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

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 wi th increasing of Al2O3 content except the development of the 500 cm(-1) ban d which is assigned to the three-dimensional network structural unit consis ted of Si4+ and Al3+ ions. Also it is found that Al3+ ions possibly interac t with monomers and form three-dimensional structures. In the metasilicate systems, intensities of 950 and 850 cm(-1) bands gradua lly decrease while that of 1 100 cm(-1) band increases with the increase of Al2O3 content. According to the following equilibrium reaction in the meta silicate composition, O--Si-O- (chain-like unit)double left right arrow Si-O2- (monomer)+ O--Si-O -0 (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 stru ctural units. On the other hand, it has a high tendency to interact with Si O44- and construct three-dimensional network units involving Al3+ and Si4ions 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+.