Hexameric poly-fluoroberyllophosphate Na4Be2PO4F5 with moderate birefringence and deep-ultraviolet transmission as a potential zero-order-waveplate crystal

Abstract

Deep-ultraviolet (deep-UV λ: <200 nm) zero-order waveplate crystal materials are currently scarce but much-needed. The exploration of promising deep-UV zero-order waveplate materials is extremely difficult because of strict building unit and structural framework requirements. Herein, the first poly-fluoroberyllophosphate crystal, Na₄Be₂PO₄F₅, with a zero-dimensional (0D) structure was rationally designed and synthesized through facile hydrothermal technology. Na₄Be₂PO₄F₅ features a novel [Be₄P₂O₈F₁₀] hexameric isolated group possessing three kinds of building units: BeO₂F₂, BeOF₃, and PO₄. For comparison, the newly-synthesized mono-fluoroberyllophosphate crystal KBe[PO₃(OH)]F features a _∞¹[BePO₃(OH)F] chain composed of BeO₃F and PO₃(OH) building units. Na₄Be₂PO₄F₅, with deep-UV transmission below 190 nm, exhibits more suitable birefringence of 0.003 at 1064 nm to act as a promising deep-UV zero-order waveplate material compared to the deep-UV crystal KBe[PO₃(OH)]F, which shows remarkably-enlarged birefringence of 0.025; the birefringence difference is mainly attributed to the different polarizabilities and stacking densities of the building units. Further, a systematic investigation into alkali beryllophosphates demonstrates that BeO₄ and PO₄ tetrahedra show a unique interconnection mode, and the 0D structure of Na₄Be₂PO₄F₅, caused by the cut-off effect of F atoms, is the optimal choice for deep-UV zero-order waveplate materials.