Polonide
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A polonide is a
- 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
Naturally occurring polonides
Lead polonide (PbPo) occurs naturally, as lead is produced in the alpha decay of polonium.[5]
Ionic polonides
The polonides of the most electropositive metals show classic ionic structural types, and can be considered to contain the Po2− anion.
Formula | Structure | Lattice parameter |
Ref. |
---|---|---|---|
Na2Po | anti-Fluorite | 747.3(4) pm | [4][2] |
CaPo | halite (NaCl) | 651.0(4) pm | [4][2] |
BaPo | halite (NaCl) | 711.9 pm | [4][3] |
With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater
Formula | Structure | Lattice parameter |
Ref. |
---|---|---|---|
MgPo | nickeline (NiAs) | a = 434.5 pm c = 707.7 pm |
[4][3] |
BePo | sphalerite (ZnS) | 582.7 pm | [4][2] |
CdPo | sphalerite (ZnS) | 666.5 pm | [4][3] |
ZnPo | sphalerite (ZnS) | 628(2) pm | [2] |
The effective radius of the polonide ion (Po2−) can be calculated from the Shannon (1976) ionic radii of the cations:[8] 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.[8]
The lanthanides also form sesquipolonides of formula Ln2Po3, which can be considered to be ionic compounds.[9]
Intermetallic polonides
The lanthanides form very stable polonides of formula LnPo with the halite (NaCl) structure. Many of them seem to involve trivalent lanthanides (though Sm, Eu, and Yb with more stable +2 oxidation states are exceptions), making them resemble electrides. They are isostructural to the lanthanide sulfides, selenides, and tellurides.[10] 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.[4][9] 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.[9]
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,[4][2][11] while bismuth and polonium are completely miscible.[3] No reaction is observed between polonium and aluminium, carbon, iron, molybdenum, tantalum or tungsten.[3]
References
- ISBN 0-85404-438-8. pp. 69, 260. Electronic version..
- ^ doi:10.2172/4367751, TID-5221.
- ^ ISBN 978-0-12-023604-6.
- ^ ISBN 978-0-08-022057-4..
- .
- S2CID 99161358.
- .
- ^ .
- ^ a b c Heat Sources for Thermoelectric Generators (PDF), Miamisburg, Ohio: Monsanto Research Corporation Mound Laboratory, 1963.
- .
- .