Cationic Substitution Engineering Induced New Quaternary Chalcogenide in A–MIIIA–MIVA–Q System Used for Optical Crystal

A new quaternary chalcogenide in A–MIIIA–MIVA–Q System, CsGaGe3S8, was synthesized via a high-temperature solid-state reaction. It crystallizes in the monoclinic system (space group P21/c) and features 2D ∞2 [(Ga/Ge)8S16]²⁻ layers with Cs⁺ ions in the interlayer regions. The material exhibits 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 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 elucidate its structure-property relationship.