Properties of metal phosphates and phosphides in glass-ceramic waste forms

Condensed glass-ceramic waste forms were developed by partial vitrification in a hot isostatic press. The calcined nuclear waste simulants, inclusive of volatiles, are partitioned among a variety of crystalline phases and glass. Phosphorus occurs largely in orthophosphate and phosphide crystals. X-ray powder diffraction analysis reveals changes in the ‘c/a’ lattice parameters coincident with cadmium and cerium substitutions for calcium in fluor-apatite. The chemical durability of fluor-apatite is not affected by the changes in the c/a ratio. The limited solubility of Ce and Cd in calcium fluor-apatite is attributed to differences in Cd–O and Ce–O hybrid bonding orbitals. Similarly, the covalent linkage between phosphorus and oxygen in the PO43− tetrahedra is conceptualized in terms of hybrid bonding. In addition to phosphates, metal phosphides occur dispersed in the glass matrix and their compositions vary with changes in partial pressure of oxygen. Hybrid electron orbitals are proposed for the coordinating linkage between metals (M) and phosphorus (P). In (Fe, Cr)P4, (Cd, Zr)P4 and Zr3P4 phases, the M and P hybrid orbitals overlap more than in the phase (B, Ca, Sr, Cs, K, Na)4P and contribute to a greater durability for the former phases. In both phosphates and phosphides, the hybrid bonding concept leads to the speculation that non-bonding itinerant electrons originate from phosphorus and are expectably localized by the negatively charged oxygen barriers in the phosphate.