Berkelium metal

Some properties of berkelium metal have been reported. Thus, its melting point is 986 + 25 °C and its volatility, relative to its congeners, is in the order Cm < Bk < Am < Cf. Its chemical behaviour is described as somewhat similar to Sm, and it does not correspond, as a metal, to Tb or Lu. It reacts with hydrogen at 225 °C to give BkH2, which is isomorphous with other lanthanide and actinide hydrides of the type MH2+ (x < 1). BkO may be formed as an impurity in the production of metallic Bk. 

The first bulk samples of berkelium metal were prepared in early 1969 by the reduction at about 1300 K of BkF3 with lithium metal vapor (119). The BkF3 samples were suspended in a tungsten wire spiral above a charge of Li metal in a tantalum crucible. A photomicrograph of the first isolated bulk (1.7 jug) sample of berkelium metal is shown in Fig. 7. 

Later berkelium metal samples of up to 0.5 mg each have been prepared via the same chemical procedure (120). Elemental berkelium can also be prepared by reduction of BkF4 with lithium metal and by reduction of Bk02 with either thorium or lanthanum metal. The latter reduction process is better suited to the preparation of thin metal foils unless multimilligram quantities of berkelium are available. 

Fig. 7. A photomicrograph of the first isolated bulk (1.7 /tg) sample of berkelium metal in a quartz X-ray capillary.

Berkelium metal exhibits two stable crystallographic modifications, double hexagonal closest packed (dhcp) and face-centered cubic (fee). Thus it is isostructural with the two preceding elements, all of which exhibit the fee structure at high temperature. The room-temperature lattice constants of the dhcp form are ao = 0.3416 0.0003 nm and c0 = 1.1069 0.0007 nm, yielding a calculated density of 1.478 x 104 kg/m3 and a metallic radius (CN = 12) of 0.170 nm (119). The room-temperature fee lattice parameter is a0 = 0.4997 0.0004 nm from which the... 

In the first experiments to measure the vapor pressure of metallic Bk, using Knudsen effusion target-collection techniques, the preliminary data were fitted with a least-squares line to give a provisional vaporization equation for the temperature range 1326-1582 K, and Ai 298 was calculated to be 382 18 kJ/mol (128). The crystal entropy of berkelium metal at 298 K (S s) had been estimated earlier to be 76.2 1.3 J K 1 mol-1 (129), and later, to be 78.2 1.3 J K 1 mol-1 (124). 

Fig. 8. Unit-cell volume of berkelium metal as a function of pressure. Adapted and reprinted with permission from the J. Phys. F Met. Phys. 14, U. Benedict, J. R. Peterson, R. G. Haire, and C. Dufour, 1984, The Institute of Physics, London (126).

Later data on the vapor pressure of berkelium metal over the temperature range 1100-1500 K, obtained by using combined Knudsen effusion mass spectrometric and target collection techniques, have been published in 1982 (124). The vaporization equations obtained were... 

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 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... 

Studies of berkelium metal under pressure should be continued to... 

Berkelium metal dissolves rapidly in aqueous mineral adds liberating hydrogen gas and forming Bk(iii) in solution. 

In 1972 a hybridized non-degenerate 6d and 5f virtual-bound-states model was used to describe the properties of the actinide metals, including berkelium [138]. It accounted for the occurrence of localized magnetism in Bk metal. In 1974 a review of the understanding of the electronic properties of berkelium metal as derived from electronic band theory was published [139]. Included was the relativistic energy band structure of face-centered cubic Bk metal (5f 6d 7s ), and the conclusion was that berkelium is a rare-earth-like metal with localized (ionic) 5f electrons resulting from less hybridization with the 6d and 7s itinerant bands than occurs in the lighter actinides. 

A phenomenological model based on crystal structure, metallic radius, melting point, and enthalpy of sublimation has been used to arrive at the electronic configuration of berkelium metal [140]. An energy difference of 0.92 eV was calculated between the 5f 7s ground state and the 5f 6d 7s first excited state. The enthalpy of sublimation of trivalent Bk metal was calculated to be 2.99 eV (288 kJmol ), reflecting the fact that berkelium metal is more volatile than curium metal. It was also concluded that the metallic valence of the face-centered cubic form of berkelium metal is less than that of the double hexagonal close-packed modification [140]. 

Table 10.2 Selected crystallographic data for berkelium metal and compounds. 

The preparation of berkelium hydride has been accomplished by treatment of berkelium metal at 500 K with H2 gas derived from thermal decomposition of UH3 [121]. The product exhibited a face-centered cubic (fee) structure with lattice parameter Uq = 0.523 0.001 nm determined from nine observed x-ray diffraction lines. By analogy with the behavior of the lanthanide hydrides [176], the superdihydride stoichiometry, BkH2 + c (0 < 1). was assigned. Later studies... 

A summary of the reported crystallographic data for californium metal is given in Table 11.3. Based on an extrapolation of data for trivalent americium, curium, and berkelium metals, californium metal would be expected to have a double hexagonal close-packed (dhcp) low-temperature phase, with parameters of approximately Oq = 0.34 and Cq = 1.10 nm, and a face-centered cubic (fee) high-temperature phase with an Oq 0.49 nm. Based on other extrapolations [75], a divalent form of californium metal would be expected to be cubic and have a larger lattice parameter than a trivalent cubic form. From the values in Table 11.3, the dhcp form with parameters Uq = 0.3384 and c = 1.1040 nm [71,72] is accepted to be the low-temperature form of trivalent californium metal. The fee material, with a = 0.494 nm [72], is very likely a high-temperature form of the trivalent metal, comparable to the fee forms of americium, curium, and berkelium metals. The second fee structure listed in Table 11.3, with Oo = 0.574 nm [70,72], has been observed by other workers using different preparative techniques. 

There has been only one reported value for the melting point of californium metal, which was estimated to be 900 30°C from the puddling of metal particles in a thin film of the metal [70]. This melting point is lower than those reported for americium, curium or berkelium metals, but it is in accord with the higher volatility of californium metal, and the increasing trend toward divalency across the actinide series. 

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