Properties and structures of alkali molybdenum iron phosphate glasses for nuclear waste immobilization

The development of glasses to immobilize nuclear wastes requires a detailed understanding of how composition affects the critical properties required to design wasteforms, including thermal stability and chemical durability. Those properties depend on the molecular-level structures of the glasses. The principal objective of this research was to develop a comprehensive understanding of the composition-structure-property relationships, including the effects of processing conditions, for glasses in the alkali- molybdenum-iron-phosphate systems that could then be used to inform the development of wasteforms of interest to the US Department of Energy. The molecular-level structures of the alkali molybdenum iron phosphate glasses were analyzed by high-pressure liquid chromatography, Raman spectroscopy, and Mossbauer spectroscopy. Alkali ions (Cs+ and Na+) are associated with both phosphate and molybdate anionic species, and the polymerization of the phosphate network with increasing MoO3 content correlates with the formation of isolated Mo6+O6 octahedra in the Mo-rich glasses. The coordination environment of iron is affected both by the addition of large Cs+ ions into glass structure and by the reduction of Fe3+ to Fe2+; the latter is sensitive to the choice of oxidizing or reducing raw materials. Mo5+ ions are incorporated in highly distorted Mo5+O5 octahedral sites, which are associated with crosslinked Mo5+OPO4 structural units that affect properties like molar volume and the glass transition temperature. The dissolution kinetics are sensitive to iron contents, with the most durable glasses having the highest iron contents and smallest phosphate anions. Glasses with isolated Mo6+O6 octahedra in their structures are less chemically durable --Abstract, page iv