Berkelium compounds

In the actinides, the element curium, Cm, is probably the one which has its inner sub-shell half-filled and in the great majority of its compounds curium is tripositive, whereas the preceding elements up to americium, exhibit many oxidation states, for example -1-2, -1-3. -1-4, -1-5 and + 6, and berkelium, after curium, exhibits states of -1- 3 and -E 4. Here then is another resemblance of the two series. 

Neither californium nor its compounds are found in nature. All of its isotopes are produced artificially in extremely small amounts, and all of them are extremely radioactive. All of its isotopes are produced by the transmutation from other elements such as berkelium and americium. Following is the nuclear reaction that transmutates californium-250 into cahfornium-252 Cf + (neutron and A, gamma rays) — Cf + (neutron and A, gamma rays) —> Cf. 

The chemical properties of berkehum are rare earth-like character because of its half-filled 5/ subsheU and should be simdar to cerium. The element readily oxidizes to berkelium dioxide, Bk02 when heated to elevated temperatures (500°C). In aqueous solutions, the most common oxidation state is -i-3 which may undergo further oxidation to +4 state. A few compounds have been synthesized, the structures of which have been determined by x-ray diffraction methods. These include the dioxide, Bk02 sesquioxide, Bk203 fluoride,... 

Chalcogenides, 5 94-96 berkelium, 28 49, 53-54 lattice energies of, 1 192, 193 ligands, 45 16 Chalcogen(II) compounds binary halides, 35 274—280 complexes with Lewis bases, 35 293-295 halo-chalcogenates(ll), 35 280-288 mixed-valence compounds, 35 288-293 cationic species, 35 291-293... 

The solubility properties of berkelium in its two oxidation states are entirely analogous to those of Lite actmide and lanLlianide elements in the corresponding oxidation states, Thus in the tripositive state such compounds as the fluoride and the oxalate arc insoluble in add solution, and the tetrapositive slate has such insoluble compounds as the lodate and phosphate in acid solution. The nitrate, sulfate, halides, perchlorate, and sulfide of both oxidation states are soluble,... 

The first compound of berkelium of proven molecular structure was isolated in 1962 by Cunningham and Wtillman. A small quantity (0,004 microgram) of berkelium (as berkelium-249) dioxide was used to determine structure by x-ray diffraction. 

The references given in Table I are those describing the preparation of a given compound the reference may or may not contain information on the behavior of the compound with time Note that the compounds have been synthesized in different oxidation states and different crystal structures where possible Not shown in the table are einsteinium, berkelium, and californium phosphates which have also been prepared and are being studied at present (11) ... 

Bulk-Phase Compounds Some of our results in the studies of the bulk-phase compounds have been published (3-7) These studies have shown that oxidation state is preserved for these actinides in either a or fT decay Trivalent einsteinium will transmute to trivalent berkelium which transmutes to trivalent californium It has also been observed that divalent einsteinium yields divalent californium. It is interesting to note in this latter case that it has not yet been possible to synthesize divalent berkelium in the bulk phase Berkelium(II) has not been observed in our aged einsteinium(II) compounds either, but it would be logical to assume it has been produced there. Our inability to observe Bk(II) could be related to weak absorption intensities and/or interference by absorption bands of einsteinium(II) or... 

The first structure determination of a compound of berkelium, the dioxide, was carried out in 1962 (5). Four X-ray diffraction lines were obtained from 4 ng of BkC>2 and indexed on the basis of a face-centered cubic structure with a0 = 0.533 0.001 nm. 

Line lists of the absorption bands of two organoberkelium compounds, Bk(C5H5)3 (116) and [Bk(C5H5)2Cl]2 (117), have also been published. For additional information (118) and discussion of the development of the theoretical treatment of berkelium spectra, the reader is referred to other sources (83,106). 

Berkelium metal dissolves rapidly in aqueous mineral acids, liberating hydrogen gas and forming Bk(III) in solution (120,133). Undoubtedly it forms alloys and/or intermetallic compounds with a number of other metals. 

Selected Crystallographic Data for Berkelium Metal and Compounds... 

The berkelium monopnictides have been prepared on the multimicrogram scale by direct combination of the elements (138). In all cases, the lattice constants of the NaCl-type cubic structures were smaller than those of the corresponding curium monopnictides but comparable to those of the corresponding terbium compounds. This supports the semimetallic classification for these compounds. One additional report of BkN has appeared (139). The lattice parameter derived from the sample exhibiting a single phase was 0.5010 0.0004 nm, whereas that extracted from the mixed-phase sample of BkN resulting from incomplete conversion of a hydride was 0.4948 0.0003 nm. Clearly, additional samples of BkN should be prepared to establish more firmly its lattice constant. 

The only other crystallographic result reported for a berkelium chal-cogenide besides those summarized in Table II is a cubic lattice parameter of 0.844 nm for Bk2S3 (155). The microscale synthesis of the brownish-black sesquisulfide was carried out by treatment of berkelium oxide at 1400 K with a mixture of H2S and CS2 vapors. In later work (136,137), the higher chalcogenides were prepared on the 20- to 30-jug scale in quartz capillaries by direct combination of the elements. These were then thermally decomposed in situ to yield the lower chalcogenides. The stoichiometries of these compounds have not been determined directly. 

The preparation and characterization of intermetallic compounds and alloys of berkelium should be pursued, as well as the determination of the stability constants of Bk(IV) complexes. The range of oxidation states accessible to berkelium might be expanded by stabilizing Bk(II) and/or Bk(V) in highly complexing aqueous, nonaqueous, or even molten salt media and/or in appropriate solid-state matrices. 

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