Structural Transition in Oxidized Ca₂N Electrenes: CaO/CaN 2D Heterostructures

Based on first-principles calculations, we show that the oxidation of ultrathin films of Ca₂N electrides (electrenes) drives a hexagonal → tetragonal structural transition. The ground-state configuration of the oxidized monolayer (ML) and bilayer (BL) systems can be viewed as CaO/CaN and CaO/(CaN)₂/CaO two-dimensional (2D) heterostructures. In both systems, we found nearly free electron states lying near the vacuum level, and the spatial projection reveals that they are localized above the oxidized CaO surface. Focusing on the magnetic properties, we find that the nitrogen atoms of the oxidized Ca₂N becomes spin-polarized (∼1 μB/N-atom), where we found an energetic preference for the ferromagnetic phase. We show that such a magnetic preference can be strengthened upon mechanical compression. Further electronic structure calculations reveal that the FM CaO/(CaN)₂/CaO presents half-metallicity, where the metallic channels project (predominantly) on the N-2pₓ,y orbitals. In addition to the total energy results, molecular dynamic and phonon spectra calculations have been done in order to verify the thermal and structural stabilities of the oxidized systems. These findings suggest that CaO/(CaN)₂/CaO is quite an interesting and structurally stable 2D FM heterostructure characterized by half-metallic bands sandwiched by NFE states lying on the oxidized surfaces.