Crystal Structures and Physicochemical Properties of Be2N and Mg2N as Electride Materials

Abstract

Electrides are a special family of materials with exotic physical properties and various crystal structures in different dimensions. Among them, alkaline-earth metal nitrides (${\mathrm{AE}}_{2}\mathrm{N}$) have attracted enormous attention, since the synthesis of ${\mathrm{Ca}}_{2}\mathrm{N}$ as a two-dimensional (2D) electride. However, apart from ${\mathrm{Ca}}_{2}\mathrm{N}$, the physicochemical properties of other ${\mathrm{AE}}_{2}\mathrm{N}$ are not well understood, and in some cases, their crystal structures remain unknown. To overcome these issues, here, we conduct systematic investigations into the crystal structures, electronic structures, and optical properties of two typical ${\mathrm{AE}}_{2}\mathrm{N}$, i.e., ${\mathrm{Mg}}_{2}\mathrm{N}$ and ${\mathrm{Be}}_{2}\mathrm{N}$, based on first-principles calculations. By symmetry analysis, we derive a crystal structure with $R\overline{3}m$ symmetry for ${\mathrm{Be}}_{2}\mathrm{N}$, which is energetically more stable than all the other structures proposed previously. Our calculations show that $R\overline{3}m$ ${\mathrm{Be}}_{2}\mathrm{N}$ is a 2D electride with the anionic electrons located in the interstitial space, forming a 2D Kagome lattice. Moreover, distinct from traditional 2D electrides, it exhibits anomalous physicochemical properties, such as having a small interstitial space, a high work function, and a large cleavage energy, mainly due to the relatively strong bonding effect between anionic electrons and cationic framework. For ${\mathrm{Mg}}_{2}\mathrm{N}$, on the other hand, we find that the most stable structure has R3m symmetry, which shows a total energy of 0.005 eV per formula unit lower than the previously predicted Cmcm structure. Moreover, different from $R\overline{3}m$ ${\mathrm{Be}}_{2}\mathrm{N}$, R3m ${\mathrm{Mg}}_{2}\mathrm{N}$ is identified as a zero-dimensional (0D) electride with a semiconducting band structure, since its anionic electrons are confined in separated 0D cavities and cannot interact with each other. These studies thus provide a deeper understanding of the crystal structures and physicochemical properties of ${\mathrm{Be}}_{2}\mathrm{N}$ and ${\mathrm{Mg}}_{2}\mathrm{N}$ as potential electride materials.