Oxygen vacancy concentration, effect

On analyzing the variation of the magnetic susceptibility with temperature for these compositions, it appears that, when the temperature is increased, the following factors begin to have an effect 1) increase in the oxygen vacancy concentration 2) increase in the fraction of pentavalent antimony ions 3) increase (mainly) in the radii of the anion-forming atoms. These factors are influential in different temperature ranges. 

In such an oxide the oxygen vacancy concentration may also be determined by the presence of dopants or a sufficiently large level of impurities with negative effective charge. If the dopant is doubly negatively charged (as, for instance, Ca2+, an acceptor dopant, in Zr02), then... 

In this oxygen excess region, the formation of cation vacancies will have the effect of strongly decreasing the oxygen vacancy concentration through the Schottky equilibrium. 

However, at low temperatures a constant water concentration has to be taken into account which then, in the form of [OHq], adds to the effective C-parameter (as it is the case for the oxygen vacancy concentration ). 

If this concentration is larger than the oxygen vacancies created by thermal effects, then the conductivity from the motion of the doubly charged ions is directly proportional to the concentration of CaO (eq. 15). 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

Very direct evidence for the existence of bound spin polarons is provided by the work of Torrance et al (1972) on the metal-insulator transition in Eu-rich EuO At low temperatures, when the moments on the Eu ions are ferromagnetically aligned, the electrons in the oxygen vacancies cannot form spin polarons and are present in sufficient concentration to give metallic conduction. Above the Curie temperature the conductivity drops by a factor of order 10 , because the electrons now polarize the surrounding moments, forming spin polarons with higher effective mass. 

Manganese at the 1 % level in air-fired dielectrics acts as a palliative. It is present as both Mn4+ and Mn3 +. The former ion must be expected to act as an effective electron trap since it is readily converted into Mn3 +. However, the presence of Mn3+ will result in a corresponding concentration of oxygen vacancies. 

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