STRUCTURE AND DYNAMICS OF CAAL2O4 FROM LIQUID TO GLASS - A HIGH-TEMPERATURE AL-27 NMR TIME-RESOLVED STUDY

The free cooling of an aerodynamically levitated liquid CaAl2O4 droplet from 2400 K to supercooled liquid and glass in a few seconds has been monitored by time-resolved Al-27 NMR. The containerless setup avoids heterogeneous nucleation and allows CaAl2O4 liquid to vitrify with an average cooling rate of 200 K s(-1). In all the observed temperature range, the Al-27 spectra of the liquid phase have Lorentzian line shapes with line widths of a few hundred hertz which have been verified as being due to the aluminum relaxation T-1 time in the extreme narrowing regime. The Al-27 chemical shift of the liquid sample increases linearly with decreasing temperature between 2400 and 1700 K (d delta/dT = -6.0 ppm/1000 K negative slope). The observed isotropic chemical shift position of the glass measured by MAS NMR at room temperature but plotted at T-g (measured by differential scanning calorimetry) falls on this straight line. This continuous evolution is attributed to the progressive dissociation of the AlO4 tetrahedral network of the glass to form AlO5 and AlO6 in the liquid with an increase of the mean coordination number of 0.2 per 1000 K, in agreement with previous ion dynamic simulations. Assuming a quadrupolar relaxation mechanism for Al-27, th, correlation time can be described as a function of temperature. It closely matches the correlation time derived from macroscopic shear viscosity measurements with the same temperature dependence (in this range 1700-2400 K). They are both related to the same microscopic fluctuations.