Coordinated assembly of alkali and alkaline earth metals with perfluorinated [AlF6] group to design deep-ultraviolet zero-order waveplate materials

The capability of zero-order waveplates to manipulate the polarization of fundamental-frequency light has garnered significant attention in light of the rapid advancements in laser technology. In particular, zero-order waveplate materials in the deep-ultraviolet range (DUV; λ < 200 nm) are in urgent and short supply. In this study, three aluminum fluorides, AlF3 (P6/mmm), BaAlF5 (I4/m), and Li2KAl2F9 (C2/m), have been successfully designed and synthesized through a strategy that combines perfluorinated [AlF6] group with alkali and alkaline-earth metals. The results of experimental and theoretical calculations indicate that all three new aluminum fluorides have a short cutoff edge (λ < 200 nm) and small birefringence (0.0006 ~ 0.0056 @1064 nm). Theoretical calculations indicate that AlF3 (P6/mmm) has the shortest wavelength of 125 nm, which is comparable to the shortest wavelength of the commercially available MgF2. However, its birefringence of 0.0006 @1064 nm is about 20 times lower than that of the MgF2 (0.012 @546 nm), making it highly advantageous for fabricating deep-ultraviolet zero-order waveplate materials. Further microscopic analysis reveals that the [AlF6] group exhibits a substantial band gap of 9.17 eV and small polarizability anisotropy, indicating that the aluminum fluorides are potential candidates for designing suitable compounds for DUV zero-order waveplate materials.