From NaSc3Ga2Q8 (Q = S, Se) to KGa2In3S8: substitution of Sc3+ with In3+ to achieve doubled birefringence

Abstract

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, AM₅Q₈ (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 3D {[M₅Q₈]⁻}_∞ 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 (Δn), observed from NaSc₃Ga₂Q₈ (Q = S, Δn = 0.050@532 nm; Q = Se, Δn = 0.088@532 nm) to KGa₂In₃S₈ (Δn = 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, KGa₂In₃S₈ exhibits a photocurrent density of ∼30 nA cm⁻². This work provides novel insights for improving the birefringence performance of materials.