Decoupled atomic contribution boosted high thermoelectric performance in mixed cation spinel oxides ACo2O4

Abstract

Decoupling the interdependence of various transport parameters in materials has been an intractable challenge in designing efficient thermoelectric materials. Using the first-principles density functional theory and the semi-classical Boltzmann transport theory, we demonstrate that the crucial criteria of obtaining suitable electronic and thermal transport have been achieved by utilizing the presence of mixed cations in spinel oxides. Differently coordinated cations present in spinel oxides lead to decoupled cationic contribution to the electronic and thermal transport properties. While electronic transport properties are controlled by tetrahedrally coordinated cation B (Co), the octahedrally coordinated cations A (Zn/Cd) only contribute to the thermal transport of the system. The combination of heavy bands in the electronic dispersions and tetrahedrally coordinated environment of Co results into an enhanced power factor. Additionally, the substitution of Cd leads to one order of magnitude reduction in the lattice thermal conductivity (κl) without affecting the electronic transport properties. The significant reduction in κl has been attributed to the large mass difference, and remarkably strong anharmonic phonon scattering introduced by Cd. Simultaneously achieved high power factor and low lattice thermal conductivity result in a maximum figure of merit of 1.68 in CdCo2O4 spinel oxide. The approach of decoupling atomic contributions utilizing various cationic sites demonstrates a potential route to enhance thermoelectric performance.