mixedanion

Mixedanion refers to a class of compounds in which two or more different anions occupy the crystal structure. In solid-state chemistry this commonly means oxynitrides (containing oxide O2- and nitride N3-), oxysulfides (O2- and S2-), and oxyfluorides (O2- and F-), though other combinations exist. The anions can occupy distinct lattice sites in an ordered pattern or be distributed non-uniformly (anion disorder).

The introduction of multiple anions provides a route to tune structural, electronic, and chemical properties beyond what single-anion compounds can achieve. For example, mixing O with N can lower the band gap and enable visible-light activity; mixing O with F can modify accessibility to reactive sites. Mixed-anion materials are investigated for photocatalysis, energy storage, catalysis, and optoelectronic applications.

Synthesis and challenges: Mixed-anion compounds are typically synthesized by solid-state reactions at high temperatures or by solution-based methods, sometimes requiring careful control of atmosphere and precursor chemistry to fix the desired anion composition and ordering. Controlling whether anions are ordered or disordered, and achieving phase stability, are major research concerns.

Characterization and example: Techniques such as X-ray and neutron diffraction, X-ray photoelectron spectroscopy, and vibrational spectroscopy are used to determine anion occupancies and local structure. A well-known oxynitride example is LaTiO2N, a perovskite-related oxide–nitride compound studied for visible-light photocatalysis.