Question

Position of octahedral voids in fcc structure is/are the:
This question has multiple correct options
A. Edge-centres of unit cell
B. Body-centre of unit cell
C. Corners of unit cell
D. Face centered unit cell

Answer 1

Octahedral voids are formed when the triangular voids of the first layer coincides with the triangular voids of the layer above as a result void is formed by enclosing six spheres. In FCC, atoms are present in corners and the center of all faces present in the cube.

Complete step by step answer:
The unit cell is the smallest repeating unit in the crystal lattice. The unit cell is the building block of the crystal.
Face centered cubic cell is a type of unit cell where the atoms are present at the corners of the crystal lattice and at the centre of all the faces of the cube. The atoms present at the face centered are shared by two unit cells present adjacent to each other and half of each atom belongs to each cell. Only 12 atoms belong to a unit cell.
The voids are the gap present in the solid crystal between the constituent particles in the closed packed structure. The close packing is done in 1D close packing, 2D close packing and 3D close packing.
In the 2D close packing, the atoms are arranged in square close packing and hexagonal close packing where we can see empty gaps between the atoms. These gaps are known as voids which are present in triangular shapes which are known as triangular voids.
There are two types of interstitial voids known as Tetrahedral void and Octahedral void. Octahedral voids are formed when the triangular voids of the first layer coincides with the triangular voids of the layer above as a result void is formed by enclosing six spheres. This void is known as an octahedral void.
In FCC face centered cubic cells, one octahedral void is present as body centre, and another octahedral void is present at the centre of each of the 12 edges surrounded by 6 atoms, four atoms belong to the same unit cell and two atoms are shared by adjacent cells.
Thus, the position of octahedral voids in fcc structure are the edge-centres of unit cell and body-centre of unit cell.

Therefore, the correct option is A and B.

Note: The number of octahedral voids present in a crystal is equal to the number of closed packed particles and the tetrahedral void present in the crystal are double of octahedral voids.