Mixed-valence phase

Decomposition of WO3, M0O3, Ti02, etc., yielding mixed-valence phases is another reaction of interest ... 

A single phase requires specific conditions to be highly active and selective. It has to possess a thermodynamically possible redox with regard to the reactants. It must be a mixed valence phase at the steady state. It must possess a particular morphology exhibiting statistically the selective lattice planes at the surface and must have optimal semiconductivity by "small polarons". 

Oxides. TijOa has been investigated by X-ray spectroscopy and by e.p.r. spectroscopy and magnetic susceptibility measurements - there has been an improved MO calculation of its band structure. A band model has been proposed to account for the origin of the magnetic moment and metallic transition of vanadium-doped Ti203- According to e.p.r. measurements the Ti ions in mixed valence phases of the system Ti 0 mined. 

This principle makes it possible to predict the sign of conduction of a mixed-valence phase, to explain the appearance of electrical conduction, and to select reliably ways of preparing materials with prescribed thermoelectric properties. The problem of the simultaneous introduction of several impurities into a semiconductor is considered. The role of cation and anion vacancies is determined. The thermo-emf power is estimated from the possibility of the appearance of ions of "abnormaT valence and from the role of the resultant polar bonds. The limits of the validity of Vetwey s principle are considered. 

Whilst no magnetic scattering has been identified in the mixed valence phase of SmS, it has been seen in Sm 25S by Mook et al. (1978) who observed a very broad peak with a FWHM of 15 meV at 30 meV. The addition of yttrium has a similar effect to that of pressure in contracting the lattice. At this composition, the valence changes with temperature from 2.3 to 2.45 at about 200 K (Weber et al. 1989). This means that the compound is at a configuration crossover with the f level effectively pinned at the Fermi level Ep, — Ep-i = p) such that the ground state consists of a hybridised mixture of the two configurations. The valence is then determined by thermodynamic considerations in which the ionic excitation... 

The specifics of the copper chemistry in aqueous systems can be manifested in an interesting reaction when the mixed-valence phase dissolves in aqueous ammonia (Fjellvag et al. 1987)... 

With M/(Fe + M) >0.15, a spinel phase (MFe204) formed in all cases and when this ratio exceeded 0.33, Cu and Ni precipitated as separate phases (Tab. 14.3). Formation of a spinel phase requires a threshold level of in the system. It is considered that the spinel phase nucleates in the water layer adsorbed on or adjacent to, the surfaces of the ferrihydrite particles and that these nuclei grow by addition of soluble M-Fe-hydroxo complexes released by the dissolving M-ferrihydrite (Cornell Giovanoli, 1987, 1989 Giovanoli Cornell, 1992). Tronc et al (1992) suggested that when the ratio is very low, a different mechanism operates a mixed valence... 

Although there are many features common to synthetic oxides and minerals, fundamental studies of the charge-transfer processes in mixed-valence compounds can only be systematically carried out on synthetic oxides of controlled stoichiometry and impurity concentration. However, with the exception of Seebeck coefficients, transport measurements require single-crystal data if quantitative interpretations are to be made. Nevertheless, conductivity data for polycrystalline samples of cubic phases are useful if the samples are dense and care has been taken to eliminate any segregation of impurities into the grain boundaries. 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

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