Structural dependence of crystallization in phosphorus-containing sodium aluminoborosilicate glasses

The article reports on the structural dependence of crystallization in Na2O-Al2O3-B2O3-P2O5-SiO2-based glasses over a broad compositional space. The structure of melt-quenched glasses has been investigated using B-11, Al-27, Si-29, and P-31 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, while the crystallization behavior has been followed using X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy. In general, the integration of phosphate into the sodium aluminoborosilicate network is mainly accomplished via the formation of Al-O-P and B-O-P linkages with the possibility of formation of Si-O-P linkages playing only a minor role. In terms of crystallization, at low concentrations (<= 5 mol.%), P2O5 promotes the crystallization of nepheline (NaAlSiO4), while at higher concentrations (>= 10 mol.%), it tends to suppress (completely or incompletely depending on the glass chemistry) the crystallization in glasses. When correlating the structure of glasses with their crystallization behavior, the MAS NMR results highlight the importance of the substitution/replacement of Si-O-Al linkages by Al-O-P, Si-O-B, and B-O-P linkages in the suppression of nepheline crystallization in glasses. The results have been discussed in the context of (1) the problem of nepheline crystallization in Hanford high-level waste glasses and (2) designing vitreous waste forms for the immobilization of phosphate-rich dehalogenated Echem salt waste.

Automated summary

The article explores the structural dependence of crystallization in Na2O-Al2O3-B2O3-P2O5-SiO2-based glasses and the impact of phosphate at different concentrations on crystallization. Phosphate is found to integrate into the glass network mainly through the formation of Al-O-P and B-O-P linkages.