We have measured the x-ray structure factor S(Q) for ${\mathrm{SiO}}_2$ glass to 42 GPa in the diamond-anvil cell using new experimetnal techniques for studying amorphous materials at high pressures. Large changes in the first sharp diffraction peak of S(Q) with increasing pressure provide evidence for changes in the medium-range order of the glass (length scales of ∼4–10 Å). Average pair correlation functions at high pressure reveal significant changes in the nearest-neighbor geometry of the glass with compression. Between 8 and 28 GPa the coordination of Si increases, and at 42 GPa the value is close to six.

• Received 19 May 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1387