Correlated evolution of dual-phase microstructures, mutual solubilities and oxygen vacancies in transparent La2-Lu Zr2O7 ceramics

The A2B2O7 series of ternary oxides are derivatives of fluorite structure over a wide range of rA/rB. Competing by two rare-earths the A-site, La2-xLuxZr2O7 ceramics were found transparent only in pore-free microstructures with similar grain sizes of pyrochlore (PY) and defective fluorite (DF) phases. Mutual solubilities of Lu and La in both phases were found by imaging and energy-dispersive spectroscopy analysis in scanning electron microscope. The dual-phase microstructures were developed with liquid-phase resulted from the exothermal reactions, creating a miscibility gap between two structures to moderate their competing grain growth. Change in grain growth behaviors in liquid-phase is described by a nucleation line in La2Zr2O7‒Lu2Zr2O7 phase diagram. Variations of solution levels in DF grains and co-existing of dual-phase grain clusters in common orientation were revealed in transparent ceramics by electron backscattered diffraction, resulted by epitaxial relation of two phases promoted by the liquid-phase. Oxygen vacancies and various hole states common in both phases were revealed by characteristic cathodoluminescence peaks. The collective effects of pores, phase and grain boundaries, oxygen vacancies on scattering or absorption of visible light enables to establish a hierarchical microstructure‒transparency relationship in such complex oxide ceramics, which could be tailored or further optimized by controllable sintering.