The similarities in electronegativity, polarizability, ionic radii and coordination numbers of oxygen and nitrogen allow the formation of the same structural types when combined with cations as well as the mutual substitution of both anions at the same crystallographic sites. This can result in the formation of solid solutions where the formal valence of one or more cations may change according to the O/N ratio. The design of new nitrides based on phases and crystal structures already known for oxides is a useful tool for exploring and/or modifying a large variety of physical properties, either through the formation of completely new compounds or of solid solutions.
We have investigated hafnium nitride halide superconductors that show high critical temperatures within non oxidic compounds. We investigate the effect of nitrogen on the properties of important materials such as CeO2 and TiO2, and we design new oxynitrides with magnetic, electrical, photocatalytic and luminescent properties. Oxynitrides of transition metals, alkaline earth metals and rare earth metals are an important group of materials to expand and tune the properties of oxides. The differences in polarizability, electronegativity and anion charge of nitrogen and oxygen induces changes in the physical and chemical properties by nitrogen introduction. The effects on properties arise from the higher covalency of metal-nitrogen bond and the changes in the energies of electronic levels, and are important in slightly doped nitrogen metal oxides as in stoichiometric oxynitrides. The stabilization of new perovskite oxynitrides, with the oxidation states of cations tuned by O/N stoichiometry, has lead to new photocatalysts, magnetic and dielectric materials. The lower electronegativity of nitrogen and larger crystal field splitting induced by N3- shifts the emission wavelenghts of phosphors to the red, and oxynitridosilicates have been investigated as components of white LED’s.