{{Short description|Chemical compound containing polonium}} [[File:Magnesium-polonide-xtal-1960-3D-SF.png|thumb|right|A space-filling representation of the crystal structure of magnesium polonide: Mg<sup>2+</sup> ions are shown in green, while Po<sup>2−</sup> ions are shown in brown.]] thumb|Polonide A '''polonide''' is a chemical compound of the radioactive element polonium with any element less electronegative than polonium.<ref>{{RedBook2005|pages=69, 260}}.</ref> Polonides are usually prepared by a direct reaction between the elements at temperatures of around 300–400&nbsp;°C.<ref name="AEC-chem">{{citation | last = Moyer | first = Harvey V. | contribution = Chemical Properties of Polonium | pages = 33–96 | title = Polonium | url = http://www.osti.gov/bridge/servlets/purl/4367751-nEJIbm/ | editor-last = Moyer | editor-first = Harvey V. | id = TID-5221 | doi = 10.2172/4367751 |osti=4367751 | year = 1956 | location = Oak Ridge, Tenn. | publisher = United States Atomic Energy Commission| doi-access = free }}.</ref><ref name="Bagnall">{{citation | first = K. W. | last = Bagnall | title = The Chemistry of Polonium | journal = Adv. Inorg. Chem. Radiochem. | year = 1962 | volume = 4 | pages = 197–229 | url = https://books.google.com/books?id=8qePsa3V8GQC&pg=PA197 | isbn = 978-0-12-023604-6 | doi=10.1016/S0065-2792(08)60268-X| series = Advances in Inorganic Chemistry and Radiochemistry | url-access = subscription }}.</ref> They are amongst the most chemically stable compounds of polonium,<ref name="G&E">{{Greenwood&Earnshaw1st|page=899}}.</ref> and can be divided into two broad groups: *ionic polonides, which appear to contain the Po<sup>2−</sup> 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== Lead polonide (PbPo) occurs naturally, as lead is produced in the alpha decay of polonium.<ref>{{cite journal |last1=Weigel |first1=F. |year=1959 |title=Chemie des Poloniums |journal=Angewandte Chemie |volume=71 |issue=9 |pages=289–316 |doi=10.1002/ange.19590710902|bibcode=1959AngCh..71..289W }}</ref>

==Ionic polonides== The polonides of the most electropositive metals show classic ionic structural types, and can be considered to contain the Po<sup>2−</sup> anion.

{| class="wikitable" |- ! Formula ! Structure ! Lattice<br/>parameter ! Ref. |- | Na<sub>2</sub>Po | anti-fluorite | 747.3(4) pm | <ref name="G&E"/><ref name="AEC-chem"/> |- | CaPo | halite (NaCl) | 651.0(4) pm | <ref name="G&E"/><ref name="AEC-chem"/> |- | BaPo | halite (NaCl) | 711.9 pm | <ref name="G&E"/><ref name="Bagnall"/> |- |}

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:<ref name="Bagnall"/> MgTe has a wurtzite structure,<ref>{{citation | first = W. | last = Zachariasen | journal = Z. Phys. Chem. | year = 1927 | volume = 128 | pages = 417–20 |title=Über die Kristallstruktur des Magnesiumtellurids |doi=10.1515/zpch-1927-12830| s2cid = 99161358 }}.</ref> although a nickeline-type phase has also been reported.<ref>{{citation | journal = J. Phys. Chem. Solids | volume = 67 | issue = 8 | year = 2006 | pages = 1668–73 | doi = 10.1016/j.jpcs.2006.02.017 | title = High pressure study of structural and electronic properties of magnesium telluride | first1 = D. | last1 = Rached | first2 = M. | last2 = Rabah | first3 = R. | last3 = Khenata | first4 = N. | last4 = Benkhettou | first5 = H. | last5 = Baltache | first6 = M. | last6 = Maachou | first7 = M. | last7 = Ameri|bibcode = 2006JPCS...67.1668R }}.</ref>

