Octahedral voids

There are two ways to further reduce the X Me ratio to a value lower than 3. The first is to arrange simple similar to coordination type structure that requests steric similarity of all cations so as to be able to occupy octahedral voids. This method will be discussed separately. The second way is to arrange the linking of the octahedrons so as to render additional structural elements, namely pentagonal "blocks". 

KNbF6 complexes TaF6 or NbF6 LiSbF6 type structure. All cations are located in octahedral voids. 

The type of crystal structure depends on the ratio X Me, where X is the total number of anions (oxygen and fluorine) and Me is the total number of all cations that can fit into/occupy octahedral voids (tantalum, niobium, lithium and other metals with similar ionic radii). 

X Me Second cation present, with coordination number greater than 6 (second cation s ionic radius > tantalum/niobium ionic radius) Only cations that can fit into/occupy octahedral voids are present (second cation s ionic radius tantalum/niobium ionic radius)... 

Figure 3. Schematic drawing of the crystal structure of f-MnOj. The manganese atoms are randomly distributed in the octahedral voids of the hexagonal dose packing of oxygen atoms (adapted from [47]).

The manganese atoms are distributed in a more or less ordered manner at the slightly distorted octahedral voids in the hexagonally close-packed oxygen atoms (as described above). 

Phase Trigonal prismatic void in 2c Metal host lattice atoms in 12k -f- 6h(l) -1- 6h(2) Icosahedral center in 2a Octahedral void in 6g ... 

Phase Octahedral void I in 16c Octahedral void 11 in 8a Icosahedral center in 16d Metal host lattice atoms in 48f + 32e... 

Large cell Base of the triple cell. The z coordinates of the spheres refer to c = c. The dots labeled , (2) and mark six octahedral voids atz = 0 andZ= ... 

Group-subgroup relations from hexagonal closest-packing of spheres to some MX3 and M2X3 structures. The boxes represent octahedral voids, with the coordinates as given at the top left. The positions of the octahedron centers are labeled by their Wyckoff letters. Gray boxes refer to occupied voids. The dots indicate how the atoms Ru, P and N are shifted from the octahedron centers parallel to c... 

Space group position of the spheres centers of the octahedral voids centers of the tetrahedral voids c/a... 

Many papers on the Ni, S system have been published. Figure 1.34 shows the phase diagram of the Ni-S system determined by Kullerud and Yund the region denoted aNij. S is discussed here. The Nij. S phase has the NiAs-type structure, in which cations occupy all of the octahedral voids formed by hexagonal close packed anions. 

The Hf5CuSri3 structure (Rieger et al., 1965) is a filled version of the binary MnsSis type (Aronsson, 1960). It is also possible, that the X component fills the octahedral void. This has first been observed for TisGa4 (Schubert et al., 1962 Potzschke and Schubert, 1962) and often, the ternary filled versions are referred to this structure type in the literature. 

The octahedral voids in the MnsSi3 type structure can be filled not only by a late transition metal. As an example Guloy and Corbett (1994) have tested this possibility for LasPb3. The voids could be filled with P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, and Ag atoms and also partially with boron or carbon. Filling of the voids leads to a small increase of the lattice parameters. On the other hand, lead can also act as the interstitial element on the octahedral site. The structural relationships of various M5X4 structures have recently been discussed by Guloy and Corbett (2005). 

The famous Heusler type structure can be considered as a ternary ordered variant of the BiF3 type. On the other hand, the YPd2Pb structure can also be derived from a cubic closest packing of the lead atoms, where the octahedral voids are filled with yttrium (rocksalt substructure) and the tetrahedral voids are filled by the palladium atoms. 

The diameter of the octahedral void is 0.4142 a0 and that of the tetrahedral void 0.2247 a0, where a0 is the distance of nearest approach of atoms in the unit cell. With a0 = 3.227 A. for the hexagonal unit cell of a-Zi, the diameter of the bonded atom which can be accommodated in the octahedral void would be 1.336 A. and in the tetrahedral void 0.725 A. The diameters of the bonded deuterium atoms as derived from the nearest metal-metal and metal-deuterium atom distances in the crystal structures of each of the deuteride phases on the other hand are as given in Table VI. 

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