Polonide
A polonide is a chemical compound of the radioactive element polonium with any element less electronegative than polonium. Polonides are usually prepared by a direct reaction between the elements at temperatures of around 300–400 °C. They are amongst the most chemically stable compounds of polonium, and can be divided into two broad groups:
Some polonides are intermediate between these two cases and others are non-stoichiometric compounds. Alloys containing polonium are also classed as polonides. As polonium is immediately below tellurium in the periodic table, there are many chemical and structural similarities between polonides and tellurides.
Lead polonide (PbPo) occurs naturally, due to the fact that lead is produced in the alpha decay of polonium.
The polonides of the most electropositive metals show classic ionic structural types, and can be considered to contain the Po2− anion.
With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater covalency in the bonding. Magnesium polonide is unusual as it is not isostructural with magnesium telluride: MgTe has a wurtzite structure, although a nickeline-type phase has also been reported.
The effective radius of the polonide ion (Po2−) can be calculated from the Shannon (1976) ionic radii of the cations: 216 pm for 4-coordination, 223 pm for 6-coordination, 225 pm for 8-coordination. The effect of the lanthanide contraction is clear, in that the 6-coordinate telluride ion (Te2−) has an ionic radius of 221 pm.
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