STRUCTURES OF CERAMICS

1.4 STRUCTURES OF CERAMICS
The structure of ceramics can be considered by dividing them into three groups—oxides, silicates, and glasses. Both oxides and silicates can form crystalline and amorphous structures. The crystalline form will be considered separately for oxides and silicates. The amorphous forms of both will be considered under glasses.

1.4.1 Oxide Structures
Oxide ceramics are those in which the nonmetal is oxygen. These ceramics Possess different structures. These structures are discussed in the following text.

1- Rock Salt Structure
In this structure, the large anions are arranged in cubic close packing, and all the octahedral interstitial positions are filled with cations. The structure is shown in Figure 9.1. Oxides having this structure are MgO (Mg2+/O2? = 0.51), CaO, SrO, BaO, CdO, MnO, FeO, CoO, and NiO. For stability, the radius ratio should be between 0.732 and 0.414, and the anion and cation valences should be the same.
Na+ ion has an ionic radius of 0.102 nm (1.02 A?); Cl- has a radius of 0.181 nm (1.81 A?). This large anion to cation size ratio is typical of many common ionic structures. This size difference is large enough so that Na+ cations fit comfortably in all octahedral interstices of the closed Cl- sublattice. The lattice constant for this structure is predicted to be ? = 2(RNa + RCl) = 0.566 nm (5.66 A?).