High‐Temperature lonic Conduction in Multicomponent Solid Oxide Solutions Based on HfO ₂

The parameter p e' of HfO 2 –CaO, HfO 2 –Y 2 O 3 , and HfO 2 –RE 2 O 3 (where RE is a rare‐earth element) solid solutions with high dopant concentrations was measured at 1200°–1600°C. In the oxide systems, the cubic phase was identified using X‐ray diffractometry. All dopant oxides formed wide cubic solid‐solution regions with HfO 2 . In the HfO 2 –Y 2 O 3 system, the 15 mol% Y 2 O 3 composition featured the lowest parameter p e' and, thus, represented the optimum composition for a solid electrolyte. For the investigated RE 2 O 3 ‐doped HfO 2 solid solutions, the parameter p e' decreased as the dopant radius decreased. The lowest parameter p e' values among the RE 2 O 3 ‐doped systems were obtained for HfO 2 –Dy 2 O 3 or HfO 2 –Yb 2 O 3 at1600°C. Furthermore, parameter p e' values were determined for the fluorite‐ and pyrochlore‐type structures in the HfO 2 –Gd 2 O 3 system and ternary oxide solutions based on HfO 2 . Additional investigations confirmed the good reproducibility of the parameter p e' values using two different measuring methods and different sintering conditions for the HfO 2 –30 mol% Y 2 O 3 composition. A comparison between the parameter p e' values of doped HfO 2 and ZrO 2 systems revealed that the parameter p e' values of HfO 2 systems were one to three orders of magnitude lower than those of ZrO 2 systems in the 1200°–1600°C temperature range.