Anion-Centered Tetrahedra in Inorganic Compounds

Tetrahedral structures are common in silicates, phosphates, aluminosilicates, aluminophosphates in compounds, where units of cationcentered tetrahedra constitute basis of crystalline compounds. As a rule, the cation-centered tetrahedra are corner-linked, and one corner belongs to no more than two tetrahedra. This is the consequence of the high valence of the central cations forming tetrahedral complexes. In the case of anion-centered tetrahedra, the situation is different, as the central ions are relatively low-charged, and the repulsion between adjacent tetrahedral centers is not so strong as is in the case of cationcentered tetrahedra. In general, there is a hierarchical relation between crystallographic, configurational, and topological equivalence. Configurationally equivalent tetrahedra are always topologically equivalent, but topologically equivalent tetrahedra may be configurationally nonequivalent. Crystallographically equivalent tetrahedra are always topologically and configurationally equivalent.