Strong Intra- and Interchain Orbital Coupling Leads to Multiband and High Thermoelectric Performance in Na$_2$Au$X$ ($X$ = P, As, Sb, and Bi)

Abstract

The intrinsic coupling among electrical conductivity ($\sigma$), Seebeck coefficient ($S$), and lattice thermal conductivity ($\kappa_{\mathrm{L}}$) imposes a fundamental limit on the dimensionless figure of merit $ZT$ in thermoelectric (TE) materials. Increasing band degeneracy can effectively balance $\sigma$ and $S$, enabling a high power factor (PF, $S^{2}\sigma$). However, compounds with intrinsically large band degeneracy are scarce. Here, we present an unconventional strategy to realize elevated band degeneracy in zigzag-chain Na$_2$Au$X$ ($X$ = P, As, Sb, Bi) compounds by harnessing strong intra- and interchain orbital coupling. Pronounced hybridization between Au-$d_{z^{2}}$ and $X$-$p_{z}$ orbitals along the Au--$X$ zigzag chains, together with unexpectedly strong interchain $X$-$p_{x}/p_{y}$ coupling, produces a highly dispersive, multivalley valence band structure that supports an exceptional PF. Concurrently, the intrinsically weak interchain interactions arising from the quasi-one-dimensional framework, together with the weakened Au--$X$ and Au--Au bonds within the chains due to filling of $p$-$d^{*}$ antibonding states, result in an ultralow $\kappa_{\mathrm{L}}$. First-principles calculations combined with Boltzmann transport theory predict that $p$-type Na$_2$AuBi achieves a PF of $63.9\,\mu\mathrm{W}\,\mathrm{cm}^{-1}\,\mathrm{K}^{-2}$, an ultralow $\kappa_{\mathrm{L}}$ of $0.49\,\mathrm{W}\,\mathrm{m}^{-1}\,\mathrm{K}^{-1}$, and a maximum $ZT$ of $4.7$ along the zigzag-chain direction at $800\,\mathrm{K}$. This work establishes a new design paradigm for high-efficiency TE materials by exploiting substantial orbital overlap in structurally weakly bonded, quasi-one-dimensional systems, opening promising avenues for the discovery and engineering of next-generation high-performance TE materials.