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

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 Q_P², Q_P³, and Q_P⁴ (with the superscript denoting the number of bridging oxygens), the Q_P⁴ 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 Q_P² units necessitates an alternative charge compensation scheme for these AlO₄ units via formation of Q_P⁴ 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.