Raman and X-ray absorption spectroscopy studies of chromium–phosphorus interactions in high-bismuth high-level waste glasses

High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated using Raman and X-ray absorption spectroscopy (XAS). The novel and practically important occurrence of foaming on cooling of these melts is associated with P and Cr from the HLW. In response, glasses were synthesized where Bi₂O₃ and P₂O₅ contents were varied independently. Relationships between P and Cr were found, where as P₂O₅-content increases, chromate Cr–O stretch Raman modes diminish intensity, while Cr XAS shows that Cr reduces, from 50% Cr⁶⁺ + 50% Cr³⁺ to nearly 20% Cr⁶⁺ + 80% Cr³⁺, explaining the chromate mode behavior. In the most P₂O₅-rich glass, the chromate Cr–O distance increases by approximately 0.10 Å, which may indicate bonding between CrO₄ and PO₄ tetrahedra, similar to that in chromo-phosphates. The presence of chromo-phosphate domains in HLW melts can be linked to oxygen generation as a source of the foaming