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.

The characterization of aluminosilicate glasses is highly relevant in geosciences and for engineering applications such as reinforcement fibers or touchscreen covers. The incorporation of phosphate as a third network-forming species into these glasses offers unique opportunities for fine-tuning glass properties via changes in glass structure and polymerization. In this work, we studied melt-quenched aluminosilicate glasses within the system SiO 2 -Al 2 O 3 -Na 2 O-P 2 O 5 with 50-70 mol% SiO 2 and up to 7.5 mol% P 2 O 5 . All glasses were metaluminous (Al:Na = 1) in order to maximize the degree of polymerization. Increasing the phosphate content at the expense of NaAlO 2 led to reduced glass polymerization and density, resulting in a decrease in elastic moduli and hardness and an increase in strain-rate sensitivity. When increasing the silica content by substituting SiO 4 for AlO 4 tetrahedra, network polymerization remained mostly unchanged, as confirmed by nearly constant hardness. Densification upon indentation was analyzed by Raman spectroscopy and finite element analysis. We find that the elastic properties and hardness of metaluminous phospho-aluminosilicate glasses are governed by changes in density and network polymerization. Other mechanical properties underlie more complex changes in glass structure.

Melt-derived metaluminous (Al/Na = 1) aluminosilicate glasses in the system SiO 2 −Al 2 O 3 −Na 2 O−P 2 O 5 were prepared with P 2 O 5 and SiO 2 contents varying from 0 to 7.5 and 50 to 70 mol %, respectively. The glass structure was investigated by X-ray absorption near edge structure, far-and medium-infrared, and polarized Raman spectroscopic techniques. The results indicate the incorporation of phosphate into the aluminosilicate network not only as partially depolymerized groups but also as fully polymerized groups charge-balanced by aluminate units in Al−O−P bonds. A new analysis method based on polarized Raman spectra in the bending frequency range indicates a preference of phosphate to reorganize the smallest ring structures. Changes in the glass transition temperature with the increase in phosphate content were found to be consistent with the depolymerization of the network structure shown by spectroscopy. By contrast, increasing the silica content by substituting SiO 4 for AlO 4 tetrahedra, while keeping the phosphate content constant, was found to have a negligible effect on network polymerization. Still, the glass transition temperature decreased and correlated with a far-infrared sodium band shift to higher frequency. This was interpreted as local changes in bond strength caused by complex interactions between the different network formers and sodium ions.