High-pressure solid-state metathesis (HPSSM) reaction is an effective route to novel metal nitrides. A recent advance in HPSSM reactions is presented for a number of examples, including 3d transition metal nitrides (ε-Fe3N, ε-Fe3−xCoxN, CrN, and Co4Nx), 4d transition metal nitrides (MoNx), and 5d transition metal nitrides (Re3N, WNx). Thermodynamic investigations based on density functional theory (DFT) calculations on several typical HPSSM reactions between metal oxides and boron nitride indicate that the pressure could reduce the reaction enthalpy ΔH. High-pressure confining environment thermodynamically favors an ion-exchange process between metal atom and boron atom, and successfully results in the formation of well-crystalized metal nitrides with potential applications.

Electrides are an emerging class of materials with excess electrons localized in interstices and acting as anionic interstitial quasi-atoms (ISQs). The spatial ion–electron separation means that electrides can be treated physically as ionic crystals, and this unusual behavior leads to extraordinary physical and chemical phenomena. Here, a completely different effect in electrides is predicted. By recognizing the long-range Coulomb interactions between matrix atoms and ISQs that are unique in electrides, a nonanalytic correction to the forces exerted on the matrix atoms is proposed. This correction gives rise to a longitudinal acoustic-transverse acoustic splitting in the acoustic branch of lattice phonons near the zone center, similar to the well-known longitudinal optical–transverse optical splitting in the phonon spectra of ionic compounds. The factors that govern this splitting are investigated, with isotropic fcc-Li and anisotropic hP4-Na as the typical examples. It is found that not all electrides can induce a detectable splitting, and criteria are given for this type of splitting. The present prediction unveils the rich phenomena in electrides and could lead to unprecedented applications.