Surface and grain-boundary amorphization: Thermodynamic melting of coesite below the glass transition temperature

Coesite, a high-pressure SiO₂ polymorph, becomes amorphous during isothermal annealing below the glass transition temperature, Tg, at one-bar pressure. Transmission electron microscopy was used to examine this fusion process (vitrification) below Tg. The transformation is dominated by a heterogeneous nucleation-and-growth controlled process above the thermodynamic melting temperature, Tₘ (875 K), but below Tg (1480 K). Amorphous domains nucleate at free surfaces and grain boundaries, and the amorphous-crystalline interface propagates into the interior of the crystal. This ``interface-mediated'' amorphization (vitrification) is the same as ``interface-mediated'' melting, based on the thermodynamic, microstructural, and mechanistic aspects of the transformation. This amorphization process parallels electron-irradiation and pressure-induced amorphization in coesite. {} 1996 The American Physical Society.