Temperature and time-dependent changes of structure in phosphorus containing aluminosilicate liquids and glasses: in situ Raman spectroscopy at high temperature

The structural role of phosphorus in glasses and liquids of composition Na2 Si2O5 and Na1.8Al0.2Si2O5.1 has been studied by in situ Raman spectroscopy in the temperature range 300-1500 K. Spectra collected at room temperature indicate that the majority of phosphorus atoms occur in Na3PO4 and Na4P2O7 complexes, as confirmed by bands at similar to 935 and similar to 1025 cm(- 1), respectively. The amplitude of the band at 935 cm(-1) decreased relativ e to that at 1025 cm(-1) with the addition of phosphorus. However, for addi tions of P2O5 > 4 mol%, the relative proportions of these two bands are obs erved to be dependent on the thermal history of the sample in the molten st ate. In situ measurements on samples at temperatures above the glass transi tion demonstrate that temperature-dependent structural changes occur and, f urthermore, that there are time-dependent charges at fixed temperature. The principal time-dependent change observed is an increase in the amplitude o f the 935 cm(-1) band relative to that at 1025 cm(-1) with increasing time. At lower temperature the intensity of 935 cm(-1) band increases immediatel y upon cooling, whereas at higher temperature no changes are detected initi ally, but change occurs after periods of up to 1 h. The kinetics of these t ime-dependent changes are interpreted to be a consequence of liquid-liquid immiscibility. (C) 2000 Elsevier Science B.V. All rights reserved.