Entropy-assisted stabilization of cubic bixbyite phases in multicomponent rare-earth oxides

Rare-earth oxides provide a model platform for examining how configurational entropy reshapes the interplay between crystal structure, lattice dynamics, and redox chemistry. Here, we compare single-component Ln $$_2$$ 2 O $$_3$$ 3 sesquioxides (Ln = Gd, Dy, Y, Yb, Lu) with two equimolar high-entropy oxides (HEO-Lu and HEO-Tb) designed to differ only in the identity of a single cation. While the reference sesquioxides adopt the cubic bixbyite structure and exhibit smooth thermal and optical responses, terbium oxide behaves fundamentally differently: The electronic accessibility of the Tb $$^{3+}$$ 3 + $$\leftrightarrow$$ ↔ Tb $$^{4+}$$ 4 + redox couple stabilizes oxygen-rich, mixed-valence phases with triclinic symmetry and localized electronic states. Under nitrogen, however, terbium remains fully trivalent, and both high-entropy oxides form single-phase cubic solid solutions with no evidence of symmetry lowering, oxygen release, or redox transitions. Laboratory XRD and synchrotron total-scattering measurements confirm that the cubic framework is preserved at both the average and local scale, demonstrating that configurational entropy contributes to hinder the structural instabilities intrinsic to terbium oxide. Notably, HEO-Tb accommodates terbium in a chemically flexible environment capable of hosting excess oxygen without triggering local distortions, revealing an oxygen-buffering behavior that is inaccessible in the binary oxide. Calorimetric measurements show positive deviations from ideal mixing in the molar heat capacity, consistent with phonon broadening and enhanced low-frequency vibrational activity induced by chemical disorder. Optical absorption spectra further support this entropy-assisted homogenization: Both HEO-Lu and HEO-Tb exhibit widened band gaps relative to their single-component analogs, reflecting the suppression of defect-related absorption tails and the elimination of localized states associated with mixed valence in terbium oxide. Overall, our results demonstrate that multicationic cubic structures can be synthesized even when incorporating chemically complex or intrinsically unstable constituents, yielding rare-earth oxides with homogenized vibrational, electronic, and redox landscapes. These findings highlight high-entropy design as a robust strategy for engineering oxide materials with enhanced structural stability, tunable phonon spectra, and chemically flexible oxygen-handling capabilities.