Let say, we have Brouwer‘s diagram for nonstoichiometric oxide e.g. CeO2.
1. Why does concentration of electron decrease with increasing oxygen pressure if concentration of oxygen vacancies is constant? I am talking about the region near stoichiometric point.
2. Why concentration of holes is smaller than concentration of oxygen interstitials (low oxygen pressure)?
Example or Brouwer’s diagram: J. Maier. Physical Chemistry of Ionic Materials: Ions and Electrons in Solids. p. 166
https://books.google.lt/books?id=-Nv750USYhAC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
There are multiple defects in oxides and again the defects have multiple charge states. For example, oxygen vacancies can exist in 3 charge states: 0, +1, +2. The charged defects along with free carriers (electrons and holes) have to obey the charge neutrality constraint. With changing oxygen partial pressure, the chemical potentials of oxygen and metal also change, which will lead to a change in Fermi level and defect formation energies. Sometimes the change in chemical potential is very minimal. As a result the concentrations of charged defects along with those of the free carriers change.
Even though oxygen vacancy concentration looks like not changing, the charge of the oxygen vacancy may be changing. Or the other defects are influencing the electron concentration. Based on the Brouwer plot, you can predict what type of defects are dominating.
Dear Mantas Sriubas:
It is easy to get the answer from the defect chemistry reaction and the electrocal conductivity/electron concentration versus oxygen partial pressure as I wrote in the attached file.
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