Oxide nitrides: From oxides to solids with mobile nitrogen ions

The possibility of fast nitrogen ion conduction in solids is reviewed. Promising electrolytes based on three different base compounds are in the focus of this contribution: Zirconium oxide nitrides, tantalum oxide nitrides and mayenite-based materials. All aspects ranging from preparation methods, crystal structures (ideal and defect structure, also at elevated temperatures), transport properties (ionic and electronic conductivity, transference numbers, diffusion) and correlations between structure and physical properties are presented and discussed, in part also in relation to theoretical calculations. Fluorite-type quaternary oxide nitrides of zirconium are proven to be the first known materials with high nitrogen ion mobility. They can be described as fast mixed oxygen/nitrogen conductors but are limited due to the low maximum nitrogen/oxygen ratio achievable. Corresponding phases based on stabilized tantalum oxide nitrides have a superior N/O ratio but show poor thermal stability. For the development of a pure nitrogen ion conductor a different approach has also been investigated: Some cage compounds, in particular mayenite, allow the substitution of oxygen anions not tightly bound in the framework by nitrogen ions. Some of the obtained N-containing phases exhibit an outstanding electrical conductivity at low temperatures. Possible devices and applications such as a new type of a nitrogen sensor and an ammonia-producing fuel cell are introduced and discussed.