Theoretical investigation of metastable Al2SiO5 polymorphs

Using theoretical simulations based on density functional theory within the generalized gradient approximation, a series of metastable phase transitio ns occurring in low-pressure Al2SiO5 polymorphs (andalusite and sillimanite ) are predicted; similar results were obtained using semiclassical interato mic potentials within the ionic shell model. Soft lattice modes as well as related structural changes are analysed. For sillimanite, an isosymmetric p hase transition at ca 35 GPa is predicted; an incommensurately modulated fo rm of sillimanite can also be obtained at low temperatures and high pressur es. The high-pressure isosymmetric phase contains five-coordinate Si and Al atoms. The origin of the fivefold coordination is discussed in detail. And alusite was found to transform directly into an amorphous phase at ca 50 GP a. This study provides an insight into the nature of metastable modificatio ns of crystal structures and the ways in which they are formed. Present res ults indicate the existence of a critical bonding distance, above which int eratomic interactions cannot be considered as bonding. The critical distanc e for the Si-O bond is 2.25 Angstrom.