Zero-Dimensional Oxyselenide Ba₁₂[(Ga₂OSe₅)₃(Si₂O₇)]: Enhanced Optical Anisotropy through Heteroanionic Engineering

The rational design of chalcogenides with exceptional optical properties remains a significant challenge. Here, we employ heteroanionic engineering to incorporate selenium into the celsian-type BaGa₂Si₂O₈, synthesizing a novel zero-dimensional oxyselenide, Ba₁₂[(Ga₂OSe₅)₃(Si₂O₇)]. It crystallizes in the hexagonal space group P6₃/m and features isolated [Si₂O₇] and [Ga₂OSe₅] clusters, balanced by Ba²⁺ cations. The [Ga₂OSe₅] unit, reported for the first time in oxychalcogenides, marks a structural breakthrough. The compound exhibits a wide bandgap (E_g = 3.14 eV), broad IR transmission (0.39-20.4 μm), and high thermal stability (up to 1100 K). Theoretical calculations reveal that the compound exhibits a 18-fold enhancement in birefringence, increasing from 0.0038 to 0.068 at 1064 nm, compared to its parent structure. This significant improvement is largely due to the role of the [Ga₂OSe₅] unit in enhancing optical anisotropy. This work highlights the potential of heteroanionic engineering for designing advanced birefringent materials, paving the way for functional optical crystals.