NMR studies of aluminum speciation in tellurite glasses

We report results from solid state nuclear magnetic resonance (NMR) studies of the aluminum coordination environments in TaAl tellurite glasses containing from 60 to 95 cation-% TeO2. These data show the presence of 4-, 5- and 6-coordinate aluminum in all glasses. The aluminum speciation is independent of the Ta/Al ratio, but is affected by the TeO2 content. In TeO2-rich glasses, aluminum is primarily in octahedral coordination. For glasses with smaller amounts of TeO2, the aluminum speciation shifts towards a larger tetrahedral population. The percentage of 5-coordinate aluminum remains constant in all TaAl tellurite glasses. The average coordination number of aluminum is also affected by the thermal history of the glasses, as this number increases after annealing of quenched glasses, consistent with a coordination change for aluminum during the liquid to glass transition.