X-ray absorption studies of the local environment of Zr in high-zirconia borosilicate glasses

Zirconium X-ray absorption spectroscopic (XAS) data were collected and analyzed for a series of zirconia borosilicate glasses developed as potential formulations for immobilization of nuclear fuel reprocessing wastes. For the first time, XAS are presented for silicate glasses with zirconia concentrations as high as 21 wt% and for zektzerite (LiNaZrSi6O15), a crystalline standard that is chemically similar to the glasses. Two other glass series were investigated where the CaO and ZnO contents were varied individually, while the Zr content was held constant. The analysis results indicate that Zr4+ is predominantly in octahedral sites in the glasses. Trends in the data also show that the Zr environments in the glasses are sensitive to changes in ZrO2 concentration,but are less sensitive to changes in Zn or Ca content. With addition of zirconia, the Zr site population shifts From mostly six-coordinated with minor amounts of seven-coordinated Zr, to having a larger fraction of seven-coordinated Zr sites, while maintaining a majority of octahedral Zr. The seven-coordinated Zr atoms probably act as nucleation sites in the glass; as the zirconia content increases to 15 wt% and higher, the numbers of these sites become large enough to promote ZrO2 baddeleyite crystallization. There is no evidence of Zr-Zr pair correlations, similar to those observed in baddeleyite, in any of the crystal-free glasses. (C) 1999 Elsevier Science B.V. All rights reserved.