ABSTRACT
Re03, rutile, corundum, and sodium chloride are built of coordination octahedra linked in different ways. In the Re03 structure the octahedra share only vertices as shown in Fig. 1.3. When octahedra share all edges, the sodium chloride structure appears. In the rutile structure (Fig. 1.4) each octahedron shares two opposite edges with the neighboring octahedra. Chains of octahedra are formed which are linked together through the remaining free vertices of the octahedra. Because of the high positive charge on metal ions the repulsive forces between them play an important role and often cause a distortion of the octahedra, resulting in an increase of the intercationic distance. In the rutile lattice the surrounding of the cation has a form of a distorted octahedron, in which two oxygen ions are slightly further than the other four. Some dioxides crystallize with a less symmetric variant of the rutile structure, in which successive pairs of metal atoms in the string of octahedra are alternately closer together and further apart. In some transition metal oxides, such as Mo03, this distortion is assisted by the formation of 1T bonds between oxide ions and the metal cations, resulting in a decrease of cation-cation repulsion and in the appearance of a strong tendency to form a layer structure, as in the case of Mo03 described below [17].
