1.7.1 Rocksalt Structure

• MX Type Compounds Based on NaCl or Rocksalt Structure

• Anions (X) form the cation sub lattice with FCC structure.

• Cations (M) fill the octahedral sites.

• 100% occupancy of sites according to the stoichiometry since there will be one octahedral site per anion.

• Radius ratio, r_c/r_a is typically between 0.414 - 0.732 with some exceptions.

• Examples of ceramic materials with such structure as NaCl, MgO, NiO, FeO etc.

Figure 1.23 Schematic of structure of Rocksalt structured compounds

• Lattice type: FCC and motif will be M at 0 0 0 and X at ½ 0 0

• Four formula units per unit cell.

• Some of the radius values of cations for selected rocksalt structured compounds are given below:

• We can verify the bond-strength of ions in NaCl
  Bond Strength of cations i.e. Na⁺ =
  Valence of anions
  which is the valence of chlorine and hence, proposed packing is appropriate.

• The octahedrons shares at the edges. If there was corner sharing of polyhedra, co-ordination number will be 2 for anions, which won’t maintain the stoichiomentry as you can verify using bond-strength relationship.

• Schematic representation of atomic arrangement on (110) plane of rock-salt structure compounds shows the empty rows of tetrahedral voids along [001]-direction.

Figure 1.24 (110) plane of a Rocksalt structured compound

• You can see that if both octahedral and tetrahedral voids were filled, this would bring cations quite close to each other, leading to large electrostatic repulsion as like neighbours do not make an energetically preferred configuration.

• Another way of looking at this structure is to visualize hexagonal arrays of cations and anions stacked along [111]-direction repeating in an alternative fashion.

• The complete structure can be viewed as two FCC lattices, one of Na and another of Cl, interpenetrating into each other.