Phononic origin of the infrared dielectric properties of RE₂O₃ (RE = Y, Gd, Ho, Lu) compounds

Understanding the phononic origin of the infrared (IR) dielectric properties of yttria (Y 2 O 3 ) and other rare-earth sesquioxides (RE 2 O 3 ) is a fundamental task in the search of appropriate RE 2 O 3 materials that serve particular IR optical applications. We herein investigate the IR dielectric properties of RE 2 O 3 (RE = Y, Gd, Ho, Lu) using density functional theory-based phonon calculations and Lorentz oscillator model. The abundant IR-active optical phonon modes that are available for effective absorption of photons result in high reflectance of RE 2 O 3 , among which four IR-active modes originated from large distortions of REO 6 octahedra are found to contribute dominantly to the phonon dielectric constants. Particularly, the present calculation method by considering one-phonon absorption process is demonstrated with good reliability in predicting the IR dielectric parameters of RE 2 O 3 at the far-IR as well as the vicinity of mid-IR region, and the potential cutoff frequency/wavelength of its applicability is disclosed as characterized by the maximum frequency of IR-active longitudinal phonon modes. The results deepen the understanding on IR dielectric properties of RE 2 O 3 , and aid the computational design of materials with appropriate IR properties.