Crystallization Transition Pathways in Neutron-Irradiated Quartz: Insights from Atomistic Simulations

Neutron irradiation of a-quartz induces amorphization accompanied by a reduction in density. Experimental studies have shown that this density reduction slows at elevated temperatures, a phenomenon interpreted as the ``thermal healing effect'' of neutron-irradiated a-quartz, i.e., a crystallization transition. However, the atomic rearrangement mechanisms underlying the recovery of long-range order remain poorly understood. Here, we reproduce amorphized a-quartz using neutron irradiation simulations and capture the crystallization transition at the atomic scale. a-quartz structures with crystallinities of 80%, 60%, and 40% are generated through irradiation simulations, and their thermal healing behavior is investigated using annealing-based and well-tempered metadynamics (WTMetaD) simulations. In the annealing-based simulations, local structural defects in SiO4 units are partially repaired, but restoration of long-range order is not achieved. In contrast, WTMetaD simulations successfully reproduce structural transitions up to nearly 100\% crystallinity. Analysis of atomic rearrangements during these transitions reveals that Si atoms diffuse through void channels aligned with the threefold helical axis of irradiated a-quartz. This diffusion pathway allows Si atoms to occupy crystalline sites, thereby driving the thermal healing effect.