Planar defects grain boundaries

The mechanism by which defects concentrate impurities is a subject of research that has important bearing on crystal growth, especially related to formation of crystalline materials for use in the electronics industry. Besides imperfections associated with isolated impurities (i.e., point defects), the other major types of structural defects are line defects (both edge and screw), planar defects, grain boundaries, and structural disorder (Wright 1989). The connection between defect formation and impurity uptake is evident in two of these defects in particular the edge defect and point defect. 

Point defects have zero dimension line defects, also known as dislocations, are onedimensional and planar defects such as surface defects and grain boundary defects have two dimensions. These defects may occur individually or in combination. 

Finally, two-dimensional defects can occur in crystals. There are two categories of planar defects stacking faults and grain boundaries. 

The introduction to this chapter mentions that crystals often contain extended defects as well as point defects. The simplest linear defect is a dislocation where there is a fault in the arrangement of the atoms in a line through the crystal lattice. There are many different types of planar defects, most of which we are not able to discuss here either for reasons of space or of complexity, such as grain boundaries, which are of more relevance to materials scientists, and chemical twinning, which can contain unit cells mirrored about the twin plane through the crystal. However,... 

The lattice defects are classified as (i) point defects, such as vacancies, interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms, (ii) line defects, such as edge, screw, and mixed dislocations, and (iii) planar defects, such as stacking faults, twin planes, and grain boundaries. 

The driving forces necessary to induce macroscopic fluxes were introduced in Chapter 3 and their connection to microscopic random walks and activated processes was discussed in Chapter 7. However, for diffusion to occur, it is necessary that kinetic mechanisms be available to permit atomic transitions between adjacent locations. These mechanisms are material-dependent. In this chapter, diffusion mechanisms in metallic and ionic crystals are addressed. In crystals that are free of line and planar defects, diffusion mechanisms often involve a point defect, which may be charged in the case of ionic crystals and will interact with electric fields. Additional diffusion mechanisms that occur in crystals with dislocations, free surfaces, and grain boundaries are treated in Chapter 9. 

Gu, H., Ceh, M., Stemmer, S., Miillejans, H. and Ruhle, M., (1995), A quantitative approach for spatially resolved electron-energy-loss spectroscopy of grain boundaries and planar defects on a subnanometer scale , Ultramicroscopy, 59 (1/4), 215-227. 

Of these four types, (a)-(c) are observed at the atomic level, whereas bulk defects are easily observed by the naked eye, or using a light microscope. These bulk defects are produced through the propagation of the microscopic flaws in the lattice. For crystals with a planar defect such as polycrystalline solids, the grain boundary marks the interface between two misaligned portions of the bulk crystal (Figure 2.23). 

To go further towards understanding the concepts of inorganic chemistry we need to consider the subject of defects in solids. They are a key to the behaviour of many materials. They are of central importance to diffusion, phase transformations and reactivity of solid compounds. Defect structures show up as the local occurrence of a grain boundary or sites of a structure building operation. The formation of a structure using translational defects is easily understood by a planar example, illustrated in Figs. 2.3. 

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

Planar defects include grain boundaries, stacking faults, and twins. These defects are formed during ion implantation and thermal and processing. All three types of planar defects are enclosed by a single dislocation or by an array of dislocations separating the faulted area from the normal area or delineating the misorientation between various areas of the semiconductor. 

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