STRUCTURE AND DYNAMICS IN CALCIUM ALUMINATE LIQUIDS - HIGH-TEMPERATURE AL-27 NMR AND RAMAN-SPECTROSCOPY

Al-27 NMR spectra of CaO-Al2O3 liquids at 2500 K have been obtained us ing a recently developed technique that involves CO2 laser heating and sample levitation inside the NMR magnet. The spectra consist of singl e, narrow lines (100 to 200 Hz FWHM), indicating that rapid chemical e xchange among molecular species occurs in the liquids. Isotropic chemi cal shifts vary linearly with composition, becoming more shielded with increasing Al2O3 content. This is most likely due to an increase in t he average nearest-neighbor coordination of Al, as the proportion of h igher coordinate Al species (AlO5 and AlO6) has been observed to incre ase with alumina content in glasses along this join. This is also in a greement with ion dynamics simulations of liquids along the CaO-Al2O3 join, which show the average Al coordination to range from 4 at low al umina content to 5 for Al2O3 liquid. Correlation times for Al-27 spin- lattice relaxation estimated from the NMR line widths are in agreement with shear relaxation times determined from experimental viscosity me asurements. This may indicate that oxygen exchange between neighboring aluminate polyhedra is a key component in the mechanism for viscous f low. Viscosities determined from oxygen ion diffusion coefficients via ion dynamics simulation agree well with extrapolation of experimental data, in further support of this. We have also obtained high-temperat ure Raman spectra of CaAl2O4 and Ca12Al14O33 glasses and liquids up to 1928 K. The liquid spectra contain structure similar to those of glas ses, indicating that the aluminate liquids consist of AlO(n) polyhedra l units on the vibrational time scale, rather than simple ionic specie s.