Layered Transition Metal Electride Hf_2Se with Coexisting Two-Dimensional Anionic d-Electrons and Hf–Hf Metallic Bonds

Electrides are unique materials, whose anionic electrons are confined to interstitial voids, and they have broad potential applications in various areas. In contrast to the majority of inorganic electrides, in which the anionic electrons primarily consist of s-electrons of metals, electrides with anionic d-electrons are very rare. Based on first-principles electronic structure calculations, we predict that the layered transition metal chalcogenide Hf_2Se is a novel electride candidate with anionic d-electrons. Our results indicate that the anionic electrons confined in the Hf_6 octahedra vacancy between Hf_2Se layers mainly come from the Hf-5d orbitals. In addition, the anionic electrons coexist with the Hf–Hf multiple-center metallic bonds located in the center of neighboring Hf_4 tetrahedra. The calculated work function (3.33 eV) for the (110) surface of Hf_2Se is slightly smaller than that of Hf_2S, which has recently been reported to exhibit good electrocatalytic performance. Our study of Hf_2Se will enrich the electride family, and promote further research into the physical properties and applications of electrides.