The structure of ZrO₂ phases and devitrification processes in a Ca-Zr-Si-O-based glass ceramic: a combined a-XRD and XAS study

The structure of the Zr atomic environment in a CaO-ZrO₂-SiO₂ glass ceramic as a function of thermal treatments has been studied, combining X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and anomalous XRD (a-XRD) techniques. The analysis of XRD patterns demonstrates that the devitrification process proceeds through the partial segregation of Zr-depleted phases (wollastonite-like) and Zr-rich phases (Zr oxides). The XAS and a-XRD measurements at the Zr K-edge have been exploited in order to obtain a closer insight into the atomic structure around the Zr atoms. In the as-quenched glass the Zr atom is sixfold coordinated to O atoms in an amorphous environment rich in Ca and Si. Thermal treatment firstly (T = 1273-1323 K) causes partial segregation of Zr in the form of an oxide with a tetragonal zirconia (t-ZrO₂) crystalline structure. Raising the temperature (T = 1373 K) causes the formation of ZrO₂ crystallites in the monoclinic crystallographic phase (baddeleyite, m-ZrO₂). Analysis of the XAS data shows that a considerable amount of Zr remains in an amorphous calcium silicate phase.