what r the differences between tetrahedral and octahedral void?

tetrahedral void occupies 2N no. of total voids


Octahedral voids occupy N no. of total voids

tetrahedral void is formed by the 4 sphere 

octahedral void is formed by the 6 sphere

Ok, we know tetrahedron is a sturcture formed by four units in which three units represent base and one to the top. So remember that tetahedral voids are formed when such a situation arises. Octahedral voids arise when two triangles are placed with their apex faced opposite sides.

So memory tip:

Place three balls (this is first layer). Here apex faces north (lets say)

I place a ball from the second layer (which shall be in top of first) on the void which is created by the three balls in first layer, oh well boy! you have a tetrahedral void. Accordingly all the rest units from the second layer will arrange itself (you can just draw after making this first tetrahedral)

If in the second layer placing, I place three balls such that apex of second layer balls (3 in no.) faces opposite (in our case it faces south) such that center of the triangle from layer I and II lie on same axis (just place on exact top) we get a void in between called as OCTAHEDRAL void.

Additional Information which may be helpful:

Let the number of close packed spheres be N, then:

The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N

A void is defined as an empty or unoccupied space that is present between the atoms or ions in a crystal lattice. Octahedral voids are those which are created by six spheres in contact (or are surrounded by six spheres). Tetrahedral voids are those which are created by four spheres in contact ( or are surrounded by four spheres).

As that the space x is surrounded by six spheres (which are also in contact with each other), hence it is an octahedral void. Similarly void y is surrounded by 4 spheres (which are also in contact with each other), therefore it is a tetrahedral void.