Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy
Alina Nizamutdinova, Tobias Uesbeck, Thilo Grammes, Delia S. Brauer, Leo van Wüllen
Abstract
In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P 2 O 5 (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— 27 Al{ 31 P}-, 27 Al{ 29 Si}-, 29 Si{ 31 P}-, and 31 P{ 29 Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31 P{ 27 Al}- and 29 Si{ 27 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 P 2, Q P 3, and Q P 4 (with the superscript denoting the number of bridging oxygens), the Q P 4 units can be safely identified with the help of 31 P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO 4 units that is needed for charge compensation of the Q P 2 units necessitates an alternative charge compensation scheme for these AlO 4 units via formation of Q P 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.