Octahedra sharing faces only

As already noted only the sharing of a pair of opposite faces has to be considered an infinite chain of composition AX3 is then formed. A number of crystalline trihalides consist of infinite molecules of this type, and chain ions of the same kind exist in BaNi03 and CsNiCl3. In the Zrl3 structure the metal atoms occupy one-third of the octahedral holes in a close-packed assembly of halogen atoms to form infinite chains perpendicular to the planes of c.p. halogen atoms  

In BaNi03 there is close packing of Ba + 3 0, and the Ni atoms occupy octahedral holes between 6 O atoms to form an arrangement similar to that of the Zr and I atoms in Zrl3. 

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The isolated Mo06 octahedron is known only in condensed units, where the Mo06 polyhedra share corners, edges and faces in combi-... 

In order to construct an MeX-type compound (X Me = 1) using only octahedral elements, the octahedrons must be linked via their faces i.e. by sharing of three angles. This arrangement of the octahedral polyhedrons yields a rock-salt type structure (NaCl type structure). 

In the RuBr3 type a succession of face-sharing octahedra form a strand in the c direction. The metal atoms in adjacent octahedra are shifted in pairs from the octahedron centers, forming metal-metal bonds (Fig. 16.10, p. 175). This seems to be the condition for the existence of this structure type, i.e. it only occurs with transition metals that have an odd-numbered d electron configuration. 

Further reduction of the nonmetal to metal ratio creates a qualitatively new situation as now not only nonmetal atoms but also metal atoms have to be shared between clusters. Such condensation can occur via corners, edges, or faces of the M( octahedron. In fact, numerous structures of metal rich compounds composed of d metals and p elements can be interpreted in terms of condensed clusters which form characteristic partial structures. A more comprehensive treatise of the concept has been given earlier. [102] At that time, emphasis was laid on a scheme designed to order a large body of related structures. The concept was looked upon as a rather didactic tool. [103] In the years since, the concept... 

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