Interstitial anionic electrons in correlated transition metal compounds

Electrides are a unique family of ionic solids, which feature excess electrons confined at interstitial sites and serving as anions. The excess electrons are donated by cationic elements with a low electronegativity, standard valence, and typically a closed shell, such as alkali and alkaline earth metals. As a result, 3d-transition metals with partially filled d orbitals are not conventionally considered a suitable condition for the formation of electrides. In this work, nevertheless, we through ab-initio calculations show that magnetic 3d-transition metal compounds can also host the electride phase. We mainly focus on the oxygen-defective transition metal monoxides, MnO and NiO. With oxygen defects, the excess electrons do not transfer to the 3d orbital of Mn or Ni, but reside at oxygen vacancies and dominate the electronic structure around the Fermi level as driven by the correlation effects of 3d electrons. The resulting defective system remains Mott insulating instead of being metallic. Interestingly, a fairly high work function is obtained (3.4 eV and 3.9 eV for the defective MnO and NiO, respectively), unlike that in typical electrides (2-3 eV in general), suggesting their high chemical stability and hence a broad range of applications. Aside from defective systems, we also apply our idea to stoichiometric transition metal materials, as evidenced by M₁₀(PO₄)₆ (M = Mn, Ni) and Mn₃N₂, which are shown to be promising electrides too.