Californium metal

Fig. 15. Apparatus used for preparing early samples of californium metal.

Then, curium metal is antiferromagnetic and its paramagnetic effective moment definitively supports the picture of a localized 5 f configuration the same is true for what is known about berkelium and californium metals ... 

Californium tribromide has been studied by spectroscopic and X-ray powder diffraction methods between 25 and 700 °C and was found to undergo thermal reduction to californium(ii) with increasing temperature. Despite this ingrowth of cali-fornium(ii) into californium tribromide, all the powder diffraction patterns showed the existence of only the monoclinic aluminium trichloride type structure of CfBr3. A mechanism which involved a reversible shift of electron density from the bromide ions to californium(iii) ions with temperature was postulated. Studies on the californium metal system have indicated the existence of both a bivalent and higher valence form. The use of or Es as dopants in strontium dichloride, and the temperature... 

There has been no evidence for the existence of a monoxide of Cf, although other compounds containing divalent Cf are known e.g., dihalides (Haire 1986). It is possible that a monoxide can be attained by high-pressure reaction of californium metal and californium oxide, as has been used for the lanthanide monoxides (Leger et al. 1980) but this reaction has not been tried with Cf. An NaCl-type lattice parameter of 0.48-0.50 would be expected for the monoxide. 

A fused salt-molten metal process has also been examined for separating californium [36, 37]. In principle, it may also be possible to reduce actinide oxides with thorium metal and distill away the more volatile californium metal (see Section 11.6.1). The separation of californium oxide from curium oxide using a vacuum sublimation procedure has also been reported [38]. 

The first attempt to prepare californium metal was reported in 1969 [67]. Subsequently, several additional attempts have been made to prepare and study this metal [68-72]. The relatively high volatility of californium metal has made its preparation and study on the microscale more difficult than the first three transplutonium metals. The possibility that the metal may exist in two different metallic valence states has made it an interesting candidate for study, but it has also complicated the full understanding of californium s metallic state. 

These limited quantities of californium metal place restrictions on the amount of analytical data that can be obtained for the products normally, analyses for hydrogen, nitrogen, and oxygen contents are not performed. The quality of the metal products has been determined by spark-source mass spectrometry, x-ray diffraction analysis, physical properties, appearance, and behavior in an experiment (such as the rate and extent of dissolution for heat-of-solution measurements). 

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. 

Table 113 Crystallographic data reported for californium metal.

Additional support for this latter structure being a form of metallic californium is that samples exhibiting this structure have been converted by thermal treatment to the fee structure having the parameter Oq = 0.494 nm, and vice versa [72], The second dhep structure [71] listed in Table 11.3 and the other hexagonal structure [70] may represent the same material. If they are metallic californium, they would represent a hexagonal structure for the divalent form of the metal. They are at present not well-established structures for the metal. The fee structure with the parameter Uq = 0.540 nm [68,69] has been observed in earlier work, where very small quantities of californium were prepared. In a later study [70], this structure (oq == 0.540 nm) was also observed in thin films of californium metal that had been heated to 200-300°C in air. It should be noted that poorly crystallized samples of californium sesquioxide (Cf203. c, body-centered cubic, Oo = 1.080-1.083 nm see Section 11.7.2) can be indexed as an fee strueture with Go = 0.540-0.542 nm. If the fee structure with Oo = 0.540 nm was indeed a metallic phase of californium, the lattice parameter would imply that the metal had an intermediate valence between 2 and 3. 

If it is assumed that californium metal does exhibit two metallic valences, then two phases for each valence form can be selected from the crystallographic data in Table 11.3. For the trivalent form, metallic radii of 1.69 and 1.75 A would be obtained, which is in accord with the radii obtained for the first three transplutonium metals, when allowing for a small systematic decrease in radius in going across the series. The divalent form would yield a radius of 2.0 A, which is similar to the radii of divalent europium or ytterbium metals. The trivalent form of californium metal is well-established if a divalent form does exist, it is favored in small quantities (thin films) obtained from higher temperatures (quendted from the vapor or molten states). Only a limited effort has been made to establish transition temperatures for californium metal [72]. 

Although bulk samarium metal is trivalent, divalent surface states have been demonstrated for this metal [76]. The next lanthanide metal, europium, is a divalent metal. It may be possible that californium metal is similar to samarium metal, and that the divalent state for californium metal may only be stable in thin films, very small samples, or in surface layers of atoms, as found for samarium. 

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. 

The first attempts to examine californium metal by high-pressure x-ray diffraction, on samples assumed to represent both the trivalent and divalent metal forms, did not provide additional insight into the valence state of the metal the compressibility data obtained from this study were not conclusive [77]. A subsequent study on the dhep (trivalent) form of californium metal found that this structure transformed into an fee structure under pressures of 10-16 GPa... 

