  The structure and transport properties of SiO2-Al2O3 melts containing 13 mol%
and 47 mol% Al2O3 are investigated by means of large scale molecular dynamics
computer simulations. The interactions between the atoms are modelled by a pair
potential which is a modified version of the one proposed by Kramer et al. [J.
Am. Chem. Soc. 64, 6435 (1991)]. Fully equilibrated melts in the temperature
range 6000 K >= T > 2000 K are considered as well as glass configurations, that
were obtained by a rapid quench from the lowest melt temperatures. Each system
is simulated at two different densities in order to study the effect of
pressure on structural and dynamic properties. We find that the Al atoms are,
like the Si atoms, mainly four-fold coordinated by oxygen. However, the packing
of the AlO4 tetrahedra is very different from that of the SiO4 tetrahedra,
which is reflected by the presence of triclusters (O atoms surrounded by three
cations) and edge--sharing AlO4 tetrahedra. On larger length scales, a
micro-segregation occurs, resulting in an Al-rich network percolating through
the Si-O network. This is reflected in a prepeak of concentration-concentration
structure factors around 0.5 A^-1 (both in the system with 47 mol% and 13 mol%
Al2O3!). We also address the interplay between structure and mass transport. To
this end, the behavior of the selfdiffusion constants for the different
compositions and densities is studied.
