Americium Compounds

Americium compounds are compounds containing the element americium (Am). These compounds can form in the +2, +3, and +4, although the +3 oxidation state is the most common. The +5, +6 and +7 oxidation states have also been reported.

Oxides

Three americium oxides are known, with the oxidation states +2 (AmO), +3 (), and +4 (). Americium(II) oxide was prepared in minute amounts and has not been characterized in detail. Americium(III) oxide is a red-brown solid with a melting point of 2205 °C.Wiberg, p. 1972 Americium(IV) oxide is the main form of solid americium which is used in nearly all its applications. Like most other actinide dioxides, it is a black solid with a cubic ( fluorite) crystal structure.Greenwood, p. 1267

The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula . Upon heating in vacuum, it loses water at 240 °C and starts decomposing into at 300 °C, the decomposition completes at about 470 °C.Penneman, p. 4 The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L.Penneman, p. 5

Halides

Halides of americium are known for the oxidation states +2, +3, and +4,Wiberg, p. 1969 where the +3 is most stable, especially in solutions.

Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides , , and . They are very sensitive to oxygen and oxidize in water, releasing hydrogen and converting back to the Am(III) state. Specific lattice constants are:
* Orthorhombic : ''a'' = , ''b'' = and ''c'' =
* Tetragonal : ''a'' = and ''c'' = . They can also be prepared by reacting metallic americium with an appropriate mercury halide , where X = Cl, Br, or I:Greenwood, p. 1272
: (at 400–500 °C)

Americium(III) fluoride () is poorly soluble and precipitates upon reaction of and fluoride ions in weak acidic solutions:
:

The tetravalent americium(IV) fluoride () is obtained by reacting solid americium(III) fluoride with molecular fluorine:Greenwood, p. 1271
:

Another known form of solid tetravalent americium fluoride is .Penneman, p. 6 Tetravalent americium has also been observed in the aqueous phase. For this purpose, black was dissolved in 15- M with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of but differed from other oxidation states of americium. Heating the Am(IV) solution to 90 °C did not result in its disproportionation or reduction, however a slow reduction was observed to Am(III) and assigned to self-irradiation of americium by alpha particles.

Most americium(III) halides form hexagonal crystals with slight variation of the color and exact structure between the halogens. So, chloride () is reddish and has a structure isotypic to uranium(III) chloride (space group P6₃/m) and the melting point of 715 °C. The fluoride is isotypic to (space group P6₃/mmc) and the iodide to (space group R). The bromide is an exception with the orthorhombic -type structure and space group Cmcm. Crystals of americium chloride hexahydrate () can be prepared by dissolving americium dioxide in hydrochloric acid and evaporating the liquid. Those crystals are hygroscopic and have yellow-reddish color and a monoclinic crystal structure.

Oxyhalides of americium in the form , , , and can be obtained by reacting the corresponding americium halide with oxygen or , and AmOCl can also be produced by vapor phase hydrolysis:
:

Other inorganic compounds

Hydroxide

The only known hydroxide of americium is , which is the first compound of americium, discovered in 1944 as part of the Manhattan project. Americium hydroxide is a pink solid which is sparingly soluble in water.

Due to self-irradiation, the crystal structure of decomposes within 4 to 6 months ( has a half-life of 432.2 years); for the same process takes less than a day ( has a half-life of 18.11 years).

When ozone is bubbled through a slurry of americium(III) hydroxide in 0.03 M potassium bicarbonate at 92 °C, hexagonal (potassium dioxoamericium(V) carbonate) can be obtained. Potassium carbonate can also be used. The resulting reacts with dilute acids to produce americium dioxide:
:

Chalcogenides and pnictides

The known chalcogenides of americium include the sulfide , selenides and , and tellurides and . The pnictides of americium (²⁴³Am) of the AmX type are known for the elements phosphorus, arsenic, antimony, and bismuth. They crystallize in the rock-salt lattice.

Silicides and borides

Americium monosilicide (AmSi) and "disilicide" (nominally , with 1.87 < x < 2.0) were obtained by reduction of americium(III) fluoride with elementary silicon in vacuum at 1050 °C (AmSi) and 1150−1200 °C (). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. has a bright silvery lustre and a tetragonal crystal lattice (space group ''I''4₁/amd), it is isomorphic with and . Borides of americium include and . The tetraboride can be obtained by heating an oxide or halide of americium with magnesium diboride in vacuum or inert atmosphere.

Organoamericium compounds

Analogous to uranocene, americium forms the organometallic compound amerocene with two cyclooctatetraene ligands, with the chemical formula . A cyclopentadienyl complex is also known that is likely to be stoichiometrically .

Formation of the complexes of the type , where BTP stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing and ions has been confirmed by EXAFS. Some of these BTP-type complexes selectively interact with americium and therefore are useful in its selective separation from lanthanides and another actinides.

See also

* Europium compounds
* Curium compounds

References

{{Americium compounds

Americium
Americium compounds
Chemical compounds by element