Crystal Structure:
The manner in which atoms are arranged in space is called crystal structure of metals. Atoms are placed in a regular and repetitious pattern. Most of the metals crystallize in mainly three structures.
Body Centered cubic (BCC) structure: A BCC unit cell has one atom placed in the centre and one atom each at all the corners. The corner atom is shared by the neighboring body center cubes. There are two atoms in each unit cell.
The atomic packing factor for BCC is 0.68. Iron possesses BCC structure at room temperature. The co-ordination number for BCC structure is 8.
Face centered cubic (FCC) structure: The FCC has an atom at each corner of the cube and one atom at each of the six faces of the cube. There are a total of 4 atoms in each unit cell.
It has a packing factor of 0.74 so it is densely packed than BCC. Copper, Aluminum, Lead and Nickel have FCC structure. The co-ordination number for FCC structure is 12.
Hexagonal Closed Packing (HCP) structure: HCP structure has one atom each at the corner of the top and bottom hexagon, one atom each at the centre of both hexagonal faces and one atom at the centre of the line connecting the perpendicular which combines the hexagonal closed packed structure.
The unit cell of HCP can’t be specified by lattice parameter ‘a’ only and is specified by ‘c’ which is the distance between basal planes and from there we have axial ratio given by c/a. The atomic packing fraction for HCP is 0.74. Zinc, Cadmium and Magnesium possess HCP structure.
