Structure and diffusion in amorphous aluminium silicate: A molecular dynamics computer simulation

TL;DR

This study uses large-scale molecular dynamics simulations to explore the structure and diffusion properties of amorphous aluminium silicate (AS2), revealing a disordered tetrahedral network with microphase separation and faster diffusion than silica.

Contribution

It provides detailed atomic-level insights into the structure and dynamics of AS2, including the role of Al in ring formations and microphase separation, supported by comparison with experimental data.

Findings

AS2 has a disordered tetrahedral network structure.

Al atoms participate in small rings and triclusters, influencing network topology.

Diffusion of Al and O is significantly faster than in pure silica.

Abstract

The amorphous aluminium silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100K >= T >= 2300K as well as glass configurations that were obtained from cooling runs from T=2300K to 300K with a cooling rate of about 10^12K/s. Already at temperatures as high as 4000K, most of the Al and Si atoms are four-fold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wavenumber q=0.5\AA^{-1}. The comparison of experimental X-ray data with the results from the simulation shows a good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.