The rutile type and its derivatives†

Rutile type crystal structures and their derivatives are based on the same topology of primary bonds: straight chains of edge-sharing coordination octahedra are connected via corner-sharing to four surrounding chains. Twenty derivatives are known. Some are based on an ordering of cations, others on an ordering of anions, others still on distortions of their coordination polyhedra. The overwhelming majority of all the compounds reviewed here crystallize in the aristotypic rutile type. The trirutile type, an ordering variant, and the CaCl2 type, a distorted rutile, are also numerous. The other rutile type derivatives are known only from one, two, or just a few examples. Most of the known compounds obey Pauling's postulate, as expressed in his second rule for complex ionic compounds, according to which the sum of the bond strengths received by an anion should be balanced for stable compounds by the formal valence of that anion. More than one half of all naturally occurring chemical elements have been observed to be present in the rutile type or its derivatives.