Stoichiometry, Defect Populations and Partial Pressures

The same analysis can be applied to compounds with a more complex formula. For example, the oxide LaCoCL, which adopts the cubic perovskite structure, usually shows a large positive Seebeck coefficient, of the order of +700 jjlV K-1, when prepared in air (Hebert et al., 2007). This indicates that there are holes present in the material. The La ions have a fixed valence, La3+, hence the presence of holes must be associated with the transition-metal ion present. Previous discussion suggests that LaCo03 has become slightly oxidized to LaCoCL+j, and contains a population of Co4+ ions (Co3+ + h or Coc0)- Each added oxygen ion will generate two holes, equivalent to two Co4+  

Using Eq. (7.2), the value of nd, the number of holes (or Co4+ ions) present, is 3 x 10-4 of the total population, n0, of Co3+. The approximate formula of the oxide can be written LaCoOs+x/2, where x is the number of holes. With x = 3 x 10-4, the material has a formula LaCoOs.ooois- 

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