Oxidation of two-dimensional electrides: Structural transition and the formation of half-metallic channels protected by oxide layers

Abstract

Based on first-principles calculations, we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized ${A}_{2}B$ electrenes, with the following combinations: (i) $A$ = Ca, Sr, and Ba for $B$ = N; (ii) $A$ = Sr and Ba for $B$ = P; and ${\mathrm{Y}}_{2}\mathrm{C}$ and ${\mathrm{Ba}}_{2}\mathrm{As}$. We have considered one side oxidation of single layer electrenes (O/${A}_{2}B$), and two side oxidation of bilayer electrenes (O/${({A}_{2}B)}_{2}$/O). We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the (fully) oxidized systems. Our total energy results reveal an exothermic structural transition from hexagonal to tetragonal ($\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$) geometry, resulting in layered tetragonal structures [(${A\mathrm{O}AB)}^{\mathrm{t}}$ and ${(A\text{O}{(AB)}_{2}A\text{O})}^{\mathrm{t}}$]. Phonon spectra calculations and molecular dynamic simulations show that the O/${A}_{2}B$ and O/${({A}_{2}B)}_{2}$/O systems, with $A$ = Ba, Ca, Sr, and $B$ = N, become dynamically and structurally stable upon such a $\mathrm{h}\ensuremath{\rightarrow}\mathrm{t}$ transition. Further structural characterizations were performed based on simulations of the near edge x-ray absorption spectroscopy at the nitrogen $K$ edge. Finally, the electronic band structure and transport calculations reveal the formation of half-metallic bands spreading out through the $A\mathrm{N}$ layers, which in turn are shielded by oxide $A\mathrm{O}$ sheets. These findings indicate that ($A\mathrm{O}A\mathrm{N}$)${}^{\mathrm{t}}$ and ${(A\mathrm{O}{(A\mathrm{N})}_{2}A\mathrm{O})}^{\mathrm{t}}$ are quite interesting platforms for application in spintronics; since the half-metallic channels along the $A\mathrm{N}$ or ${(A\text{N})}_{2}$ layers (core) are protected against the environment conditions by the oxidized $A\text{O}$ sheets (cover shells).