Silicon nitride

The wide applications of silicon nitride, Si₃N₄, as a ceramic and refractory material and in the form of whiskers  justify its inclusion here. It is a white, chemically inert amorphous powder, which can be formed by or by combining Si and N₂ above 1650 K.

  The two main polymorphs, α- and β-Si₃N₄, possess similar infinite chain lattices in which Si and N are in tetrahedral and approximately trigonal planar environments, respectively. Recently, a denser, harder polymorph, γ-Si₃N₄, has been obtained by high-pressure and -temperature (15 GPa, >2000 K) fabrication. This polymorph has the spinel structure (see Box 12.6): the N atoms form a cubic closepacked structure in which two-thirds of the Si atoms occupy octahedral holes and one-third occupy tetrahedral holes. The oxide spinels  contained metal ions in the ‏2 and ‏3 oxidation states, i.e. (AII)(BIII)₂O₄. In γ-Si₃N₄, all the Si atoms are in a single (‏4) oxidation state. Another new refractory material is Si₂N₂O, made from Si and SiO₂ under N₂/Ar atmosphere at 1700 K; it possesses puckered hexagonal nets of alternating Si and N atoms, the sheets being linked by Si^_O^_Si bonds.