Polonium
Po
Description
Polonium ($\text{Po}$) is a rare, highly radioactive metal discovered by Marie and Pierre Curie in 1898. Its most remarkable crystallographic feature is that its alpha phase ($\alpha$-Po) is the only known element to crystallize in a simple cubic lattice at standard temperature and pressure.
In the simple cubic structure, a single Polonium atom sits at each of the 8 corners of the cubic unit cell. Because each corner atom is shared among 8 adjacent unit cells, the total number of atoms per unit cell is exactly: $$ Z = 8 \times \frac{1}{8} = 1 \text{ atom/cell} $$
The simple cubic structure of $\alpha$-Po transforms into a rhombohedral structure ($\beta$-Po) when heated above $36^\circ \text{C}$. The simple cubic phase is stabilized by strong spin-orbit coupling effects and relativistic corrections affecting its electron orbitals.
Properties
Atomic and Physical Properties:
- Atomic Number: $84$
- Atomic Mass: $209 \text{ u}$ (most stable isotope)
- Density ($\alpha$-phase): $9.196 \text{ g/cm}^3$
- Lattice Constant: $a = 3.352 \text{ \AA}$
Geometrical Relationships: For $\alpha$-Polonium, the atoms touch along the edge of the cube. Thus, the metallic radius ($r$) can be derived directly from the lattice constant ($a$): $$ r = \frac{a}{2} = \frac{3.352}{2} = 1.676 \text{ \AA} $$
Volume Calculation: The volume of the $\alpha$-Po unit cell is simply the cube of the lattice constant: $$ V = a^3 = (3.352 \times 10^{-8} \text{ cm})^3 \approx 3.766 \times 10^{-23} \text{ cm}^3 $$ This theoretical volume aligns perfectly with its macroscopic density measurements.
Applications
Because of its extreme radioactivity and high toxicity, Polonium has very specialized, niche applications:
Heaters in Spacecraft: $1 \text{ gram}$ of $^{210}\text{Po}$ generates approximately $140 \text{ watts}$ of thermal power due to its intense alpha decay. This heat can be converted into electricity using thermoelectric cells in satellites and lunar rovers.
Static Eliminators: Polonium brushes and industrial tools are used to eliminate static electricity in environments where delicate photographic film or sensitive optics are handled. The alpha particles ionize the surrounding air, neutralizing static charges.
Neutron Sources: When mixed with Beryllium ($\text{Be}$), Polonium emits alpha particles that strike the Beryllium atoms, causing them to release neutrons. This was historically used in early nuclear weapons and scientific experiments.
Unit Cell Axes
a=3.35 b=3.35 c=3.35
Unit Cell Angles
α=90° β=90° γ=90°
Grid Expansion
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