Two-dimensional phase-change chalcohalides

Two-dimensional (2D) materials exhibiting structural phase transitions have a wide range of potential applications, yet 2D phase-change materials have been mostly restricted to binary chalcogenides. Here, we report the computational discovery of a new class of mixed-anion 2D phase-change Janus chalcohalides InSeX (X = Cl, Br, I), in which the lateral displacements of the halogen atoms can be induced by external stimuli such as biaxial strain, electric field, or chemical vacancies, leading to a structural phase transition from an octahedrally (O) coordinated phase to a unique tetrahedrally (T) coordinated phase. The O-to-T phase transition not only leads to an indirect-to-direct bandgap transition, but also results in a drastic reduction of the electron effective mass from 0.31 to 0.81 m(0) to similar to 0.19 m(0). These results highlight the potential for discovering novel phase-change materials in mixed-anion compounds.