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

Neutron irradiation of α-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 α-quartz using neutron irradiation simulations and capture the crystallization transition at the atomic scale. α-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 metadynamics (MetaD) simulations to analyze the structural changes accompanying the phase transition. 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, MetaD 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 α-quartz. This diffusion pathway allows Si atoms to occupy crystalline sites, thereby driving the thermal healing effect.