Sesquioxide, thermodynamic properties

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. 

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. 

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). 

Physical and thermodynamic properties of rare earth sesquioxides. 

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 ... 

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