Nonstoichiometric oxygen-deficient oxides

For nonstoichiometric oxides the concentration of electronic defects is determined by the deviation from stoichiometry, the presence of native charged point defects, aliovalent impurities and/or dopants. The concentration of electronic defects can be evaluated from proper defect structure models and equilibria. Various defect structure situations have been described in previous chapters and at this stage only one example - dealing with oxygen deficient oxides with doubly charged oxygen vacancies as the prevalent point defects - will be described to illustrate the electrical conductivity in nonstoichiometric oxides. 

Nonstoichiometry can be caused by oxygen deficiency (or excess) or by fractional valences of the metal components. For example, the existence of Cu " in nonstoichiometric cuprates has been widely discussed [9,10]. It is essential that in nonstoichiometric oxides the microscopic fluctuations of the composition should proceed (the so-called phase separation). The characteristic size of heterogeneities induced can exceed atomic dimensions by an order of magnitude. This phenomenon is attributable to the fact that the electron-nonuniform state of such chemically singlephase materials appears to be energetically more advantageous. 

An n-type cation interstitial metal-excess oxide semiconductor has an excess of interstitial cations or oxygen anion vacancies in the crystal lattice. An excess of electrons maintains the neutrality and electrical conductivity. The oxygen vacancies are created from single-point defects. The nonstoichiometric oxide MO2-X with large cations is oxygen deficient and creates anion vacancies. 

At 20 nm, the SiOa layers are practically homogeneous, and the ratio f ii6o/f i25o is independent of the thickness. For a thinner SiOa layer, the band at 1250 cm is broader, its maximum is shifted toward lower frequencies, and the ratio Rim/Rnso is increased. These effects are all a result of an oxygen deficiency at the initial stages of thermal silicon oxide formation, as in the case of anodic oxides, which leads to the appearance of a nonstoichiometric SiO layer, as observed in the IRRAS spectra [23]. 

Real oxide films are typically nonstoichiometric due to an excess of metal ions or a deficiency of oxygen ions in the film and are often amorphous or nanocrystalline. In the presence of water, hydrated oxides or hydroxides often form, such as Al(OH)3 or AlOOH in the passive layer of Al and Fe203-H20 or y-FeOOH in the passive layer of Fe. Furthermore, the migration or diffusion of defects within the oxide leads to transport of ions within the film and to ion transfer reactions (ITRs) that take place at the oxide-electrolyte interface. Defect concentrations in passive films usually range from 10 to 10 cm [15]. Thus, as CPs are ion exchange polymers, ion transfer across CP-metal oxide interfaces is likely. 

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