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how to find formula of a compound $ number of voids filled ? give an example
LOCATING TETRAHEDRAL AND OCTAHEDRAL VOIDS

    The close packed structures have both octahedral and tetrahedral voids. In a ccp structure, there is 1 octahedral void in the centre of the body and 12 octahedral void on the edges. Each one of which is common to four other unit cells. Thus, in cubic close packed structure.

 Octahedral voids in the centre of the cube =1
    Effective number of octahedral voids located at the 12 edge of = 12 × 1/4 = 3
    ? Total number of octahedral voids = 4

 In ccp structure, there are 8 tetrahedral voids. In close packed structure, there are eight spheres in the corners of the unit cell and each sphere is in contact  with three groups giving rise to eight tetrahedral voids.                

 

                  416_tetrahedral.JPG

 

                      436_octahedral.JPG
 

Circles labelled T represent the centers of the tetrahedral interstices in the ccp arrangement of anions.  The unit cell "owns" 8 tetrahedral sites.
 

Circles labelled O represent centers of the octahedral interstices in the ccp arrangement of anions (fcc unit cell).  The cell "owns" 4 octahedral sites.


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