Actinide thermodynamics

After the description of binary alloys the focus will be on ternary alloys with a rare earth metal as one constituent which exhibit extraordinary properties. In the last decades the so-called Heavy Fermion Systems (HFS) [16-18] attracted a growing interest in sohd state physics constituting a widely studied class of strongly correlated systems. These systems are binary or ternary compounds with lanthanides or actinides. Thermodynamic investigations of these materials result... 

Table 52 reports the enthalpies and entropies of formation of As-based alloys. With rare-earth elements the enthalpies of formation are known for the 1 1 composition. For the actinides thermodynamic data are known only for the U-As system. The enthalpies of formation of all Ln-As compounds are nearly the same, the enthalpy of formation of U-As being clearly less negative (fig. 135). 

The rare earths (except Eu and Yb) and actinides exhibit strong interactions with many metallic elements, Be and Mg, the late transition elements (columns headed by Mn, Fe, Co and Ni), and with the elements of columns IB to VB. The corresponding phase diagrams exhibit intermetallic compounds. With elements of column lA and column IIA from calcium to barium, comparison of rare earth and actinide thermodynamic behavior is difficult because many phase diagrams are unknown or subject to caution. With early transition metals, rare earths exhibit positive deviations from ideality, characterized in the phase diagram by a miscibility gap in the liquid phase, while actinides are completely miscible in these elements in the liquid state. 

Szabb Z, Toraishi T, Vallet V, Grenthe I (2006) Solution coordination chemistry of actinides thermodynamics, stmcture and reaction mechanisms. Coord Chem Rev 250(7-8) 784—815... 

Many experimental and theoretical studies of thermochemical and thermophysical properties of thorium, uranium, and plutonium species were undertaken by Manhattan Project investigators. Some of these reports appeared in the National Nuclear Energy Series [1]. These papers, and others in the literature through 1956, formed the basis for Table 11.11 Summary of thermodynamic data for the actinide elements of the first edition of this book. That table, completed by J. D. Axe and E. F. Westrum Jr, listed 126 species, of which the properties of 40 were estimates. A fair measure of the progress in actinide thermodynamics is the number of subsequent research papers and reviews another measure is the 731 species included in Table 17.14 of this chapter, few of which are estimates. 

Until recently, the reviews of actinide thermodynamics lagged behind the reports of these measurements themselves. In 1952 two monumental works... 

Critical efforts to compile and to assess actinide thermodynamic properties have improved in more recent years. Krestov [10] prepared an extensive compilation of rare-earth and actinide thermochemical properties. Rand [11] comprehensively and critically reviewed thorium thermodynamics, and the thermodynamics group of the US National Bureau of Standards [12] published the final volume of the Technical Note 270 series, which included the elements actinium through uranium. At nearly the same time the parallel compendium of Glushko et al. [13] was published in the USSR. The most contemporary and thoroughly annotated compilation is the fourteen-part series issued under the auspices of the International Atomic Energy Agency, 7%e Chemical Thermodynamics of Actinide Elements and Compounds, of which nine volumes [14-21, 354] have been published as of the time of writing. 

An important correlation between trivalent f-block ions and their Trivalent atoms (f" ds ) is the P(M) function proposed by Nugent et al. [29]. This function has been utilized for predicting enthalpies of sublimation of metals and enthalpies of formation of aqueous ions. David et al. [27] used heavy-actinide thermodynamic properties to establish a P(M) function relating all of the actinide metals and their 3 + aquo ions. Morss and Sonnenberger [ 103] used newer data to refine this P(M) and to develop similar P(M) plots relating f-block metals and their sesquioxides and trichlorides (Figs 17.5 and 17.6). 

Bratsch SB, Lagowski JL (1986) Actinide thermodynamic predictions. 3. Thermodynamics of compounds and aquo-irms of the 2+, 3+, and 4+ oxidation states and standard electrode potentials at 298.15 K. J Phys Chem 90 307-312 (1987) Predicted and expraimcaital standard electrode potentials in liquid ammonia at 25°C. J Solution Chem 16 583-601... 

F. L. Getting, M. H. Rand, and R. J. Ackermaim, ia F. L. Oettiag, ed.. The Chemical Thermodynamics of Actinide Elements and Compounds, Part 1, The Actinide Elements, SHlPDBj424j 1, IAEA, Vienna, Austria, 1976. 

A critical assessment of the chemical thermodynamic properties of the actinides and their compounds is presently being prepared by an international team of scientists under the auspices of the International Atomic Energy Agency (Vienna). As a result of this effort, four publications (1, 2, 3, 5) have already become available and a further ten 6-T4), including the halides (8) and aqueous complexes with Tnorganic ligands (12),... 

Complex chlorides of plutonium (34, 41) do not present such a wide range of formulae as the complex fTuorides but we have at hand thermodynamic information on two important species which have also been characterized with other actinides. In table II we have disregarded the complex halides for which no thermodynamic data are available. The enthalpy of formation of Cs2NaPuClg(c) (55) and Cs2PuClg(c) (56) have been obtained from enthalpy of solution measurements."The selected (8) values are AHf(Cs2NaPuCl6,c) =... 

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. 

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. 

Equilibrium vapor pressures were measured in this study by means of a mass spectrometer/target collection apparatus. Analysis of the temperature dependence of the pressure of each intermetallic yielded heats and entropies of sublimation. Combination of these measured values with corresponding parameters for sublimation of elemental Pu enabled calculation of thermodynamic properties of formation of each condensed phase. Previ ly reported results on the subornation of the PuRu phase and the Pu-Pt and Pu-Ru systems are correlated with current research on the PuOs and Pulr compounds. Thermodynamic properties determined for these Pu-intermetallics are compared to analogous parameters of other actinide compounds in order to establish bonding trends and to test theoretical predictions. 

Reliable data on the thermodynamic and phase relationships of actinide oxide systems are essential for reactor safety analysis. This paper reviews certain aspects of thermodynamic data currently available on the nonstoichiometric Pu-0 system, which may serve as a basis for use in reactor safety analysis. Emphasis is placed on phase relationships, vaporization behavior, oxygen-potential measurements, and evaluation of pertinent thermodynamic quantities. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Fuger, J. Oetting, F.L. "The Chemical Thermodynamics of Actinide Elements and Compounds. Part 2. The Actinide Aqueous Ions" International Atomic Energy Agency Vienna, 1976. 

Manes L, Benedict U (1985) Structural and Thermodynamic Properties of Actinide Solids and Their Relation to Bonding. 59/60 75-125... 

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

The ionic radii of the commonest oxidation states are presented in Table 2. There is evidence of an actinide contraction of ionic radii as the 5/ orbitals are filled and this echoes the well established lanthanide contraction of ionic radii as the 4/orbitals are filled. Actinides and lanthanides in the same oxidation state have similar ionic radii and these similarities in radii are obviously paralleled by similarities in chemical behaviour in those cases where the ionic radius is relevant, such as the thermodynamic properties observed for halide hydrolysis. 

A guiding principle for the solvent extraction chemist is to produce an uncharged species that has its maximum coordination number satisfied by lipophilic substances (reactants). Eor trivalent lanthanides and actinides (Ln and An, respectively), the thermodynamic data suggest a model in which addition of one molecule of TBP displaces more than one hydrate molecule ... 

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. 

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