An enhanced resolution of the structural environment of zirconium in borosilicate glasses

The short- and medium-range local environment of zirconium was determined by Zr L2,3-edge and K-edge XANES and by Zr K-edge EXAFS in borosilicate glasses with 1 to 8 mol% ZrO2. Regardless of the ZrO2 content of the glass, Zr is six-coordinated in octahedra with ZrO distances of 2.09 Å. In the glasses containing 8 mol% ZrO2, the octahedra are more distorted than at lower ZrO2 content. Enhanced resolution in the real space is achieved by recording Zr K-edge EXAFS spectra over a broad energy range, enabling to get access k-values up to 20 Å− 1 with a high signal to noise ratio. This allows discriminate between the various second neighbor contributions. A correct fit is based on four Si and two B. This local structure around Zr derived from that of zirconosilicates such as elpidite, in which two Si are replaced by two B. The ZrSi distances increase from 3.63 ± 0.02 to 3.67 ± 0.01 Å, as ZrB distances remain constant within uncertainties, as ZrO2 content increases. Two Na are situated at 3.64 ± 0.03 Å from Zr, ensuring charge compensation of the ZrO6 sites. Increasing the CaO content from 4 to 8 mol% does not lead to any structural change around Zr.