A Tri-Alkali/Alkaline-Earth Metal Fluorophosphate Deep-Ultraviolet Birefringent Crystal with Coexisting PO3F and PO2F2

Deep-ultraviolet (deep-UV) birefringent crystals capable of generating or modulating deep-UV polarized lights are essential for modern laser technologies. However, there are few commercial deep-UV birefringent crystals, except MgF₂, which exhibits tiny birefringence of 0.012@532 nm. Herein, by incorporating multiple alkali/alkaline-earth metal elements, the first tri-alkali/alkaline-earth metal fluorophosphate KBaSr(PO₂F₂)(PO₃F)₂ (KBSPF) with the coexistence of (PO₃F)^2- and (PO₂F₂)^- has been successfully synthesized using the hydrothermal method. Experimental results indicate that KBSPF not only shows a wide transparency window down to the deep-UV spectral region, but also exhibits relatively large birefringence of 0.042@550 nm, exceeding that of commercial MgF₂ and most alkali/alkaline-earth metal phosphates or fluorophosphates. Theoretical calculations reveal that (PO₃F)^2- and (PO₂F₂)^- anions are responsible to the optical properties of KBSPF. Our work not only provides a promising deep-UV birefringent crystal but also offers new insights to enrich the solid-state chemistry of fluorophosphates.