Valence sesquioxides

Figure 8.23A shows a simplified Eh-pH diagram for the Mn-O-H system. Within the stability field of water, manganese occurs in three valence states (2+, 3 +, and 4+). Figure 8.23A shows the condensed phases relative to the three valence states as the hydroxide pyrochroite Mn(OH)2 (2+), multiple oxide haus-mannite Mu304 (2+, 3 + ), sesquioxide Mu203 (3 + ), and oxide pyrolusite Mn02 (4+). 

The most stable oxidation state of thuhum is -i-3. Only the tripositive Tm3+ ion is encountered in aqueous media. The metal also forms compounds in +2 and +4 valence states, but there is no evidence of Tm2+ and Tm + existing in aqueous phase. Thuhum is relatively stable in air at ambient temperature. However, it combines with oxygen on heating forming its sesquioxide, Tm203. 

The XPS valence band spectra for the dioxides of the transuranium elements (from Np to Bk) have been presented in an extensive and pioneering work that also includes core level spectra and has been for a long time the only photoemission study on highly radioactive compounds. High resolution XPS spectra (AE = 0.55 eV) were recorded on oxidized thin metal films (30 A) deposited on platinum substrates with an isotope separator. (The oxide films for Pu and the heavier actinides may contain some oxides with lower stoichiometry, since starting with Pu, the sesquioxides of the heavier actinides begin to form in high vacuum conditions.)... 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

Sometimes the magnetic method reveals the existence of valence bonds (p. 38) between adjacent positive ions, such as between iron and iron in hydrous (p. 80) or supported (p. 68) iron sesquioxide. 

Electronic stmctures of some sesquioxides were calculated from the special interests on these materials properties. For example, Skorodumova et al. [5] calculated the electronic structure of Ce203 by FP-LMTO method for the purpose of clarifying the role of oxygen-poor Ce203 in catalytic property and its band gap and magnetic property were discussed on the basis of valence-band and core-state... 

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. 

The lanthanide oxides find important applications in the catalysis, lighting, and electronics industries. In particular, the design of advanced devices based upon the integration of lanfhanide oxides with silicon and other semiconductors calls for a defailed undersfanding of the bonding, electronic, and dielectric properties of fhese materials (Scarel et al., 2007). Here, we use the SIC-LSD to address the issue of the lanthanide valence in the dioxides RO2 and sesquioxides R2O3, for R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, and Ho. 

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