Mechanism of Ultra-Low Thermal Conductivity and Insulating-to-Semiconducting Transition Driven by Configurational Entropy

High entropy oxides (HEOs) have garnered attention for their emergent properties, displaying distinct thermal and electronic behavior from their parent phases. These emergent properties could greatly expand the flexibility and possible applications of HEOs, so understanding HEOs' emergent properties is critical to their further development. In this talk, we will report the mechanism of the ultra-low thermal conductivity and unusual insulating-to-semiconducting transition in the HEO compound (Mn,Fe,Co,Ni,Cu,Zn)WO₄ (A⁶WO₄) [1]. Through integrated experimental and theoretical studies, we find the increased crystal field splitting and electronegativity variation of mixed cations, the preservation of spilt-off states, cation charge exchange induced in-gap states, and lattice distortion driven degeneracy lifting all combine to drive the transition from the large gap insulators (2.5-3.8 eV) parent phases to a small gap (0.24 eV) semiconductor for A⁶WO₄. These mechanisms are tied to the high configurational entropy of the system and most have not been previously reported in a HEO system. Furthermore, our thermal conductivity calculations indicate that the ultra-low thermal conductivity in A⁶WO₄ results from a 6 order-of-magnitude increase in phonon-defect scattering resulting from the increased configurational entropy.

1. Katzbaer, Rowan R., et al. Inorganic Chemistry (2023).