Comparative High-Pressure Study on Rare-Earth Entropy Fluorite-Type Oxides

We report a comparative high-pressure study of two fluorite-type rare-earth oxides with increasing configurational entropy, (CePr)-O₂ _- _δ and (CePrLa)-O₂ _- _δ. Synchrotron-based powder X-ray diffraction and Raman spectroscopy were carried out up to 30 and 20 GPa, respectively. Both compounds retain the cubic fluorite structure throughout the pressure range explored, although an anomaly is observed between 9 and 16 GPa, characterized by a compressibility plateau and changes in vibrational modes. This behavior is attributed to local lattice distortions and a progressive bond angle bending rather than abrupt phase transitions. In (CePrLa)-O_2-δ, the onset of amorphization is observed above 22 GPa, highlighting its reduced structural stability. The bulk modulus of both systems shows a slight decrease after the onset of the anomaly, suggesting subtle lattice softening. Raman spectroscopy reveals suppression of the F₂g mode intensity with increasing cationic disorder, and under compression, partial reordering is evidenced by an increase in the RE-O mode intensity. Our results highlight the complex interplay between configurational entropy, cation size, and pressure in determining the structural stability and vibrational properties of rare-earth high-entropy oxides and provide insight into the mechanisms governing their resilience and local disorder under extreme conditions.