Broadband optical phonon scattering reduces the thermal conductivity of multi-cation oxides

Multicomponent oxides, such as many minerals and high entropy oxides, show promise as materials for protection in extreme environments. Similar to other phononically dominated materials, the spectrum of vibrational carriers and phonon scattering heavily influences thermal transport in multi-cation oxides. In this work, we experimentally and computationally investigate the nature of phonon scattering and thermal transport in a series of single and multi-cation rare earth sesquioxides and zirconates. A reduction in thermal conductivity was observed from the single to multi-cation oxides, which is directly correlated to measured optical mode lifetimes. Via spectroscopic ellipsometry, we observe red shifting of the optical modes from local bonding distortion. Density functional theory calculation was used to evaluate how bonding distortions influence the phononic scattering rate observed through modal broadening and reduced thermal conductivity. Compared to single-cation oxides, the multi-cation oxides, especially those with larger cation size variance, exhibited lower effective coordination number and greater bond distortion.