Oxygen-defect structure

There are two basic questions which can be decided only by experiments. First, we must know whether the metal or the oxygen is present in excess, and second, we must know how the excess component is incorporated in the oxide lattice. In connection with the latter question we have to remember that a non-stoichiometric crystal remains electrically neutral (except in narrow regions near the surfaces), so that if the excess component is present in the crystal as ions, lattice defects with charges of opposite sign must necessarily be present also (see Figs. 1.77 and 1.78). The most important defect structures will be discussed in this section. 

In a number of studies a correlation was seen between the amount of nonstoi-chiometric oxygen in the spinel and the spinel s activity. It appears that excess oxygen consolidates the spinel s defect structure with a large number of active sites. Strong anodic polarization leads to ordering of this structure and thus to a decrease in catalytic activity. 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

Figure 8.15 Calculated composition versus oxygen stoichiometry curves for Lai- SrjCoCb-s. [The two experimental points are taken from data in A. N. Petrov, V. A. Cherepanov, and A. Y. Zuev, Thermodynamics, Defect Structure and Charge Transfer in Doped Lanthanum Cobaltites An Overview, J. Solid State Electrochem., 10, 517-537 (2006).]...

Various defect models have been proposed to explain the defect structure of the doped LaMn03 oxides particularly in the oxygen-excess region. At high oxygen... 

A natural question to ask is whether this two-regime theory is consistent with the known properties of LSM. As recently reviewed by Poulsen, the defect structure of LSM has some similarities with other more reducible perovskites such as LSG and LSF. Like these other perovskites, LSM has electrical properties on the border between that of a p-type semiconductor and a metaP and becomes oxygen substoichiometric at high temperature and low as shown in Figure 35. However, unlike its more reducible cousins (which may have significant vacancy concentration at atmospheric Pq ), LSM maintains a nearly full perovskite stoichiometry above atm and in fact becomes superstoichio-... 

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