1.5 Structure of Covalent Ceramics

Most ceramic materials are neither purely covalently or ionically bonded materials. In most ionically bonded materials, there is a significant level of covalency which is decreases as the difference between the electronegativities of cations and anions increases. While covalent bonding is prevalent among the group IV solids such as diamond and many other compound semiconductors, most ceramics such as NaCl, MgO, BaTiO₃, Fe₃O₄ etc are predominantly ionically bonded. Covalent bonding, as we saw in preceding sections, arises from the sharing of orbitals and as a result materials with this type of bonding are characterized by significant hybridization of orbitals and directionality of the bonds which play a crucial role in determining the crystal structure. In contrast, ionically bonded solids are predominantly based on the size difference between the cations and the anions and the formation of structures in them is determined by a set of rules called as Pauling’s Rules which we will see later in this module.
In this section, we will understand the structures of a few covalently bonded materials with emphasis on the Diamond structure.