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 degrees 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.