Variable valence cation

The electrons might be free or else may be associated with variable valence cations, chemically transforming M2+ ions (Mm) to M+ ions (MV) ... 

Ulibarri, M. A., Fernandez, J. M., Labajos, F. M. and Rives, V. (1991). Anionic clays with variable valence cations synthesis and characterization of... 

The introduction of small amounts of variable-valence cations, such as iron or... 

Ionic conductivity is the transport of cations and/or anions across the perovskite under the influence of an electric field. As with diffusion, for ionic conductivity of cations and anions in perovskites to occur the structure must either contain open regions or a significant population of vacancies on the appropriate sublattice to allow ionic movement. Substitution is again widely used to create vacancies in perovskites with approximately cubic structures so as to increase conductivity. A further requirement, for strictly ionic conductivity, is the absence of cations with a variable valence. In cases where variable valence cations are present, electronic conductivity may also occur and in such cases will invariably dominate, in magnitude, the ionic conductivity (Sections 5.4 and 5.5). 

When a d-metal atom loses electrons to form a cation, it first loses its outer s-electrons. However, most transition metals form ions with different oxidation Variable valence is discussed further states, because the ( -electrons have similar energies and a variable number can be... 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

The variable valency of sulfur allows the formation of S-oxides (11) which unlike thiabenzene 1-oxides are non-aromatic and exist in half-boat conformations (78CC197). On the other hand, acid treatment of 6-hydroxy-6H- 1,2-oxazines (12) yields cations (13) which are analogous to pyrylium salts (74ZOR1513). 

If the cations of variable valency (e.g., Fe2+/Fe3 + ) are present in not too low concentrations, the crystals will be semiconductors. In non-equilibrium vermiculites, the internal electric field is then strongly influenced by their electronic conductivity, as explained in Section 4.4.2. If we start with an equilibrium crystal and change either pH, ae, aor a, (where i designates any other component), coupled transport processes are induced. The coupling is enforced firstly by the condition of electroneutrality, secondly by the site conservation requirements in the T-O-T blocks (Fig. 15-3), and thirdly by the available free volume in the (van der Waals) interlayer. It is in this interlayer that the cations and the molecules are the more mobile species. However, local ion exchange between the interlayer and the relatively rigid T-O-T blocks is also possible. 

The copper-porphyrin complex gives cation radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation radical, which belongs to the -rr type (Ehlinger Scheidt 1999). A new bifunctional tetrathiafulvalene-type donor molecule (D-ct-D) with a copper iodine bridge has recently been synthesized. Its cation radical salt, (D-variable valence (Ramos et al. 1997). 

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 ratio between anions and cations can vary from a simple integral value because of the variable valence of a cation. Thus manganese dioxide is a... 

Secondly, superconductivity relates to the variable valence of Cu and Bi ions. From XPS analysis mentioned above, in Figure 3(b), the Cu 2Pyj have two peaks, indicating that the Cu cations exist as both valence states, Cu and Cu in the high Tc 2223 phase. Figure 3(c) also shows that there are two valence states of Bi, Bi and Bi. The two kinds of ions, and are of the variable valence, and exist as mixed valence states,... 

In the K2NiF4 series of phases described above, composition variation can take place by disorder in the width of the perovskite slabs, and the solid contains planar defects. In cases where the components of the mtergrowth contain cations with a variable valence, additional defect... 

The two initiation methods most efficient for grafting of monomer and polymer to cellulose involve oxidation of the matrix by variable valency metal ions and chain transfer from the initiator radicals to cellulose. The oxidant cations or the initiator radicals in... 

For cations with variable valence, the oxidation number is used. 

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 NASICON structure was chosen because it can be readily synthesized, is thermally very stable, and can accommodate a large fraction of vacancies and cation substitutions [9-12], In addition, this structure possesses two features which should be important for the catalyst design as envisioned above. First, it is a phosphate and hence expected, owing to its acidic nature, to stabilize the lower oxidation states of transition metals, e.g., V second, owing to its structure, layered octahedral metal centers with variable valence are separated from each other by redox inactive tetrahedral phosphate groups, i.e., the structure provides for isolation of descrete layers. 

With increasing oxidation degree the solubility of their cations noticeably declines as, for instance, at oxidation of Cu to Cu or Fe to Fe. However, with valence increase they are capable of forming stable oxian-ion, which facilitates their increasing content in water. That is why Mn behaves as cation and Mn forms anion MnO. Similar properties have chromium (CrO ), arsenic (AsO ), uranium (UO ) and other elements with variable valence. 

By non-stoichiometry it is implied that the metal to non-metal atom ratio is not exactly that given by the chemical formula, even though the compound is electrically neutral. These can only be reconciled by assuming that either the anion or the cation exhibits variable valency on its sub-lattice. It is much more likely that the metal or... 

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