Abstract
It is documented that composition and crystal ordering of oxide surfaces and grain boundaries differ from the bulk as a result of segregation of lattice defects. Theoretical approaches on segregation in metals and limitations of their application to ionic solids are discussed. Several experimental approaches to evaluate segregation in ionic solids are considered. The approaches involve mainly the studies of surface (near-surface) versus bulk studies of several structural and composition-sensitive parameters as diffusion coefficient, Fermi energy level and chemical composition data. The oxygen-pressure exponent of defect-related parameters such as electrical conductivity, Seebeck coefficient, work function, weight change is also a sensitive probe in studies of segregation. Finally when the grain size decreases below a certain critical value, which is comparable to dimensions of the near-surface or grain boundary layer, then most of physical and chemical parameters are partially or entirely sensitive to properties of the layer. Examples are given to illustrate segregation in nonstoichiometric oxides and their solid solutions. The results are discussed in term of different bulk versus near-surface miscibility ranges and resulting impact on interpretation of phase diagrams for ionic solids. It is shown that segregation leads to formation of surface or grain boundary bidimensional phases which exhibit extraordinary properties. Segregation and thus formed strong electric fields lead to the formation of the near-surface diffusive resistance for transport of charged defects across interfaces. The resistance may have a significant effect on the rate of heterogeneous processes even at high temperatures. Practical aspects of segregation on chemical processes and materials properties are discussed.
Published Version
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