Optical Absorption and Excitonic States in Ytterbium-based Oxides: Role of Local Symmetry and Disorder

Rare-earth-based ceramic oxides, particularly those containing trivalent ytterbium (Yb³⁺), have attracted significant interest for their unique optical properties and high-temperature phase stability, making them promising candidates for advanced thermal barrier coatings (TBCs) in gas turbine engines. Understanding the fundamental mechanisms governing optical absorption in these ceramics is essential for predicting radiative heat transfer and excitation behavior under extreme conditions. We investigate the optical response of Yb-based ceramic oxides using first-principles density functional theory (DFT) and many-body perturbation theory (MBPT). The systems studied include cubic Yb₂O₃ (Ia-3), monoclinic Yb₂Si₂O₇ (C2/m), and cubic Yb₂Zr₂O₇ in both ordered pyrochlore (Fd-3m) and disordered fluorite (Fm-3m) structures. The calculated optical absorption spectra reproduce features characteristic of 4f → 4f transitions observed in experimental measurements within the near-infrared range. Excitonic analysis based on G₀W₀ and Bethe–Salpeter Equation (BSE) calculations shows that Yb³⁺ ions at non-centrosymmetric point groups C₂ and C₁ exhibit enhanced 4f oscillator strengths through parity mixing, while those at centrosymmetric point groups S₆ and D_3d yield optically dark excitons. Structural disorder in Yb₂Zr₂O₇ further relaxes selection rules, broadening and intensifying absorption bands. These results connect local point group symmetry and exciton localization to tunable optical activity in Yb-based ceramic oxides, highlighting pathways to tailor optical absorption in these materials for thermal barrier coating applications.