Wed March 06, 2013 By: Shivendra Upadhyay

sir, it is said that hcp type structure ccp type structure & fcc type structure are equivalent . Is this true ? why/why not ?

Expert Reply
Thu March 07, 2013
hcp and ccp (or fcc) arrangements are equivalent in these ways:
1. In hcp and ccp arrangements, a sphere is in direct contact with 6 other spheres in its own layer. The coordination number in both hcp and ccp arrangment is 12.
2. Both hcp and ccp arrangement are equally efficient and occupy 74% of the available volume.
But the major difference between them is:
First layer: A
Second layer: B
hcp arrangement is ABABA....Here the third layer (c) is build by placing spheres over  tetrahedral voids. Here, third layer becomes exactly identical to the first layer. (A).
Whereas, in case of ccp arrangment, the third layer (c) is build by placing spheres over octahedral voids. This arrangement is ABCABCA..type. It is also called fcc arrangement.
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