Structure and viscosity of phase-separated BaO–SiO₂ glasses

Barium silicate glasses with 0-40 mol% BaO were synthesized either by aerodynamical levitation and laser heating (at low barium content) or by conventional melting and quenching process. Characterization by means of Raman scattering spectroscopy and scanning transmission electron microscopy reveals a structural transition between glasses with low BaO content (<10 mol%) showing an atomic network resembling the one of amorphous silica, and glasses with a BaO content larger than 10 mol%, which exhibit the typical signature of a binary silicate glass with Q₂ and Q₃ units. Viscosity curves show a marked increase of the viscosity as the BaO content decreases below 20 mol%. Barium is found to easily diffuse and promote phase separation while silicon remains homogeneously distributed. A dramatic increase in the viscosity is observed as phase separation proceeds, resulting in the formation of Ba-rich nodules in a percolating SiO₂-rich matrix at low barium content, or in Ba-poor nodules in a BaO-rich matrix at large barium content.