Actinide sesquioxides

Isostructural compounds include some other lanthanide and actinide sesquioxides and oxide-chalcogenides such as La202X (X = S or Se) as well as Th2N2X (X = O, S, Se), Th2NOX (X = P, As) and analogous uranium compounds. In all cases the third anion is the one in octahedral coordination. (Antistructures include N2Li2Zr.)... 

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. 

Of the six known actinide sesquioxides (e.g., sesquioxides of Pu, Am, Cm, Bk, Cf and Es) only those of Cm, Cf and Es are resistent to air oxidation to ambient temperature. Due to the stability of their dioxides, the sesquioxides of Pu, Am and Bk readily take up oxygen, and require special storage (e.g., sealed containers) to avoid oxidation. All three oxides are oxidized readily when heated in oxygen-containing atmospheres below 1000°C. Above this temperature, oxygen is lost from AmOj. Heating higher oxides of Cm or Cf in air or in vacuum above 900°C produces the sesquioxides. 

The actinide sesquioxides have many similarities with the lanthanide sesquioxides, such as crystal structures (A, B and C forms), lattice parameters, etc., but there are also some significant differences. One notable difference is their melting points it appears that the transcurium sesquioxides have significantly lower (several hundreds of degrees C) melting points and display a different melting point trend with Z than for comparable lanthanide sesquioxides. The similarities and differences of the two f-series oxides are discussed in more detail in the following section. 

The best characterized einsteinium oxide is the cubic (C-type) sesquioxide, prepared by calcining nanograms of a nitrate salt and rapidly analyzing the product by electron diffraction (Haire and Baybarz 1973). A monoclinic form of the sesquioxide was observed when thin films of einsteinram metal were oxidized in oxygen-containing atmospheres at temperatures of 800 1000°C (the direct oxidation of other actinide metals can also yield the monoclinic form of their sesquioxides at temperatures lower than that required for transformation of their C- to the B- forms). This low temperature for the monoelinic form of einsteinium sesquioxide is therefore not comparable to the established transition temperatures reported for the other actinide sesquioxides. The... 

With regard to the actinide sesquioxides, excluding AC2O3, the first sesquioxide occurs at Pu in the series and the last experimentally established sesquioxide is ES2O3. 

The sesquioxides of the lanthanide and actinides are multiphasic. Figure 19 is a plot of phase formation as a function of temperature and radius of the lanthanide sesquioxides, and is based on a published plot (Chikalla et al. 1973, Schulz 1976). Included in a section of the plot are the radii for the first six actinide sesquioxides (Pu forms the first sesquioxide in the series if Ac is excluded) placed above comparable radii of the lanthanides, as opposed to their positions in the periodic table. If the very high temperature phases of the lanthanides (e.g., X, H phases) and the melting-point behaviors are excluded, there is a reasonable agreement between the expected and observed actinide sesquioxide behaviors based on radii. The X, H phases as such have not been reported for the actinide sesquioxides and there is a discrepancy with the melting points the latter is discussed below. 

Fig. 19. Phase diagram of the lanthanide and actinide sesquioxides as a function of ionic radius.

A summary and a comparison of the different phases of the lanthanide and actinide sesquioxides is given in fig. 22 taken from Baybarz and Haire (1976), where the molecular volumes of the hexagonal, monoclinic and cubic forms are plotted. There is a considerable densification in going from the cubic (six coordinated) to the monoclinic (six and seven coordinated) and finally to the hexagonal (seven coordinated) forms of these oxides. It is evident that the monoclinic form has been observed at a larger molecular volume in the lanthanide sesquioxide series than in the actinide sesquioxide... 

The behavior and the structural details for the so-called intermediate oxides (1.5 < O/M < 2.0) are not as well established for the actinides as they have been for the lanthanides. Given the range of the above O/M ratios, the five actinides which qualify as potentially being able to exhibit such intermediate oxides are Pu through Cf. This limitation arises as Ac and the transcalifomium elements do not have known dioxides, and the first actinide sesquioxide appears at the position of Pu in the series. 

Fig. 23. Lattice parameters of the lanthanide and actinide sesquioxides and mononitiides, and the atomic volumes of the respective metals.

A cycle for yielding the enthalpy of solution of the lanthanide and actinide sesquioxides has been used and is described by Morss. A graphical approach (see fig. 24) has been devised using data for the different crystal forms of the lanthanide and actinide sesquioxides, which is useful for predictive purposes. The enthalpies of formation and... 

The first actinide sesquioxide encountered is plutonium sesquioxide (if actinium sesquioxide is excluded vaporization data for it are not available) and Cf is the highest sesquioxide in the actinide series for which vaporization/decomposition data have been reported (Haire and Gibson 1992). Thus a comparison between the vaporization behaviors of the sesquioxides of the two f series can be made only between the lanthanides and the first six transneptunium oxides (Haire 1994). 

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 ... 

The enthalpy of solution of lanthanide and actinide sesquioxides is plotted as a function of molar volume in Fig. 17.4. Molar volume was chosen as a parameter because there are three different sesquioxide structures with different coordination numbers and numbers of molecules per unit cell the plot is referenced to one mole of M (MO 15 rather than M2O3) because trichloride data are shown on a similar plot and will be discussed below. Ionic radii [44] could have been used, since these are tabulated as a function of coordination number, but often they are reliable to only two significant figures. It is evident that, for all three structure types, the enthalpies of solution of actinide sesquioxides are significantly less exothermic than for structurally similar lanthanide sesquioxides. 

For predictive purposes, the enthalpies of solution of the other known sesquioxides (PU2O3 and Bk203) were estimated from Fig. 17.4. Enthalpies of solution and molar volumes of expected structural types of neighboring actinide sesquioxides were also estimated. Table 17.5 shows these estimates and also the... 

Table 17.5 Enthalpies of solution and formation of lanthanide and actinide sesquioxides. 

Fig. 20.4 Unit cells of the three forms of actinide sesquioxides A, hexagonal B, monoclinic and C, cubic. Only those atoms are included which are needed to show the coordination of the metal ions in each instance.

Morss, L.R. and Sonnenberger, D.C. (1985) Enthalpy of formation of americium sesquioxide systematica of actinide sesquioxide thermochemistry, /, Nucl. Mater., 130, 266-272,... 

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