Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides

High pressure is able to modify profoundly the chemical bonding and generate new phase structures of materials with properties not accessible at ambient conditions. We here report an unprecedented phenomenon on the pressure-induced formation of semi-conducting electrides via compression of layered alkaline-earth-metal subnitrides that are conducting electrides with loosely confined electrons in the interlayer voids at ambient pressure. Our structure searches identified the high-pressure semiconducting electride phases of a tetragonal I-42d structure for Ca₂N and a monoclinic Cc structure shared by Sr₂N and Ba₂N, both of which contain atomic-size cavities with paring electrons distributed within. These electride structures are validated by the excellent agreement between the simulated X-ray diffraction patterns and the experimental data available. We attribute the emergence of the semiconducting electride phases to the p−d hybridization on alkaline-earth-metal atoms under compression and the filling of the p−d hybridized band due to the interaction between Ca and N. Our work provides a unique example of pressure-induced metal-to-semiconductor transition in compound materials and reveals unambiguously the electron-confinement topology change between different types of electrides.