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Question

What is the difference between ccp and hcp?


Solution

1. Stacking Sequence of Layers
-HCP layers cycle between two equivalent shifted layers in an ABABAB sequence
-CCP layers cycle among three different layers in an ABCABC sequence
2. Shape of the Unit Cells
- HCP structures have hexagonal unit cells characterized by a diamond shaped or hexagonal base with sides of equal length
-CCP structures have cubic unit cells
3. Volume of Unit Cells
-HCP unit cells have a volume which can be calculated with the equation V=24*sqrt(2)*r^3 in which r = the atomic radius
-CCP unit cells have a volume which can be calculated with the equation V=8*sqrt(8)*r^3 in which r = the atomic radius
4. Number of Atoms per Unit Cell
-HCP unit cells contain 6 atoms
-CCP unit cells contain 4 atoms
 

In hcp(hexagonal closed packing), the third layer of spheres is same as the first layer and covers tetrahedral voids.Therefore, the stacking may be represented as a-b-a-b-a-b type.

Whereas, in ccp(cubic closed packing), the fourth layer of spheres is same as the first layer and covers octahedral voids.Therefore, the stacking may be represented as a-b-c-a-b-c type.









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