| We have investigated by 27Al Nuclear Magnetic Resonance spectroscopy some composi-tions in the Ln2O3–Al2O3–SiO2 (Ln=Y or La) ternary phase diagram containing more than 60 mol% of SiO2. One– and two–dimensional high–field (17.6 T) high–speed (30 kHz) Magic Angle Spinning experiments have been performed along with simulations of the spectra to quantify the amount of penta–coordinated aluminum present in those glasses as a function of composition. Very high–temperature experiments have allowed to follow selected samples from 2200°C down to 1700°C and hence to characterize the aluminum coordination state and dynamics in those liquids. The present study re–enforces the current view that “minor” species such as penta–coordinated aluminum are actually present in considerable amount in aluminosilicate glasses and high–temperature liquids at and above the charge compensation join. The high–field strength of Y3+ and La3+ reveal, for the first time in glasses, different mean electric field gra-dient perceived by the tetra– and penta–coordinated aluminum environments. The movements responsible for the NMR relaxation of aluminum in the high–temperature liquid are showed to be uncorrelated with the movements responsible for the macroscopic shear viscosity. Results obtained both on glasses and in–situ at high–temperature suggest a preferential localization of Ln3+ nearby tetra–coordinated aluminum species, with possible formation of tricluster and/or Ln3+ coordination changes. |
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