Polonide

 ( Polonides ) 

A polonide is a chemical compound of the radioactive element polonium with any Chemical 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. containing polonium are also classed as polonides. As polonium is immediately below tellurium in the periodic table, there are many periodic table between polonides and tellurides.

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Naturally occurring polonides Lead polonide (PbPo) occurs naturally, as lead is produced in the alpha decay of polonium.

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Ionic polonides The polonides of the most electropositive metals show classic ionic structural types, and can be considered to contain the Po²⁻ anion.

sodium polonide	anti-fluorite	747.3(4) pm
calcium polonide	sodium chloride	651.0(4) pm
barium polonide	sodium chloride	711.9 pm

With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater covalent bond 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	a = 434.5 pm c = 707.7 pm
BePo	Sphalerite	582.7 pm
cadmium polonide	Sphalerite	666.5 pm
zinc polonide	Sphalerite	628(2) pm

The Ionic 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 lanthanides also form sesquipolonides of formula Ln₂Po₃, which can be considered to be ionic compounds..

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Intermetallic polonides The form very stable polonides of formula LnPo with the Halite. Many of them seem to involve trivalent lanthanides (though Sm, Eu, and Yb with more stable +2 oxidation states are exceptions), making them resemble . They are isostructural to the lanthanide sulfides, selenides, and tellurides.. . These compounds are stable to at least 1600 °C (the melting point of thulium polonide, TmPo, is 2200 °C), in contrast to the ionic polonides (including the lanthanide 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 NiPo _x ( 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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See also

• Polonium hydride, also known as hydrogen polonide

Categories: Polonides, Anions

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