Americium
Halides

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

Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides AmCl, AmBr and AmI. They are very sensitive to oxygen and oxidize in water, releasing hydrogen and converting back to the Am(III) state. Specific lattice constants are:

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

The tetravalent americium(IV) fluoride (AmF) is obtained by reacting solid americium(III) fluoride with molecular fluorine:

Another known form of solid tetravalent americium fluoride is KAmF. Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH) was dissolved in 15- M NH 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 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) is reddish and has a structure isotypic to uranium(III) chloride (space group P6 /m) and the melting point of 715 °C. The fluoride is isotypic to LaF (space group P6 /mmc) and the iodide to BiI (space group R 3). The bromide is an exception with the orthorhombic PuBr -type structure and space group Cmcm. Crystals of americium(III) chloride hexahydrate (AmCl ·6H 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 O X, Am O X, Am OX and Am OX can be obtained by reacting the corresponding americium halide with oxygen or Sb O, and AmOCl can also be produced by vapor phase hydrolysis:
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