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:
- ionic polonides, which appear to contain the Po2− anion;
- intermetallic polonides, in which the bonding is more complex.
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.
Naturally occurring polonides
The polonides of the most electropositive metals show classic ionic structural types, and can be considered to contain the Po2− anion.
|CaPo||halite (NaCl)||651.0(4) pm|||
|BaPo||halite (NaCl)||711.9 pm|||
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.
|MgPo||nickeline (NiAs)||a = 434.5 pm
c = 707.7 pm
|BePo||sphalerite (ZnS)||582.7 pm|||
|CdPo||sphalerite (ZnS)||666.5 pm|||
|ZnPo||sphalerite (ZnS)||628(2) pm|||
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.
The lanthanoids also form sesquipolonides of formula Ln2Po3, which can be considered to be ionic compounds.
The lanthanoids form very stable polonides of formula LnPo with the halite (NaCl) structure: as the +2 oxidation state is disfavoured for most lanthanoids, these are probably best described as intermetallic compounds rather than charge-separated ionic species. These compounds are stable to at least 1600 °C (the melting point of thulium polonide, TmPo, is 2200 °C), in contrast the ionic polonides (including the lanthanoid sesquipolonides Ln2Po3) which decompose at around 600 °C. The thermal stability and non-volatility of these compounds (polonium metal boils at 962 °C) is important for their use in polonium-based heat sources.
Mercury and lead also form 1:1 polonides. Platinum forms a compound formulated as PtPo2, while nickel forms a continuous series of phases NiPox (x = 1–2). Gold also forms solid solutions with polonium over a wide range of compositions, while bismuth and polonium are completely miscible. No reaction is observed between polonium and aluminium, carbon, iron, molybdenum, tantalum or tungsten.
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