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Hexagonal Close-packed Structure

Key idea: H3 Solid State Physics: Hexagonal Close-packed Structure — key ideas and exam-focused notes on bonding, crystal structures, conduction models, and band ideas.

  • Advanced Physics
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Learning objectives

  • Use advanced reference material to reinforce mathematical, optical, material, and modern-physics models.

Hexagonal close-packed (HCP) is a crystal packing where identical atoms are stacked in an ABAB… sequence, with the third layer sitting directly above the first.

Core concept

Both HCP and face-centred cubic (FCC), also called cubic close-packed (CCP), are close-packed structures, but they differ in stacking order:

  • HCP: ABAB…
  • FCC / CCP: ABCABC…

Both ideal equal-sphere structures have coordination number 12 and packing fraction about 0.74. Their different stacking sequences give different crystal symmetry and available slip systems.

Schematic side projections compare HCP layers A-B-A, where the third layer lies directly above the first, with FCC or CCP layers A-B-C, where the third layer uses a new registration. Both have coordination number 12 and packing fraction about 0.74.
HCP repeats after two layer positions; FCC/CCP repeats after three.

Worked interpretation and common mistake

If a question states “third layer directly above layer A”, you should identify the structure as HCP immediately. A common mistake is to label any close-packed structure as FCC and miss the ABAB clue in diagrams.

Next steps

Continue with the next resource in this course.

Course and syllabus information
Course
Advanced Physics
Edition
Advanced Physics