Achieving the decoupling of lattice thermal conductivity and electrical transport remains a central challenge in thermoelectric materials. Here, based on first-principles calculations and anharmonic lattice dynamics, we show that lone-pair-driven orbit-lattice coupling in quasi-one-dimensional halides $\mathrm{Tl}{X}_3$ ($X=\mathrm{Br},\mathrm{I}$) simultaneously enables ultralow lattice thermal conductivity and high thermoelectric performance. We reveal that the stereochemically active Tl-$6s^2$ lone-pair electrons, through $s\text{−}p$ hybridization and antibonding interactions, induce off-center distortions via a pseudo-Jahn-Teller mechanism, leading to highly polarizable bonding, softened interatomic force constants, and pronounced lattice anharmonicity. This electronically driven lattice instability strongly suppresses phonon group velocities and lifetimes, resulting in ultralow lattice thermal conductivities of 0.13 and 0.16 ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ along the $b$ axis for ${\mathrm{TlI}}_3$ and ${\mathrm{TlBr}}_3$ at 300 K, respectively. Concurrently, spin-orbit coupling induces band convergence near the valence band maximum, enhancing valley degeneracy and improving the $p$-type Seebeck coefficient without completely suppressing carrier transport, thereby leading to moderate but beneficial power factors. As a result, peak $ZT$ values of 2.22 (700 K) for $p$-type ${\mathrm{TlBr}}_3$ and 1.91 (800 K) for $p$-type ${\mathrm{TlI}}_3$ are achieved, while $n$-type ${\mathrm{TlI}}_3$ exhibits a competitive $ZT$ of 0.85 at 800 K. Our results establish orbit-lattice coupling as a unifying mechanism for achieving phonon-glass-like thermal transport and high thermoelectric performance, providing useful design insights for the development of next-generation thermoelectric materials.

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• Electronic structure of atoms & molecules
• Lattice dynamics
• Thermal conductivity
• Thermoelectric effects
• Thermoelectrics
• Ab initio molecular dynamics