| Abstract
| - Ab initio calculations have been carried out for the three polymorphs of SiAl2O5 in order to study the 17ONMR characteristics of tricoordinated O atoms, O[3], and tetracoordinated O atoms, O[4], that are possiblypresent in the structure of aluminosilicate glasses. We present δiso, CQ, and η calculations using the densityfunctional theory-based GIPAW method for all silicon, aluminum, and oxygen sites and in particular for the14 O[3] atoms, which are bonded to three Al or Si atoms in the andalusite, sillimanite, and kyanite polymorphsof SiAl2O5. The O[4] parameters calculated in kyanite are also compared with the corresponding site in α-Al2O3.The calculated values for the 29Si and 27Al isotropic chemical shift values as well as 27Al quadrupolar couplingconstants (CQ) and asymmetry parameters (η) are in good agreement with experimental data. Apart from inalumina and in grossite CaAl4O7, 17O NMR parameters have not been measured experimentally yet but theCQ and η values obtained for all sites are consistent with those previously calculated, and the correspondingchemical shift values are reported for the first time. These calculations suggest that it will be difficult todistinguish O(SiAl2) tricoordinated oxygen atoms from O[2] ones in aluminosilicate glasses on the basis of17O NMR data alone since positions of the signals overlap either in a two-dimensional (δ, CQ) space or in a3Q-MAS spectrum. It should be noticed that the tricoordinated O atoms calculated are not surrounded by anycations such as Ca2+ or Na+ while 17O chemical shift values for SiOAl and SiOSi show considerable variationswith cation type. Nonetheless, the steric hindrance around highly coordinated oxygen sites is important, andwe can assume that the first neighbors of the O atoms are usually only Si and Al atoms. Only OAl4 sites withhexacoordinated aluminum, exhibiting a smaller CQ and relatively high isotropic chemical shift values, canbe clearly distinguished from O[2] sites in a 3Q-MAS spectrum.
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