Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth Metal Subnitrides

High pressure is able to to modify profoundly the chemical bonding and generate new phase structures of materials with chemical and physical properties not accessible at ambient conditions. We here report an unprecedented phenomenon on the pressure-induced formation 9f semi-conducting electrides via compression of layered alkaline-earth subnitrides Ca2N, Sr2N, and Ba2N that are conducting electrides with loosely confined electrons in :the interlayer voids at ambient pressure. Our extensive first-principles swarm structure searches identified the high-pressure semiconducting electride phases of a tetragonal 142d 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 validted 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 as Well as 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.