Crystallization of gadolinium- and lanthanum-containing phases from sodium alumino-borosilicate glasses

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 their highest concentrations in these glasses, lanthanide silicate crystals with an apatite structure were found. In this paper, we characterize these crystals using powder X-ray diffraction (XRD), electron microscopy, energy dispersive spectroscopy (EDS), and selective area diffraction (SAD) to evaluate baseline glass composition effect and mixed La/Gd effect on the structure and chemistry of these crystals. When different lanthanide elements (lanthanum and gadolinium) co-exist in the glasses, complete lanthanide silicate solid solution is observed. Small amounts of boron can enter the gadolinium silicate structure if aluminum is present in the melt. The boron is probably substituting for the silicon in the crystal lattice. This substitution will cause a decrease in the unit cell parameters a(0) and c(0). A small amount of Na can also enter the crystal lattice, causing a decrease in the cell parameter a(0), but an increase in c(0). These results may help us to develop better understanding on the solution mechanism of lanthanide oxides in these glasses. (C) 2000 Published by Elsevier Science B.V. All rights reserved.