Thermal-induced structural behavior in CaO-doped ZrO2 nanocrystals: A high-temperature synchrotron XRD and XAS study

The crystal and local structures of nanocrystalline undoped and CaO-doped ZrO2 were investigated using high-temperature synchrotron X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS). Nanocrystalline ZrO2 was synthesized via a co-precipitation method, whereas CaO-doped ZrO2 was prepared through mechanochemical wet milling, using CaO derived from natural limestone as the dopant. High-temperature synchrotron XRD analysis showed the transformation of undoped and CaO-doped ZrO2 from an amorphous state to a tetragonal phase, stable up to 1100 °C. The CaO-doped ZrO2 required higher temperatures to achieve a fully tetragonal transformation compared to the undoped sample. At equivalent temperatures, Ca doping induced larger lattice parameters, reduced tetragonality, and slower unit-cell volume contraction. However, CaO-doped ZrO2 with 5.0 mol% CaO dopant concentration following fast cooling at a rate of 50 °C/min induced the formation of minor phases, specifically m-ZrO2 and CaZrO3. Furthermore, in situ extended X-ray absorption fine structure (EXAFS) analysis at 700 and 800 °C revealed that the Ca dopant elongated Zr-OI bonds by substituting Zr4+ with larger Ca2+ ions and forming oxygen vacancies, effectively suppressing atomic vibrations. The results reported here point out the structural adaptability of CaO-doped ZrO2 nanocrystals, reinforcing their suitability for high-temperature applications.