Amorphous-amorphous phase transition in simulated amorphous SiO2 nanoparticles

Abstract

Via molecular dynamics (MD) simulation we found the transition from low density to high density states in amorphous SiO2 nanoparticles under compression from 2.20 g/cm3 to 5.35 g/cm3. Simulations were done in a spherical model containing 2214 atoms under non-periodic boundary conditions. However, unlike those observed in the bulk (i.e. it was found a clear transition from a tetrahedral to an octahedral network structure under compression over the same density range) we found just the transition from a tetrahedral to a pentahedral network structure in nanoparticle, i.e. the mean coordination number for Si-O pair increases from 4.00 to 5.10. This indicates surface effects on the pressure-induced phase transition in nanoscaled systems. The changes in coordination number for different atomic pairs in the core and at surface of nanoparticles upon compression have been found and discussed. The atomic density profile for both Si and O in nanoparticles presents a possibility of pressure-induced partial crystallization in amorphous SiO2 nanoparticles.