ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on rare earths environment and crystallization
Résumé
The effect of adding increasing ZrO 2 content on the environment of Nd 3+ ions in a glass belonging to the SiO 2-B 2 O 3-Al 2 O 3-Na 2 O-CaO system was investigated both by optical absorption spectroscopy and Nd-EXAFS (L III-edge). In agreement with the evolution of the structure of the glassy network and more particularly of the distribution of Na + and Ca 2+ ions with ZrO 2 addition put in evidence in a previous study, it is shown here that the average Nd-O distance continuously increases whereas the bond covalency decreases with zirconium content. This result can be explained both by the decrease of the amount of non-bridging oxygen atoms (NBOs) and by the increase of the proportion of Ca 2+ ions acting as charge compensators in the neighborhood of neodymium polyhedra in the (Daniel Caurant) 2 depolymerized regions of the glass structure. This evolution is due to a competition in favor of zirconium between Zr and Nd for Na + cations charge compensators. The local structural evolution around neodymium is probably responsible for the evolution of the crystallization tendency-increase of Nd-rich apatite (Ca 2 Nd 8 (SiO 4) 6 O 2) crystallization in the bulk-observed in this work both during melt cooling and glass heating by increasing ZrO 2 content. It is proposed that the nucleating effect of ZrO 2 on apatite crystallization put in evidence here is mainly due to the changes that are indirectly induced by zirconium in the neighborhood of Nd 3+ ions (destabilization) rather than to the formation of Zr-rich crystals that would then act as nucleating phase for apatite.
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