Close Packed Structures: fcc and hcp

Close Packing of Spheres

Two Dimensions

One can easily see that the closest packing of spheres in two dimensions is realised by a hexagonal structure: Each sphere is in contact with six neighboured spheres.

Three Dimensions

In three dimensions one can now go ahead and add another equivalent layer. However, for ideal packing it is necessary to shift this layer with respect to first one such that it just fits into the first layer's gaps.

Now the third layer can be either exactly above the first one or shifted with respect to both the first and the second one.

So there are three relative positions of these layers possible (denoted by A, B and C). These could in principle be combined arbitrarily but in nature the sequences A-B-A-B-A-B and A-B-C-A-B-C are most common. In the following we will see that the lattice that forms the latter one is just the fcc lattice which is one of the 14 Bravais lattices we encountered before. The other one is called hcp (hexagonal close packing) but not a Bravais lattice because the single lattice sites (lattice points) are not completely equivalent! There are two different types of lattice sites which have different environments. Therefore the hcp structure can only be represented as a Bravais lattice if a two-atomic basis is added to each lattice site.

The fcc Structure

Conventional Unit Cell

The conventional unit cell is a cube with edge length $a$ and 8 lattice sites at the corners and 6 additional ones at the faces of the cube.

What is the number of lattice points per unit cell>? The 8 vertices are shared by 8 cells and contribute therefore only with $\frac{1}{8}$ each (think of them as extended spheres). The 6 additional points at the faces are shared by two cells and therefore contribute with $\frac{1}{2}$ respectively. So this adds up to a total of \begin{align} n = 8 \cdot \frac{1}{8} + 6 \cdot \frac{1}{2} = 4 \end{align} points per unit cell.^[1][2]

Packing Density

The packing density $\varrho$ is the ratio of the volume filled by the spherical atoms within a unit cell to the total volume $V_\text{uc}$ of the unit cell. Overall there are $n=4$ atoms per unit cell with a volume of $V_\text{sph} = \frac{4}{3} \pi r^3$ each. Thus for the packing density one obtains \begin{align} \varrho &= \frac{n \cdot V_\text{sph}}{V_\text{uc}} = \frac{ 4 \cdot \frac{4}{3} \pi \cdot \left( \frac{\sqrt{2}}{4} \right)^3 a^3}{a^3} \nonumber \\[1ex] &= \frac{\sqrt{2} \pi}{6} \approx 74\% \end{align} which is the highest possible for spherical objects.^[4][5]

Coordination Number

In the fcc structure each atom has $c_1 = 12$ nearest neighbours (coordination number) at a distance of \begin{align} d_{c_1} = 2r = \frac{a}{\sqrt{2}} \approx 0.707a \end{align} and $c_2 = 6$ next-nearest neighbours at a distance of \begin{align} d_{c_2} = a \approx 2.83r \approx 1.415 \, d_{c_1} . \end{align} ^[6][7]

The hcp Structure

The hcp structure is characterised by two nested hexagonal lattice that are shifted by the vector $\left( \frac{2}{3},\frac{1}{3},\frac{1}{2} \right)$ (in the conventional unit cell basis) against each other. The undelying lattice is not a Bravais lattice since the individual lattice points are not equivalent with respect to their environments. But it can be looked at as a hexagonal Bravais lattice with a two-atomic basis with the atoms sitting at the positions $\left( 0, 0, 0, \right)$ and $\left( \frac{2}{3},\frac{1}{3},\frac{1}{2} \right)$.^[8]

Even though the term hcp is used for all those lattices, a close-packing of equal atoms is only obtained for a certain ratio $\frac{c}{a} = \sqrt{\frac{8}{3}}$ of the lattice constants.^[9]