The key difference between FCC and HCP is that FCC structure cycles among three layers whereas HCP structure cycles among two layers.
FCC is a face-centred cubic close-packed structure while HCP is a hexagonal close-packed structure. We talk about these structures under the field of crystal lattices.
CONTENTS
1. Overview and Key Difference
2. What is FCC
3. What is HCP
4. Side by Side Comparison – FCC vs HCP in Tabular Form
5. Summary
What is FCC?
FCC is a face-centred cubic close packing structure of lattices. It is a space-efficient composition of crystal structures. The coordination number of this structure is 12, while the number of atoms per unit cell is 4. Here, the coordination number is the number of atoms the unit cell touches. Importantly, this structure efficiently occupies 74% of the space; thus, the empty space is 26%.
Figure 01: FCC Structure
In this structure, the unit cell contains atoms at the centre of the faces of the unit cell, and thus, it leads to naming it as face-centred. Further, in a cubic close packing structure, FCC is the simplest repeating structure. FCC layers cycle among three layers. It has three types of planes which are different from each other. Some examples for metals having this structure include aluminium, copper, gold, lead, silver, platinum, etc.
What is HCP?
HCP is a hexagonal close packing structure of lattices. It is also a space-efficient composition of crystal structures. The coordination number of this structure is 2, while the number of atoms per unit cell is 6. The structure occupies 74% of total space; thus, the empty space is 26%. Here, HCP layers cycle among two layers. That means; the third layer of the structure is similar to the first layer.
Figure 02: HCP Crystal Structure
We can describe this stacking as “a-b-a-b-a-b”. Some metals having hexagonal close-packed crystal structures include cobalt, cadmium, zinc, and α phase of titanium.
What is the Difference Between FCC and HCP?
FCC is a face-centred cubic close packing structure of lattices while HCP is hexagonal close packing structure of lattices. The key difference between FCC and HCP is that FCC structure cycles among three layers whereas HCP structure cycles among two layers.
Moreover, one other difference between FCC and HCP is that in FCC, the third layer is different from the first layer while in HCP, the third layer is similar to the first layer. Aluminium, copper, gold, lead, silver, platinum, etc. are some examples for FCC while examples for HCP include cobalt, cadmium, zinc, and the α phase of titanium.
Summary – FCC vs HCP
In summary, FCC is a face-centred cubic close packing structure of lattices and HCP is hexagonal close packing structure of lattices. The key difference between FCC and HCP is that FCC structure cycle among three layers whereas HCP structure cycle among two layers.
Image Courtesy:
1. “FCC crystal structure” By User:ARTE – Own work (Public Domain) via Commons Wikimedia
2. “HCP crystal structure” By User:ARTE – Own work (Public Domain) via Commons Wikimedia