Defect ordering

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

VO also possesses a high proportion of vacancies just like TiO, but the defect ordering at low temperature is probably quite different from that in low-temperature... 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

Anion-deficient nonstoichiometry in AB03 x perovskites is not accommodated by the CS mechanism. The reason probably is that the constant A/B ratio required by the composition of perovskites. prevents formation of CS planes. Defect-ordering in AB03 oxides involves a conservative mechanism in the sense that the vacancies are assimilated into the structure resulting in large supercells of the basic perovskite structure. The type of superstructure formed depends however on the identity of the B-cation. 

Such defect-driven structural transformations are effectively investigated by powder diffraction analysis of samples kept in reactive atmospheres. As solid catalysts are dynamic systems, the phase inventory and the defect ordering (real structure) may well change as a result of changes of chemical potential of a constituent in a reactive environment. Some of the changes are irreversible and can be detected by pre- and postoperation analysis of catalysts, but many are reversible and will not be evident in such experiments. 

These elements of disorder will not lead to a shift of a sharp peak. Shifts occur only if defects order in such a way as to modify all unit cells of the crystal in the same way (Figure 4). This effect is frequently encountered in solid solutions (Abd Hamid et al., 2003 Langford and Louer, 1996 Valtchev and Bozhilov, 2004), in which atoms of different sizes occupy the same lattice positions. In many cases there are linear... 

Disorder is not mere chaos it implies defective order ... 

Bogicevic, A., Wolverton, C., Crosbie, G.M., and Stechel, E.B., Defect ordering in aliovalently doped cubic zirconia from first principles. Physical Review B, 2001, 64, 014106. 

Crystallographic Aspects of Defect Interaction Examples of Defect Ordering Phenomena... 

Defective Order Forms Errors or Omissions on DEA Form 222 ... 

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