Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy.

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of PO (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments-Al{P}-, Al{Si}-, Si{P}-, and P{Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as P{Al}- and Si{Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMR-allow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as Q, Q, and Q (with the superscript denoting the number of bridging oxygens), the Q units can be safely identified with the help of P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO units that is needed for charge compensation of the Q units necessitates an alternative charge compensation scheme for these AlO units via formation of Q units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P-O-Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P-O-Al bonding, the formation of Si-O-P bonding plays only a minor role.