Why silicon is hard.

Compared with pure metals and ionic salts, covalent solids such as silicon are hard and brittle because dislocations do not move in them except at high temperatures. A satisfactory explanation for this behavior has been lacking in spite of its great importance for the mechanics of materials and structures. It is shown here that the critical atomic process leading to the observed brittleness is analogous to a chemical substitution reaction. Analysis of this analogy with the aid of a correlation diagram yields the observed high resistive stress and high activation energy. When a kink on a dislocation line moves, it breaks the atomic bonding symmetry, a forbidden process.