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 P2O5 (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27Al­{31P}-, 27Al­{29Si}-, 29Si­{31P}-, and 31P­{29Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31P­{27Al}- and 29Si­{27Al}-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 QP 2, QP 3, and QP 4 (with the superscript denoting the number of bridging oxygens), the QP 4 units can be safely identified with the help of 31P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO4 units that is needed for charge compensation of the QP 2 units necessitates an alternative charge compensation scheme for these AlO4 units via formation of QP 4 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.