Wigner–Seitz radius

The Wigner–Seitz radius $r_{\mathrm{s}}$, named after Eugene Wigner and Frederick Seitz, is the radius of a sphere whose volume is equal to the mean volume per atom in a solid (for first group metals). In the more general case of metals having more valence electrons, $r_{\mathrm{s}}$ is the radius of a sphere whose volume is equal to the volume per a free electron. This parameter is used frequently in condensed matter physics to describe the density of a system. $r_{\mathrm{s}}$ is typically calculated for bulk materials.

Formula

In a 3-D system with $N$ free valence electrons in a volume $V$, the Wigner–Seitz radius is defined by

$$\frac{4}{3}\pi r_{\mathrm{s}}^3=\frac{V}{N}=\frac{1}{n}$$

where Template:Mvar is the particle density. Solving for $r_{\mathrm{s}}$ we obtain

$$r_{\mathrm{s}}=\sqrt[3]{\frac{3}{4\pi n}}.$$

The radius can also be calculated as $$r_{\mathrm{s}}=\sqrt[3]{\frac{3M}{4\pi \rho N_V{N}_{\mathrm{A}}}}$$ where Template:Mvar is molar mass, Template:Mvar is the count of free valence electrons per particle, Template:Mvar is the mass density, and Template:Mvar is the Avogadro constant, Template:Physconst.

This parameter is normally reported in atomic units, i.e., in units of the Bohr radius.

Assuming that each atom in a simple metal cluster occupies the same volume as in a solid, the radius of the cluster is given by $$R_0=r_s{n}^{1/3}$$ where n is the number of atoms.

Values of $r_{\mathrm{s}}$ for the first group metals:

Element	Template:Mvars in [[Bohr radius|Template:Mvar0]]
Template:SimpleNuclide	3.25
Template:SimpleNuclide	3.93
Template:SimpleNuclide	4.86
Template:SimpleNuclide	5.20
Template:SimpleNuclide	5.62

Wigner–Seitz radius is related to the electronic density by the formula $$r_s=0.62035{\rho }^{1/3}$$ where ρ can be regarded as the average electronic density in the outer portion of the Wigner-Seitz cell.

See also

• Wigner–Seitz cell
• Wigner crystal

References

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