Phase transitions in Si and Ge at high pressure and deformation under pressure

Under normal conditions, germanium and silicon have a diamond-type lattice (Ge: a = 5.657 Å, Si: a = 5.4307 Å [3.1]). Some properties of Ge and Si are shown in Table 3.1 [3.1-3.3]. Phase transitions under pressure have been studied by many authors using different methods (Fig. 3.1 [3.2, 3.3]). The best known is the ‘semiconductor-metal polymorphic transition’ (SC ↔ Me, I → II, the lattice type β-Sn), which was observed in a number of works. The strong influence on this transition of shear stress has been noted. The transition is characterized by a large hysteresis, and by reducing the pressure part the Ge (Si) phase of the type β-Sn can be converted into Kasper phases: tetragonal phase III with a = 5.910 Å, a = 6.973 Å) [3.4] in Ge and in the body-centered cubic phase III with a = 6.636 Å, and Z = 16 [3.5]) – in Si.