Chemical-Bonding Unrelated Asymmetric F-Orbital in Rare Earth Oxides Suppresses Lattice Thermal Conductivity

In electrically insulating materials, quantized atomic vibration (≡ phonon) predominantly carries heat. Phonons in crystalline materials can propagate over thousands of lattice constants but atomic vibration is localized, not collectively excited, in amorphous materials because of lack of long-range order in chemical bonds. For example, quartz crystal shows rather high thermal conductivity (κ) but silica glass shows low κ, suggesting the degree of disorder in the atomic bonds affect the κ. Here, we show that nonspherical, randomly oriented f orbital significantly suppresses κ. We investigated κ in three rare earth oxides of CeO_x, PrO_x, and TbO_x (1.5 ≤ x ≤ 2) crystallizing in the identical structure with different number of f electrons. The experimental results show that κ in CeO₂ (f⁰) and TbO₂ (f⁷) is high (12−13 W/mK) whereas κ in PrO₂ (f¹) is low (6 W/mK). Further, κ in CeO_1.5 (f¹), PrO_1.5 (f²), and TbO_1.5 (f⁸) is low ~2 W/mK. The theoretical calculation supports the experimental data. Although the f-orbitals are unrelated to the chemical bonding, the orbital calculations suggest that nonspherical nature of f-orbitals except f⁰ (closed shell) and f⁷ (half-closed shell) would suppress κ. The present finding would be useful for exploring thermoelectric materials and active materials for thermal transistors.^[1-3]

[1] Q. Yang, M. Yoshimura et al., Adv. Funct. Mater. 33, 2214939 (2023).
[2] Z. Bian, M. Yoshimura et al., Adv. Sci. 11, 2401331 (2024).
[3] A. Jeong, M. Yoshimura et al., Science Adv. in press (2024).