Abstract:
Although it is customary to substitute cations in metal oxides, sulfides, and other materials to modify their structure and properties, effects of anion substitution have not been investigated sufficiently. This is particularly true of materials cosubstituted by two anions (such as N3- and F- in place of O2- or P3- and Cl- in place of S2-). Substitution of a trivalent anion along with a monovalent anion helps to eliminate defects, the three anions being isoelectronic and of nearly the same size. Furthermore, such aliovalent anion substitution gives rise to marked changes in the electronic structure and properties. Isovalent anion substitution (e.g., S2- in place of O2- or Se2- in place of S2-) does not bring about such changes. In this Perspective, we examine the electronic structures and properties of several oxides involving cosubstitution of N and F for oxygen. The oxides discussed are TiO2, ZnO, Cr2O3, and BaTiO3. Aliovalent anion substitution decreases the band gaps of the oxides and affect the magnetic and ferroelectric transitions. Sulfides such as CdS and ZnS where sulfur is substituted by P and Cl also show a large decrease in band gaps. Unlike in cation substitution where the conduction band is mainly affected, in the aliovalent anion-substituted materials the p-states of the trivalent anions (N3- and P3-) dominate the top of the valence band, with the metal-trivalent anion (N, P) bond being shorter and the metal halogen bond longer. Such materials with high visible absorption extending to long wavelengths may indeed find uses.
