Hexagonal Close Packed to obtain a three dimensional structure, more layers of spheres must be stacked over the base layer. Hexagonal close packing to understand the staking of spheres over the base layer, the sphere and the depressions (also called voids) in the base layer are labelled as follows:
- Formation of Second layer: To obtain a closet packed structure maintaining hexagonal close packed pattern, the spheres must be placed in the depressions of the base (first) layer. Now, if a sphere is placed at position B, then the position c cannot be utilized for accommodating another sphere. Thus, either all B locations or all C locations can be occupied by the spheres of the second layer.
- Formation of Third Layer: After the second layer is formed by placing spheres in the B voids, there are two different types of voids in the second layer.
- Void just above the void C in the first layer, and
- Void just above the particles A in the first layer.
So, now if third layer is generated using the void above the void C, then a sequence of the type ABC ABC ABC … is obtained. This packing mode is called cubic close packing (ccp). The cubic close packing gives a face – centred cubic (FCC) lattice.
On the other hand, if the void just above the particle a in the base layer is used, then a sequence of the type AB AB AB AB.. .. is obtained. The structure so obtained is called as hexagonal close packed structure (hcp).When a sphere is placed above the void of the first layer a tetrahedral void is formed. When the triangular voids in the second layer are above the triangular voids in the first layer, the void created is called octahedral void.
- Hexagonal close packed
- Hexagonal close packing
- Hexagonal close packed structure