Line-defects, oxygen vacancies

Thermodynamic considerations imply that all crystals must contain a certain number of defects at nonzero temperatures (0 K). Defects are important because they are much more abundant at surfaces than in bulk, and in oxides they are usually responsible for many of the catalytic and chemical properties.15 Bulk defects may be classified either as point defects or as extended defects such as line defects and planar defects. Examples of point defects in crystals are Frenkel (vacancy plus interstitial of the same type) and Schottky (balancing pairs of vacancies) types of defects. On oxide surfaces, the point defects can be cation or anion vacancies or adatoms. Measurements of the electronic structure of a variety of oxide surfaces have shown that the predominant type of defect formed when samples are heated are oxygen vacancies.16 Hence, most of the surface models of... 

Pure monocrystalline V205 is diamagnetic, and the EPR spectra can be recorded only in the presence of paramagnetic V5+ ions. The Ay values were determined approximately twice smaller than in the rest of the cases [177, 179], and the spectrum pattern included 15 equally spaced lines. This was serious evidence that there were certain defect centers in the matrix, in which V4+-V5+ or V4+-V3+ pairs were located. The results were interpreted in terms of a model by which an unpaired electron interacted with two equivalent nuclei separated by an oxygen vacancy. A self-consistent mechanism has been proposed for the formation of the low-temperature form of non-stoichiometry in V205 [179],... 

Oxygen vacancies created by annealing Oxygen vacancies created by other means Line defects Impurities... 

Figure 27. Self-diffusion of O and in Fe304 exchanged with variable partial pressures of oxygen at 1150°C (dry). The dashed line represents the theoretical curve with a consideration of oxygen vacancy defects and interstitials (To + <9i). The sohd curve represents the theoretical curve with consideration of both oxygen vacancies and stable cluster defects involving oxygen and iron [To + (FoTpe)]. Modified from Millot and Niu (1997). At 1150°C, Ni-NiO and the QFM (Quartz-Fayalite-Magnetite) buffers lie at P02 values of roughly 10 and 10, respectively.

These studies were examined critically and it was concluded that they were inconsistent with interstitial cerium atoms but consistent with oxygen vacancies if in addition there were other anion and cation vacancies. They also indicated that their results could be interpreted in terms of defect-defect interaction forming extended defects which must be considered a real possibility. The X-ray line profiles did not show evidence of deviation from the cubic structure. 

Figure 13. Oxygen Is photoemission spectra (left panel) and oxygen 2s plus valence band photoemission spectra (right panel) of MgO(lOO) after sequential interactions with water vapor (3 min exposure) at different p(H20) (reported in torr to left of each spectrum) showing growth of low kinetic energy feature at 81 eV in the Is spectra and at 47 and 60 eV in the 2s + VB spectra. Also shown are Is and 02s + VB spectra of MgO(lOO) after immersion in liquid water. The 2s + VB spectrum of brucite is also shown. A schematic view of the MgO (100) surface is shown on the left before and after reaction with water vapor. Edge and comer defects are shown together with a vacancy, which are the most reactive sites on the MgO (100) surface. The structure of bmcite is shown at the upper left. These data were taken on SSRL beam line 10-1. (after Liu et al. 1998a)...

Figure 10.7. Calculated equilibrium defect diagrams for a binary oxide MO with Schottky defect pairs. In case (a) the equilibrium constant for vacancies is taken to be much larger than for electronic disorder (Kg = 10 Ki) case (b) gives the concentrations if the Schottky disorder is the smaller. The defect concentrations in regions I and III have a power dependence on the oxygen pressure with the exponent Region II in case (a), which includes the electrolytic domain has an exponent of the defect lines of in case (b) the exponent is...

Figure 10.24 Defect diffusivities of O and Ti vacancies, and Dv versus oxygen activity at 1200°C. The solid lines indicate the average values.

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