Impact of Glass Compositions on Molybdate Crystallization in Borosilicate Glasses

Molybdate crystals tend to precipitate in nuclear waste glasses, significantly compromising their chemical and thermal stability, thereby rendering them unsuitable for long-term storage. However, the mechanisms by which glass composition influences the precipitation of molybdate crystals remain poorly understood. This study investigated this influence by preparing three series of molybdenum-doped sodium–calcium mixed aluminum borosilicate glasses using the melt-quenching technique. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy, supplemented by Raman spectroscopy, was utilized to examine the glass structure at the atomic scale to reveal composition-dependent structural impacts on crystallization, while transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to identify the precipitated crystals. The results demonstrate that increasing the Al2O3 content effectively suppresses molybdate crystal precipitation. It has been proven that high-valence cations differ in their ability to capture free oxygen, with the order of strength being Al3+ > Mo6+ > B3+ and Si4+. It is the strong ability of Al3+ to capture free oxygen and the formation of Al[4]–Ca2+–Mo[6] linkages that are responsible for inhibiting molybdate crystallization in the glass. An intriguing and important abnormal crystallization behavior was observed: a slight substitution of Na2O with CaO resulted in CaMO4 crystal precipitation, whereas larger substitutions paradoxically suppressed it. The findings reveal that in CaO–Na2O mixed aluminum borosilicate glasses, Al[4] preferentially attracts Na+ over Ca2+ to compensate for its negative charge. Meanwhile, Ca2+ ions are capable of forming an Al[4]–Ca2+–Mo[6] linkage, which Na+ ions cannot achieve. This fundamental difference results in the abnormal precipitation of CaMO4 crystals.