Transition metal compounds phase stability

One of the most successful applications of crystal field theory to transition metal chemistry, and the one that heralded the re-discovery of the theory by Orgel in 1952, has been the rationalization of observed thermodynamic properties of transition metal ions. Examples include explanations of trends in heats of hydration and lattice energies of transition metal compounds. These and other thermodynamic properties which are influenced by crystal field stabilization energies, including ideal solid-solution behaviour and distribution coefficients of transition metals between coexisting phases, are described in this chapter. 

Chapter 7 discusses some of the thermodynamic properties of transition metal compounds and minerals that are influenced by crystal field effects. The characteristic double-humped curves in plots of thermodynamic data for suites of transition metal-bearing phases originate from contributions from the crystal field stabilization energy. However, these CFSE s, important as they are for explaining differences between individual cations, make up only a small fraction of the total energy of a transition metal compound. In the absence of spectroscopic data, CFSE s could be evaluated from the double-humped curves of thermodynamic data for isochemical compounds of the first transition series. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Electronic transitions like insulator-metal transitions, magnetic order-disorder transitions, spin transitions and Schottky-type transitions (due to crystal field splitting in the ground state in/element-containing compounds) profoundly influence the phase stability of compounds. A short description of the main characteristics of these transitions will be given below, together with references to more thorough treatments. 

The second mode of action of a modifier is direct reaction with the analyte to convert it into a phase with greater thermal stability, that is, to reduce analyte volatility. In this way, the charring stage can be carried out at higher temperatures, allowing a more efficient removal of the matrix but without the loss of analyte. Examples of this type of matrix modifier include transition metal ions (mainly Pd), which form thermally stable intermetallic compounds with analytes, and magnesium nitrate, which thermally decomposes to magnesium oxide, and in the process traps analyte atoms in its crystalline matrix it is thermally stable until 1100°C. In fact, the most frequently reported mixture for matrix modification consists of Pd(N03)2 and Mg(N03)2, proposed by Schlemmer and Welz as a universal chemical modifier.17... 

There are several approaches for obtaining spectral data for low-abundance transition metal ions, rare minerals and crystals of small dimensions. Data for a transition element in its chemical compounds, such as hydrates, aqueous solutions, molten salts or simple oxides, may be extrapolated to minerals containing the cation. Such data for synthetic transition metal-doped corundum and periclase phases used to describe principles of crystal field theory in chapter 2, appear in table 2.5, for example. There is a growing body of visible to near-infrared spectral data for transition metal-bearing minerals, however, and much of this information is reviewed in this chapter and the following one. These results form the data-base from which crystal field stabilization energies (CFSE s) of most of the transition metal ions in common oxide and silicate minerals may be estimated. 

The correlation between the valence electron counts and the stabilities of intermetallic phases and stmctures were also espoused by others, like the physical chemists Neds N. Engel (b. 1904) and Leo Brewer (1919-2005), although Hume-Rothery found their result somewhat controversial. The Engel-Brewer theory asserts that the crystal stmctures of transition metals and their intermetallic compounds are determined solely by the number of valence s and p electrons. For example, Engel suggested in 1949 that the BCC stmcture correlated with (where n is the total number of valence... 

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