{| class="wikitable" |- ! Formula ! Structure ! Lattice<br/>parameter ! Ref. |- | MgPo | nickeline (NiAs) | ''a'' = 434.5 pm<br/>''c'' = 707.7 pm | <ref name="G&E"/><ref name="Bagnall"/> |- | BePo | sphalerite (ZnS) | 582.7 pm | <ref name="G&E"/><ref name="AEC-chem"/> |- | CdPo | sphalerite (ZnS) | 666.5 pm | <ref name="G&E"/><ref name="Bagnall"/> |- | ZnPo | sphalerite (ZnS) | 628(2) pm | <ref name="AEC-chem"/> |- |}

The effective radius of the polonide ion (Po<sup>2−</sup>) can be calculated from the Shannon (1976) ionic radii of the cations:<ref name="Shannon">{{citation | first= R. D. | last = Shannon | title = Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides | journal = Acta Crystallogr. A| year = 1976 | volume = 32 | pages = 751–67 | doi = 10.1107/S0567739476001551|bibcode = 1976AcCrA..32..751S | issue= 5 | doi-access = }}.</ref> 216&nbsp;pm for 4-coordination, 223&nbsp;pm for 6-coordination, 225&nbsp;pm for 8-coordination. The effect of the lanthanide contraction is clear, in that the 6-coordinate telluride ion (Te<sup>2−</sup>) has an ionic radius of 221&nbsp;pm.<ref name="Shannon"/>

The lanthanides also form sesquipolonides of formula Ln<sub>2</sub>Po<sub>3</sub>, which can be considered to be ionic compounds.<ref name="Mound">{{citation | url = https://www.osti.gov/opennet/servlets/purl/16137309-oYiakP/16137309.pdf | title = Heat Sources for Thermoelectric Generators | publisher = Monsanto Research Corporation Mound Laboratory | location = Miamisburg, Ohio | year = 1963}}.</ref>

==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.<ref>{{citation | journal = J. Inorg. Nucl. Chem. | volume = 28 | issue = 8 | year = 1966 | pages = 1581–88 | doi = 10.1016/0022-1902(66)80054-4 | title = Rare earth polonides | first1 = C. J. | last1 = Kershner | first2 = R. J. | last2 = DeSando | first3 = R. F. | last3 = Heidelberg | first4 = R. H. | last4 = Steinmeyer}}. {{citation | journal = J. Inorg. Nucl. Chem. | volume = 32 | issue = 9 | year = 1970 | pages = 2911–18 | doi = 10.1016/0022-1902(70)80355-4 | title = Promethium polonide synthesis and characterization | first1 = C. J. | last1 = Kershner | first2 = R. J. | last2 = Desando}}.</ref> These compounds are stable to at least 1600&nbsp;°C (the melting point of thulium polonide, TmPo, is 2200&nbsp;°C), in contrast to the ionic polonides (including the lanthanide sesquipolonides Ln<sub>2</sub>Po<sub>3</sub>), which decompose at around 600&nbsp;°C.<ref name="G&E"/><ref name="Mound"/> The thermal stability and non-volatility of these compounds (polonium metal boils at 962&nbsp;°C) is important for their use in polonium-based heat sources.<ref name="Mound"/>

Mercury and lead also form 1:1 polonides. Platinum forms a compound formulated as PtPo<sub>2</sub>, while nickel forms a continuous series of phases NiPo<sub>''x''</sub> (''x''&nbsp;= 1–2). Gold also forms solid solutions with polonium over a wide range of compositions,<ref name="G&E"/><ref name="AEC-chem"/><ref>{{citation | title = The Preparation and Identification of some Intermetallic Compounds of Polonium | first1 = W. G. | last1 = Witteman | first2 = A. L. | last2 = Giorgi | first3 = D. T. | last3 = Vier | journal = J. Phys. Chem. | year = 1960 | volume = 64 | issue = 4 | pages = 434–40 | doi = 10.1021/j100833a014}}.</ref> while bismuth and polonium are completely miscible.<ref name="Bagnall"/> No reaction is observed between polonium and aluminium, carbon, iron, molybdenum, tantalum or tungsten.<ref name="Bagnall"/>

== See also == *Polonium hydride, also known as hydrogen polonide

==References== {{reflist}} {{Polonium compounds}} {{Monatomic anion compounds}}

* Category:Polonides Category:Anions