Curium fluoride

A number of compounds of curium have been produced, including two forms of curium oxide (Cm203 and Cm02), two forms of curium fluoride (CmF3 and CmF4), curium chloride (CmCl3), curium bromide (CmBr3), and curium hydroxide (Cm(OH)3). 

Its stability then decreases progressively until we reach curium where aqueous solutions containing the tetra-positive state must be complexed by ligands such as fluoride or phosphotungstate. Even then, they oxidize water and revert to cur-ium(lll). The expected drop in I4 between curium and berkelium provides Bk" (aq) with a stability similar to that of Ce (aq), but the decrease in stability is then renewed, and beyond californium, the +4 oxidation state has not yet been prepared [2, 10, 15]. 

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. 

Many compounds of curium are known. They include the oxides, Cm02 and Cm203, fluorides CmF4 and CmFs, other halides, CmXa hydroxide, Cm(OH)3, and oxalate Cm2(C204)3. The oxide, hydroxide, fluoride, and oxalate salts are insoluble in water and may be obtained by precipitation reactions. 

ACTINIDE CONTRACTION. An effect analogous to the Lanthanide contraction, which lias been found in certain elements of the Actinide series. Those elements from thorium (atomic number 90) to curium (atomic number 96) exhibit a decreasing molecular volume in certain compounds, such as those which the actinide tetrafluoiides form with alkali metal fluorides, plotted in Eig. 1. The effect here is due to the decreasing crystal radius of the tetrapositive actinide ions as the atomic number increases. Note that in the Actinides the tetravalent ions are compared instead of the trivalent ones as in the case of the Lanthanides, in which the trivalent state is by far the most common. 

The solubility properties of curium(111) compounds are in every way similar to those of Ihe other tripositive Actinide elements and the tnpositive Lanthanide elements. Thus the fluoride and oxalate tire insoluble in acid soluliun, while the nitrate, halides, sulfate, perchlorate, and sullide are all soluble. 

Solid curium irifluoride has been prepared by drying the fluoride, which precipitates from dilute UNO, upon the addition of hydrofluoric acid,... 

In research al the Institute or Radiochemistry. Karlsruhe, West Germany during Ihe early 1970s. investigators prepared alloys of Curium with iridium, palladium, plalinum. and rhodium. These alloys were prepared by hydrogen reduction of the curium oxide or fluoride in the presence of finely divided noble metals. The reaction is called a coupled reaction because the reduction of the metal oxide can be done in the presence of noble metals. The hydrogen must be extremely pure, w ith an oxygen content of less than 10 -s Inrr. 

Sill et al. [26] have discussed a spectrometric method for the determination of americium and other alpha-emitting nuclides, including curium and californium, in potassium fluoride-pyrosulfate extracts of soils. Sekine [27] used a-spectrometry to determine americium in soils with a chemical recovery of 60-70%. Joshi [28] and Livens et al. [29] have discussed methods for the determination of241 americium in soils. 

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. 

Neptunium, plutonium and americium metals have been isolated by reduction of their fluorides with barium at 1200° C. Like uranium they are base metals and do not resemble the platinum group. The metal curium has not yet been described. 

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... 

To date, the hahdes represent by far the most extensively characterized class of curium compounds (refer to Table 9.2). The complete CmXs series (X = F, Q, Br, I), as well as CmF4 and several complex Cm(iv) fluorides, have been prepared and studied. Several reviews deal specifically with actinide halides and for further information (especially for cross-comparisons of Cm with other actinide halides) the reader is referred to these articles [2, 3, 58, 59]. 

There is no evidence for an oxidation state in solution less than ii. A fluoride complex of tetravalent curium was obtained in aqueous solution when separately prepared CmF4 was dissolved in concentrated (15 m) MF solution (M = alkali-metal ion) [21,159]. Even under these conditions, and using the self-... 

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