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LOCATION OF VOIDS IN THE CRYSTAL:-

In a closed packed structure [ccp or hcp] if there are n spheres (atoms or ions) in the packing then
No. of octahedral voids = n
No. of tetrahedral voids = 2n

Example:

In cubic close packing (ccp)
No. of atoms = 4
So No. of octahedral voids = 4
No. of tetrahedral voids = 8

a. Octahedral Voids:-

One octahedral void is present at the body centre of cube and 12 octahedral voids are present on the centres of the 12 edges of the cube. But void on each edge is shared by 4 unit cells so its contribution is = 1/4

b. Tetrahedral Voids:-

There are & tetrahedral voids are present in the ccp structure at different location thus:-
In ccp total no. of voids per unit cell = 8 + 4 = 12
In hcp total no. of voids per unit cell = 12 +6 = 18

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