Polonide
A polonide is a chemical compound of polonium with an element from groups 1–15 of the periodic table (including hydrogen, the lanthanoids and the actinoids).[1] Polonides are amongst the most stable compounds of polonium,[2] 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.
As well as polonides which are intermediate between these two cases, there are also non-stoichiometric polonides and alloys of polonium. As would be expected from periodicity, polonides are often structurally and chemically similar to tellurides. Polonides are usually prepared by a direct reaction between the elements.[3]
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 | [2][3] |
| CaPo | halite (NaCl) | 651.0(4) pm | [2][3] |
| BaPo | halite (NaCl) | [2] |
With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater covalancy in the bonding.
| Formula | Structure | Lattice parameter |
Ref. |
|---|---|---|---|
| MgPo | nickeline (NiAs) | [2] | |
| BePo | sphalerite (ZnS) | 582.7 pm | [2][3] |
| CdPo | sphalerite (ZnS) | [2] | |
| ZnPo | sphalerite (ZnS) | 628(2) pm | [3] |
The lanthanoids also form sesquipolonides of formula Ln2Po3 which can be considered to be ionic compounds.[4]
Intermetallic polonides
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.[2][5] 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.[2][4] 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.[4]
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.[2][3][6]
References
- ↑ Nomenclature of Inorganic Chemistry; IUPAC Recommendations 2005; Royal Society of Chemistry: Cambridge, 2005; pp 69, 260. ISBN 0-85404-438-8, <http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf>.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; p 899. ISBN 0-08-022057-6.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Moyer, Harvey V. Chemical Properties of Polonium. In Polonium; Moyer, Harvey V., Ed.; United States Atomic Energy Commission: Oak Ridge, Tenn., 1956; pp 33–96. TID-5221. doi:10.2172/4367751, <http://www.osti.gov/bridge/servlets/purl/4367751-nEJIbm/>.
- ↑ 4.0 4.1 4.2 Heat Sources for Thermoelectric Generators; Monsanto Research Corporation Mound Laboratory: Miamisburg, Ohio, 1963, <https://www.osti.gov/opennet/servlets/purl/16137309-oYiakP/16137309.pdf>.
- ↑ Kershner, C. J.; DeSando, R. J.; Heidelberg, R. F.; Steinmeyer, R. H. Rare earth polonides. J. Inorg. Nucl. Chem. 1966, 28 (8), 1581–88. DOI: 10.1016/0022-1902(66)80054-4. Kershner, C. J.; Desando, R. J. Promethium polonide synthesis and characterization. J. Inorg. Nucl. Chem. 1970, 32 (9), 2911–18. DOI: 10.1016/0022-1902(70)80355-4.
- ↑ Witteman, W. G.; Giorgi, A. L.; Vier, D. T. The Preparation and Identification of some Intermetallic Compounds of Polonium. J. Phys. Chem. 1960, 64 (4), 434–40. DOI: 10.1021/j100833a014.
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