Thermodynamic of actinide metals

This treatment aiming to evaluate thermodynamically the orbital character of the bond in actinide metals, follows closely the general features illustrated above and has a particular value inasmuch as it is accompanied by a fairly comprehensive survey of the chemical and physical properties of actinide metals known at that time. In it, the metallic radius and the crystal structures are taken as valence indicators AH nd Tm as the bonding indicators . The metallic valence, however, is not taken as constant throughout the actinide series, but rather allowed to vary. The particular choices are justified by physical and chemical arguments, which are taken in support of the hypothesis chosen. 

In the treatment of Johansson and Rosengren, no attempt is made, as in the previous treatment, to deduce fractional degrees of participation of different orbitals in the bonding of actinide metals, but, rather, attention is focussed on the correct attribution of metallic valences and on the discrepancy found between the thermodynamically calculated Ecoh in a certain valence state and the experimental one. This difference is attributed to the bonding character of the 5 f orbitals due to itineracy. 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

Comparable recent detailed reviews of the actinide halides could not be found. The structures of actinide fluorides, both binary fluorides and combinations of these with main-group elements with emphasis on lattice parameters and coordination poly-hedra, were reviewed by Penneman et al. (1973). The chemical thermodynamics of actinide binary halides, oxide halides, and alkali-metal mixed salts were reviewed by Fuger et al. (1983), and while the preparation of high-purity actinide metals and compounds was discussed by Muller and Spirlet (1985), actinide-halide compounds were hardly mentioned. Raman and absorption spectroscopy of actinide tri- and tetrahalides are discussed in a review by Wilmarth and Peterson (1991). Actinide halides, reviewed by element, are considered in detail in the two volume treatise by Katzet al. (1986). The thermochemical and oxidation-reduction properties of lanthanides and actinides are discussed elsewhere in this volume [in the chapter by Morss (ch. 122)]. 

Table 17.1 Thermodynamic properties of actinide metals and aqueous ions at 25°C. 

Table 19.3 Heats of sublimation and crystal entropies of actinide metals (see Chapter 17 for complete listings of thermodynamic data).

Relatively few thermodynamic studies have been performed on compounds involving Th, U and Pu with noble metals. Most of the previous work has involved electrochemical cell determinations of free energies of formation, hence little has been published concerning the sublimation behavior of actinide intermetallics. 

These considerations have brought actinide researchers to develop models for actinide metals which are based not only on structural but also on thermodynamic properties (Brewer-type models). Here, the 5 f participation in the conduction band is usually taken into consideration. A review of these models appears in Chap. C. 

A concept related to the localization vs. itineracy problem of electron states, and which has been very useful in providing a frame for the understanding of the actinide metallic bond, is the Mott-Hubbard transition. By this name one calls the transition from an itinerant, electrically conducting, metallic state to a localized, insulator s state in solids, under the effect of external, thermodynamic variables, such as temperature or pressure, the effect of which is to change the interatomic distances in the lattice. 

In Chap. C, the thermodynamic and structural outlook of the bond, which had been the matter of discussion in Part A of this chapter, is further developed, and the model formalism, which takes advantage of the well known Friedel s model for d-transition metals but is inspired by the results of refined band calculations, is presented for metals and compounds. Also, a hint is given of the problems which are related to the nonstoichiometry of actinide oxides, such as clustering of defects. Actinide oxides present an almost purely ionic picture nevertheless, covalency is present in considerable extent, and is important for the defect structure. 

Examples of Semi-Theoretical Thermodynamic Treatments for the Actinide Metal Series... 

It is possible to estimate the metallic radii (or volumes) and the bulk moduli of the light actinide metals by the well known thermodynamic equations ... 

From a thermodynamic viewpoint, we may imagine that, in an actinide metal, the model of the solid in which completely itinerant and bonding 5 f electrons exist and that in which the same electrons are localized, constitute the descriptions of two thermodynamic phases. The 5f-itinerant and the 5 f-localized phases may therefore have different crystal properties a different metallic volume, a different crystal structure. The system will choose that phase which, at a particular T and p (since we are dealing with metals, the system will have only one component) has the lower Gibbs free-energy. A phase transition will occur then the fugacity in the two possible phases is equal e.g. the pressure. To treat the transition, therefore, the free energies and the pressures of the two phases have to be compared. We recall that ... 

Control of the particle valence/conduction band oxidation/reduction potential is not only achieved through a judicious choice of particle component material band edge redox thermodynamics of a single material are also affected by solution pH, semiconductor doping level and particle size. The relevant properties of the actinide metal are its range of available valence states and, for aqueous systems, the pH dependence of the thermodynamics of inter-valence conversion. Consequently, any study of semiconductor-particle-induced valence control has to be conducted in close consultation with the thermodynamic potential-pH speciation diagrams of both the targeted actinide metal ion system and the semiconductor material. 

The predicted waste inventory for the repository indicates that potentially significant quantities of the organic ligands—acetate, citrate, oxalate, and EDTA—will be present (US DOE, 1996). Actinide interactions with these compounds were not considered in the speciation and solubility modeling, as calculations suggested that they would be mostly complexed by transition metal ions (Fe, Ni " ", Cr, and Mn " ") released by corrosion of the steel waste containers and waste components. A thermodynamic model of actinide-ligand interactions appropriate to brines will be included in solubility calculations for WIPP recertification. 

A method is being developed to transform actinide ions in the near surface environment to less soluble, less reactive, thermodynamically stable phosphate minerals phases through application of organophosphorus complexants. These complexants decompose slowly, releasing phosphate to promote the formation of stable phosphate mineral phases, particularly with the more soluble trivalent, pentavalent, and hexavalent actinide ions. The complexant of choice, myo-inositol(hexakisphosphoric acid) or phytic acid, is a natural product widely used as a nutritional supplement. We have determined that phytic acid decomposes slowly in the absence of microbiological effects, that crystalline phosphate minerals are formed as a consequence of its decomposition, and that the formation of actinide (lanthanide) phosphates reduces the solubility of trivalent and hexavalent metal ions under environmental conditions. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

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