Thermodynamics lanthanide oxides

Table VII. Thermodynamics of Formation of Other Lanthanide Oxides at 298.15...

A review of the binary lanthanide oxides has already appeared in an earlier volume of this series (Eyring 1979). A review of the Seltenerdelemente appeared in Gmelin Handbuch der Anorganischen Chemie (1974). The preparative methods and characterization of the many phases of the lanthanide oxides have recently been published (Eyring 1991). The structures and transformations of the rare-earth oxides have recently been reviewed (Schweda et al. 1991, Schweda 1992). Some aspects of these reviews are summarized and brought up-to-date here. A review of thermodynamic properties can be found in the chapter by Morss (ch. 122) in this volume. 

The thermodynamic properties of the lanthanide oxides 1.4.1. The derived Junctions... 

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. 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

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. 

Manes, L. A new method of statistical thermodynamics and its application to oxides of the lanthanide and actinide series, in Non-stoichiometric Oxides (ed. Sorensen, O. T.), Academic Press, New York, Chapt. 3, p. 99 (1981)... 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

Based on the fluorite-type module theory the thermodynamic properties, hysteresis, and reactions between the homologous series can be elucidated and the structures of homologous series experimentally discovered may be modeled. Using these principles a wide range of non-stoichiometric ternary lanthanide higher oxides from RO2 to R2O3 were founded. 

Several review articles and books on the lanthanide higher oxides, which include thermodynamic properties, have been published (Eyring, 1979 Haire and Eyring, 1994 Trovarelli, 2002 Adachi and Imanaka, 1998 Adachi et al., 2005). The systematic thermodynamic data of the cerium, praseodymium, and terbium oxides can be found in Bevan s and Eyring s papers (Hyde et al., 1966 Hyde and Eyring, 1965 Bevan and Kordis, 1964). 

Of the 15 experimentally known phases of the higher oxides only five of them have been determined by X-ray and neutron diffraction using the Rietveld refinements method. To understand the thermodynamic behavior and phase reactions it is helpful to have a model of the undetermined structures. Using the experimental electron diffraction data it is possible to determine the symmetry of the unit cell and develops a transformation matrix between the fluorite and ten of the intermediate phases as shown in Table 2. The module theory provides a method for modeling the unknown structures of the homologous series of the lanthanide... 

The lanthanide higher oxides have not only peculiar thermodynamic properties, but also unique physical and chemical properties. The physical and chemical properties are presented as a macroscopic parameter, such as the electrical conductivity, the coefficient of expansion, and the conversion rate of a catalysis process. Due to the lack of knowledge of the wide range of non-stoichiometry of the oxygen-deficient fluorite-related homologous series of the lanthanide higher oxides, the macroscopically measured data of the physical and chemical properties are scattered, and therefore, based on the structural principle of the module ideas a deep understanding the relationship between the properties and structures is needed. 

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