Berkelium dioxide

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,... 

A simplistic picture of the situation is to have a relationship between the efifective moments of the f-element materials with the probable ion configuration. In this situation, localized f electrons in the metal would have the same moment as localized f electrons in a compound. The moment would depend on the number of such localized electrons regardless of the particular f-element s chemical form. Thus, the number of localized f electrons in Gd metal is seven (4f with three electrons in a ds conduction band), as it is in Cm metal (Sfconfiguration) there are also seven localized f electrons in both gadolinium and curium sesquioxides. Further, terbium and berkelium dioxides have seven localized f electrons. All six materials should have the same moment based on seven, unpaired free-ion electrons. 

Americium, californium, and einsteinium oxides have been reduced by lanthanum metal, whereas thorium has been used as the reductant metal to prepare actinium, plutonium, and curium metals from their respective oxides. Berkelimn metal could also be prepared by Th reduction of Bk02 or Bk203, but the quantity of berkelium oxide available for reduction at one time has not been large enough to produce other than thin foils by this technique. Such a form of product metal can be very difficult to handle in subsequent experimentation. The rate and yield of Am from the reduction at 1525 K of americium dioxide with lanthanum metal are given in Fig. 2. 

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 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. 

Calculations of the expected XPS spectra for the actinide dioxides uranium through berkelium were reported by Gubanov et al. (10). Results for UO2 are shown in Fig. 3 along with experimental spectra. These calculations, extending about 30 eV below the Fermi level, are based on a one-electron molecular-cluster approach. 

The quantity of information available for the berkelium oxygen system is far less than that for the previous actinides, due mainly to the scarcity and the shorter half-life (325 days) of the Bk-249 isotope used in solid-state studies. The sesquioxide and the dioxide are the two well-established oxides of Bk, and the dioxide is much more stable than the dioxides of Cm and Am. This stability can be attributed in part to the fact that the Bk(IV) state results in a half-filled 5f state, as found for TbOj. However, BkOj is much more stable and is formed more easily than is TbOj. 

The sesquioxide is formed by reduction of the dioxide in hydrogen or CO/COj atmospheres at elevated temperatures. Some care must be used to assure that reduction is complete (e.g., the O/M ratio reached is 1.50). The dioxide of Bk (black/brown) is readily obtained by decomposition of a variety of berkelium salts (e.g., nitrate, oxalate, etc.) in air or oxygen-containing atmospheres. In fact, precautions must be used to avoid the uptake of oxygen by the sesquioxide, even at room temperature. Heating lower oxides of Bk to 500°C in air is sufficient to produce the stoichiometric dioxide. The dioxide crystallizes in the fluorite structure (see table 25) and is isostructural with the earlier actinide dioxides. 

In a study of the berkelium oxygen system (Turcotte 1980) it was concluded that the growth of the Cf daughter in berkelium oxides gave different effects, depending upon the amount of Cf. For a Cf content up to 25 mol %, the Cf in the matrix was apparently oxidized to a dioxide when the value reached 64 mol%, the Cf content controlled the Bk oxidation and the stoichiometry of the mixed cation product was limited to the stoichiometry of An70i2- The behavior of the mixed oxides is in accord with the behaviors known for the two pure oxide systems. 

In comparing berkelium and terbium, the situation with their configurations seems clearer both form a half-filled f orbital in their dioxides, although there is a large difference in ease of formation and stability of these two dioxides. The dioxide of Bk is very stable and readily forms in air, whereas the formation of TbOj requires highly oxidizing conditions and Tb02 is less stable thermally. 

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