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Crystal Structure

Hexagonal Close Packing (HCP): Structure, Layers, and Properties

Understand hexagonal close-packed crystal structures. Covers the ABABAB layer sequence, coordination number, packing efficiency, and comparison with CCP/FCC.

V
Vectora Team
STEM Education
7 min read
2025-10-11

What Is Hexagonal Close Packing?

Hexagonal Close Packing (HCP) is one of two ways to achieve maximum packing efficiency with equal-sized spheres (the other being CCP/FCC). In HCP, close-packed layers are stacked in the sequence ABABAB... — every other layer is directly above the first.

Like CCP, HCP achieves a packing efficiency of 74% and a coordination number of 12.

Learning Goals: By the end of this guide, you should be able to:

  1. Describe the HCP unit cell and ABABAB layer sequence.
  2. Calculate atoms per unit cell.
  3. Compare HCP with CCP and BCC structures.
  4. Identify elements with HCP structures.

The HCP Unit Cell

The conventional HCP unit cell is a hexagonal prism:

FeatureValue
Atoms per unit cell6 (in the hexagonal cell)
Coordination number12
Packing efficiency74%
Layer sequenceABABAB
Ideal c/ac/a ratio8/3≈1.633\sqrt{8/3} \approx 1.633

Atom Positions

  • 12 corner atoms (each shared among 6 cells) → contributes 12×16=212 \times \frac{1}{6} = 2
  • 2 face atoms (top and bottom hexagonal faces, shared between 2 cells) → 2×12=12 \times \frac{1}{2} = 1
  • 3 interior atoms → 3×1=33 \times 1 = 3

Total: 6 atoms per unit cell.

ABABAB vs ABCABC

The key difference between HCP and CCP is how the third layer is placed:

  • HCP: Layer C is placed directly above Layer A → ABABAB
  • CCP: Layer C is offset from both A and B → ABCABC

Both give the same packing efficiency and coordination number, but result in different unit cell shapes and different slip systems (affecting ductility).

HCP Crystal Explorer

View the ABABAB layer stacking in 3D. Compare with CCP side by side, count coordination neighbours, and explore the hexagonal unit cell.
Explore HCP in 3D

Real-World Examples

ElementStructureProperties
MagnesiumHCPLightweight, less ductile than Al (FCC)
ZincHCPBrittle compared to Cu (FCC)
TitaniumHCP (α-Ti)Strong, used in aerospace
CobaltHCP (low-T)Transitions to FCC at 417°C

HCP metals tend to be less ductile than FCC metals because they have fewer slip systems (3 vs 12), making it harder for layers to slide past each other.

Worked Example: Density Calculation

Given: Magnesium (HCP), a=320a = 320 pm, c=520c = 520 pm, M=24.3M = 24.3 g/mol.

Volume of hex cell: V=322a2c=32/2×(3.20×10−8)2×5.20×10−8V = \frac{3\sqrt{2}}{2}a^2c = 3\sqrt{2}/2 \times (3.20 \times 10^{-8})^2 \times 5.20 \times 10^{-8}

ρ=6×24.3V×6.022×1023=1.74 g/cm3\rho = \frac{6 \times 24.3}{V \times 6.022 \times 10^{23}} = 1.74\ g/cm^3

Common Mistakes

  1. Confusing HCP with CCP packing efficiency — They're the same (74%). The difference is in the layer sequence, not the efficiency.

  2. Wrong number of atoms per unit cell — HCP has 6 atoms per hexagonal cell, not 4 (that's FCC) or 2 (that's BCC).

  3. Assuming all close-packed metals are ductile — HCP metals (Mg, Zn) are less ductile than FCC metals (Cu, Au) due to fewer slip planes.

Frequently Asked Questions

Why do some metals prefer HCP over CCP?

The preference depends on electronic structure, atomic size, and bonding. The energy difference between HCP and CCP is often very small — some metals even transition between them at different temperatures (e.g., cobalt, iron).

What is the c/a ratio?

The c/ac/a ratio is the height-to-width ratio of the hexagonal unit cell. The ideal value for perfect sphere packing is 8/3≈1.633\sqrt{8/3} ≈ 1.633. Deviations indicate non-ideal packing.