Transplutonium metals

We assume, in this case, that the conduction band has become normal (that is, it has no longer any 5 f character). Thus, physical properties may be usefully compared with those of the lanthanides. In Table 5 we report known basic properties (metallic radii, crystal structures, melting temperatures and enthalpies of sublimation) of the transplutonium metals. 

Caution is indicated with regard to interpretations (or speculations) on the basis of fee high temperature modifications of the rare transplutonium metals fee phases of similar lattice... 

The polymorphism of the lighter actinides reflects the existence of numerous bonding (including 5f) electron states of almost identical energies. The observation of dhcp structures for the transplutonium metals indicates only a slight participation of the predominantly localized 5f electrons in the bonding. 

Johansson (17) expects the transplutonium metals to transform to a bcc phase immediately before melting in this bcc phase, the 5f wave function overlap is reduced, hence, also the 5f electron contribution to the bond. 

The major breakthrough occurred in 1953 when the Ames Laboratory team (Daane et al. 1953) reported the preparation of samarium, europium and ytterbium in high purity and high yields by the reduction of their oxides with lanthanum metal in a vacuum. With the preparation of samarium metal, finally, 126 years after the first rare earth element was reduced to its metallic state, all of the naturally occurring rare earths were now available in their elemental state in sufficient quantity and purity to measure their physical and chemical properties. The success of this reaction is due to the low vapor pressure of lanthanum and the extremely high vapor pressures of samarium, europium and ytterbium (Daane 1951, 1961, Habermann and Daane 1961). It is interesting to note that this same technique has been the method of choice for the preparation of some transplutonium metals (Cunningham 1964). 

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. 

Ideally, a larger quantity of metal would be prepared, characterized, and then used for a number of scientific measurements or experiments. But with californium (and some of the other transplutonium metals), the preparation of the metal often becomes an integral part of a subsequent study, and either the major portion or the entire preparation is needed and often consumed (i.e. dissolution in add) in an experiment. 

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]. 

A correlation [93] relating crystal entropy to metallic radius, atomic wei t, magnetic properties, and electronic structure has permitted the accurate calculation of unknown entropies for these elements. This approach does require a defined electronic structure in order to predict accurate entropy values. Thermodynamics for the transplutonium metals have been sumnmrized [94,95]. 

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. 

Recently there has been interest in the sorptive behavior of natural clays toward metal ions potentially present in radioactive wastes. Initial studies of the transplutonium elements have been carried out to define their sorption behavior with such materials ( ). However, it is also important to understand the stability of the clay-actinide product with regard to radiation damage and to be able to predict what changes in behavior may occur after exposure to radiation, so that accurate transport models may be constructed. 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

In Table I are summarized some generally useful chemical reactions for the preparation of transplutonium element metal and some compounds. For simplicity, and because of variable oxidation states, the equations are not necessarily balanced. 

Table I. Preparative Chemistry for Transplutonium Element (An) Metal and Some Compounds...

Transplutonium(VI) complexes aqua,1220 carbonates, 1220 carboxylates chelating, 1220 halogens, 1220 monocarboxyiates, 1220 nitrato, 1220 oxides, 1220 oxoanions, 1220 Triethanolamine alkali metal complexes, 23 Triethylamine, 2,2, 2"-trimethoxy-alkali metal complexes, 24 Trimolybdates, 1032 Trithioarsenates, 249 1,3,6,2-Trithioarsocane, 2-chloro-, 249 Trithioraolybdates, 1378 Trivanadates, 1027... 

On the topic of lanthanide/actinide separation, few reviews have dealt in detail with the most difficult aspect of this field, separation of the lanthanides from the trivalent transplutonium actinides. Jenkins (1979,1984) reviewed ion exchange applications in the atomic-energy industry. Relatively short sections of these reviews dealt with the separation of the trivalent metal ions. Symposium volumes entitled Actinide Separations (Navratil and Schulz 1980) and Lanthanide/Actinide Separations (Choppin et al. 1985) are collections of papers from several authors covering all aspects of lanthanide/actinide separation, some of which deal with the purification of the trivalent metal ions. 

Thermodynamic measurements that have been made on the actinide metals are low- and high-temperature heat capacities, properties of phase transitions, and vapor pressures. At least one of these measurements has been made on each element through einsteinium unfortunately, however, none has been made on actinium, so that even its enthalpy of vaporization must be estimated. As of the time of writing (February 1986) vapor-pressure measurements have been made through einsteinium [22] and low-temperature heat-capacity measurements through americium [23] by innovative microscale methods, innovative microscale methods have been applied to determine vapor pressures and very low-temperature heat capacities of transplutonium actinides. 

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