Nearest-neighbour atoms

The EXAFS technique is used primarily for investigations of disordered materials and amorphous solids. Figure 8.35(b) shows how interference occurs between the wave associated with a photoelectron generated on atom A and the waves scattered by nearest-neighbour atoms B in a crystalline material. 

EXAFS spectra of platinum metal, having a face-centred cubic crystal stmcture, have been obtained at 300 K and 673 K. Explain what qualitative differences you might expect. How many nearest-neighbour atoms are there in this stmcture Illustrate your answer with a diagram. 

Wheland and Pauling (1959) tried to explain the inductive effect in terms of ar-electron theory by varying the ax and ySxY parameters for nearest-neighbour atoms, then for next-nearest-neighbour atoms and so on. But, as many authors have also pointed out, it is always easy to introduce yet more parameters into a simple model, obtain agreement with an experimental finding and then claim that the model represents some kind of absolute truth. 

In simple solutions such as binary alloys, the components are distributed on a single lattice. More complex solutions may consist of two or more sub-lattices, and in a solution of simple ionic salts like NaCl and NaBr there is one sub-lattice for cations and one for anions. In these cases the interactions considered in the models are between next neighbouring pairs of atoms rather than nearest neighbour atoms, as is the case with a single lattice. Two sub-lattice models can also be applied to... 

A first set of interatomic distances (and the corresponding coordination, taken as the number of nearest-neighbour atoms) which can be considered in the AuCu3 phase is that related to the Au coordination around Au atoms (see Fig. 3.29(a)) ... 

In fact, the converse is observed. The main features of the spectra of transition metal ions in solution are very similar to those for crystal lattices where the same donor atom is present as an anion. Further, the spectra differ little between solids provided the nearest-neighbour atom is unchanged, even if it is part of a multi-atom species and even if the symmetry of the crystal structure is low. The spectra of the first transition series carbonates, for example, are not markedly different from those of their oxides, nor from those of the ions in aqueous solution. In each case the nearest-neighbour atom is oxygen and six of these surround the metal atom in approximately octahedral positions. 

The number of nearest neighbour atoms, viz. the crystal co-ordination number, is 8 in (i). A further 6 atoms, almost as close, makes 14 near neighbours. Obviously tlicre are insufficient electrons to account for the... 

Figure 4.5. Schematic representation of the range of Si chemical shifts in Si-O compounds of differing coordination and with different nearest-neighbour atoms.

The repulsive interaction U/j is assumed to be short-range, normally taken over nearest neighbour atoms 1 and J — it replaces ail the terms in (11) which are added on to the one-electron energy. Termination of the... 

The chemistry of Xe is much the most extensive in this group and the known oxidation states of Xe range from -f-2 to -f-8. Details of some of the more important compounds are given in Table 18.2. There is clearly a rich variety of stereochemistries, though the description of these depends on whether only nearest-neighbour atoms are considered or whether the supposed disposition of lone-pairs of electrons is also included. Weaker secondary interactions in crystalline compounds also tend to increase the number of atoms surrounding a central Xe atom. For example, [XeFs] [AsF6] has 5 F at 179-182 pm and three further F at 265-281 pm, whereas [XeFsJ+iRuFe]" has 5 F at 179-184 pm and four further F at 255-292 pm. If only the most closely bonded atoms are counted, then Xe is known with all coordination numbers from 0 to 8 as shown schematically in Table 18.3. 

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