Solid-state disordering and melting of silica stishovite : the role of defects

Molecular dynamics simulations are conducted to investigate disordering of stishovite (a high-pressure polymorph of silica) and the role of defects including grain boundaries, vacancies and free surfaces. It is shown that pre-existent defects initiate or facilitate solid-state disordering and melting. As illustrated in the case of vacancies, melting precedes solid-state disordering for stishovite with low defect concentrations, while defect-rich stishovite may transform into high-density vitreous silica which then undergoes a (quasi-)continuous transition into melt. In sharp contrast, melting of ordinary glass at the same heating rate is sluggish yet evidently first order. Disordering on free surfaces is anisotropic, being more pronounced on (101), (011) and (111) than on (100), (010), (001) and (110) crystallographic planes.