Coexistence of two states in optically homogeneous silica glass during the transformation in short-range order

Silica glass is an optically homogeneous material with many practical applications. Unlike crystalline materials, it remains optically homogeneous during transformations. However, high-pressure in situ small-angle x-ray-scattering measurements indicate an increase in the scattering intensity during the transformation in short-range order between fourfold-coordinated and sixfold-coordinated amorphous polymorphs, providing strong evidence for heterogeneity. Detailed analyses suggest that silica glass consists of optically invisible subnanometer-scale domains for the two amorphous polymorphs and boundary region between them. The boundary region, which likely has an intermediate structure, is comparable in size to the domains, and works as a buffer to prevent the glass from cracking during the transformation. The transformation is more drastic and the domains are more distinct upon decompression than compression.