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-O_I 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.