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

Deep-ultraviolet (deep-UV lambda: <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)(infinity)[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.