ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on rare earths environment and crystallization

The effect of adding increasing ZrO2 content on the environment of Nd3+ ions in a glass belonging to the SiO2-B2O3-Al2O3-Na2O-CaO system was investigated both by optical absorption spectroscopy and Nd-EXAFS (LIII-edge). In agreement with the evolution of the structure of the glassy network and more particularly of the distribution of Na+ and Ca2+ ions with ZrO2 addition put in evidence in a previous study, it is shown here that the average NdO 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 Ca2+ ions acting as charge compensators in the neighborhood of neodymium polyhedra in the 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 (Ca2Nd8(SiO4)6O2) crystallization in the bulk - observed in this work both during melt cooling and glass heating by increasing ZrO2 content. It is proposed that the nucleating effect of ZrO2 on apatite crystallization put in evidence here is mainly due to the changes that are indirectly induced by zirconium in the neighborhood of Nd3+ ions (destabilization) rather than to the formation of Zr-rich crystals that would then act as nucleating phase for apatite.