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Close-packed Structures

! Both FCC and HCP crystal structures have atomic packing factors of 0.74 (maximum possible value)

! Both FCC and HCP crystal structures may be generated by the stacking of close-packed planes

! The difference between the two  structures is in the stacking sequence

      FCC: Stacking Sequence ABCABCABC...

   Third plane is placed above the “holes” of the first plane not covered by the second plane

HCP: Stacking Sequence ABABAB...

Third plane is placed directly above the first plane of atoms

Density Computations :

  A knowledge of the crystal structure of a metallic solid permits computation of its theoretical density ? through the relationship:

Thus, the formula for the density is:

 Where:           

             n : No. of atoms associated with each unit cell.

             A: Atomic weight

                  Vc :Volume of the unit cell

                  N_A: Avogadro number = 6.023 X10²³ atoms/mol