What are tetrahedral and octahdral voids ? How are their radii related to the radii of the spheres in the close packed arrangement ?

If in a closest packed arrangement r is the radius of the sphere respresenting the tetrahedral void and R is the radius of the spheres in closest packed arrangement then

What are interstitial sites ? Discuss tetrahedral and octahedral sites in close packed arrangement :

Which of the following statement is correct for a closet packed structure? (i) Each tetrahedral void is surrounded by 4 spheres and each sphere is surrounded by 8 tetrahedral voids. (ii) Each octahedral voids is surrounded by 6 spherus and each sphere is surrounded by 6 octahedral voids. (iii) The number of tetrahedral voids in a closest-packed arrangement is twice the number of spheres. (iv) The number of octahedral voids in a closest-packed arrangement is equal to the number of spheres.