TRAPDOR NMR investigations of phosphorus-bearing aluminosilicate glasses.
The local environment of phosphorus in aluminosilicate glasses of variable sodium/aluminum ratio has been investigated using double-resonance magic angle spinning (MAS) NMR spectroscopy, TRAPDOR (transfer of populations in double resonance) NMR. The studied glasses have the general compn. xNa2O-(1-x)Al2O3-2SiO2 and contain approx. 4 mol% P2O5. The short-range dipolar couplings of phosphorus atoms to aluminum and sodium are used to deduce the connectivities of phosphate units to AlO4 tetrahedra and/or sodium by observing the effects on a 31P MAS echo spectrum caused by simultaneous irradn. of the 27Al or 23Na resonance frequency. In this way, different phosphorus environments were identified. The obsd. resonances can be assigned to Na3PO4 and Na4P2O7 units, as well as phosphate tetrahedra coordinated to n aluminum atoms in the aluminosilicate framework (with n varying from 1 to 3). The results provide unambiguous evidence for the interaction of P with aluminum even in peralkaline compns., confirming a recently proposed model describing the soln. of phosphorus in aluminosilicate melts.
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