Direct Cation-Cation Interactions in Primarily Ionic Solids

It is pointed out that there is considerable experimental evidence for direct cation-cation interactions in several primarily ionic solids containing transition-element cations with outer-electron configuration ${\mathrm{nd}}^m$ , where $\text{m≤5}$ . Competitive indirect (cation-anion-cation) interactions are dominant if $\text{m\hspace{0.17em}=\hspace{0.17em}5}$ . Nevertheless, if cation-occupied octahedra share a common face, as in the corundum and NiAs-type structures, the direct (cation-cation) interactions may also significantly influence the physical properties of the material. If octahedral-site cations have $\text{m≤3}$ , the cation-anion-cation interactions are weak and cation- -cation interactions may be dominant. The consequences of cation- -cation interactions are described and shown to be quite distinct from those of cation-anion-cation interactions so that the dominant mechanism can usually be distinguished. Data for several illustrative materials are presented.