Strong Intra- and Interchain Orbital Coupling Leads To Multiband and High Thermoelectric Performance In Na₂AuX (X =...

The coupling among electrical conductivity ( $\sigma$ ), Seebeck coefficient ( $S$ ), and lattice thermal conductivity ( ${\kappa }_L$ ) fundamentally limits thermoelectric performance. Increasing band degeneracy can effectively balance $\sigma$ and $S$ to achieve a high-power factor (PF, $S^2\sigma$ ), yet highly degenerate electronic structures are uncommon, particularly in low-symmetry materials. In this work, we propose an unconventional strategy to enhance band degeneracy in zig-zag-chain ${\mathrm{Na}}_2\mathrm{Au}$ $X$ ( $X=$ P, As, Sb, and Bi) compounds. Strong intra-chain hybridization between ${\mathrm{Au}}_{d_{z^2}}$ and $X_{p_z}$ orbitals, together with unexpectedly strong inter-chain coupling of $X_{p_x}$ states, generates a highly dispersive multivalley valence band that supports large PF. Concurrently, the quasi-one-dimensional framework's inherently weak inter-chain interactions, together with the softened Au- $X$ and Au-Au bonds within the chains due to the antibonding $p$ - $d^{\ast }$ states, lead to a substantial reduction in ${\kappa }_L$ . First-principles calculations, integrated with Boltzmann transport theory, confirm that these unique structural and electronic attributes enable $p$ -type ${\mathrm{Na}}_2\mathrm{AuBi}$ to exhibit high thermoelectric performance. This work establishes a new design paradigm for high-efficiency thermoelectric materials by harnessing substantial orbital overlap within weakly bonded, quasi-one-dimensional systems. The findings open promising avenues for discovering and engineering high-performance thermoelectric materials.