Pressure-induced amorphization and mechanical instability

Pressure-induced amorphization of solids has received considerable attention recently in physical and material sciences, although the first reports of the phenomenon appeared in 1963 in the geophysical literature (actually amorphization on reducing the pressure [18]). During isothermal or near isothermal compression, some solids, instead of undergoing an equilibrium transition to a more stable high-pressure polymorph, become amorphous. This is known as pressure-induced amorphization. In some systems the transition is sharp and mimics a first-order phase transition, and a discontinuous drop in the volume of the substance is observed. Occasionally it is strictly not an amorphous phase that is formed, but rather a highly disordered denser nano-crystalline solid. Here we are concerned with the situation where a true amorphous solid is formed. 

In the two-state model [20,21] the two different species interact and the interaction can be expressed using the regular solution model. Thus the Gibbs energy of the liquid is 

Here Q is the regular solution constant and xSiB the fraction of Si atoms in silicon state B. By noting that xSiA =1 - xSi B, equation (5.31) becomes 

Equation (5.33) can alternatively be written in terms of the equilibrium constant for reaction (5.29) as 

The relative populations of the two states vary with temperature and pressure. The species A which dominates at low temperature and low pressure has the larger molar volume, while the denser species B becomes increasingly more favoured at high pressures. 

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