Characterization of irradiation-induced novel voids in α-quartz

The macroscopic volume expansion of minerals subjected to high-energy irradiation typically occurs because of structural disordering. However, the mechanisms involved in this swelling associated with structural disordering have not been fully clarified. In particular, the role of the voids resulting from the aggregation of defects (which, in contrast to irradiated materials, are not observed in disordered melt-quenched amorphous glass) is still poorly understood. Here, we employ molecular dynamics simulations of α-quartz to examine a model that involves three stages of amorphization and volume expansion. The collapse of the crystalline structure is directly evaluated in terms of structural ordering based on symmetry operations, which enables the estimation of isolated defects. In the first stage, with increasing deposited energy, crystallinity decreases sharply compared with the decrease in density, which is linked to the formation of under- and over-coordinated atomic structures. Large voids (≥7.0 Å), which are not present in melt-quenched glass, are created at a deposition energy of 4 eV/atom, and in the second stage, the volume fractions of the large voids increase during subsequent irradiation from this energy up to 25 eV/atom. In the final stage, at higher deposited energies, the volume fraction of the large voids and the density fluctuate and become saturated owing to the balance between generation and annihilation of the large voids.