Americium oxide fluorides

Only a few compounds of americium exist. The most important is americium oxide (AmOj), whose main use is in the preparation of other compounds. Americium can also form compounds with the halogens, similar to other transuranic elements—for example, americium iodide (Aml ), americium fluoride, and americium chloride (AmF and ArnGl ). 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Treatment of the starting mixtures of plutonium, americium, or curium with elemental fluorine at 350 °C. for — 16 hours was necessary to form good x-ray samples of LiF XF4 or 7NaF 6XF4 (X = Pu, Am, Cm). No oxidation to pentavalent plutonium was observed under these conditions. When heavier alkalis are present (Rb or Cs), fluorination of such mixtures yields Pu( V) fluorides. 

Pearson s principle simplifies the treatment of the results of investigations performed by Lambertin et al. [40]. The authors of this work examined the effect of the addition of fluoride ion on the equilibrium between different oxidation states of americium in the molten KCl-LiCl eutectic at 743 K. The increase of fluoride ion concentration was shown to result in a shift to the right of the following reaction ... 

In aqueous solution americium exists in the four oxidation states Am(III), Am(IV), Am(V), and Am(VI). In the absence of complexing agents trivalent, pentavalent, and hexavalent americium exist as Am , Am02, and Am02, usually in hydrated form. In aqueous solution tetravalent americium rapidly disproportionates, except in concentrated fluoride and phosphate solutions. 

The oxidation number +4 is not known in aqueous solutions of americium and curium. The measurement of the absorption spectra of americium and curium tetrafluorides by Asprey and Keenan (7) is thus a valuable contribution to the study of the electronic spectra of the actinide elements. Special techniques were devised to measure these spectra with microgram amounts of CmF4 and AmF4 over the region 3500-20000 A. Table XIV lists the positions and the relative intensities of the observed maxima. The agreement of the trivalent fluoride spectra with those of the corresponding... 

Stephanou and Penneman [36] found that Cm(iii) could be separated from americium by oxidizing the latter to Am(vi) with potassium persulfate and precipitating CmF3 Am(vi) fluoride is soluble under these conditions. 

The trivalent state of americium is the stable aqueous oxidation state. Although americium is the homolog of europium, the Am " radius (0.975 A) is closer to that of Nd " " (radius 0.983 A) [76]. It is a convenient rule of thumb that the radii of the light lanthanide ions are nearly identical to the radii of the corresponding actinides shifted three elements to the right in the periodic table, e.g. r(La " ) a r(U ). In some early purification schemes, fission-product promethium accompanied americium. Am(iii) is precipitated by hydroxide, fluoride, phosphate, and oxalate ions from aqueous solution. 

Nearly all of the formation constants listed in Table 8.8 are for complexes formed by Am(iii), as little work has been done on complexes of americium with oxidation states higher than iii. Color changes indicate existence of Am(vi) nitrate, sulfate, and fluoride complexes. There is also spectrophotometric evidence [299] for the existence in 1 m NaOH solution of a peroxide complex of... 

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