Direct Observation of Amorphous to Amorphous Apparently First-Order Phase Transition in Fused Quartz

We report for the first time the direct observation of an apparently first-order phase transition in amorphous silica from the low density amorphous phase to the high density amorphous phase with a discontinuous volume change of about $20\%$ from the dilatometric measurements using the piston cylinder apparatus at 36 kbar and 680 °C. Our Raman spectroscopy measurements on the retrieved samples quenched from high pressure and high temperature experiments do not show any Raman peaks indicating a large modification in the intermediate range order in the structure of amorphous silica.