Scanning Auger microscopy study of lanthanum partitioning in sphene-based glass‐ceramics

Glass-ceramics are being investigated as possible hosts for the radioactive wastes that would result from recycling irradiated nuclear fuels. For a glass‐ceramic to be a superior waste form to a single-phase glass, some of the long-lived actinides and fission products must be preferentially incorporated into the lattice of the crystalline phase. The partitioning of lanthanum in sphene-based glass‐ceramics has been studied by scanning Auger electron microscopy for lanthanum concentrations from 0.2 to 2.0 mol.%. Sphene crystals (CaTiSiOs) were located in the silica-rich glass matrix by recording digital Auger images of the calcium and titanium distributions. The sphene crystals .were typically 0.5 to 5 μn in size and occupied approximately 40% of the total specimen volume. Auger spot analyses revealed that lanthanum was strongly partitioned (by at least a factor of 10) into the sphene phase of phosphorus-free glass-ceramics; however, when a small amount of phosphorus (0.7 mol.%) was included in the glass-ceramic composition as a crystal nucleating agent, the lanthanum was concentrated in a third minor phase which also contained calcium, phosphorus and oxygen. This phase appeared to be a mixed apatite, possibly Ca4La(PO4)3O; individual features were about 2 μm in size and very widely scattered. Although the electron probe current was typically only 1 nA, some beam damage artefacts were observed, especially during extended spot analyses. Chemical shift effects in the Auger spectra of silicon, titanium and phosphorus showed evidence for electron-stimulated desorption of oxygen.