An apparently first-order polyamorphic transition has been observed with increasing pressure at ambient temperature in a molecular glass of composition ${\mathrm{Ge}}_{2.5}{\mathrm{As}}_{51.25}{\mathrm{S}}_{46.25}$. Raman spectroscopic measurements on pressure-quenched samples and in situ x-ray diffraction measurements indicate that this transition corresponds to a collapse of the ambient-pressure molecular phase to a high-pressure network phase. The high-pressure phase first appears at a pressure of $\sim 8–9\mathrm{GPa}$ and the transformation becomes complete at $\sim 14–15\mathrm{GPa}$. Calorimetric measurements indicate that the low- and high-pressure phases are thermodynamically distinct and that they coexist in the transition range.

• Received 22 April 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.025504