Silicides

  The structures of the metal silicides (prepared by direct combination of the elements at high temperatures) are diverse, and a full discussion of the structures is beyond the scope of this book.† Some examples of their solid state structural types are:

• isolated Si atoms (e.g. Mg₂Si, Ca₂Si);

•  Si₂-units (e.g. U₃Si₂);

• Si₄-units (e.g. NaSi, KSi, CsSi)

• Sin-chains (e.g. CaSi);
• planar or puckered hexagonal networks of Si atoms (e.g. b-USi₂, CaSi₂);
• three-dimensional network of Si atoms (e.g. SrSi₂, a-USi₂).

  The Si₄-units present in the alkali metal silicides are noteworthy. The [Si₄]^4- anion is isoelectronic with P₄ and the solid state structures of several group 1 metal silicides contain tetrahedral Si₄-units, but these are not isolated anions. The structure of Cs₄Si₄ comes close to featuring discrete, tetrahedral [Si₄]^4- ions, but significant cation–anion interactions exist. The silicide K₃LiSi₄ possesses tetrahedral Si₄-units linked by Li‏ ions to give infinite chains, and in K₇LiSi₈, pairs of Si₄-units are connected as shown in structure 1.1 with additional interactions involving K‏ ions.

      (1.1)

     Silicides are hard materials, but their melting points are generally lower than those of the metal carbides. Treatment of Mg₂Si with dilute acids gives mixtures of silanes. The properties of some silicides make them  useful as refractory materials (e.g. Fe₃Si and CrSi₂); Fe₃Si is used in magnetic tapes and disks to increase their thermal stability.