Hierarchical chemical bonding and multi-valley band edge-induced high performance in layered Bi6Ag2O6Se4: a theoretical study

Abstract

Exploring novel intrinsically low lattice thermal conductivity materials has become an effective strategy to obtain high thermoelectric performance. Based on first-principles calculations and Boltzmann transport theory, this study investigates the electronic band structure and thermoelectric properties of the layered oxychalcogenide Bi₆Ag₂O₆Se₄. As an indirect bandgap semiconductor, Bi₆Ag₂O₆Se₄ possesses a sandwich structure composed of positively charged [Bi₂O₂]²⁺ oxide layers and negatively charged Se²⁻ and [Ag₂Se₂]²⁻ layers stacked along the c-axis. This layered architecture imparts a distinct anisotropic crystal structure and transport properties. The hierarchical chemical bonding, weak interlayer interaction, lone pair electrons of Bi and rattler-like behavior of Ag result in strong anharmonicity and intrinsically low lattice thermal conductivity. The Ag-4d and Se-4p hybridization at the valence band maximum induces multi-valley band edges and gives rise to excellent electrical transport properties, especially along the in-plane direction. Benefiting from the synergistic effect of low thermal conductivity and multi-valley band edges, the maximum ZT of p-type doping increases from ∼0.24 at 300 K to ∼2.19 at 900 K with the optimized carrier concentration of 2.94 × 10²⁰ cm⁻³ and 4.92 × 10²⁰ cm⁻³, respectively. This study identifies Bi₆Ag₂O₆Se₄ as a promising high performance thermoelectric candidate. Moreover, understanding the relation between the anisotropic crystal structure and thermoelectric properties enables further optimization of thermoelectric performance.