Pressure-induced crystallization of vitreous ZnCl₂

In situ Raman scattering measurements have been obtained for vitreous ZnCl₂ compressed isothermally to 25 GPa. The Raman spectra indicate that, in contrast to other tetrahedral framework glasses, when compressed at room temperature glassy ZnCl₂ crystallizes to the high-pressure CdCl₂-structured phase between 2.3 and 3.0 GPa; on decompression, the high-pressure phase remains until 1.1 GPa, where it then reverts to γ-ZnCl₂. On recompression the γ-ZnCl₂ phase converts to the CdCl₂ phase at 2.5 GPa and backtransforms at 1.1 GPa upon decompression. We suggest that a density-driven phase separation in the supercooled liquid regime underlies the amorphous-to-crystalline transition of ZnCl₂ and other glassy materials.