Coordination and polyamorphism of aluminium silicate under high pressure: insight from analysis and visualization of molecular dynamics data

Abstract

The amorphous aluminium silicate (Al2O3)·2(SiO2) (abbreviated as AS2) is investigated using a molecular dynamics simulation with the Born–Mayer potential. The models of amorphous AS2 are constructed in a wide pressure range. The results show that the structure of AS2 mainly consists of the basic structural units TOx (T is Al or Si; x = 4, 5, 6). The topology of basic structural units at different pressures is identical. Two adjacent units TOx are linked to each other through common oxygen atoms and form a continuous random network of basic structural units TOx. The different aluminium silicate states result from the difference of the fraction of units TOx and their spatial distributions. The coordination units (triclusters) OAl3 and OSi3 and (tetraclusters) OAl4 and OSi4 result in AlOx-rich and SiOx-rich regions, and this is the origin of the microphase separation. Regarding the polymorphism, it can be seen that the structure of AS2 comprises three structural phases: TO4, TO5, and TO6 structural phases. The size of TO4 structural phase regions decreases and the size of TO6 structural phase regions increases as pressure increases. Inversely, the size of TO5 structural phase regions increases to a maximum value and then decreases as pressure increases. In the considered pressure range, with increasing pressure, there is a transformation from TO4 structural phase (at low pressure) to TO6 structural phase (at high pressure).