oxynitrides

Oxynitrides are a class of compounds that contain both oxide (O2−) and nitride (N3−) anions in combination with metal cations. The coexistence of two different anions within a single lattice allows a wide range of compositions and structures. A common subclass comprises ABO2N-type compounds, in which a perovskite-like framework houses two oxide ions for every nitride, maintaining charge balance and enabling diverse cation chemistries.

Structure and bonding: Many oxynitrides adopt perovskite-derived structures in which metal cations occupy octahedral sites coordinated by a mixed O/N anion lattice. The ordering of oxygen and nitrogen (for example, cis vs trans arrangements around the B-site) can vary, and this anion ordering strongly affects the electronic structure, band gap, and optical properties.

Synthesis: Oxynitrides are typically prepared by high-temperature ammonolysis or nitridation of oxide precursors under ammonia or N-containing atmospheres. Alternative routes include nitridation of mixed oxides with nitrogen sources or hydrothermal methods with nitrogen donors. Doping and cation substitutions are commonly employed to tune electronic properties and stability.

Properties and applications: Substituting N for O generally narrows the band gap relative to oxide analogs, yielding visible-light activity in many compounds. Consequently, oxynitrides are actively explored as photocatalysts for water splitting and solar-driven transformations. Stability varies with composition; some oxynitrides show good chemical robustness, while others are more susceptible to hydrolysis or decomposition under ambient conditions.

Representative members and research directions: Prominent examples include LaTiO2N and SrTaO2N, among others. Research focuses on controlling anion order, understanding structure–property relationships, and developing scalable synthesis routes to enable practical applications in photocatalysis and energy conversion.