Nominal Valence

The composition variation described in the previous chapter has a considerable impact upon the electronic properties of the solid. However, it is often difficult to alter the composition of a phase to order, and stoichiometry ranges arc frequently too narrow to allow desired electronic properties to be achieved. Traditionally, the problem has been circumvented by using selective doping by aliovalent impurities, that is, impurities with a different nominal valence to those present in the parent material. However, it is important to remember that all the effects described in the previous chapter still apply to the materials below. The division into two chapters is a matter of convenience only. 

Figure 20 contains spectra of compounds in which the chromium is of nominal valence - -6 and is tetrahedrally coordinated. The chromate ion shows a type IV spectrum extending out to about 100 ev. which is inde-... 

Fig. 8.16. Reaction scheme for photoanodic dissolution of silicon in low intensity limit illustrating the competition between hole capture steps (rate constants k to k ) and electron injection steps (rate constants k to k,). The nominal valence states of the silicon intermediates are indicated. The final product Si(IV) is the soluble hexafluorosilicate species.

A very clear distinction between the total DOS D s) and the LDOS D e,t) is shown by calculations on binary alloys [51]. Look, e.g., at the (partly hypothetical) series of isoelectronic 1 1 alloys TcTc, MoRu, NbRh, ZrPd, YAg, where the alloying partners have nominal valence differences between 0 (for pure Tc) and 8 (for YAg). As shown in Figure 5, the overall density of states curves look more or less alike for all five alloys, but the partial densities on the sites of the individual partners are very different. Such curves also show why the so-called collective electron model does not work for catalytic activity [52, p. 458], or even for alloy properties in general. 

Many elements possess the ability to form more bonds than expected based upon the nrunber of unpaired electrons that are present in their atomic ground states. The elements P, S, Cl, Kr in the p-block and the elements below them in the periodic table behave in this marmer. Compoimds involving these elements that exceed the nominal valence for a given colrurm are said to be hypervalent or hypercoordinated, terms coined by Musher [1] and Schleyer [2], respectively. 

The utilisation of bond valence sums is widely used in these rather complex structures for either the allocation of an ion with a known nominal valence to the correct coordination polyhedron or the allocation of a charge state to an ion in a known coordination polyhedron. Using the bond valence method, the valence states of the Mn ions in i -Mn203 were determined to be Mn in A, Mn in A and Mnj Mn in B, giving an average valence of Mn for the phase, as necessitated by the overall formula. Similarly, bond valence sums for the perovskite LaCUjFe Oj allow the charge state at room temperature to be assigned La Cu Fe Ojj rather than La CUj fFe FCj )0,2. 

One major advantage of the SIC-LSD energy fimctional is that it allows one to determine valencies of the constituent elements in the solid. This is accomplished by realizing different valence scenarios, consisting of atomic coidigurations with different total numbers of localized and itinerant states. The nominal valence is defined as the integer number of electrons available for band formation, namely... 

Thus, for the noble gases the nominal valencies of n, iv, vi, and viii are possible. 

Electric monopole interaction between nucleus and electrons at the nuclear site Isomer shift 6 a. Oxidation state (nominal valency) of the Mossbauer atom b. Bonding properties in coordination compounds (covalency effects between central atom/ion and ligands. Delocalization of d-electrons due to back-bonding effects, shielding of s-electrons by p-and d-electron ) c. Electronegativity of ligands... 

Electric quadrupole interaction between electric quadrupole moment of the nucleus and electric field gradient at the nuclear site Quadrupole splitting A q a. Molecular symmetry b. Oxidation slate (nominal valency) c. Spin Slate d. Bonding properties... 

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