Difference Between HCP and CCP

Key Difference – HCP vs CCP
 

The term “close packed structure” is used regarding the lattices or crystal systems. It describes crystal systems having tightly packed atoms. In crystal systems, an atom is known as a “sphere”. That is because an atom is considered as a spherical structure for the ease of describing a crystal system. Close packing of equal spheres will form a dense crystal system with minimum empty spaces or holes between these spheres. There are several types of holes that can exist in between spheres. A hole exists between three equal spheres is known as a trigonal hole because it appears as a triangle. There are several layers of spheres present on top of one layer. If the second layer is placed in such a way that a triangular hole is covered by this second layer spheres, it creates a tetrahedral hole. But if the second layer is placed uncovering the trigonal hole, then it creates an octahedral hole. There are few types of close packed crystal structures such as HCP (Hexagonal closest packed) and CCP (Cubic closest packed). The key difference between HCP and CCP is that the repeating structure of HCP has 2 layers of spheres whereas the repeating structure of CCP has 3 layers of spheres.

CONTENTS

1. Overview and Key Difference
2. What is HCP
3. What is CCP
4. Similarities Between HCP and CCP
5. Side by Side Comparison – HCP vs CCP in Tabular Form
6. Summary

What is HCP?

The term HCP stands for hexagonal closest packed crystal systems. In hexagonal closest packed crystal systems, the third layer of spheres has the same arrangement of spheres as in the first layer. Then the spheres of the second layer cover the tetrahedral holes of the first layer and the third layer.

The hexagonal closest packed crystal system has around 74% of its volume occupied by spheres or atoms whereas 26% of the volume is occupied by empty spaces. One atom or sphere in the HCP structure is surrounded by 12 neighbouring spheres. The HCP crystal system has 6 members (atoms or spheres) per unit cell.

What is CCP?

The term CCP stands for cubic closest packed crystal systems. Here the second layer of spheres is placed on half of the depressions of the first layer. The third layer is completely different from that of the first two layers. The third layer is stacked inside the depressions of the second layer. Therefore, this packing covers all the octahedral holes since the layers are not packed in line with each other. However, the fourth layer is similar to that of the first layer and hence, the structure repeats.

The cubic closest packed crystal system has around 74% of its volume occupied by spheres or atoms whereas 26% of the volume is occupied by empty spaces. One atom or sphere in the CCP structure is surrounded by 12 neighbouring spheres as same as in HCP. The CCP crystal system has 4 members (atoms or spheres) per unit cell.

What are the Similarities Between HCP and CCP?

• Both HCP and CCP has spheres with 12 neighbouring spheres.
• Both HCP and CCP crystal systems have around 74% of its volume occupied by spheres or atoms whereas 26% of the volume is occupied by empty spaces.

What is the Difference Between HCP and CCP?

Summary – HCP vs CCP

HCP and CCP are two forms of crystal structures. The difference between HCP and CCP is that, In HCP crystal systems, the third layer of spheres has the same arrangement of spheres as in the first layer; hence, spheres of the second layer cover the tetrahedral holes of first layer and the third layer whereas in CCP crystal systems, the second layer of spheres is placed on to half of the depressions of the first layer, and the third layer is completely different from that of the first two layers; the third layer is stacked in the depressions of the second layer.

Reference:

1.“Closest Packed Structures.” Chemistry LibreTexts, Libretexts, 21 Feb. 2018. Available here  
2.“Hexagonal Close Packing.” From Wolfram MathWorld. Available here
3.“Close-Packing of equal spheres.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here 

Image Courtesy:

1.’Hexagonal close-packed lattice 1′ (CC BY-SA 3.0) via Commons Wikimedia  
2.’FCC closed packing tetrahedron (4)’By TraceyR – Own work, (CC BY-SA 3.0) via Commons Wikimedia