1.7.4 Spinel Structure

• Formulae – (A²⁺)(B³⁺)₂O₄ or AB₂O₄ or AO.B₂O₃

• FCC Packing of anions

• Partial occupancy of both tetrahedral and octahedral sites i.e.1/8^th of tetrahedral and ½ of the octahedral sites are occupied.

• A spinel unit-cell is made up of eight FCC cells made by oxygen ions in the configuration 2×2×2, so it is a big structure consisting of 32 oxygen atoms, 8 A atoms and 16 B atoms.

• Depending on how cations occupy different interstices, spinel structure can be Normal or Inverse.

  
  

       1.7.4.1 Normal Spinel

• Chemical formula: (A²⁺)(B³⁺)O₄

• Examples are many aluminates such as MgAl₂O₄, FeAl₂O₄, CoAl₂O₄ and a few ferrites such as ZnFe₂O₄ and CdFe₂O₄.

• In this structure, all the A²⁺ ions occupy the tetrahedral sites and all the B³⁺ ions occupy the octahedral sites.

• Apply bond strength rule to verify the stoichiometry

• Cations: - A²⁺ - ²⁄₄ ; B³⁺ - ³⁄₆
  Oxygen valence = (²⁄₄x1)+ (³⁄₆x3) = 2
  

Figure 1.30 Schematic of spinel structure

       1.7.4.2 Inverse Spinel   B(AB)O₄

• Chemical formula: (A²⁺)(B³⁺)₂O₄ but can be more conveniently written as B(AB)O₄.

• Most ferrite follow this structure such as Fe₃O₄ (or FeO.Fe₂O₃), NiFe₂O₄, CoFe₂O₄ etc.

• In this structure, ½ of the B³⁺ ions occupy the tetrahedral sites and remaining ½ B³⁺ and all A²⁺ ions occupy the octahedral sites (now you can hopefully make sense of the formula in the previous line).