Entropy stabilized off-stoichiometric cubic γ-Cu_1−xI_x phase containing high-density Cu vacancies

Copper iodide (CuI) has gained attention as a highly conductive p-type transparent material. Here, we investigate the phase stability of I-rich cubic CuI phases with Cu-vacancy defects by performing hybrid-density functional theory calculations. In the Cu-rich equilibrium condition, the nearly stoichiometric phase is the ground state with quenched Cu-vacancy defects, and the Cu-vacancy defect is a major acceptor responsible for the intrinsic p-type conductivity. In contrast, in the I-rich condition, the off-stoichiometric Cu_1−xI_x phase (x = 0.5-0.55) containing high-density Cu vacancies is stabilized with configuration entropy from Cu vacancies. As the off-stoichiometric phases contain high-density neutral Cu vacancies, the hole transport can be hindered, and the hole mobility could be reduced.