Sb ₂ (SeO ₃ ) ₂ SeO ₄ : A Selenite–Selenate Crystal with Balanced Band Gap and Birefringence

The combination of stereochemically active [SeO3] units with Sb3+ cations effectively enhances optical anisotropy and yields sizable birefringence in antimony selenite compounds. However, the stereochemically active lone pairs often induce a red shift in the UV cutoff edge, which may limit the application of these compounds in the short-wave UV region. To overcome this limitation while preserving anisotropic building units, we introduced tetrahedral [TO4] (T = S, Se) groups into the SeO3–Sb(III) framework. Two compositionally related crystals, Sb2(SeO3)2SO4 (ASeS) and Sb2(SeO3)2SeO4 (ASeSe), were synthesized and characterized. Both maintain short-wave UV cutoffs (≤300 nm), and ASeSe exhibits sizable birefringence of 0.158 at 546 nm. Structure–property analysis attributes the performance to cooperative [SeO3]/[SbO5]/[TO4] (T = S, Se) building units, with the higher polarizability anisotropy of [SeO4] underpinning the larger birefringence in ASeSe.