Nonstoichiometric phases, lead oxides

The procedure is best illustrated by an example. Suppose that a nonstoichiometric phase of composition MA can have an existence range, which spans both sides of the stoichiometric composition, MX, oo. Assume that in this phase only vacancies are of importance, so that the stoichiometric composition will occur when the number of vacancies on the cation sublattice is exactly equal to the number of vacancies on the anion sublattice, which is, therefore, due to a population of Schottky defects. At other compositions, electrical neutrality is adjusted via mobile electrons or holes, leading to n-type or p-type semiconductivity. Thus there are four defects to consider, electrons, e, holes, h, vacancies on metal sites, Vm, and vacancies on anion sites, Vx. Finally, assume that the most important gaseous component is X2 as is the case in most oxides, halides, and sulphides. 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

In addition to alumina, our studies reveal that water has a stabilizing influence upon the nonstoichiometric oxide. Color changes produced by water adsorption upon both the oxidized and reduced forms of the catalyst attest strongly to the existence and stabilization of an intermediate oxide phase. Adsorption of water on the yellow oxidized catalyst leads to darkened areas indicating reduction, whereas water adsorption on the reduced form produces light-colored areas indicative of oxidation. 

The transition metal oxides are nonstoichiometric compounds whose compositions depend on the surrounding gas-phase environment. The formation of point defects or alteration of the linkage between polyhedra under catalytic reaction conditions, because of lattice oxygen incorporation in the adsorbed hydrocarbon, leads to the reorganization of the surface and to the formation of extended defects in crystallographic shear planes. This has been demonstrated by Gai [16] for VPO... 

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