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

Electrides are a special family of materials with exotic physical properties and various crystal struc-tures in different dimensions. Among them, alkaline-earth metal nitrides (AE2N) have attracted enormous attention, since the synthesis of Ca2N as a two-dimensional (2D) electride. However, apart from Ca2N, the physicochemical properties of other AE2N 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 AE2N, i.e., Mg2N and Be2N, based on first-principles calculations. By symmetry analysis, we derive a crystal structure with R3 over bar m symmetry for Be2N, which is energetically more stable than all the other structures proposed pre-viously. Our calculations show that R3 over bar m Be2N 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 Mg2N, 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 R3 over bar m Be2N, R3m Mg2N 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 under-standing of the crystal structures and physicochemical properties of Be2N and Mg2N as potential electride materials.