rapidly grows in strength and red shifts with increasing x with a power- law of p$_2$= 1.05(5) characteristic of IPs observed earlier$^a$ in other 3D covalent and ionic networks. In addition, many other modes are observed, some blue-shift, some red-shift and some remain unchanged with x. These data will be discussed in relation to glass structure evolution with composition.\\[4pt] D.Novita et al. J. Phys. Condens. Matter 21, 205106 (2009)