Class 12 Chemistry The Solid State Locating Octahedral Voids

Locating Octahedral Voids

  • Take a unit cell of ccp or fcc.
  • The body center of the cube, C is not occupied but surrounded by six atoms on face centers.
  • Joining the face centers creates a octahedron.
  • Thus, this unit cell has one octahedral void at the body centre of the cube.
  • There is one octahedral void at the centre of each
    of the 12 edges.
  • It is surrounded by six atoms, three belonging to the same unit cell and three belonging to
    two adjacent unit cells.
  • Each edge of the cube and octahedral void are shared between four adjacent unit cells.
  • Only ¼ of each void belongs to a particular unit cell

In cubic close packed structure octahedral void at the body-center of the cube = 1
12 octahedral voids positioned at each edge and shared between four unit cells =12 x 1/4 =3

Total number of octahedral voids = 4

In ccp structure, each unit cell has 4 atoms. Thus, the number
of octahedral voids is equal to this number.

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