Zr coordination change during crystallization of MgO-Al2O3-SiO2-ZrO2 glass ceramics

Glasses with the composition 51.9 SiO2 21.2 MgO 21.2 Al2O3 5.7 ZrO2 (in mol%) are transformed into high- and low-quartz solid solution glass ceramics, by a heat treatment at 950 °C for different periods of time. X-ray absorption near edge structure (XANES) studies at the Zr L2,3 edge show a transition of the Zr coordination number from approximately 6 in the base glass to 8 after completion of crystallization. Scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy element mapping shows that this coordination change is linked to structural alterations that occur in the samples upon annealing: in the base glass, Zr is evenly distributed, whereas in the crystalline samples, it occurs as nanocrystalline ZrO2. A comparison of the Zr L2,3 spectra of the glass ceramic samples with spectra of tetragonal ZrO2 reference samples also indicates that the ZrO2 nanocrystals in the samples are of tetragonal nature, a finding that is also backed by a comparison of the spectra with ab initio multiplet calculations of the Zr L2,3 edges from ZrO2 in different modifications.