Materials in Today's World

Hexagonal Close Packed Crystal Structure (HCP)


If you look at the figure below, you might think that hexagon close-packed crystal structure is more complicated than face-centered cubic crystal structure. In fact, it is a simpler structure.

Think back to the last section where we constructed first one layer of atoms and then a second layer of atoms for face-centered cubic structure. Now, for hexagonal close-packed crystal structure, we do not construct a third layer. Instead, the third layer is simply the first layer repeated, the fourth layer is the second layer repeated, and so on and so on as shown in the figure below.

ABABABA Structure
Hexagonal close-packed structure
Credit: Callister & Rethwisch 5e

It turns out that face-centered cubic and hexagonal close-packed crystal structures pack atoms equally tightly. Some metals with hexagonal close-packed crystal structures include cobalt, cadmium, zinc, and the α phase of titanium. A more typical representation of the hexagonal close-packed structure is shown in the figure below. In this representation a hexagon on the top and on the bottom sandwich a triangle in between the two hexagons.

Hexagons sandwiching a triangle
Hexagonal close-packed structure unit cell
Credit: Callister & Rethwisch 5e

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