Coordination of aluminum in amorphous sodium aluminosilicate films

Abstract

Amorphous sodium aluminosilicate thin films containing large amounts of Al 2O 3 were deposited on fused silica substrates by rf-sputtering, and their aluminum K-band X-ray emission spectra were measured by using an electron probe X-ray microanalyser in order to determine the coordination number of aluminum ions in the amorphous thin films. The chemical shifts for the amorphous films with Al 2O 3/Na 2O<1 were almost identical with those of tetrahedrally coordinated aluminum ions in microline. On the other hand, for the amorphous films with Al 2O 3/Na 2O>1, the chemical shifts increased with increasing Al 2O 3/NA 2OF ratio, approaching that of amorphous alumina. From the comparison with the chemical shifts of α-Al 2O 3 and mullite, the coordination state of aluminum ions in amorphous alumina was found to be about 5, and its structure was found similar to be that in crystalline Al 2O 3 with spinel-type structure. These results indicate that in amorphous sodium aluminosilicate thin films aluminum ions exist in the tetrahedrally coordinated state when the Al 2O 3/Na 2O ratio is nearly equal to or less than unity. However, when the Al 2O 3/Na 2O ratio exceeds unity, some of the aluminum ions begin to assume the octahedrally coordinated state and increase in number with increasing Al 2O 3/Na 2O ratio.