Comparative roles of Al3+ and Fe3+ ions for network construction in sodium silicate melts

Abstract

The Raman spectra of Na2O-Al2O3-SiO2 system with different Al2O3 contents (up to 25 mol%) and with Na2O/SiO2 ratio from 1 to 2 have been measured at 1 673 K. In the disilicate systems, the Raman spectra do not change significantly with increasing of Al2O3 content except the development of the 500 cm-1 band which is assigned to the three-dimensional network structural unit consisted of Si4+ and Al3+ ions. Also it is found that Al3+ ions possibly interact with monomers and form three-dimensional structures. In the metasilicate systems, intensities of 950 and 850 cm-1 bands gradually decrease while that of 1 100 cm-1 band increases with the increase of Al2O3 content. According to the following equilibrium reaction in the metasilicate composition, -O-Si-O-(chain-like unit) ⇔Si-O2- (monomer)+-O-Si-O0 (sheet-like unit) it is confirmed that Al3+ ions in the metasilicate systems do not interact with the polymerized silicate anions, such as chain-like or sheet-like structural units. On the other hand, it has a high tendency to interact with SiO44- and construct three-dimensional network units involving Al3+ and Si4+ ions randomly. So it is obvious that the role of Al3+ ions for constructing the network structure is found to be quite different from that of Fe3+.