Lanthanide sesquioxides

The second scheme involves the enthalpy of solution of the lanthanide sesquioxide and the ... 

Figure 4-1. Schematic energy diagrams of lanthanide sesquioxides. (Energy diagrams of sesquioxides of light lanthanides) (Reproduced with permission from ref.8 and 9. Copy right 1985 Springer-Verlag)...

As described in the beginning of the section, we can find several investigations focusing the electronic stmeture of a lanthanide sesquioxide such as La203 and... 

Justice and Westrum [7] measured the low-temperature heat capacity of ten lanthanide sesquioxides by adiabatic calorimetry in the 5 K to 350 K range, from which the standard entropy can be derived, though the complex electronic structure of the f-element ions result in significant contributions at very low temperatures. 

Figure 7-1. Enthalpies of solution of the lanthanide sesquioxides as a lunction of Z note that for Sm and Eu data for two crystal modifications are known.

Gschneidner [28,29] showed that the enthalpies of formation of several classes of lanthanide compounds can be correlated systematically as a function of atomic number. He pointed out [30] that the correlations for europium and ytterbium are anomalous because they are divalent in their metallic state but trivalent in the compounds. As shown in Figure 1, the enthalpies of formation of the lanthanide sesquioxides (or of any other class of compounds of R ) do not change in a smooth fashion as a function of Z or of the ionic radius of R. These enthalpies of formation correspond to the reactions that appear to be similar throughout the rare earths,... 

The entropies of most of the lanthanide sesquioxides have been determined by low-temperature heat capacity measurements, principally by Justice and Westrum Jr. [7,33-38]. Recently new measurements for Y2O3 [39] and Ce203 [24] and the first measurements of Pt203 [40] have been reported. The measurements for the lanthanide sesquioxides reveal the presence of Schottky and Kramer components related to electronic stmcture of the lanthanide ions. The trend in the lanthanide sesquioxide series, shown in Figure 7-3, is t3qjical for many lanthanide compounds, as discussed by Konings [41]. The heat capacity, as well as the entropy, can be described by a lattice and an excess component, the latter representing the Schottky... 

Figure 7-3. The entropy (top) and heat capacity (bottom) of the lanthanide sesquioxides at T = 298.15 K, showing the linear lattice component derived for the f , f and f configuration ( ) calculated values ( ) and the experimental values (o).

Figure 7-4. The heat capacity of the lanthanide sesquioxides La203, Gd203 and LU2O3 as a function of temperature.

Enthalpies of formation remain to be measured for some of the lanthanide sesquioxides, e.g. La203(bcc), Nd203(monoclinic) and Gd203(monoclinic). The enthalpy of formation of Ce203 remains in doubt because of the discrepancy between solution and combustion calorimetry values (section 4.1). The thermochemical properties of metallic monoxides should be measured, but only when stoichiometric samples that are fi ee of unreacted metal can be synthesized. 

FIGURE 18 Calculated equilibrium volumes (in A ) of the lanthanide sesquioxides, crystallizing in the hexagonal A-type structure and the cubic C-type structure. The triangles, squares, and circles refer to PAW (Hirosaki et al., 2003), SIC-LSD (present), and experimental results (Eyring, 1978), respectively. 

From experiment, we know that whereas all the lanthanide sesquioxides can be found in nature, the only lanthanide dioxides that occur naturally are Ce02, Pr02, and Tb02. On the other hand, SIC-LSD theory concludes that all the lanthanide sesquioxides prefer the trivalent ground state configuration, whereas the dioxides can be separated into tetravalent light lanthanide dioxides, including... 

The formation of lanthanide sesquioxides from the thermal decomposition of their salts (Eyring 1991). 

Fig. 8 Polymorphic transformation temperatures for the lanthanide sesquioxides (after Foex and Traverse 1966b, Warshaw and Roy 1961).

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 actinides are probably more like the lanthanides in their oxide forms than in the solid phases of their other compounds. This is especially true when considering the metallic states, where large deviations are apparent. If an actinide exists in a particular oxide stoichiometry (e.g., a sesquioxide), it is likely to have comparable chemical and physical behavior to that of a lanthanide sesquioxide that has a similar ionic radius. The first important point for these two series of oxides is whether a particular stoichiometry is formed by different members of each series. 

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. 

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... 

Fig. 24. The enthalpies of solution of actinide and lanthanide sesquioxides (three different values exist for plutonium sesquioxide).

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