High entropy fluorite oxides
Abstract Eleven fluorite oxides with five principal cations (in addition to a four principal cation (Hf0.25Zr0.25Ce0.25Y0.25)O2 d as a start point and baseline) were fabricated via high energy ball milling, spark plasma sintering, and annealing in air. Eight of the compositions, namely (Hf0.25Zr0.25Ce0.25Y0.25)O2 d, (Hf0.25Zr0.25Ce0.25)(Y0.125Yb0.125)O2 d, (Hf0.2Zr0.2Ce0.2)(Y0.2Yb0.2)O2 d, (Hf0.25Zr0.25Ce0.25)(Y0.125Ca0.125)O2 d, (Hf0.25Zr0.25Ce0.25)(Y0.125Gd0.125)O2 d, (Hf0.2Zr0.2Ce0.2)(Y0.2Gd0.2)O2 d, (Hf0.25Zr0.25Ce0.25)(Yb0.125Gd0.125)O2 d, and (Hf0.2Zr0.2Ce0.2)(Yb0.2Gd0.2)O2 d, possess single phase solid solutions of the fluorite crystal structure with high configurational entropies (on the cation sublattices) akin to those high entropy alloys and ceramics reported in prior studies. Most high entropy fluorite oxides (HEFOs) except for the two containing both Yb and Gd, can be sintered to high relative densities. These single phase HEFOs exhibit lower electrical conductivities and comparable hardness (even with higher contents of softer components such as Y2O3 and Yb2O3) in comparison with 8 mol. Y2O3 stabilized ZrO2 (8YSZ) Notably, these single phase HEFOs possess lower thermal conductivities than that of 8YSZ, presumably due to high phonon scattering by multiple cations and strained lattices.