Variable valence states

Irrespective of origin, all iron compounds tend to the stable Fe state in air. Iron itself corrodes in moist air and reacts with nonoxidising acids to yield H2 and Fe(ll) salts. Fe(II) salts are subsequently oxidised to Fe(IU) salts in air. This is a slow reaction in acidic solution and is rapid in a basic 

These confusing relations occur for most transition metals, as well as for the lanthanides and actinides. The various stability regions are most easily understood via the appropriate Pourbaix diagram. 

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The structure of WO3 is of the Re03 type, as shown in Fig. 10.4.4(a). The tungsten bronzes of the formula MxW03 (0 < x < 1) are derived from the three-dimensional network of WO3, in which the W atoms adopt variable valence states, and the large unoccupied interstices conveniently accommodate other cationic species. When a portion of the WfH cations is converted to W5+, the requisite cations M (normally Na or K but also Ca, Sr, Ba, Al, In, Tl, Sn, Pb, Cu, Ag, Cd, lanthanides, H+, and NHj) are incorporated into the structure to maintain electrical neutrality. The additional cations M are partially located at the centers of the unit cells to give the perovskite stmcture type, as shown in Fig. 10.4.4(b). 

The alnicos are polycrystalline solids, in which very complex structural disorder generally occurs, due to the wide variety of deviations from periodicity that may be present. Physical disorder corresponds to the displacement of atoms, and/or the presence of amorphous regions and non-uniform crystallite sizes, while chemical disorder corresponds to site occupation by impurity atoms, and/or the presence of vacancies accompanied by variable valence states of... 

Let us consider an anti-Frenkel disordered material, taking into account both O and v , but initially neglecting the occurrence of variable valence states. (In addition, we will assume a quasi one-dimensional situation.) Doing this we reduce the problem to a relatively trivial case. For it is immediately evident that the source terms disappear on consideration of the total ion flux densityj (or current density i)... 

If we also allow for interactions between ions and electrons, the situation becomes qualitatively different (see Fig. 56) even on the level of electronic and ionic constituents. In such cases we introduce variable valence states by permitting O and 0° as valence states, i.e.,... 

Let us now discuss the influence of variable valence states on the tracer (D ) and charge diffusion coefficients (DQ). 

In the case of oxides with variable valence state, the degree of densification may depend further on partial oxygen pressure in the atmosphere. For instance, spinel MgCr204 requires for its densification an O2 pressure lower than 10 Pa (Anderson, 1974). Maximum densities are attained at an oxygen pressure beyond which the spinel becomes unstable and volatilizes. 

As a rule, B-site cations with variable valence states should not be introduced in the solid electrolyte compositions, as these species may contribute to the electronic conduction. Note that an opposite statement may be valid for MIECs. 

The lattice parameter of the compounds R3 MC in the lanthanide series for a given element M changes linearly with the radius of the trivalent rare earth ions, following the lanthanide contraction regularity, except for europium and ytterbium which appear to have a variable-valence state. In the ytterbium-M (Al, Ga, In, Tl)-carbon system, the unit cell volume of the compound YbgMC, is larger than the value expected from the relationship mentioned above. In the europium-M(Al, Ga, In, Tl)-carbon system no compound was found. For the compounds of yttrium, as expected, the lattice parameter falls between those of the respective terbium and dysprosium compounds. 

Tellurium belongs to group 16 (formerly called VIB) of the periodic table, which also includes oxygen, sulfur, and selenium. Its atomic number is 52, atomic weight 127.6, melting point 449.8°C, and boiling point 989.9°C. Elemental tellurium is a silvery white, metallic, lustrous solid that is quite brittle. Tellurium is known to exist in two allotropic forms the hexagonal crystalline form and an amorphous powder. Chemical properties of tellurium resemble that of selenium and sulfur. However, it is more metallic than these elements. Tellurium exhibits variable valence states from 2- to 6+ the most common ones are 2-, 4+, and 6+. Elemental tellurium is insoluble in water,... 

However, since both iron and cobalt have variable valence states, it is important to know if they have the same or different chemical states in these compounds. EMS offers a unique way to probe the Co sites in these Co-containing perovskites. The 1 1 ratio of the two elements facilitates the comparison of the transmission and emission Mossbauer spectroscopy results. 

Let us now turn to the tracer (D ) and the charge diffusion coefficients (D ). If we allow for variable valence states of the oxygen defects, modifications in these parameters result simply because of the different weight of the different charges. 

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