The effect of the addition of NiO on the crystallization mechanisms has been studied for SiO2–Al2O3–MgO glass‐ceramics. A detailed Rietveld analysis has been made to determine the chemical composition and the cation distribution in the spinel phase and the evolution of Ni2+ coordination site using diffuse reflectance spectroscopy. Powder X‐ray diffraction (XRD) results show the formation of β‐quartz and spinel and the nickel fraction in the spinel phase decreasing as the plateau temperature increases. The determination of cation distribution in spinel using XRD reveals a preference of Ni2+ and Al3+ for octahedral sites and an increasing proportion of Ni2+ in tetrahedral sites for heat treatments at high temperature (1100°C–1200°C). Diffuse reflectance spectroscopy shows that Ni2+ is mainly fivefold coordinated in the starting glass, playing an important role in the crystallization of the spinel phase and it confirms the presence of tetrahedral and octahedral Ni2+ in the glass‐ceramics. The combination of these two techniques also indicates a chemical heterogeneity of spinel crystals with Ni2+‐rich and Mg2+‐rich zones.

No abstract

Selected results on the environment around Zr, Ti, and Ni in glasses and glass-ceramics using X-ray absorption spectroscopy (XAS) are presented to illustrate how this technique can provide information on the nucleation/crystallization processes. These elements actively participate to nucleation and, additionally, some of them bring new optical properties to the material. We show that extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) information, at K-edge or L 2,3 -edges, can give short and medium range structural and bonding information. Nucleation activity of transition elements cannot be concluded from their local environment within the investigated parent glasses (at least for Zr and Ti, the main nucleating agents). On the contrary, medium range organization is important to understand nucleation processes with the evidence of heterogeneities or structural fluctuations that can be directly related to the first crystallizing phases. New developments in the spatial resolution or in situ high temperature measurements show the utility of the XAS technique with environmental changes around transition elements that can be detected prior to macroscopic crystallization. This can be useful to determine whether a specific ion (e.g., Ni 2+ ) enters the crystalline phase and in which site, allowing a better control of the temperature or crystallization kinetics to optimize the material properties.

No abstract