Trigonal prismatic holes

The most important sulfides are MoSg and WSg, which possess a layered structure. The metal atoms occupy trigonal-prismatic holes in the plane between the hex-agonally arranged sulfur layers, as shown in Fig. 5.12. The layers are shifted with... 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

Omloo and Jellinek7 have described the synthesis and characterization of intercalation compounds of alkali metals with the group V layered transition metal dichalcogenides. Typically, these types of intercalation complexes are sensitive to moisture and must be handled in dry argon or nitrogen atmospheres. The alkali metal atoms occupy either octahedral or trigonal prismatic holes between X-M—X slabs. There are two principal means by which these compounds may be prepared. 

The O -orthorhombic phase has the layer structure of V2O5 itself with the alkali metals inserted in between the layers. For Na+, the substitution of what appears to be an inappropriately large cation into a small trigonal prismatic hole is quite... 

Fig. 9. (a) Arrangement of trigonal prisms in a close-packed structure showing a boron atom in one trigonal prismatic hole and the locations of the next nearest neighbor holes p, q, and r. (b) A three dimensional representation of the boron atom environment in (a), (c) A representation of the structure of the boride MB2, where all holes are filled and the boron atoms form a hexagonal net within the close-packed metal lattice. 

IrB i.35 has a crystal structure that may be described in terms of puckered layers of B and puckered double layers of Ir which also contain B atoms in trigonal-prismatic holes. There is no three-dimensional B network.331... 

The CrB and FeB structures and the CrB-FeB stacking variants belong to the large group of structures where all R atoms participate in the formation of centred trigonal prisms and where all trigonal prismatic holes are occupied by T and/or M atoms. The known ternary structures of this group are listed in table 3. 

Systematic studies of the structural properties of TaS2 by Jellinek10 revealed the presence of several polymorphic forms of the compound as a function of temperature. Within a layer, the tantalum atom sits in the holes formed between two layers of sulfur atoms in the S—Ta—S sandwich. The coordination of the tantalum is trigonal prismatic or octahedral, depending on whether the two sulfur layers lie one on top of the other or are rotated by 60°. Thus the various poly-... 

The unique TI17S12 structure (Fig. 22) is built up of units of three infinite prisms. In addition to the trigonal-prismatic bonds only two bonds in the plane of the cation can form. These two coplanar bonds are responsible for shifting all these triple prism columns in the same sense. The comers of the trigonal units create chains of octahedra. If these octahedral holes... 

If these compounds crystallized in their antistructure, which is a 2H—MoSa structure with all octahedral holes filled, semiconductivity would be feasible by assuming the cation in trigonal-prismatic coordination to be divalent and the one in octahedral coordination to be tetra-valent. 

Numerous borocarbides have been prepared, for example, those of A1 already described and compounds such as Mo2BC, in which C is octahedrally coordinated by Mo and B has trigonal prismatic coordination and forms chains as in CrB. Of special interest are the borocarbides MB2C2 formed by Sc and the 4f elements. The metal atoms are situated between plane 3-connected layers which in the 4f compounds are the 4 8 net (with alternate B and C atoms) but in SCB2C2 are the 5 7 net, necessarily with pairs of adjacent B and C atoms. The Sc atoms are in pentagonal prism holes between the layers, but there are also Sc—Sc distances very nearly the same as in h.c.p. Sc metal, so that Sc has (14 + 5)-coordination (C-C, 1-45, B-B, 1-59, B-C, 1-52-1-61 A). ... 

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