Structure and diffusion in amorphous aluminum silicate:a molecular dynamics computer simulation

摘要

The amorphous aluminum silicate (Al_2O_3)2(SiO_2)[AS2] is investigated by means of large scale molecular dynamics computer simulations.We consider fully equilibrated melts in the temperature range 6100K>=>=2300K as well as glass configurations that were obtained from cooling runs from T=2300 to 300K with a cooling rate of about 10~(12)K/s.Already at temperatures as high as 4000K,most of the Al and Si atoms are fourfold coordinated by oxygen atoms.Thus,the structure of AS2 is that of a disordered tetrahedral network.The packing of AlO_4 tetrahedra is very different from that of SiO_4 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 SiO_2 network.The latter structure gives rise to a prepeak in the static structure factor at a wave number q=0.5 A~(-1).A comparison of experimental x-ray data with the results from the simulation shows good agreement for the structure function.The diffusion dynamics in AS2 is found to be much faster than in SiO_2.We show that the self-diffusion constant for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.