Ternary structures

Ternary structures are ones with three elements exist, instances being CaCO₃ and CaTiO₃. Oxides are the most common instances of such type of structures and exemplify some of the significant principles. Two fundamentally distinct structural features are possible,  as follows:

1. Complex oxides are compounds consisting of complex ions, that appear as discrete structural units. For instance, calcium carbonate has a structure based on rocksalt with the distinct sites occupied by Ca²⁺ and CO^2-₃ ions.
2. Mixed oxides are exemplified by CaTiO₃ that, even though often called 'calcium titanate', does not have distinct titanate ions. Perovskite structure (Fig. 1. depicts a corner-sharing network of TiO₆ octahedra (necessarily the ReO₃ structure;) with Ca²⁺ occupying the large central site coordinated through 12 oxygen ions.

Though, this division is not absolute and the varied structures of silicates provide instances of intermediate cases. ZrSiO₄ (zircon) has discrete ions SiO₄^4- but silicates like CaSiO₃ do not contain individual SiO₃^2-units but are created from tetrahedral SiO₄ groups sharing corners to make rings or infinite chains.

Additional sharing of corners can make two- dimensional and three- dimensional networks. The distinct structures of silicates and carbonates reflect some typical and very significant differences in bonding preference between periods 2 and 3 in the p block.

Complex oxides are generally found when a nonmetal is exist, with oxoanions like nitrate NO₃^-,carbonate C₃^2- phosphate P₄^3-or sulfate SO₄^2- but are also occasionally formed by metals in high oxidation states (example permanganate MnO₄^- in KMnO₄). When a compound consists of two metallic elements the mixed oxide form is more normal but it is significant to notice that the compound formula itself gives very little guide to the structure (compare CaCO₃ and CaSiO₃ above). A identical structural variety is found with complex halides. For instance, the K₂NiF₄ structure is based on layers of corner-sharing NiF₆ octahedra with no discrete complex ions, where K₂PtCl₄ consists of individual square planar ions [PtCl₄]^2-. These variations reflect the bonding preferences of Ni^II and Pt^II.