Berkelium oxides

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 results of a preliminary study of a sample of berkelium oxide (Bk02, Bk203, or a mixture of the two) via X-ray photoelectron spectroscopy (XPS) included measured core- and valence-electron binding energies (162). The valence-band XPS spectrum, which was limited in resolution by photon broadening, was dominated by 5f-electron emission. 

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. 

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. 

Although the berkelium oxidation states, 0 iii, and iv are known in bulk phase, further work is required to characterize more completely the solid-state and solution chemistries of this element. The synthesis of divalent berkelium in bulk should be possible via the metallothermic reduction of its trihalides, e.g. 

One recent study examined the oxidation behavior of (Bk, Cf) oxides as a function of the californium content [12S]. The conclusion reached was that, when the californium content was up to 2S mol%, the californium was readily oxidized in air to O/M = 2, behaving like pure berkelium oxide when the californium content reached 64 mol %, the californium controlled the berkelium oxidation and limited the mixed cation product to a stoichiometry of M7O12-... 

Oxygen Decomposition Pressures and Thermodynamic Data for Nonstoichiometric Berkelium Oxide, R.P. Turcotte, T.D. Chikalla and L. Eyring, J. Inorg. Nucl. Chem., 33, 3749-3763 (1971). 

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. 

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]. 

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,... 

Benzyl alcohol, oxidation, by ruthenium oxo complexes, 39 287-289 Benzylazide, NMR of, 4 263 Benzylidene diacetate, nitration of, 6 114 Berkelium... 

Berkelium is known to exist 111 aqueous solution in two oxidation states, the (III) and the (IV) states, and the ionic species presumably correspond to Bk+3 and Bk+4. The oxidation potential for the berkelium(lll)-berkehum(IV) couple is about —1.6 V on the hydrogen scale (hydrogen-hydrogen ion couple taken as zero)... 

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 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... 

During the handling of microgram-sized samples of berkelium metal, it was observed that the rate of oxidation in air at room temperature is not extremely rapid, possibly because of the formation of a protective oxide film on the metal surface (135). Berkelium is a chemically reactive metal, and berkelium hydride (123), some chalco-genides (123, 136, 137) and pnictides (138, 139) have been prepared directly from the reaction of Bk metal with the appropriate nonmetal-lic element. 

Berkelium exhibits both the III and IV oxidation states, as would be expected from the oxidation states displayed by its lanthanide counterpart, terbium. Bk(III) is the most stable oxidation state in noncomplex-ing aqueous solution. Bk(IV) is reasonably stable in solution, undoubtedly because of the stabilizing influence of the half-filled Sf7 electronic configuration. Bk(III) and Bk(IV) exist in aqueous solution as the simple hydrated ions Bk3+(aq) and Bk4+(aq), respectively, unless com-plexed by ligands. Bk(III) is green in most mineral acid solutions. Bk(IV) is yellow in HC1 solution and is orange-yellow in H2S04 solution. A discussion of the absorption spectra of berkelium ions in solution can be found in Section IV,C. 

The first estimate of the Bk(IV)-Bk(III) potential was made in 1950, only a short time after the discovery of the element. A value of 1.6 V was reported, based on tracer experiments (3). Later, in 1959, a refined value of 1.62 0.01 V was reported for the couple, based on the results of experiments with microgram quantities of berkelium (4). The potential of the Bk(IV)-Bk(III) couple has subsequently been determined by several workers using direct potentiometry (220-224) or indirect methods (218, 225, 226). All of the above-mentioned determinations were performed in media of relatively low complexing capability. The formal potential of the Bk(IV)—Bk(III) couple is significantly shifted to less positive values in media containing anions that strongly complex Bk(IV), such as PCV- and CO32- ions (227). This behavior closely parallels that of the Ce(IV)-Ce(III) couple (228). In fact, the Bk(IV)-Bk(III) couple markedly resembles the Ce(IV)-Ce(III) couple in its oxidation-reduction chemistry. 

Additional information on the oxidation-reduction behavior of berkelium can be found in a comprehensive review (238). 

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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