Bi₂PdO₄: A Promising Thermoelectric Oxide with High Power Factor and Low Lattice Thermal Conductivity

The search for new energy harvesting materials that directly convert (waste) heat into electricity has received increasing attention. Transition metal oxides are a promising class of thermoelectric (TE) materials that can operate at high temperature due to their chemical and thermal stability. However, the high lattice thermal conductivity, poor electrical conductivity, and low thermopower have significantly impeded their applications to date. Using first-principles calculations, we predict a known oxide Bi₂PdO₄ to be a highly efficient hole-doped TE material with low lattice thermal conductivity and high power factor. These properties are due to (i) the strong anharmonicity stemming from Bi³⁺ 6s² lone pair electrons (leading to low lattice thermal conductivity) and (ii) the flat-and-dispersive valence band structure with high band degeneracy originating from the localized Pd²⁺ d_z ² orbitals in the stacked square planar ligand field (leading to a large power factor). Our results highlight the possibility of oxides as potential TE materials and also afford a novel strategy of designing TE materials by synthesizing compounds which combine a lone pair active cation with a d⁸ cation in a stacked square planar ligand field.