Alkali silicides

Table 1. Conditions of Formations of Silicon and Germanium Clathrates by Thermal Decomposition of Alkali Silicides and Germanides MSi and MGe (From Ref. 6) ...

Procedures for the preparation of silicides are also found in other sections of this book (see Alkali Silicides p. 989 f., magnesium silicide p. 921 f., calcium silicide p. 946 f., silicides of Ti, Zr and Th. p. 1249 f.). 

Alkali silicide or germanide Experimental conditions Struct. type X Lattice const. a (nm) Remarks... 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

Tungsten also forms hard, crystalline refractory borides, such as WB2, W2B and WB when heated with boron in an electric furnace. Tungsten also forms a group of silicides, hard refractory compounds of compositions WSi2, WSis and W2Si3. These silicides are attacked by hydrofluoric-nitric acid mixture or by fused alkalies. 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

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. 

Ruthenium Silicide, RuSi, results on heating a mixture of finely divided ruthenium and crystallised silicon in the electric furnace.3 The product is crushed and treated successively with alkali and a mixture of hydrofluoric and nitric acids. The silicide together with carborundum remains behind. The two may be separated with methylene iodide on account of the high density of ruthenium silicide, namely, 5-4. 

While interactions of alkali metals or magnesium with elemental silicon and germanium (or their oxides) can give various silicides or germanides, most of these products do not contain true anions and some are semiconductors. However, in Li12Si7 there are Si5 rings and trigonal planar Si-centered Si, units in the lattice. 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

Iron silicide or Iron monosilidde, FeSi, is prepared by heating a mixture of copper silicide and iron filings in an electric furnace. The resulting product is treated with 50 per cent, nitric acid to decompose any copper silicide, and washed. Obtained in this way, iron silicide occurs as tetrahedral crystals, with a brilliant metallic lustre they are extremely hard, and have a density of 6-17 at 15° C. Fluorine attacks them at ordinary temperatures, whilst chlorine and bromine decompose them at red heat. Molten alkali hydroxides attack the silicide, as also do fused mixtures of the alkali nitrates and carbonates.11... 

The silicides of the alkali and alkaline-earth metals are prepared by melting the metal with Si. To obtain pure silicides, the synthesis must be carried out in an inert atmosphere. The reactions proceed at high T, e.g., ca. 600°C and above. Because silicides react slowly with carbon and oxides, syntheses are carried out in graphite or refractory oxide crucibles. 

Molybdenum Sesquisilicide, MOjSi, is said to result when crv stallised silicon is heated in a carbon crucible in the electric furnace with the oxides of molybdenum obtained by calcining ammonium molybdate. The mass is disintegrated by electrolysis in hydrochloric acid, and the residue successively treated with aqua regia, caustic alkali, and hydrofluoric acid. The silicide burns in chlorine at 800° C., with formation of silicon tetrachloride and molybdenum pentachioride. 

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