From NaScGaQ (Q = S, Se) to KGaInS: substitution of Sc with In to achieve doubled birefringence.

The exploration of novel compounds with new crystal and electronic structures is essential for advancing the development of functional materials. Here, three novel isomorphic quaternary alkali-metal chalcogenides, AMQ (A = Na, K; M = Sc, Ga and In; Q = S, Se), with a new structure type have been obtained by a facile reactive flux assisted boron-chalcogen solid-state method, and adopt a 3 {[MQ]} open-framework architecture with A ions occupying the cavities to balance charges. Subtle modulation of the M-Q bonds induces a twofold enhancement in optical birefringence (Δ), observed from NaScGaQ (Q = S, Δ = 0.050@532 nm; Q = Se, Δ = 0.088@532 nm) to KGaInS (Δ = 0.134@532 nm). Their optical band gaps are 2.21-3.10 eV. The theoretical calculation results reveal that the replacement of Sc with In enhances the orbital hybridization between cations and anions, amplifies microscopic polarizability anisotropy and consequently doubles the birefringence. In addition, KGaInS exhibits a photocurrent density of ∼30 nA cm. This work provides novel insights for improving the birefringence performance of materials.