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 gla ss. Phosphorus occurs largely in orthophosphate and phosphide crystals. X-r ay powder diffraction analysis reveals changes in the 'c/a' lattice paramet ers coincident with cadmium and cerium substitutions for calcium in fluor-a patite. The chemical durability of fluor-apatite is not affected by the cha nges in the cia ratio. The limited solubility of Ce and Cd in calcium fluor -apatite is attributed to differences in Cd-O and Ce-O hybrid bonding orbit als. Similarly, the covalent linkage between phosphorus and oxygen in the P O43- tetrahedra is conceptualized in terms of hybrid bonding. In addition t o phosphates, metal phosphides occur dispersed in the glass matrix and thei r compositions Vary with changes in partial pressure of oxygen. Hybrid elec tron orbitals are proposed for the coordinating linkage between metals (M) and phosphorus (P). In (Fe, Cr)P-4, (Cd, Zr)P-4 and Zr3P4 phases, the M and P hybrid orbitals overlap more than in the phase (B, Ca, Sr, Cs, K, Na)(4) P and contribute to a greater durability for the former phases. In both pho sphates and phosphides, the hybrid bonding concept leads to the speculation that non-bonding itinerant electrons originate from phosphorus and are exp ectably localized by the negatively charged oxygen barriers in the phosphat e. (C) 2000 Elsevier Science B.V. All rights reserved.