Multianion

Multianion is a descriptor used in chemistry and materials science to denote systems that contain more than one type of negatively charged species. It is not a single defined compound class, but a broad label that applies to both solution-phase mixtures and solid materials in which different anions are present or occupy distinct lattice sites.

In solution, multianion environments occur whenever two or more anions coexist, such as electrolytes containing sulfate, phosphate, and carbonate. In solid-state chemistry, multianion compounds are materials in which the crystal lattice includes two or more distinct anions, for example oxynitrides (oxygen and nitrogen), oxyfluorides (oxygen and fluorine), or sulfide-chalcogenide mixtures. Mixed-anion chemistries are studied to tailor properties by adjusting anion identity, ratio, and ordering.

The interest in multianion systems stems from the ability to tune electronic structure, ionic conductivity, catalytic activity, and redox behavior. By selecting different anions and controlling their distribution, researchers aim to stabilize unusual oxidation states, modulate band gaps, or enhance ion mobility.

Synthesis approaches include conventional high-temperature solid-state reactions, nitridation or sulfidation of oxides, fluorination, hydrothermal methods, and low-temperature fluorination. Challenges include achieving precise anion ordering, maintaining phase stability, and managing differing anion sizes and charges, which can drive phase separation or defect formation.

Applications span energy storage (solid-state electrolytes and redox-active materials), photocatalysis, and optoelectronics, as well as fundamental studies of lattice dynamics and bonding in complex anion environments.

See also: mixed-anion chemistry, oxynitride, oxyfluoride, sulfide-sulfate systems, perovskite.