Atomic Structure and Symmetry-Dependent Band Structure of the Novel Spinel Phase AgIn ₂ Se ₄

Band structure fundamentally governs the functionality of solid-state materials. However, precise control without disrupting crystal symmetry remains challenging. Herein, we demonstrate symmetry-driven band-structure engineering in the Ag-In-Se system by phase manipulation under high-temperature and high-pressure conditions. A previously unreported spinel-type cubic AgIn₂Se₄ phase is synthesized and resolved by advanced electron microscopy, exhibiting a well-defined semiconducting band structure with an enlarged band gap and an upshifted position of the conduction band compared to rhombohedral and tetragonal phases. These phenomena originate from a reconstruction of the In-Se coordination environment from tetrahedral to octahedral geometry and enhanced In 5s-Se 4p orbital hybridization for promoting electron localization, thereby shifting the band edges. Our work gives an insightful understanding of symmetry-dependent band structures, affording phase-engineered symmetry control as a general strategy for band-structure optimization in optoelectronic materials with fixed composition.