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

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 Q 3 and Q 4 species are determined semiquantitatively using charge-balance constraints on 29 Si NMR results, showing that the silicate speciation in high-Ga glasses is restricted to Q 4 units, which may affect the properties relevant to nuclear-waste immobilization.