Abstract

An investigation of the structure of sodium aluminum silicate glasses [composition (x)Na2O, (1-x)Al2O3, 0.9SiO2 perturbed through the addition of 2 mol % P2O5] was performed using a combination of 27Al, 29Si, and 31P solid state NMR spectroscopy and ab initio chemical shielding calculations (GIAO method).

A physically consistent method for the deconvolution of inhomogeneously broadened 31P MAS NMR spectra using a combination of spinning sideband analysis with the results of ab initio shielding calculations. It is predicted that a minimum of 13 different phosphorous species must be present in these glasses to account for the spectral complexity apparent over the compositional range investigated. The majority of these species are oxygen bridged to the silicate lattice and are described as (Al,Si)Qpn 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 bridging species, there are non-lattice bridged Na-P type species including ortho-, pyro-, and tripolyphosphates. A sharp cross over in the predominance of Na-P species to (Al,Si) Qpn dominant species occurs at low Al2O3 content. Within the group of non-bridging (Na-P) species, increased bulk aluminum content shifts the predominance from orthophosphate towards tripolyphosphate. Within the group of bridging [(Al,Si) Qpn] species, increasing bulk aluminum content shifts the predominance towards greater bridging with the silicate lattice.

In order to highlight the perturbative effect of 2 mol. % P2O5 addition on glass structure, 29Si MAS NMR of P-bearing and P-absent glasses were compared. The speciation determined from the combined 31P MAS NMR and ab initio shielding calculations is confirmed through analysis of these 29Si MAS NMR spectra. Significantly, it is determined that ca. 8 % of the silica is directly linked to PO4 via Si-O-P linkages.

A parallel analysis of 27Al MAS NMR spectra for both P-bearing and P-absent glasses qualitatively reveal 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, consistent with an abundance of (Al,Si) Qpn species. Analysis of the 27Al inner satellite transitions reveals only a minor amount of VIAl .

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 in the physical properties of melts derived from them.