Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 42

Materials such as diamond and silicon carbide, which have very strong, directional, covalent bonds, can also be described as cp structures or eutactic structures; many have the same structures as ionic compounds. Thus, one polymorph of SiC has the wurtzite structure and it is immaterial whether Si or C is regarded as the packing atom since the net result, a 3D framework of corner‐sharing tetrahedra, is the same. Diamond can be regarded as a sphalerite structure in which half of the C atoms form a ccp array and the other half occupy T₊ sites, but again the two types of atom are equivalent. Classification of diamond as a eutactic structure is useful since in diamond all atoms are of the same size and it is unrealistic to distinguish between packing atoms and interstitial atoms.

Many structures have mixed ionic–covalent bonding, e.g. ZnS and CrCl₃; one advantage of describing them in terms of cp is that this can be done, if necessary, without reference to the type of bonding that is present.