A bulk modulus of 50 5 GPa was derived for the dhcp form of californium metal, which compares favorably with the bulk moduli for many of the lanthanide metals but is significantly lower than the moduli of thorium to plutonium metals. In recent studies of californium metal under pressures up to 48 GPa, it was found that the metal transforms into an a-uranium orthorhombic-type structure which corresponds to the onset of 5f-electron participation in the metallic bonding [79]. 

The vapor pressure and the heat of sublimation of californium metal have been measured using the Knudsen effusion technique [80]. The vaporization of californium metal (originally dhcp, trivalent form) over the temperature range 733-973 K is described by the equation ... 

The A//298 WrtS calculated to be 196.23 1.26 kjmol , and AS298 was derived to be 80.54 JK mol . The estimated boiling point for the metal is 1745 K. Nugent et al. [81] had estimated the heat of sublimation to be 163 kJ mol and David et al. [82] had predicted a value of 197 kJmol . The vapor pressure of californium metal is intermediate between that of samarium metal (trivalent) and of europium metal (divalent) [80]. The data show that the californium metal was clearly trivalent up to 1026 K, and that it is one of the most volatile actinide metals. Its high volatility precludes bulk vaporization studies above 1073 K by the Knudsen technique. No evidence by mass spectrometry was obtained in this latter work for the presence of CfO. 

Efforts have been made to determine the magnetic susceptibility of californium metal. Although it would seem that susceptibility data would be very valuable in ascertaining the metallic valence of californium metal, it turns out that this information does not differentiate between the divalent form (presumably 5f state, = 10.22 fi ) and the trivalent state (for 5f state, = 10.18 [89]). 

Californium metal forms alloys with the lanthanide metals but definite compounds have not been reported. When heated, the metal wets and appears to form an alloy with tantalum, which usually prohibits the use of tantalum containers for high-temperature work. Tungsten metal is the preferred container material. 

Several properties of californium were considered in making correlations of the 5f-electron status across the series [81,82,85-88,91-93] one prediction was that californium metal would be close to the divalent-trivalent metallic boundary [85]. Californium is the first element in the actinide series to show strong divalent tendencies, owing to the progressive stabilization of the divalent ground state [92] that is probably complete at fermium. 

The pattern of superconductivity in f-band metals has received considerable attention. It is unlikely that californium metal would show superconductivity, as a result of its magnetic moment and localized 5f electrons, but prospects for superconducting behavior in other actinides have been discussed [96, 97]. 

In addition to the oxides and halides, several other compounds of californium have been prepared and their crystallographic data reported (see Table 11.5). Some of these data represent preliminary values or results from single experiments. In some cases (pnictides, chalcogenides, etc.), the limited supply of californium metal has precluded the preparation of specific compounds, especially where close control of the stoichiometries is required (for example, CfS). The general preparative techniques for pnictides and chalcogenides of the transuranium elements has been reviewed [147]. 

There has been one report on the preparation of californium hydrides [235]. The hydrides were prepared by reaction of californium metal with hydrogen at elevated temperatures. It was believed that the stoichiometries were close to that for the dihydride (CfHj+The products exhibited fee structures with an average lattice parameter of Oo = 0.5285 nm, which is slightly larger than expected for the compound based on extrapolations of parameters for preceding actinide dihydrides. This larger parameter and the inability to prepare a trihydride of californium were believed to reflect a tendency for californium to be divalent In the lanthanide-hydrogen system, the hydrides of divalent europium and ytterbium metals deviate from the behavior of the other lanthanide hydrides [149]. 

Only a limited amount of magnetic work has been reported for californium, some of which was discussed in the earlier section on the metal (Section 11.6). The transplutonium metals with localized Sf electrons behave as though they consist of ions embedded in a sea of conduction electrons. These 5f electrons are mainly responsible for the susceptibility. With this simple model, the effective moment for californium metal can be considered to be the same as that for a californium ion that had the same number of electrons involved in bonding in a compound. In Table 11.7 are listed some metal ions and their calculated magnetic moments based on LS coupling and Hund s rule. On this basis, the moments of Cf(iv) and Tb(iii) or Bk(iii) would be the same, the moments of Cf(iii) and Dy(iii) or Es(iv) would be identical, and the moments of Cf(ii) and Es(iii) or Ho(m) would be equal. As was pointed out earlier (Section 11.6), it is unfortunate that the measured moment cannot differentiate between Cf(n) and Cf(ui), and that the calculated difference between Cf(m) and Cf(nr) is only 0.9 However, the magnetic behavior as a function of temperature and/or magnetic field still provides very useful information, which by itself may even be sufficient to differentiate between these states. 

---