Spontaneous disordering of nm-grain-sized polycrystals and clusters of silica stishovite

We investigate spontaneous structural disordering of nm-grain-sized polycrystals and clusters of stishovite at ambient conditions using molecular dynamics, and compare to disordering in stishovite nanocrystals and single-crystal α-quartz at high pressures. For nanocrystals and clusters at ambient conditions and sub-ns time scales, the extent and rate of disordering increase with the surface-area-to-volume ratio, and the structure becomes completely amorphous if the size is less than about 2 nm. Within the stability field of stishovite, disordering at the grain boundaries is less pronounced. Low-pressure disordering of stishovite structure involves Si coordination number reduction from six to five and finally to four, a process opposite to the pressure-induced amorphization of quartz.