Valence stability

Los Alamos is processing a wide variety of residues, including Pu-Be neutron sources, polystyrene-Pu02-U02 blocks, incinerator ash, Pu-U alloys and oxides, Pu-Zr alloys and oxides, Pu-Np alloys and oxides, Pu-Th alloys and oxides, etc. Processes have been developed for these scrap items (see Figure 2), but we need to know more about Pu-Np separations Pu-Th separations oxalate precipitations for both plus 3 and plus 4 valences valence stabilization dissolution methods for high-fired impure oxides in-line alpha monitors to measure extremely low concentrations of Pu and Am in HNO3 solutions and solubility of various mixtures of Pu02 and UO2 under a variety of conditions. 

A similar oxidation-reduction mechanism in the carbon monoxide oxidation reaction on oxide catalysts has been proposed by Benton (71), Bray (72), Frazer (73), and Schwab (74). In this reaction also, Mooi and Selwood (57) found that a decrease in the percentage of iron oxide, manganese oxide or copper oxide on the alumina support first increased the rate, and then at lower percentages decreased the rate, of carbon monoxide oxidation, indicating that valence stabilization is again operative in these cases. 

Among families of post-transition metals if there are two possible valences, the lower valence is most stable among the heavier members that is, the inert pair is most inert at the bottom of a group. Note that this trend in valence stability is opposite from that within the transition metal families where the lower valence states are more stable among the lighter members. 

Atomic Valence Stability by Golden Ratio Imbalance.66... 

ATOMIC VALENCE STABILITY BY GOLDEN RATIO IMBALANCE... 

Dorenbos P (2(K)5) Valence stability of lanthanide ions in intn-ganic compounds. Chem Mater 17 6452... 

Since those cathodes include the transition metal oxide, their valence state may change under polarization. This means that the interface stability depends on temperature and oxygen potential so that operation conditions should affect the interface stability. A most typical example is the LSM/YSZ interfaces. Since the tetravalent Mn ions are relatively stable in the perovskite lattice, this causes the formation of A-site deficiency in Lai yMn03. Since this is related with the valence stability, behaviors associated with the A-site deficiency is sensitively dependent on the Sr content in LSM, polarization directed and its magnitude, and temperature. 

Solid Oxide Fuel Cells, Thermodynamics, Fig. 4 Chemical potential diagrams for the La-Mn-Zr-O and the La-Co-Zr-0 system and analyses in terms of the stabilization energy and the valence stability of transition metal oxides... 

Let us now turn to the question of valence stability in EuO at higher pressures. L, edge measurements (Rohler et al. 1985) suggested a nonlinear increase of valence between 5 and 12 GPa. Mossbauer data disproved this claim since the isomer shift neither showed irregularities nor particular temperature dependences between 4.2 and 300 K... 

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. Data from temperature-programmed reduction (TPR) measurements indicate that the stability (or reducibility) of the perovskite oxides increases (decreases) with increasing size of the A ion, which would be consistent with the preferred occupancy of the larger Lrf ion in a 12-fold coordination. The trend is just the reverse of that of the stability of the corresponding binary oxides. The ease of reduction increases by partial substitution of the A ion, e.g., La by Sr. Trends in the thermodynamic stabilities of perovskite oxides have been systematized in terms of the stabilization energy from their constituent binary oxides and the valence stability of the transition metal ions by Yokokawa et al. ... 

In the heteronuclear Ru(III)-Fe(II)-Ru(II) system listed in Table 1.1, Ru-Fe transitions but not Ru-Ru are seen in the spectrum. Electrochemical studies of a series of mixed-metal/mixed-valence complexes have been used to estimate mixed-valence stabilization effects. For example, in a series M [Fe(CN)5] , where M may be an organic group or a metallo-organic group, trends in Fe(III/II) can be correlated with electrostatic interactions, but they also reveal small mixed-valence stabilization effects. 

Table 2.2 summarizes and compares various features of perovskite cathodes from the aspect of valence stability. Valence stability is directly related with chemical stability and also indirectly with electrochemical activity through oxide ion conductivity. 

The valence stability itself is defined as the thermodynamic properties, so that it is natural to expect that the chemical stability of perovskite oxides is related to the valence stability in addition to the stabilization energy of double oxides from the constituent oxides. Particularly, the reaction of perovskite oxides with YSZ has been well examined experimentally as well as thermodynamically. 

Lattice Structure, Oxygen Nonstoichiometry, and Valence Stability... 

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