Silicate-phosphate interactions in silicate glasses and melts:: I.: A multinuclear (27Al,29Si,31P) MAS NMR and ab initio chemical shielding (31P) study of phosphorous speciation in silicate glasses

An investigation of the structure of sodium aluminum silicate glasses [composition (x)Na2O, (1-x)Al2O3, 0.9SiO(2) perturbed through the addition of 2 mol.% P2O5] was performed using a combination of Al-27, Si-29, and P-31 solid state NMR spectroscopy and ab initio chemical shielding calculations (GIAO method). Speciation within these glasses was estimated using a physically consistent method for the deconvolution of inhomogeneously broadened P-31 MAS-NMR spectra by employing a combination of spinning sideband analysis with the results of ab initio shielding calculations. A minimum of 13 different phosphorus-bearing species were estimated to contribute to the spectral complexity in these glasses over the compositional range investigated. The majority of these species are oxygen-bridged to the silicate lattice. These are described as (AI,Si)Q(p)(n) species, where n corresponds to the number of bridging oxygens (n = 1-4) and where there is variation in the Al/Si ratio of the tetrahedral nearest neighbors. In addition to the species bridging to the silicate lattice, nonlattice bridged Na-P type species, including ortho-, pyro-, and tripolyphosphates, are observed or deduced based on spectral behavior. A sharp crossover in the predominance of Na-P species relative to (AI,SI) Q(p)(n) dominant species occurs at low Al2O3 content. Within the group of nonbridging (Na-P) species, increased bulk aluminum content shifts the predominance from orthophosphate toward tripolyphosphate. Within the group of bridging [(Al,Si) Q(p)(n)] species, increasing bulk aluminum content shifts the predominance toward a greater number of oxygen bridges to the silicate lattice. To highlight the perturbative effect of 2 mol.% P2O5 addition on glass structure, Si-29 MAS-NMR of P-bearing and P-absent glasses were compared. The speciation estimated from the combined P-31 MAS-NMR and ab initio shielding calculations is consistent with Si-29 MAS-NMR spectroscopic data. Analysis of Al-27 MAS-NMR spectra for P-bearing and P-absent glasses reveals extensive Al-O-P interactions evident by systematic shifts in peak maxima to lower frequencies (greater shielding). Increases in the central transition peak half widths in the P-bearing samples reflect enhanced second-order quadrupolar interactions, also consistent with an abundance of (AI,SI) Q(p)(n) species. Analysis of the Al-27 inner satellite transition sidebands reveals only a minor amount of Al. It is concluded that the addition of even a small amount of P2O5 (2 mol.%) results in a large perturbation of the structure of silicate glasses and should manifest significant effects on the physical properties of. melts derived from them. Copyright (C) 2001 Elsevier Science Ltd.