Sesquioxides thermodynamic

This paper reviews data on certain thermodynamic aspects of 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. Limited high temperature oxygen potential data obtained above the fluorite, diphasic, and sesquioxide phases In the Pu-0 system are presented. 

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 ... 

The rate of hydration is also sensitive to the preparation procedure and/or the pre-treatment applied to the oxides [111]. This suggests that the differences noted between results reported in [39] and [107,111,112] probably have a kinetic origin, and therefore, that there are no thermodynamic limitations to the bulk hydration of the whole series of rare earth sesquioxides. 

To summarize, the experimental studies commented on above show that aging-in-air is very relevant in the chemistry of the lanthanoid sesquioxides. We may also conclude that both, the intensity of the process and the actual nature of the aged phases are kinetically controlled. Consequently to this, the rare earth oxides exhibit a rich variety of behaviors, far more complex dian presumed on the basis of purely thermodynamic considerations. In this respect, it is worth recalling that, after [32], for... 

In this chapter we have shown that the thermochemistry of the rare-earth oxides is well established for the majority of the stoichiometric compounds. The thermodynamic properties follow clear trends that can be understood in terms of valence states and electronic configurations of the lanthanide ions and metals. For the sesquioxides, the principal group of rare earth compounds, the data are reliable up to 2000 K, an interval in which the A, B and C phases are stable. However, for the high-temperatures, where the H and X phases are stable, no experimental data exist. 

This section discusses and tabulates best values for important experimentally derived thermodynamic properties of free atoms and ions, aquo-ions, halides, dioxides, sesquioxides and tripositive hydroxides in an effort to characterize the properties of... 

The thermochemical and thermophysical properties of the rare earth sesquioxides were critically evaluated in 1973 (Gschneidner etal. 1973). A systematic comparison of rare-earth and actinide sesquioxides was published in 1983 (Morss 1983). Thermodynamic properties of europium oxides were assessed by Rard (1985). Since then the enthalpies of formation of AmjOj and CfjOj were determined by solution microcalorimetry. The Afif [Am (aq)] has been redetermined even more recently so the Af//°[Y (aq)] has been corrected in table 4. Recently, the enthalpy of formation of YjOj was redetermined by combustion calorimetry (Lavut and Chelovskaya 1990) and independently by solution calorimetry (Morss et al. 1993). The latter determination took advantage of a determination of Afff [Y (aq)] that used very pure Y metal (Wang et al. 1988). Assessed values are listed in table 4. 

Thus, one finds that the sesquioxides of Th, Pa, U and Np are thermodynamically unstable with regard to disproportionation to the metals and dioxide (Morss 1986). An overview of the enthalpies of formation and standard entropies for some binary actinide oxides are listed in table 26 (from Katz et al. 1986a). Morss (1986, see also ch. 122 in this volume) has described the cycle employed in obtaining the enthalpies of solution and formation for these oxides. The enthalpies of solution of the actinide oxides is expected to change slowly and smoothly as a function of ionic size of the metal ion. The enthalpies of formation of Am sesquioxide (Morss and Sonnenberger 1985), of Cm sesquioxides (Morss 1983), and of Cf sesquioxide (Morss et al. 1987, Haire and Gibson 1992) have been measured directly. 

With the lanthanide sesquioxides, the high-temperature vapor species encountered above the molten or solid oxides range from R, RO, RjO and R2O2. Thermodynamic properties of the lanthanide oxides have been given in section 1 and are therefore only reviewed briefly in this section for ease of comparison. 

Chermin, in a series of articles, discusses the thermodynamics of some classes of hydrocarbon and hydrocarbon derivatives. Edmister, in Applied Hydrocarbon Thermodynamics , includes information on equations of state, Mollier charts for pure hydrocarbons, compression and expansion charts for gases, and details of petroleum distillation calculations. Green has reviewed the thermodynamic properties of organic oxygen compounds and the thermodynamic properties of the normal alcohols Cl to Ci2. Justice has treated the thermodynamic properties and electronic energy levels of some rare-earth sesquioxides. 

B. H. Justice, Thermodynamic Properties and Electronic Energy Levels of Eight Rare-earth Sesquioxides , University of Michigan, Ph.D. thesis, 1961, University Microfilms Inc., Ann Arbor, Michigan. See also B. H. Justice and E. F. Westrum, jun., J. Phys. Chem., 1963, 67, 339, 345, 659. 

It has been generally assumed that the rare-earth sesquioxides vaporize con-gruently with almost stoichiometric composition though a few studies (White et al. 1962, Ackermann and Rauh 1971, 1975) have shown small deviations (R2O2 99 to R2O2 995) from ideal stoichiometry. But this approximation had insignificant contribution to the reliability of the thermodynamic data of monoxides. 

Unlike the 4f elements, for which sesquioxides are ubiquitous, only the sesquioxides of Ac and Pu through Es have been prepared. Sesquioxides of Th through Np are clearly thermodynamically unstable with respect to disproportionation to the metals and the much more stable dioxides. (Those of heavier actinides would be obtainable if their half-lives were much longer and nuclear yields more favorable.) An overview of known actinide oxides, and their enthalpies of formation and standard entropies, was given earlier in Table 14.9 although most of the actinide sesquioxides have been known since before 1970 (Am203 since before 1950), only in the past decade have thermophysical [98] and thermochemical [99] properties been determined. Since the optimum solvent for solution calorimetry of these sesquioxides is moderately concentrated hydrochloric acid, a systematic approach to the prediction of the enthalpies of formation of other sesquioxides is to devise a cycle yielding the enthalpy of solution in infinitely dilute add ... 

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). 

Table 14.9 showed that all the dioxides from Th02 through Cf02 are known, but that several have not been studied thermodynamically. Morss and Fuger [39-41, 157] have established an enthalpy-of-solution plot (Fig. 17.7) for f-block dioxides that is similar in principle to that discussed above for sesquioxides but shows no shift between 4f and 5f elements. (If greater covalence of 5f ions than 4f ions with... 

Physical and thermodynamic properties of rare earth sesquioxides. 

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