Partial substitution with a significant effect: coexistence of a wide band gap and large birefringence in the oxychalcogenide AEGe2O4Se (AE = Sr and Ba)

Abstract

Much effort has been devoted to the discovery of novel birefringent crystals that display considerable birefringence (Δn) in the infrared (IR) region. However, the simultaneous achievement of a wide energy gap (E_g > 3.1 eV) and a large Δn (>0.2) in a heteroanionic chalcogenide system remains a formidable challenge. To address this bottleneck, we applied the partial-substitution strategy and successfully designed and synthesized two new quaternary oxychalcogenides, namely AEGe₂O₄Se (AE = Sr and Ba). These two isomorphic compounds belong to the monoclinic space group P2₁/c (no. 14), featuring a structure composed of two-dimensional (2D) [Ge₂O₄Se]²⁻ layers with an antiparallel arrangement, which are separated by charge-balanced Ba²⁺ cations. Remarkably, they exhibit the coexistence of large Δn values (0.209 and 0.238@2050 nm based on the generalized gradient approximation) and wide E_g values (3.57 and 3.81 eV). Furthermore, theoretical calculations were performed to elucidate the interplay between optical properties and electronic structures. These results reveal that the significantly improved Δn value (approximately 15–17 times that of the parent compound BaGe₂O₅) can mainly be attributed to the newly discovered [GeO₃Se] heteroanionic motif. In brief, this study provides a simple chemical substitution method to overcome the trade-off between wide E_g and large Δn values in heteroanionic chalcogenides.