Module 1: Structure of Ceramics
  Compounds based on FCC Packing of Ions


       1.7.2 Antifluorite (A2X) and Fluorite (AX2) Structures Antifluorite

  • FCC packing of anions

  • All tetrahedral sites filled by cations

  • Coordination : Anions: 8, Cations: 4

  • Chemical formula: M2X

  • Example: Li2O, Na2O, K2O

  • Radius ratio (rc/ra): 0.225-0.414

  • Examples: r(Li+) : 0.059 nm, r(Na+) : 0.099 nm, r(O-) : 0.14 nm

Figure 1.25 Antifluorite structure
  • Lattice type: FCC

  • Motif – X: 0 0 0, M -

  • Four formula units per unit cell

  • In this structure in many cases, although rc/ra ratio predict an octahedral co-ordination, tetrahedral coordination is preferred to fulfill the stoichiometry requirements. In turn, anions are cubic coordinated by cations (CN: 8)

  • The structure shows corner sharing of tetrahedra. Fluorite Structure (CaF2 Structure)

  • Slightly bigger cations in comparison to other structures

  • Example:UO2, ZrO2, CaF2, CeO2

  • Typical representation of the structure appears as if cations make a FCC lattice and anions occupy the tetrahedral sites.

Figure 1.26 Fluorite Structure
  • While more appropriate Fluorite structure representation is shown below where eight primitive cubic unit cells made by anions are joined together to make a big cube and cations occupy the centers of four of these small cubes in an ordered fashion.

  • Figure 1.27 A more appropriate representation of fluorite structure
  • Co-ordination number: Cations - 8 ;  Anions - 4

  • Lattice: FCC

  • Motif: M – 0 0 0; X – ¼ ¼ ¼; ¾ ¾ ¾  

  • Examples of ionic radii of a few ions:

  • U4+ : 0.1 nm, Zr4+: 0.084 nm, Ce4+: 0.097 nm, O2- : 0.14 nm (observe that cations are quite large as compared to oxygen ions)

  • The structure as you can also see has a large void in the center of unit cell made by cations.

  • These empty spaces make such oxides, good ionic conductors which is useful in applications such as energy storage e.g. batteries.

  • For having some fun with the structure, we can also draw as projection of this material on (110) plane. Here you can see the row of empty octahedral sites along [110]-direction.

Figure 1.28 View of (110) plane of fluorite structure