Brillouin scattering from amorphous materials at high pressures

Recent Brillouin experiments on a-${\mathrm{SiO}}_2$ have shown that it undergoes an amorphous- to amorphous-phase transition as a function of pressure. Here we report Brillouin scattering results from a-${\mathrm{GeO}}_2$, a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$, a-${\mathrm{B}}_2$ ${\mathrm{O}}_3$, and a polymeric glass. Irreversible effects under pressure are found for a-${\mathrm{GeO}}_2$ and a-${\mathrm{B}}_2$ ${\mathrm{O}}_3$ which we also interpret as due to phase transitions. The polymeric glass and a-${\mathrm{As}}_2$ ${\mathrm{S}}_3$ show no evidence of unusual behavior.