Chemical theory of dislocation mobility

Crystal dislocations were invented (circa. 1930) to explain why pure metal crystals are soft compared with homogeneous shear strengths calculated from atomic theory. They do this very well. However, later (circa. 1945) it was found that pure semiconductor crystals (e.g., Ge and Si) have hardnesses at room temperature comparable with calculated homogeneous shear strengths. Furthermore, it was known that pure metal–metalloid crystals (e.g., TiC and WC) are very hard, although they conduct electricity like metals. How can these differences be explained? fferences just outlined cannot be explained by means of a classical mechanical model. However, they can be explained by considering the chemical bonding in differing solid crystals. In particular, hardness depends on the degree of localization of the valence (bonding) electrons. Qualitatively, this is a very old idea. What is new is that it is possible to provide quantitative theories by using the results of quantum chemistry, and relatively simple analysis. Selected sets of crystal types must be treated, of course, just as selected sets of molecules are treated in theoretical chemistry. Otherwise the rationalization becomes unmanageable.