Polymorphic transitions and phase stability of CeO2-δ·(Dy,La,Sm,Y)2O3 high-entropy oxides consolidated by spark plasma sintering

The influence of spark plasma sintering (SPS) on the phase stability and bandgap of single-phase HEO ceramics with the composition CeO2-δ·(Dy,La,Sm,Y)2O3 was studied. Single phase HEO powders were prepared by combustion synthesis. The powders were consolidated by SPS at temperatures 1300° C, 1400° C, and 1600° C to prepare dense bulk ceramics. At 1600° C, a complete cubic-to-monoclinic phase transformation occurred, driven by Ce4 + to Ce3+ reduction under reducing SPS conditions. The change in the oxidation state of Ce resulted in an increase of the average ionic radius driving the cubic-to-monoclinic phase transition. Annealing the monoclinic samples at 1200° C in air led to a reversible transformation to the cubic structure. The phase transformations coupled with the presence of oxygen vacancies resulted in a bandgap energy reduction.