High-temperature Raman spectroscopy of Na2TiSi2O7 glass and melt: coordination of Ti4+ and nature of the configurational changes in the liquid
Reynard, Bruno; Webb, Sharon L.
published: Jan 26, 1998
manuscript accepted: Sep 29, 1997
manuscript received: Mar 13, 1997
ArtNo. ESP147051001006, Price: 29.00 €
Abstract Raman spectra of Na2TiSi2O7 (NTS2) glass and melt have been recorded between ambient temperature and 1100 K in order to study the coordination of Ti4+ and its evolution with temperature. Second-order and anharmonic intensity variations were estimated in order to obtain the spectroscopic signature of configurational changes above the glass transition (Tg ≈ 860 K). At ambient temperature, the Raman spectrum of the glass is dominated by a strong peak at 895 cm-1, which is attributed to the stretching vibration of the short Ti = O bond in (Ti = O)O4 square-pyramidal, five-fold coordination sites. Above Tg, the intensity of this peak does not change significantly, indicating no or little coordination change of Ti4+ with increasing temperature. In the melt, the most important change is a large intensity decrease centered near 700 cm-1. In consistency with the EXAFS results on the same composition, in a model of glass and melt structure based on percolation domains, this is attributed to the progressive breaking of (Ti,Si)-O-Si bridges that link the (Ti = O)O4 units to network former-enriched domains, and to a decrease of the percolation-domain size with increasing temperature. This interpretation is also supported by the evolution of the linewidth of the Ti = O stretching mode. These structural changes can account for the large heat capacity anomaly at Tg in the absence of significant coordination change of Ti.