Characterization of immiscibility in calcium borosilicates used for the immobilization of Mo⁶⁺ under Au-irradiation

The aim of this paper was to assess factors affecting primary and secondary phase separation in simplified calcium borosilicate glasses studied for nuclear waste applications. Several glasses with varying [MoO₃] and [B₂O₃] were synthesized and exposed to Au-irradiation to examine compositional effects on glass structure and domain size of separated phases induced by accumulated radiation damage resulting from alpha-decay over a similar to 1000 year timeframe. The produced glasses fell within the immiscibility dome of CaO-SiO₂-B₂O₃ and showed a unique microstructure of embedded immiscibility with three identifiable amorphous phases according to electron microscopy, Raman spectroscopy, and diffraction. These glasses were then bombarded with 7 MeV Au³⁺ ions to a dose of 3 x 10¹⁴ ions/cm² creating an estimated similar to 1 dpa of damage. Several changes to the morphology, spatial distribution, and size of secondary phases were observed, indicative of significant structural reorganization and changes to the chemical composition of each phase. A general mechanism of coalescence to form larger particles was observed for [MoO₃] < 2.5 mol%, whereas segregation to form smaller more evenly distributed particles was seen for [B₂O₃] ≤ 15 mol% and [MoO₃] ≥ 2.5 mol%. These microscopic changes were concurrent to surface-bulk diffusion of Ca and/or Mo ions, where the direction of diffusion was dependent on [B₂O₃] with a barrier identified at similar to 20 mol%, as well as cross-phase diffusion of said ions. These modifications occurred in part through the formation of distorted ring structures within the borosilicate network, which enabled the increased dissolution of isolated (MoO₄)^2- units. Au-irradiation was therefore able to increase the solubility of molybdenum and alter the structure and composition of secondary phases with the extent of modification varying with [MoO₃] and [B₂O₃]/[SiO₂], though glasses notably remained heterogeneous. The collective results suggest that radiation and composition can both be used as design tools to modulate the domain size and distribution of separated phases in heterogeneous glasses. (authors)