Zr environment and nucleation role in aluminosilicate glasses

We have investigated the role of ZrO2 on the nucleation/crystallization properties of aluminosilicate glasses. A comparison between Zr-free and Zr-bearing glasses shows that adding ZrO2 favors nucleation in Li-, Mg-, Ca- and Zn-bearing glasses and has no effects in Na-bearing glasses. The Zr environment has been elucidated coupling X-ray absorption spectroscopy at both Zr K- and L2,3-edges. The Zr environment corresponds to six-fold coordinated sites (Li and Na glasses) and seven-fold coordinated sites (Mg, Ca and Zn glasses), indicating the coordination number has little influence on the ability to crystallize. Direct Zr-Zr polyhedral linkages are observed for all glasses except the Na-bearing one. Since no correlation between the local Zr site and its ability to promote nucleation can be observed, the origin of the nucleating role of Zr has been interpreted as resulting from the Zr distribution with in the aluminosilicate matrix. The poor crystallization ability for Na-bearing glasses is due to the lack of direct Zr-Zr linkages. Medium range ordering appears as a key parameter to explain properties of Zr-bearing glasses.