Module 1: Structure of Ceramics
  Structures based on HCP Packing of Ions
 

      
      1.10.2 Corundum (Al2O3) Structured Compounds

  • M2X3 type of  compounds

  • - Alumina or Sapphire (Al2O3) is the parent compound.

    • Other examples are compounds like Cr2O3, Fe2O3

  • Anions form an HCP lattice

  • Two-third of octahedral voids are occupied by the cations to maintain the stoichiometry.

  • Coordination numbers: M: 6, X: 4.

  • This arrangement preserves the charge neutrality as you can also verify using bond strength formula.

  • This can be best viewed when we look at the basal plane of (0001)-plane of the unit-cell and start filling the interstices.

Figure 1.40 Layered filling of Corundum

  • One unit-cell consists of six layers of oxygen ions.

  • A side view of the structure on  plane can be seen below where you can see columns of cations along the c-axis with 2⁄3 rd filling of octahedral sites.

Figure 1.41 View of {1010} plane of Corundum