CuI: Energy, geometry, electronic structure and optical properties of four phases from first-principles calculations

This study employs first-principles DFT calculations with the LDA/GGA functional to systematically investigate the structural, electronic, bonding, and optical properties of four copper iodide (CuI) phases: [Formula: see text], β-P3m1, and high-pressure P4/nmm and Cmcm. The [Formula: see text] phase exhibits the lowest ground-state energy. Calculated direct band gaps at G are 2.292 eV ([Formula: see text]), 2.634 eV (P3m1), 2.667 eV (P4/nmm), and 2.127 eV (Cmcm). Density of States analysis reveals similar orbital contributions across phases, with P4/nmm and P3m1 showing weak Cu-I covalent character alongside dominant ionic bonding, confirmed by charge (Mulliken, Hirshfeld) and ELF analysis (~60-70% ionicity). Optical properties display significant phase dependence: P4/nmm exhibits broad absorption, [Formula: see text] shows a steep absorption edge (peak α=2.7×10 5 cm −1 ) characteristic of direct band gaps, Cmcm has a high low-energy refractive index, and P4/nmm demonstrates strong photon localization (reflectivity peak 0.48 at 7.5 eV). Absorption intensity ranks [Formula: see text] > P3m1 > P4/nmm > Cmcm. These distinct properties guide phase selection for targeted optoelectronic applications like solar cells ([Formula: see text], P3m1), broadband detectors (P4/nmm), and optical resonators (Cmcm).