DYNAMICS OF NA IN SODIUM ALUMINOSILICATE GLASSES AND LIQUIDS

Na-23 NMR measurements on Na2Si3O7, Na3AlSi6O15, and NaAlSi3O8 glasses from room temperature to 1200 degrees C show that the dynamics and lo cal structure of sodium in silicate/aluminosilicate glasses and melts vary with composition and temperature. The peak positions decrease in frequency between room temperature and 200 degrees C indicating that t he Na sees a larger average site as temperature is increased, Between 200 degrees-300 degrees and 700 degrees C, line widths, nutation frequ encies and peak positions are consistent with motional averaging of qu adrupolar satellites, Above 700 degrees C there is little or no change in the peak positions with temperature, Chemical shifts of the materi als at 1000 degrees C (Na2Si3O7: 3.6; Na3AlSi6O15: -1.3; NaAlSi3O8: -6 .4 ppm) indicate a slight change in the average Na coordination number from 6-7 for the silicate to 7-8 for the aluminosilicates, Relaxation time (T-1) measurements show a shift in the T-1 minimum to higher tem perature with the addition of aluminum to the system. This is indicati ve of Na motion being hindered in aluminosilicates relative to silicat es. Comparison of the data to a model for spin-lattice relaxation invo lving a distribution of barrier heights to atomic hopping (Svare et al . 1993) yields average barrier heights consistent with this, as well a s indicating differences in the number of neighboring sites and amount of disorder around Na sites in the three glasses. The slopes on the h igh temperature side of the relaxation curves yield apparent activatio n energies for diffusion of 70 kJ/mol, as has been previously seen for several other alkali silicate liquids.