Oxygen-defect

Another source of departure from stoichiometry occurs when cations are reduced, as for example in tire reduction of zinc oxide to yield an oxygen-defective oxide. The zinc atoms which are formed in tlris process dissolve in the lattice, Zn+ ions entering interstitial sites and the coiTesponding number of electrons being released from these dissolved atoms in much the same manner as was found when phosphorus was dissolved in the Group IV semiconductors. The Kroger-Viirk representation of dris reduction is... 

It is well known that the electrical activity of many deep-level defects disappears when the crystal is exposed to atomic hydrogen (see Pearton et al., 1987 and Chapter 5 of this Volume). This has been attributed to complex formation with the hydrogen, and it is very common for transition-metal impurities. Unfortunately, very little theoretical work has been reported for these systems. The deactivation of second- and third-period deep-level impurities is better understood theoretically. The substitutional oxygen defect in silicon ( A center Watkins and Corbett, 1961 Corbett et al., 1961) can be deactivated by exposure to hydrogen. Recently, a theoretical study of the deactivation of substitutional sulfur through the formation of a hydrogen-sulfur pair has been reported (Yapsir etal., 1988). 

Figure 1 Idealized crystal structures of (a) La2Cu04 tilted along [110] from [010] directions, (b) Oxygen defect K2NiF4 structure of La2 xSrxCu04 (x >0.05), in [010], with c-axis vertical, and is related to Figure la a = aorth / /2. ...

Over most reported Au catalysts, CO oxidation takes place at the junction perimeter between Au NPs and the metal oxide supports. Carbon monoxide is adsorbed on the edges, corners and steps of Au NPs. Molecular oxygen is adsorbed on the support surfaces and may be activated at the oxygen defect sites at the perimeter interfaces, where the two adsorbates react to form C02 in the gas phase. At the perimeter interfaces Au is assumed to exist as Au3 +, which might be stabilized through bonding with OH-. 

In the case of perovskites, a material of interest in the context of this book, oxygen defects may possibly be the prevailing ones while cations are only minority defects. Notwithstanding the fact that the perovskite formula is AB03 and not AO, the equilibrium equations previously expressed are as well satisfied. 

---