High-entropy design of transition metal oxide semiconductors with ultra-low thermal conductivity

Metal oxides are used in a broad array of technological applications. However, only a small subset of oxide materials are semiconducting, which limits the range of chemical compositions available for engineering. Here we demonstrate a strategy for driving insulating metal oxides into a semiconducting state with ultra-low thermal conductivity (less than 1 W/m/K) by introducing configurational entropy. This change in electronic character is facilitated by cation mixing in a high-entropy phase, which activates several microscopic mechanisms in electronic and vibrational subsystems that combine to dominate the observed electronic and thermal response of the material. The electronic mechanisms include increased crystal field splitting and electronegativity differences, the preservation of split-off states from parent phases, in-gap states induced by charge transfer between mixed cations, and orbital degeneracy lifting due to lattice distortion. The ultra-low thermal conductivity is attributed to a combination of phonon-defect and crystal momentum non-conserving three-phonon scattering events, both of which arise from chemical disorder. We establish and quantify these effects through co-validated experimental and theoretical analyses of the high-entropy wolframite oxide A6WO4 (A = Mn, Fe, Co, Ni, Cu, Zn). Our analyses suggest that the proposed mechanisms are readily generalizable to a range of functional materials, and could be especially valuable in designing thermoelectric materials, which require simultaneous engineering of semiconducting and thermal properties. Transition metal oxides are interesting for advancing technology as they can combine ease of synthesis, resilience to defects, and high environmental stability, yet few exhibit semiconducting properties, limiting their engineering potential. Here, the authors transform insulating oxides into semiconductors with ultra-low thermal conductivity by introducing configurational entropy, offering a generalizable approach to designing advanced thermoelectric materials with tailored electronic and thermal properties.