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/crystallisation processes. These elements actively participate to nucleation and, additionally, some of them bring new optical properties to the material. In this paper the authors show that extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) information, at K-edge or L2,3-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 organisation 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 crystallisation. This can be useful to determine whether a specific ion enters the crystalline phase and in which site, allowing a better control of the temperature or crystallization kinetics to optimise the material properties.
