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][4]
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) | 711.9 pm | [2][4] |
With smaller cations, the structural types suggest greater polarization of the polonide ion, or greater covalancy in the bonding. Magnesium polonide is unusual as it is not isostructural with magnesium telluride:[4] MgTe has a wurtzite structure,[5] although a nickeline-type phase has also been reported.[6]
| Formula | Structure | Lattice parameter |
Ref. |
|---|---|---|---|
| MgPo | nickeline (NiAs) | a = 434.5 pm c = 707.7 pm |
[2][4] |
| BePo | sphalerite (ZnS) | 582.7 pm | [2][3] |
| CdPo | sphalerite (ZnS) | 666.5 pm | [2][4] |
| ZnPo | sphalerite (ZnS) | 628(2) pm | [3] |
The effective radius of the polonide ion (Po2−) can be calculated from the Shannon (1976) ionic radii of the cations:[7] 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.[7]
The lanthanoids also form sesquipolonides of formula Ln2Po3, which can be considered to be ionic compounds.[8]
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][9] 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][8] 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.[8]
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][10] while bismuth and polonium are completely miscible.[4] No reaction is observed between polonium and aluminium, carbon, iron, molybdenum, tantalum or tungsten.[4]
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 4.3 4.4 4.5 4.6 Bagnall, K. W. The Chemistry of Polonium. Adv. Inorg. Chem. Radiochem. 1962, 4, 197–229, <http://books.google.de/books?&lr=&id=8qePsa3V8GQC&oi=fnd&pg=PA197#v=onepage&q&f=false>.
- ↑ Zachariasen, W. Z. Phys. Chem. 1927, 128, 417–20.
- ↑ Rached, D.; Rabah, M.; Khenata, R.; Benkhettou, N.; Baltache, H.; Maachou, M.; Ameri, M. High pressure study of structural and electronic properties of magnesium telluride. J. Phys. Chem. Solids 2006, 67 (8), 1668–73. DOI: 10.1016/j.jpcs.2006.02.017.
- ↑ 7.0 7.1 Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, 32, 751–67. DOI: 10.1107/S0567739476001551.
- ↑ 8.0 8.1 8.2 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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