Transparent silica glass is an indispensable material for today's information technology society as an ultra-low loss optical fiber. Recently, further low-loss optical fibers are demanded due to rapidly expanding information. A small amount of fluorine doping is known to reduce the Rayleigh scattering further by reducing the density fluctuations of silica glass, even though the fluorine is an impurity for the pristine silica glass. This study devoted to answer the riddle of F-doping and an accurate force-matching potential optimized by machine-learning enables us to understand how fluorine remedies the disorders in silica glass. It was found that fluorine mainly replaces an oxygen in tetrahedral SiO4 unit, but also forms five-fold Si, such as SiO4F and SiO3F2 units. The former disrupts silica network, while the latter attenuates the network rigidity by loosening the five Si-O(F) bonds. Since the two local effects of fluorine spreads through the glass network, F-doping prompts silica glass to relax further even at temperature lower than the glass transition (fictive) temperature. Consequently, fluorine minimizes the density fluctuation (thus, Rayleigh scattering) of silica glass at around 1 wt%, although fluorine itself is an impurity for silica glass.
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