Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.