Controlled valence

Through the years, the development of transition metal-catalyzed methodologies, notably involving cyclotrimerization and Sonogashira or Heck cross coupling, has paved the way for rapid and efficient access to aryl glycoclusters with desired and controlled valency. 

Next, let us consider the dependence of carrier concentration on for Bi- or Ag-doped PbS. This problem is related to the method of controlled valency, developed by Verwey and Kroger and Vink. It is assumed, as observed, that Bi substitutes for Pb in the crystal and acts as a donor. The equilibrium equations for this case are the six independent ones, eqn (1.204) to eqn (1.211), which describe the chemical equilibrium in pure PbS, and the following two equations ... 

Fig. 7.12 Controlled valency (N - Nt1 ) by addition of Li ions to NiO. (From Hannay. N. B. Solid-State Chemistry Prentice-Hall Englewood Cliffs. NJ, 1967 Reproduced wilh permission.]...

The incorporation on a cation site of an impurity cation of higher charge than the host cation can stabilize a lower valence state of the host cation. The addition of titanium (as Ti02> to the spinel, zinc ferrite, leads to an B-type semi-conductor of this type of formula, Zn (Ti + Fe t28 Fe ) O. The defect centre may be described as an impurity cation of higher relative charge on a cation site plus a quasi-free electron bound on a neighbouring host cation. A necessary condition for the application of the prindple of controlled valency is that the impurity cation should be of much the same size as the host cation. 

Baf Mna" " Mn t Ou -(2 - 8) H2O. It has not been found possible to differentiate Mn + and Mn + in these structures, presumably because they are not ordered. These two minerals and cryptomelane are natural controlled-valency semi-conductors. A more detailed study of the structural relationships of the manganese oxide noinerals should provide useful data on the geochemistry of manganese and on the defect solid state in general. 

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

Verwey s controlled valence principle is applied to a large number of nontransitioi metal compounds. 

The published work and our own investigations have suggested that Verwey s controlled valence principle [4] provides a satisfactory basis for considering the doping of semiconducting compounds. 

The introduction of Me2X into FbX generates a proportional number of anion vacancies which are an obstacle to an increase in the valence of lead and, once again, p-t3q>e conduction is not obtained. The controlled valence mechanism, which increases the charge of the lead ions, is effective only if the excess sulfur, selenium, or tellurium is not less than that required to form Me2X with the same cation-anion ratio as the host substance ... 

Chen Q, Dahlgaard H, Hansen HJM, and Aarkrog A (1990) Determination of Tc in environmental samples by anion exchange and liquid-liquid extraction at controlled valency. Analytica Chimica Acta 228 163. 

C. Wagner [19] was the first to point out the possibility of the controlled introduction of defects by means of dissolving a third heterovalent component in binary ionic crystals. In a binary compound, one of two things will happen, depending upon the disorder type. Either the above case of controlled valence will occur, or the excess charge introduced by the third component will be compensated by vacancies or by interstitial ions. The latter is the case,... 

This region is followed by a stoichiometric region (R II) where the material acts as a controlled valence semiconductor and = 0. 

For compounds in which one of the components has more than one stable valence state (e.g., transition metals), charge compensation of foreign atoms may occur by a change in oxidation state of that component. Such compounds are called controlled-valency semiconductors. A well-known illustration of such a material is nickel oxide containing lithium which substitutes for nickel in the lattice. In order to maintain electroneutrality, for each Li" ion present a normal divalent Ni ion becomes converted to a Ni ion. The Ni ion in the NiO lattice behaves as a hole ... 

Since this is a controlled-valency semiconductor, the 16 electrons are actually Fe ions on Fe sites. 

---