Hexagonal Close Packing (HCP)

Layer sequence: ABAB... (alternating)

• A layers: atoms sit in hollows of each other
• B layers: atoms sit in hollow "pockets" of A-layer atoms (triangular hollows)
• Third layer (A): exactly above the first layer

Unit cell (full hexagonal):

• 12 corner atoms × (1/6) = 2
• 2 face atoms (top + bottom) × (1/2) = 1
• 3 interior atoms × 1 = 3
• Total Z = 6 atoms per hexagonal unit cell

Key parameters:

• CN = 12 (4 in same layer + 4 in layer below + 4 in layer above)
• Packing efficiency = 74% (same as FCC)
• Ideal axial ratio: $\frac{c}{a} = \sqrt{\frac{8}{3}} \approx 1.633$

Real metal axial ratios:

Examples: Mg, Zn, Ti, Be, Co (at low T), Cd

HCP vs FCC comparison: