Crystal Lattice Structures
Previous - Next
Introduction

Simple Cubic

Body Centered Cubic

Face Centered Cubic

Primitive FCC

Simple Hexagonal

Hexagonal Closest Packing

HCP Coordination

Cubic Closest Packing

CCP Coordination

Rhombohedral

2- & 3-Layer repeats

4-layer repeats

Tetrahedral Holes

Octahedral Holes

CsCl

NaCl

Halite

Fluorite

Zinc Blende

Exercises

Face Centered Cubic
  In the Face Centered Cubic (FCC) unit cell there is one host atom at each corner and one host atom in each face.  Since each corner atom contributes one eighth of its volume to the cell interior, and each face atom contributes one half of its volume to the cell interior (and there are six faces), then Z = 1/8.8 + 1/2.6 = 4.
The corner and face atoms touch along the face diagonal, and it is easy to show that the cube edge (a) is about 2.8r. Thus, the corner atoms do not touch one another.

The packing efficiency is about 74%.  This is the maximum packing efficiency for spheres of equal radius and is call closest packing.  Thus a face centered lattice of atoms is also called Cubic Closest Packing (CCP).

Previous - Next
Steven F. Watkins, Department of Chemistry, Louisiana State University