# 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 (Am$_2$O$_3$), and +4 (AmO$_2$). 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. 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.
The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula Am$_2$(C$_2$O$_4$)$_3$·7H$_2$O. Upon heating in vacuum, it loses water at 240 °C and starts decomposing into AmO$_2$ at 300 °C, the decomposition completes at about 470 °C. The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L.

## Halides

Halides of americium are known for the oxidation states +2, +3, and +4, where the +3 is most stable, especially in solutions.
| Oxidation state | F | Cl | Br | I |
| +4 | Americium(IV) fluoride AmF$_4$ pale pink | | | |
| +3 | Americium(III) fluoride AmF$_3$ pink | Americium(III) chloride AmCl$_3$ pink | Americium(III) bromide AmBr$_3$ light yellow | Americium(III) iodide AmI$_3$ light yellow |
| +2 | | Americium(II) chloride AmCl$_2$ black | Americium(II) bromide AmBr$_2$ black | Americium(II) iodide AmI$_2$ black |
Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides AmCl$_2$, AmBr$_2$, and AmI$_2$. 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 AmCl$_2$: a = 896.3±0.8 pm, b = 757.3±0.8 pm and c = 453.2±0.6 pm
* Tetragonal AmBr$_2$: a = 1159.2±0.4 pm and c = 712.1±0.3 pm. They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX$_2$, where X = Cl, Br, or I:
    Am + HgX$_2$ (mercury halide) → AmX$_2$ + Hg (at 400–500 °C)
Americium(III) fluoride (AmF$_3$) is poorly soluble and precipitates upon reaction of Am+3 and fluoride ions in weak acidic solutions:
    Am+3 + 3F$^−$ → AmF$_3$↓
The tetravalent americium(IV) fluoride (AmF$_4$) is obtained by reacting solid americium(III) fluoride with molecular fluorine:
    2AmF$_3$ + F$_2$ → 2AmF$_4$
Another known form of solid tetravalent americium fluoride is KAmF$_5$. Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)$_4$ was dissolved in 15-M NH$_4$F with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of AmF$_4$ 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 (AmCl$_3$) is reddish and has a structure isotypic to uranium(III) chloride (space group P6$_3$/m) and the melting point of 715 °C. The fluoride is isotypic to LaF$_3$ (space group P6$_3$/mmc) and the iodide to BiI$_3$ (space group R3). The bromide is an exception with the orthorhombic PuBr$_3$-type structure and space group Cmcm. Crystals of americium chloride hexahydrate (AmCl$_3$·6H$_2$O) 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 Am$^{VI}$O$_2$X$_2$, Am$^V$O$_2$X, Am$^{IV}$OX$_2$, and Am$^{III}$OX can be obtained by reacting the corresponding americium halide with oxygen or Sb$_2$O$_3$, and AmOCl can also be produced by vapor phase hydrolysis:
    AmCl$_3$ + H$_2$O → AmOCl + 2HCl

## Other inorganic compounds
### Hydroxide

The only known hydroxide of americium is Am(OH)$_3$, 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 $^{241}$Am(OH)$_3$ decomposes within 4 to 6 months ($^{241}$Am has a half-life of 432.2 years); for $^{244}$Cm(OH)$_3$ the same process takes less than a day ($^{244}$Cm 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 KAmO$_2$CO$_3$ (potassium dioxoamericium(V) carbonate) can be obtained. Potassium carbonate can also be used. The resulting KAmO$_2$CO$_3$ reacts with dilute acids to produce americium dioxide:
    O$_3$ + Am(OH)$_3$ + KHCO$_3$ + H$_2$O → KAmO$_2$CO$_3$ + 3H$_2$O + O$_2$

### Chalcogenides and pnictides

The known chalcogenides of americium include the sulfide AmS$_2$, selenides AmSe$_2$ and Am$_3$Se$_4$, and tellurides Am$_2$Te$_3$ and AmTe$_2$. The pnictides of americium ($^{243}$Am) 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 AmSi$_x$, 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$_x$). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. AmSi$_x$ has a bright silvery lustre and a tetragonal crystal lattice (space group I4$_1$/amd), it is isomorphic with PuSi$_2$ and ThSi$_2$. Borides of americium include AmB$_4$ and AmB$_6$. 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 (η$^8$-C$_8$H$_8$)$_2$Am, but it is still hypothetical up to date. An anionic complex KAm(COT)$_2$ can be prepared by reacting K$_2$COT and AmI$_3$ in THF. A cyclopentadienyl complex is also known that is likely to be stoichiometrically AmCp$_3$.
Formation of the complexes of the type Am(n−C$_3$H$_7$−BTP)$_3$, where BTP stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing n−C$_3$H$_7$−BTP$^−$ and Am+3 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.