Pressure-Induced Irreversible Metallization Accompanying Phase Transition of Chalcopyrite Cu(In_0.7Ga_0.3)Se₂

Chalcopyrite copper-indium-gallium diselenides (CIGS) have emerged as promising materials with remarkable electronic properties and potential applicability to high-efficiency solar cells. The crystal and electronic structures of CIGS can be continuously tuned from their initial states under pressure. Although pressure-induced band gap closure in CIGS has been predicted in extensive theoretical studies, it has not been supported by experimental evidence. Here, we comprehensively investigate the pressure-dependent optical, electronic, and structural properties of Cu(In_0.7Ga_0.3)Se₂ up to 42.6 GPa. Our experimental results reveal an irreversible electronic transition from the semiconducting to the metallic state at 14.3 GPa. Under compression, the Cu(In_0.7Ga_0.3)Se₂ structure evolves from a tetragonal I4̅2d phase to an orthorhombic Pna2₁ phase, which has not been previously reported in chalcopyrite. More intriguingly, the Pna2₁ phase is irreversible and possesses smaller Cu-Se and In/Ga-Se bond lengths and a smaller Cu-Se-Cu bond angle than the I4̅2d phase. Density functional theory calculations indicate a lower enthalpy of the Pna2₁ phase than that of the I4̅2d phase at pressures above 10.6 GPa. Meanwhile, density of states calculations illustrate that metallization arises from the overlap of the Se p and Cu d orbitals as the bond length reduces. This pressure-induced behavior could facilitate the development of novel devices with various phenomena involving strong coupling of the mechanical, electrical, and optical properties of chalcopyrite.