The heart of crystalline materials

In covalent solids, the number n of adjacent atoms to a given atom is equal to \(8 - N\) where \(N\)

is the number of electrons participating in the bond (or the number of the column in the periodic table that the element belongs to).

For example, in the case of carbon diamond \((\ce{2s^2 2p^2})\) or silicon \((\ce{3s^2 3p^2})\), each atom is surrounded by four adjacent atoms (angle between bonds: 109.5°). This structure belongs to the \(\ce{FCC}\), lattice and is commonly called a diamond cubic structure.

In the case of silica (\(\ce{SiO2}\)) the base unit is a tetrahedron composed of one silicon atom and four oxygen atoms. To ensure electrical neutrality, there must be twice as many oxygen ions (\(\ce{O^{2-}}\)) as silicon ions (\(\ce{Si^{4+}}\)). An oxygen ion must therefore be shared by two tetrahedrons joined at their summit. The figure below presents two allotropic forms of silica (quartz - hexagonal and cristobalite \(\beta\) / - cubic). The stability of these two forms depends on temperature and cooling conditions. Note that if the rate of cooling during solidification is sufficiently high, the tetrahedrons will not have time to arrange themselves and the result is vitreous silica, which is amorphous.

Silicates, with great industrial importance (raw material for ceramic), have structures derived from that of silica. The basic unit remains the tetrahedron \(\ce{(SiO4)^{4-}}\), linked by metal ions that give up their electrons to establish bonds.