What is the Difference Between Hexagonal Close Packing and Cubic Close Packing?

The main difference between hexagonal close packing (HCP) and cubic close packing (CCP) lies in the arrangement of atoms in their respective unit cells. Both HCP and CCP are efficient ways to pack atoms in a lattice structure, but they have distinct features:

• Hexagonal Close Packing (HCP):
• Contains two layers of spheres placed one on the other, forming tetrahedral and octahedral holes.
• Has a coordination number of 12 and contains 6 atoms per unit cell.
• The hexagonal unit cell has one large octahedral hole per sphere surrounded by six smaller octahedral holes and two tetrahedral holes for each sphere.
• The third layer is placed in such a way that the atoms in the third layer lie directly above the atoms in the first layer.
• Cubic Close Packing (CCP):
• Contains three layers of spheres placed one on the other, covering all the octahedral holes by a third layer of spheres.
• Has a coordination number of 12 and contains 4 atoms per unit cell.
• The cubic unit cell has tetrahedral holes but no octahedral holes.
• The third layer is placed in a way that the atoms in the third layer are not in line with those in the first layer, and the structure does not repeat until a fourth layer is added.

In summary, the main difference between HCP and CCP is the arrangement of atoms in the third layer and the presence of octahedral holes in HCP, while CCP has no octahedral holes in its unit cell and a different stacking pattern in the second and third layers.

Comparative Table: Hexagonal Close Packing vs Cubic Close Packing

The main differences between hexagonal close packing (hcp) and cubic close packing (ccp) are the arrangement of atoms in the layers and the coordination number. Here is a table comparing the two structures:

Both hexagonal and cubic close packing structures have a coordination number of 12 and efficiently fill up 74% of space. However, the arrangement of atoms in the layers and the number of atoms per unit cell differ between the two structures.