Americium halides

Americium halides. Americium forms binary halides in the oxidation states of III and IV. The trifluoride is prepared by hydrofluorination of Am02 with HF at 400 to 500 C and by precipita-... 

Low-Temperature Optical Absorption of Americium Halides, R.G. Pappalardo, W.T. Carnall, and PR. Fields, J. Chem. Phys. 51, 1182-1200 (1969). 

Runnals [85] patented a method for making americium-aluminum alloys in which a mixture of aluminum metal and an americium halide is heated in a vacuum of 10 torr at 700-1200°C until the americium is reduced and alloyed. Homogeneous ameridum-aluminum alloys suitable for irradiation of americium can be prepared by reaction of aluminum, Am02, and NajAlF (cryolite) at 1100-1200°C [86]. d-Plutonium and jS-americium form a continuous series of fee solid solutions that are stable at room temperature in the composition range from about 6 to 80 at % americium [294]. Other alloys and intermetallic compounds are listed in Table 8.3. 

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. 

Halides of truly divalent americium, however, can be prepared ... 

Alkenes, Cyclic Alkenes, and Dienes Alkenes, Cyclic Alkenes, and Dienes Ethylene Propylene Alkyl Halides Alkynes Aluminum Americium Amides Amides Carbofuran Dimethyl Acetamide Amines Antimony... 

In its precipitation reactions ameiicium(IIl) is very similar to the other tripositive actinide elements and to the rare earth elements. Thus the fluonde and the oxalate are insoluble and the phosphate and iodate are only moderately soluble in acid solution, whereas the nitrates, halides, sulfates, sulfides, and perchlorates are all soluble. Americium(VI) can be precipitated with sodium acetate giving crystals isostructural with sodium uranyl acetate,... 

Relatively few complexes of americium have been characterized those that have tend to resemble the corresponding compounds of the three previous metals. Many are halide complexes, such as (NH4)4 [AmFg], which resembles the U analogue, Cs2NaAmCl6, and (Ph3PH)3AmX6 (X = Cl, Br). 

Tn reviewing the chemistry of the actinides as a group, the simplest approach is to consider each valence state separately. In the tervalent state, and such examples of the divalent state as are known, the actinides show similar chemical behavior to the lanthanides. Experimental diflB-culties with the terpositive actinides up to plutonium are considerable because of the ready oxidation of this state. Some correlation exists with the actinides in studies of the lanthanide tetrafluorides and fluoro complexes. For other compounds of the 4-valent actinides, protactinium shows almost as many similarities as dijSerences between thorium and the uranium-americium set thus investigating the complex forming properties of their halides has attracted attention. In the 5- and 6-valent states, the elements from uranium to americium show a considerable degree of chemical similarity. Protactinium (V) behaves in much the same way as these elements in the 5-valent state except for water, where its hydrolytic behavior is more reminiscent of niobium and tantalum. 

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 oxyhalides AmOCl, AmOI, and CmOCl are isostructural PbClF type (hexagonal), and contain nine coordinate An surrounded by four oxygens and five halides. Americium(III) oxyboride has also been prepared, but the structure is unconfirmed. 

Isomorphism among compounds of the actinides is common and only a few examples need be given. The dioxides, MO2, of thorium, uranium, neptunium, plutonium and americium all have a fluorite lattice. The trihalides of the transuranic elements are isomorphous not only with the corresponding trihalides of actinium and uranium but also with those of the lanthanides. Isomorphism is also exhibited in many complex halides thus thorium, ura-... 

Barium reduces the oxides, halides, and sulfides of most of the less reactive metals, thereby producing the corresponding metal. It has reportedly been used to prepare metallic americium via reduction of americium trifluoride (13). However, calcium metal can, in most cases, be used for similar purposes and is usually preferred over barium because of lower cost per equivalent weight and nontoxicity (see ACTINIDES AND TRANSACTINIDES). 

Curium halides. Like americium, curium forms binary halides in the oxidation states of III and... 

The (III) state is by far the most stable oxidation state for americium, and it is possible only by the use of fluorine to prepare a higher halide. Americium tetrafluoride has been prepared by direct fluorination, but attempts to prepare AmF5 or AmF have so far been fruitless. It is a reasonable presumption that AmF6 does not exist as a stable compound under ordinary conditions. 

Truly divalent actinide halides are known only for americium and californium. AnX2 species for Es have been identified by their absorption spectra. For Fm, Md, and No, AnX2 halides should be possible if sufficient amounts of these metals could be obtained. Thl2 is also known, but crystallographic studies of this compound reveal the true formulation to be Th(IV), 21 , and 2e . This compound has some metallic character, including its luster and electrical conductivity. 

One of the main differences, therefore, between actinides and rare earths is the steeper change of the f-d energy difference along the actinide series (fig. 3). This first of all makes americium trivalent, but also implies that a series of elements at the end of the actinide series will be divalent. Indeed, the divalent state of nobelium is so relatively stable that, of the possible halide compounds, the only trivalent compound it will be able to form is the trifluoride (Johansson 1977a). 

A two-stage, countercurrent molten-salt extraction process is used to extract Am from many kilograms of aged plutonium metal in which Am has grown-in by beta decay of Pu. The purification scheme, based partly on molten-halide/molten-metal studies at the Los Alamos Scientific Laboratory and Argonne National Laboratory, ronoves about 90% of the americium from plutonium metal, typically containing 200 2000 ppm Am [16]. 

Known halides are described in Table 8.3 and have been reviewed extensively [3, 4, 10-12], The only remarkable halides are those of extreme oxidation states. Dihalides of americium cannot be prepared by hydrogen reduction of trihalides, although hydrogen reduction is successful for the lanthanides Sm, Eu (the 4f analog of americiumX and Yb. Instead, americium metal must be reacted with a halide such as Hgl2 [329] or Hg02 [100]. It is likely that the reaction... 

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