Silicides structural units

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

Gold is an example of a metal that does not form a silicide, and one may therefore expect the Au/Si interface to be abrupt. The structures of Au on Si (111) are interesting in that the unit cell is much smaller than that of the well known (7 X 7)... 

The structure of type I hydrates is repeated in melanophlogite 61), and that of hydrates of types I and II is repeated in clathrate silicides or germa-nides of Na, K, Rb, or Cs 59, 60). The alkali metal atoms are enclosed in cages of Si or Ge atoms and are thereby protected from attack by atmospheric oxygen. The limiting composition is 8G 46Si or 24G-136Si, or the same with Si replaced by Ge however, neither of these compositions is reached since some voids do not contain an alkali metal atom. The unit cells vary as follows with the bond distances between pairs of vertices in the polyhedra. 

Silicon is usually found in oxides and silicates. With very electropositive metals it forms silicides. The compound CaSi2 can be viewed as containing Ca2+ and Si ions. The crystal structure is trigonal, Dgft, R3m, a = 3.8259, and c = 15.904 A with three molecules in the unit cell. The Si ion has the same valence shell as As, and layers of... 

Slightly more reduced but smaller in Mg content is a new ternary strontium silicide with the composition Sr,Mg2Si,o [31] and a mean formal charge of q = -2.6 on silicon. There are two isolated Si anions per formula unit while the other eight silicon atoms form a unit of a linear chain Sij , which is again completely planar. The structure is displayed in Fig. 5b. 

The structural properties of RuSia have been calculated for three crystalline phases high-temperature tetragonal P4lmmm) a-phase, orthorhombic (Cmcd) 3-phase, and metastable pseudomorphic cubic (Fm3m) y-phase with 3, 24 and 3 atoms in the unit cell, respectively [9]. Table 1 summarizes the optimized lattice constants of the mthenium silicides. 

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