CsGaGe₃Se₈: Enhanced optical anisotropy and bandgap via cation-substitution-driven dimensionality reduction

A strategic dimensionality reduction approach is demonstrated through controlled Cs⁺ incorporation into the three-dimensional (3D) AgGaGe₃Se₈ framework, yielding the new layered compound CsGaGe₃Se₈ while preserving stoichiometry. Single-crystal X-ray diffraction reveals its crystallization in P2₁/c space group, exhibiting a pronounced 2D layered structure distinct from the parent 3D network. Comprehensive characterization shows that CsGaGe₃Se₈ possesses remarkable thermal stability (stable up to 600 °C) and a direct bandgap of 2.41 eV, a 15 % increase compared to the parent phase (2.10 eV). The compound exhibits broad-spectrum transparency (0.43–25 μm) and a moderate birefringence (Δn_cal = 0.05), representing an about 2.5-fold enhancement over AgGaGe₃Se₈ (Δn_cal ≈ 0.020). These findings establish that controlled cationic substitution-driven dimensionality reduction can concurrently modulate bandgap expansion and optical anisotropy. This study not only enriches the family of selenides with a high-performance optical material but also provides a generalizable structural-design paradigm for developing infrared birefringence optical crystals.