Two Short-Wave UV Beryllium Selenites Exhibiting Diverse Optical Properties Stemming from Functional Group...

The arrangement of functional groups exerts a crucial role in determining the characteristics of compounds. In this study, we synthesized two novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals: KBe₂(SeO₃)₂(OH)·H₂O and K₂Be(SeO₃)₂. Interestingly, the two compounds show the same SeO₃ triangular pyramids and K-O polyhedra. However, the two compounds exhibit distinct beryllium-oxygen anion groups: BeO₃(OH) for KBe₂(SeO₃)₂(OH)·H₂O and BeO₄ for K₂Be(SeO₃)₂. This results in the SeO₃ groups within the structure having different orientations, ultimately leading to the two compounds exhibiting completely different optical properties. KBe₂(SeO₃)₂(OH)·H₂O displays a large second harmonic generation (SHG) effect equivalent to 2× KH₂PO₄ (KDP), coupled with a large birefringence of 0.078 at 546 nm. In contrast, the SHG effect and birefringence of K₂Be(SeO₃)₂ are only 0.33× that of KDP and 0.024 at 546 nm, respectively. Structural analyses and theoretical calculations indicate that these pronounced differences in optical properties stem from variations in the arrangement of the SeO₃ functional groups. This study not only sheds light on the correlation between crystal structure and optical behavior but also presents a hopeful avenue for the advancement of materials in the short-wave UV spectrum.