Enhanced optical anisotropy of six-coordinated silica polymorphs via high-pressure hydrothermal treatment

Abstract

Optical anisotropy determines the performance of birefringent crystals, used in various photonic and optoelectronic instruments. High optical anisotropy often arises from broken crystal symmetry and preferential exciton direction, both of which are readily tuned by applying external stress. In this study, we investigated the birefringence of three silica polymorphs with distinct Si–O polyhedral arrangements via high-pressure hydrothermal treatment combined with polarized optical microscopy and first-principles calculations. Our findings revealed that the birefringence of ambient-stable quartz can be substantially enhanced by approximately 5 times through pressure-controlled polymorphism and doping, reaching up to 0.044 in rutile-type hydrous stishovite and 0.041 in CaCl₂-type Al-doped hydrous post-stishovite. The enhanced optical anisotropy stems from the parallel chain-like structures featured in these dense six-coordinated silica. Pressure-induced phase transition may provide a predictive and controllable approach to improve the optical properties of Earth's abundant materials such as silica.