Coordinated assembly of alkali and alkaline earth metals with the perfluorinated [AlF ₆ ] 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; <i>λ</i> < 200 nm) are in urgent and short supply. In this study, three new aluminum fluorides, AlF₃ (<i>P</i>6/<i>mmm</i>), BaAlF₅ (<i>I</i>4/<i>m</i>), and Li₂KAl₂F₉ (<i>C</i>2/<i>m</i>), have been successfully designed and synthesized through a strategy that combines the perfluorinated [AlF₆] group with alkali and alkaline-earth metals. The results of experimental and theoretical calculations indicate that three new aluminum fluorides have a short cutoff edge (<i>λ</i> < 200 nm) and small birefringence (0.0006-0.0056@1064 nm). In addition, the AlF₃ (<i>P</i>6/<i>mmm</i>) has the shortest wavelength of 125 nm based on theoretical calculations, 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.