Americium oxides, reactions

Investigations of the solid-state chemistry of the americium oxides have shown that americium has properties typical of the preceding elements uranium, neptunium, and plutonium as well as properties to be expected of a typical actinide element (preferred stability of the valence state 3-j-). As the production of ternary oxides of trivalent plutonium entails considerable difficulties, it may be justified to speak of a discontinuity in the solid-state chemical behavior in the transition from plutonium to americium. A similar discontinuous change in the solid-state chemical behavior is certainly expected in the transition Am Cm. Americium must be attributed an intermediate position among the neighboring elements which is much more pronounced in the reactions of the oxides than in those of the halides or the behavior in aqueous solution. 

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. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

The extent of the first reaction is about 67%, which means that some of the MgCl2 remains in the bulk salt. In a typical 4.5-kg run containing 3000 ppm americium, 90% of the americium is oxidized at the expense of approximately 100 g plutonium. A typical product weighs 4400 g and contains 98% of the feed plutonium. ... 

There is limited information available regarding the abiotic transformation of americium in the atmosphere. Oxidation is the most common reaction that occurs in the atmosphere. This would not be expected for americium compounds, which are generally present in the +3 oxidation date under environmental conditions. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Cranium, Neptunium, Plutonium, and Americium in Aqueous Solution, TID-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

From these values of the free energy of formation, it is reasonable to expect that both PuCl3 and MgCl2 would oxidize americium metal according to the reaction shown in Equations 1 and 2. 

Salt and metal insoluble impurities, such as Pu02, associated with plutonium metal are taken up by the salt in Stage 1. Stage 2 is essentially free of these impurities. Strickland, et al. (14), reported that plutonium oxide extracts americium from molten plutonium metal in a molten salt media. Because these salt and metal insoluble impurities are present in sizable amounts only in Stage 1, the side reaction between americium and these impurities occurs only in Stage 1. The side reaction term (B) is introduced to quantify the side reaction caused by the presence of impurities such as in Stage 1. 

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

A general method for preparation of all An metals is by reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca, or Ba at 1100 to 1400°C the chlorides or oxides are sometimes used. There are some special methods such as the preparation of Th or Pa from their tetraiodides by the van Arkel-de Boer process, or the following reaction for the relatively volatile americium ... 

Ternary and polynary oxides of trivalent to hexavalent americium are obtained by solid-state reactions of Am20s and Am02 with the oxides of various elements. Compounds of pentavalent and hexavalent americium, which are iso-structural with the corresponding ternary oxides of Pa, U, Np, and Pu, are formed only with the oxides of alkali metals and alkaline earth metals. By reaction of Am02 with oxides of tetravalent elements—e.g., SiO, ZrO, or... 

Th02—ternary oxides or oxide phases with tetravalent americium are stabilized. The solid-state reaction of Am02 with most group V elements yields compounds with trivalent americium which are isostructural with the analogous rare earth compounds. In the last types of reactions americium exhibits a typical actinide behavior. 

III and V elements. It was to be expected that americium would behave as a typical actinide element—i.e., that the majority of the ternary oxides to be expected contain Am (III). This assumption is based upon the analogous behavior of Ce02, PreOn, and Tb407 found in reactions with oxides of group V elements as well as upon the excellent thermal stability and the high lattice symmetry of compounds like and... 

All investigations were carried out in glove boxes with quantities of 5-20 mg. 2 Am02. The experimental results were evaluated mainly by x-ray analysis. In special cases, differential thermal analysis (DTA) and the thermogravimetric analysis (TGA) (Figure 1) were employed, respectively. The valence of americium in the ternary oxides with Am(V) and Am (VI), after dissolution of the reaction product in diluted HCIO4, was determined by recording an absorption spectrum of the solution obtained. 

The Reaction of Am02 and Ani203 with the Oxides of Group V Elements. In its reactions with group V elements americium behaves like a typical rare earth element. When using Am02 as the americium component there is always loss of oxygen and the formation of ternary oxides... 

This study is concerned with the chemistry of the actinides in saturated KF solution. The areas examined are solubilities, absorption spectra, oxidation-reduction reactions, and solid compounds that can be produced in this medium. This paper reports work with neptunium which is essentially complete, and also includes work with uranium and americium. 

Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium... 

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