Which of the following is correct?

Hexagonal closed pack structure:

• In a hexagonal closedpacked structure the third layer has the same arrangement of spheres as the first layer.
• Since the structure repeats itself after every two layers the stacking for hcp may be described as "ababab".
• The atoms in a hexagonal closed packed structure efficiently occupy 74% of space while 26% is empty space.
• Ex: Mg, Zn

Coordination Number and Number of Atoms Per Unit Cell:

• The hexagonal closed packed (hcp) structure has a coordination number of 12 and contains 6 atoms per unit cell.
• The facecentered cubic (fcc) structure has a coordination number of 12 and contains 4 atoms per unit cell.
• The bodycentered cubic (bcc) structure has a coordination number of 8 and contains 2 atoms per unit cell.
• The simple cubic structure has a coordination number of 6 and contains 1 atom per unit cell.

Crystal structure of Material

FCC: Ni Cu Ag Pt Au Pb Al Austenite or Ƴiron

BCC: V Mo Ta W Ferrite or αiron δferrite or δiron

HCP: Mg Zn

Cobalt: HCP < 420°C FCC > 420°C

Chromium: HCP < 20°C BCC > 20°C

Glass: Amorphous

Important Points

In the crystal structure the atomic packing factor (APF) or packing efficiency or packing fraction is the fraction of volume in a crystal structure that is occupied by constituent particles. It is a dimensionless quantity and always less than unity.

The atomic packing factor of different crystal structures is given in the table below: