Brillouin and Raman spectroscopy were performed on ${\mathrm{B}}_2{\mathrm{O}}_3$ glass compressed to 57 GPa at 273 K. Upon compression the sound velocities increase smoothly and the boroxol ring Raman mode vanishes by 11 GPa. Upon decompression the sound velocities follow a different path and at 3 GPa a discontinuity of $3\mathrm{km}/\mathrm{s}$ in $V_p$ and $2\mathrm{km}/\mathrm{s}$ in $V_s$ returns the velocities to the values seen on compression. After the transition, the boroxol ring Raman mode reappears. A second pressure cycle produces the same behavior, suggesting the 3 GPa transition occurs between vitreous polymorphs with different boron coordination.

• Received 16 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.215701