Defects, biographical

Two German physical chemists, W. Sehottky and C. Wagner, founded this branch of materials seience. The story is very clearly set out in a biographical memoir of Carl Wagner (1901 1977) by another pioneer solid-state chemist, Hermann Schmalzried (1991), and also in Wagner s own survey of point defects and their interaction (Wagner 1977) - his last publieation. Sehottky we have already briefly met in connection with the Pohl school s study of colour centres... 

In such an approach there is no fundamental difference between the chemisorbed particles and the biographical structural defects which are always present on any real surface. The only difference is that the chemisorbed particles are free to leave the surface for the gaseous phase and to return to the surface from this phase, whereas the biographical defects must be considered as rigidly bound to the surface and incapable of interchange with the gaseous phase. 

States due to different biographical structural defects existing on any real surface and playing the part of local disturbances in the strictly periodic structure of the surface (Sec. IX,A). These include vacant lattice sites in the surface layer of the lattice, atoms or ions of the lattice ejected onto the surface, and foreign atomic inclusions in the surface of the lattice (surface impurities). 

The totality of defects in a unit volume of the crystal is termed the disorder of the crystal. This disorder is assumed to be small and composed of defects having either a biographical or a thermal origin. The biographical disorder, denoted by X, is irreversible and preserved... 

In the enumeration of the types of defect in 1.2 some effort was made to produce a rational classification. The placing of a defect solid in one or other class is undoubtedly somewhat arbitrary, particularly as one solid of definite composition may well contain defect systems of different kinds and these may interact. Moreover, some types of defect have temperature-dependent concentrations and would disappear in the true thermodynamic equilibrium state at 0 i they may be referred to as thermal, indicating their origin. Defects of other kinds that are inherent in the particular solid would be present in the equilibrium state at they have been referred to as biographical defects. Defects of all types... 

Even with such a classification, some difficulty arises in assigning certain defect solids to one class or another. There can be no dispute that defects of the Schottky and Frenkel type in stoicheiometric crystals are thermal in origin there can equally be no dispute that the stoicheiometric dual-valency compounds are biograpliical, but the situation with regard to non-stoicheiometric compounds and anomalous solid solutions of various types is by no means as clear. Various authors have stated that non-stoicheiometric compounds are biographical in type this is incorrect, as the departure from stoicheiometry (or inversely the range of existence of a non-stoicheiometric phase) is a function of temperature which tends to zero as For example, zinc oxide... 

Only the point defects are thermodynamically equilibrium defects. All others, which sometimes are referred to as biographical defects , depend on the prehistory of each sample - that is, on the method and conditions of preparation, heat treatments, and so on. Therefore, a thermodynamic approach can be applied basically for the point defects. 

The physical nature of the biographical (intrinsic or a priori) non-uniformity, advanced originally by Langmuir and Taylor can be attributed to the difference in the properties of the different crystal faces and the occurrence of dislocations, defects and other disturbances. 

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