Three Deep-UV Phosphate Optical Materials with Interesting Structural Evolution

In this work, three novel phosphates – LiRb(PO3)2, NaCs(PO3)2, and RbSr2(PO3)5 – were synthesized via high-temperature solution methodology. NaCs(PO3)2 and RbSr2(PO3)5 crystallize in the centrosymmetric (CS) space groups P21/c and P1̅, respectively, while LiRb(PO3)2 crystallizes in the noncentrosymmetric (NCS) I4̅ space group. It was also found that different cations can flexibly regulate the linking modes of the P–O framework between cyclic and chain. Performance tests show that these three compounds have large band gaps and deep ultraviolet (deep-UV) cutoff edges below 200 nm. First-principles calculations suggest that P–O units contribute mainly to their optical properties. The second harmonic generation (SHG) of LiRb(PO3)2 and the birefringence of the title compounds were also studied. This work provides a valuable guide for designing phosphates with different structural types and deep-UV transmitting optical materials.