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

Dispersion, ionic bonding and vibrational shifts in phospho-aluminosilicate glasses

Understanding the relationships between structure and properties of aluminosilicate glasses is of interest in magmatic studies as well as for glass applications as mechanical or optical components. Glass properties may be tailored by the incorporation of additional elements, and here we studied the effect of phosphate incorporation on refractive index and the degree of ionic bonding in aluminosilicate glasses. The studied glasses in the system SiO2-Al2O3-Na2O-P2O5 had a metaluminous composition (Al:Na = 1) with the content of SiO2 ranging from 50 to 70 mol% and of P2O5 from 0 to 7.5 mol%. Refractive index was measured at four wavelengths from visible to near-infrared and found to decrease both with increasing P2O5 content (at the expense of NaAlO2) and with increasing SiO2 content (by substitution of SiO4 for AlO4 groups). This trend correlated with a decrease in density while, additionally, the formation of Al-O-P bonds with an SiO2-like structure may account for this change. The degree of ionic bonding, assessed via optical basicity and oxygen polarisability, decreased with increasing P2O5 and SiO2 content. Despite the complexity of the studied glasses, oxygen polarisability and optical basicity were found to follow Duffy's empirical equation for simple oxide glasses. In the high frequency infrared and Raman spectra, band shifts were observed with increasing P2O5 and SiO2 content. They indicated changing average bond strength of the glass network and showed a linear correlation with optical basicity.

Structure and dissolution of silicophosphate glass

The solubility of P2O5–SiO2–Na2O–CaO glasses was suppressed by the coexistence of CaO and Na2O, attributed to the delocalization of the electron distribution of P in QP3 units coordinated to the six-fold-coordinated Si.

A modified random network model for P2O5-Na2O-Al2O3-SiO2 glass studied by molecular dynamics simulations

We investigated the short- and medium-range structural features of sodium aluminosilicate glasses with various P₂O₅ (0–7 mol%) content and Al/Na ratios ranging from 0.667 to 2.000 by using molecular dynamics simulations. The local environment evolution of network former cations (Si, Al, P) and the extent of clustering behavior of modifiers (Na⁺) is determined through pair distribution function (PDF), total correlation function (TDF), coordination number (CN), Q_xⁿ distribution and oxygen speciation analysis. We show that Al–O–P and Si–O–Al linkage is preferred over other connections as compared to a random model and that Si–O–Si linkage is promoted by the P₂O₅ addition, which is related to structural heterogeneity and generates well-separated silicon-rich and aluminum–phosphorus-rich regions. Meanwhile, due to the relatively high propensity of Al to both Si and P, heterogeneity can be partly overcome with high Al content. A small amount of Si–O–P linkages have been detected at the interface of separated regions. Clustering of Na⁺ is also observed and intensified with the addition of P₂O₅. Based on the simulated structural information, a modified random network model for P₂O₅-bearing sodium aluminosilicate glass has been proposed, which could be useful to optimize the mobility of sodium ions and design novel functional glass compositions.

(A) A modified structural model proposed for P₂O₅-bearing sodium aluminosilicate glasses. (B) Degree of preferred connection (DPC) of different T–O–T network linkage for LAP, MAP and HAP glass compositions with various P₂O₅ content.