Explain the Hexagonal Close Packing $(hcp)$ and Cubic Close Packing $(ccp)$ or Face-Centered Cubic $(fcc)$ structures.

(A) When the third layer is placed over the second layer,there are two possibilities:
- $(i)$ Covering Tetrahedral Voids: When the third layer spheres cover the tetrahedral voids of the second layer,the spheres of the third layer align exactly with those of the first layer. This pattern of spheres repeats alternately and is written as the $ABAB$ pattern. This structure is called the Hexagonal Close Packed $(hcp)$ structure. Example: $Zn$ and $Mg$ metals exhibit this structure.
- $(ii)$ Covering Octahedral Voids: When the third layer spheres cover the octahedral voids of the second layer,the spheres of the third layer do not align with those of the first or second layer. This arrangement is called type $C$.
- When the fourth layer is placed,the spheres align exactly with the first layer. This pattern of layers is written as $ABCABC...$.
- This is called the Cubic Close Packed $(ccp)$ or Face-Centered Cubic $(fcc)$ structure. Example: Metals like $Cu$ and $Ag$ crystallize in this structure.
- Both types of close packing are highly efficient and occupy $74 \%$ of the crystal space. Here,each sphere is in contact with $12$ other spheres. Thus,the coordination number in both structures is $12$.