Partial pressure and defect populations

Defect populations and physical properties such as electronic conductivity can be altered and controlled by manipulation of the surrounding atmosphere. To specify the exact electronic conductivity of such a material, it is necessary to specify its chemical composition, the defect types and populations present, the temperature of the crystal, and the surrounding partial pressures of all the constituents. Brouwer diagrams display the defect concentrations present in a solid as a function of the partial pressure of one of the components. Because the defect populations control such properties as electronic and ionic conductivity, it is generally easy to determine how these vary as the partial pressure varies. 

Preparations under higher oxygen partial pressure would be expected to increase hole concentrations (CoCo ) and under lower partial pressures to increase the concentration of vacancies (Vo ). The exact defect population in a sample will depend upon temperature, oxygen partial pressure, and the rate at which samples are cooled. 

With decreasing temperature, as we have seen, the intrinsic defect population decreases exponentially and, at low T, extrinsic disorder becomes dominant. Moreover, extrinsic disorder for oxygen-based minerals (such as silicates and oxides) is significantly alfected by the partial pressure of oxygen in the system (see section 4.4) and, in the region of intrinsic pressure, by the concentration of point impurities. In this new region, term Qj does not embody the enthalpy of defect formation, but simply the enthalpy of migration of the defect—i.e.,... 

A consequence of the presence of oxygen vacancies results in the appearance of a balancing population of electrons (Equation (5.2)), which will give rise to electronic (n-type) conductivity. Within the limits of the point defect equUibrium model, the electronic conductivity will be proportional to the number of electrons and the partial pressure of oxygen as... 

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