Cationic substitution engineering induces a new quaternary chalcogenide in the A–MIIIA–MIVA–Q system for optical crystal applications

Abstract

A new quaternary chalcogenide in the A–M^IIIA–M^IVA–Q system, CsGaGe₃S₈, was synthesized via a high-temperature solid-state reaction. It crystallized in the monoclinic system (space group P2₁/c) and featured 2D ∞² [(Ga/Ge)₈S₁₆]²⁻ layers, with Cs⁺ ions occupying the interlayer regions. The material exhibited excellent thermodynamic stability (up to 600 °C) and a broad UV-vis-IR transmission range (0.33–23 μm), with a band gap of 3.38 eV and a birefringence of 0.043 at 546 nm. Compared to AgGaS₂, CsGaGe₃S₈ offers a wider transmission range and larger birefringence, making it a promising candidate for infrared birefringent crystals. First-principles calculations further elucidated its structure–property relationship.