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Ab initio studies of possible fluorine-bearing four- and fivefold coordinated Al species in aluminosilicate glasses

Center for High Pressure Research and Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, U.S.A.

Correspondence: ^* E-mail: hanna.nekvasil{at}sunysb.edu

Ab initio NMR gauge-including atomic orbital (GIAO) calculations were used to constrain assignments of resonances in ²⁷Al NMR spectra of F-bearing alkali aluminosilicate glasses. The effect of bond angles within the range 126–150° on the chemical shift was investigated using cluster models of next-nearest atoms that are charge balanced by hydrogen atoms. GIAO calculations used geometries obtained through optimization at fixed Al-O-Si bond angles. The calculated peak positions for all of the 4-fold coordinated Al species yielded calculated ²⁷Al NMR peak positions in good agreement with the experimental data, suggesting that any or all of the species AlF₄^–, AlF₃O(SiH₃)^–, AlF₂O₂(SiH₃)₂^–, and AlFO₃(SiH₃)₃^– may be present. Three of the investigated 5-fold coordinated species AlF₅^2–, AlF₃O₂(SiH₃)₂^2–, and AlF₂O₃(SiH₃)₃^2– fit the experimental requirements well, whereas the remaining 5-fold coordinated species that were tested [AlF₄O(SiH₃)^2–, and AlFO₄(SiH₃)₄^2–] did not.

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