PNNL

Abstract

Lanthanide-containing glasses, commonly used for optical and laser applications, are also important in the vitrification of actinide-bearing radioactive wastes. In previous studies, we measured the glass forming regions of La2O3 and Gd2O3 in some sodium alumino-borosilicate glasses. Above the lanthanide oxide solubility, lanthanide silicate crystals with an apatite structure were found. In this study, we characterized these crystals with powder X-ray diffraction (XRD), electron microscopy, energy dispersive spectroscopy (EDS), and selective area diffraction (SAD) to evaluate the baseline glass compositions and mixed La/Gd effect on the structure and chemistry of these crystals. When both lanthanide elements (lanthanum and gadolinium) are added to the glasses, complete lanthanide solid solution is observed in the lanthanide silicate that forms above the solubility limit. Small amounts of boron can enter the gadolinium silicate structure if aluminum is present in the melt. Boron is probably substituting for silicon in the crystal lattice. This substitution will cause a decrease in the unit cell parameters a0 and c0. A small amount of Na can also enter the crystal lattice, causing a decrease in a0, and an increase in c0. these results may help us to develop better understanding of the mechanism by which lanthanide oxides dissolve in these glasses.