Heteroanion-introduction-driven birefringence enhancement in oxychalcogenide Ba3MIIGe3O2S8 (MII = Mn, Cd)

Abstract

Birefringent crystals play a crucial role in regulating the polarization of light and are widely used in optoelectronic fields. However, the effective design of novel infrared (IR) birefringent crystals with large birefringence (Δn) still face significant challenges. In this study, we present the rational design and successful synthesis of two novel quinary oxychalcogenides with the formula Ba₃M^IIGe₃O₂S₈ (M^II = Mn, Cd), employing a heteroanion-introduction strategy via high-temperature solid-state reactions. Ba₃M^IIGe₃O₂S₈ (M^II = Mn, Cd) crystallized in the monoclinic space group P2₁/n (no. 14) and the structures comprised one-dimensional (1D) [M^IIGe₃S₈O₂]⁶⁻ chains arranged in an antiparallel manner and separated by Ba²⁺ cations. The coexistence of multiple heteroanionic ligands ([M^IIOS₅] octahedra, [GeOS₃], and [GeO₂S₂] tetrahedra) in one material was surprisingly discovered for the first time in the realm of oxychalcogenides. It was revealed that the heteroanion-introduction strategy not only leads to a reduction in the structural dimensionality but also enhances the optical anisotropy significantly. Notably, Ba₃M^IIGe₃O₂S₈ (M^II = Mn, Cd) demonstrated large Δn values of 0.11 and 0.14, which represent a remarkable improvement compared to the three-dimensional (3D) parent AE₃M^IIM^IV₂Q₈ system (Δn = 0). Furthermore, theoretical calculations suggest that the significant Δn of Ba₃M^IIGe₃O₂S₈ (M^II = Mn, Cd) resulted primarily from the combination of polarizabilities from the various heteroanionic groups. Overall, these results highlight the potential of the heteroanion-introduction strategy for designing novel IR birefringent materials for optoelectronic applications.