Crystalline Ceramics

CRYSTALLINE CERAMICS

These have simple crystal structure, such as aluminium oxide (corundum), magnesium oxide, silicon carbide. Most of the oxides can be considered packing of oxygen ions with the cations occupying the tetrahedral and / or octahedral sites in the structure. 

Magnesium oxide is used in refractory furnace lining for steel making. Silicon carbide is used for cutting tools.

The crystal structure of ceramic is, more complex, since atom of different size and electronic configuration are assembled together.

Common crystal structures found in crystalline ceramics particularly those of the oxide type are briefly described below:

Cesium Chloride Structure

It is possible for ceramic compounds to have simple cubic structure that are not found among metals. Cesium chloride is a prototype for this case.

In this structure, chlorine ions are arranged in a simple cubic structure and all interstices are occupied by cesium ions. The co-ordination number is eight (Fig.4.14).

Rock Salt Structure

Most of the oxides and halides crystallize in the closed packed cubic structure similar to that of a rock salt (sodium chloride). The structure can be considered as consisting of the FCC anions with smaller cations filling all available interstitial positions.

Here, each metal atom is surrounded by six non-metallic atoms and vice versa (Fig. 4.15). Thus, atomic coordination (CN) is 6. Other examples are MgO, CaO, BaO, CdO, MnO, FeO, CeO and NiO.

Zinc Blende Structure

Two of the more cubic ceramic compounds which have atoms in the 4-fold sites, are zinc blende (ZnS), silicon carbide (B-SiC). The atomic coordination is 4. (Fig. 4.16)

• Each type of atom form an FCC structure of its own.

• Only half of the available tetrahedral interstices are filled with the small cations.

• The structure is the same as the diamond cubic except that alternate atoms are of different elements.

This structure also includes cadmium sulphide (CdS) and aluminium phosphide (AIP).

Perovskite Crystal Structure

It is also possible for ceramic compounds to have more than one type of cation. For example Barium titanate (BaTiO₃), having both Ba²⁺ and Ti⁴⁺ cations, falls into this classification.

This material has a perovskite crystal structure. A unit cell of this structure is shown in fig. 4.17. Ba²⁺ ions are situated at all eight corners of the cube and a single Ti ⁴⁺is at the cube center. The 0^2- ions located at the center of each of the six faces of the unit cell.