α-Polonium (simple cubic) 1: introduction

To go directly to the unit cell structure, click the link below to page 3.

Polonium is extremely rare; trace amounts can be found with uranium ores. About 100 g are synthesised each year in nuclear reactors. All of its isotopes are radioactive. Polonium-210 is its main isotope, which is a very powerful alpha emitter. As such it has been used as a fail-safe heater in space craft (one gram of polonium will self-heat up to 500°C). It is also therefore extremely toxic if ingested - it has been shown to be 5 million times more toxic than hydrogen cyanide. The first case of poisoning from polonium was Marie Curie's daughter Irène following the explosion of a sealed capsule of the element on her laboratory bench.

α-Polonium atoms are arranged in a cubic manner but they are not "close packed" (see copper for what this means). Simple trigonometry shows that only 52% of space is occupied by hard spheres packed in a simple cubic arrangement, even less than in body-centred cubic packing. That can't be appreciated looking at the structure above as the atoms are drawn with half their actual radius, so they are not touching. This is so that you can see inside the structure, which is impossible if the atoms are shown to be in contact. A space-filling representation is shown on the next page. Each atom in the bulk is in contact with six neighbours, which are arranged along the cartesian axes around the original atom. The bonds in the structure to the left connect atoms that are touching in reality. Note that the number of nearest neighbours (the "coordination number") is less than it is for sodium (8) and copper (12). The lower the coordination number the less efficient the packing in terms of the fraction of space that is filled. The structure on the left shows a segment of the bulk of α-polonium.

α-Polonium is only stable under 36°C. Above this temperature it is more stable as β-polonium, which has a trigonal unit cell.

Go to page 2 to look at a space-filling representation of the structure of α-polonium.

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