Network Formation in Borosilicate Glasses with Aluminum or Gallium: Implications for Nepheline Crystallization

Abstract

The evolution of the network connectivity of borosilicate glasses with the progressive addition of aluminum is analyzed using solid-state multinuclear magnetic resonance (NMR) spectroscopy to better understand the structural mechanism by which boron inhibits nepheline crystallization in nuclear-waste glasses. The [4]Al units exert a stronger influence on Na+ than do the [4]B units, resulting in the reorganization of tetrahedral to trigonal-planar boron ([4]B → [3]B) with increasing Al. The [3]B units preferably bond to [4]Al and interfere with the Si–Al ordering critical to nepheline formation, thereby inhibiting nucleation. Moreover, as Na+ charge compensates both [4]B and [4]Al units, the presence of anionic borate units confers a heterogeneous chemical environment about Na+ and prevents its transformation into a nepheline-like crystalline environment. The NMR results for aluminoborosilicate glasses are compared with those of their gallium analogues to assess the structural similarities of Al and Ga in silicate-based glasses. Gallium integrates into the borosilicate network as tetrahedral units similar to Al but is less effective in competing with [4]B for Na+, leading to higher [4]B ratios. The relative fractions of silicate Q3 and Q4 species are determined semiquantitatively using charge-balance constraints on 29Si NMR results, showing that the silicate speciation in high-Ga glasses is restricted to Q4 units, which may affect the properties relevant to nuclear-waste immobilization.

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