Covalency-Driven Framework Contraction Weakens the Coupling Between Charge and Phonon Transport in Tunnel-Type...

Framework thermoelectrics are promising for decoupling charge and phonon transport, yet a general structural strategy for simultaneously enhancing carrier mobility and suppressing lattice thermal conductivity (κL) remains unclear. Here, a comparative survey of representative layered, tunnel-type, and cage-like framework thermoelectrics reveals an empirical trend within an isomorphic family: reduced unit-cell volume is generally accompanied by enhanced apparent weighted mobility (μw) and power factor (PF), whereas κL shows no corresponding dependence on unit-cell volume. This weakened coupling suggests that framework contraction is a promising method for achieving excellent thermoelectric performance. Among tunnel-type thermoelectric materials MCu5Se3 (M = Tl, K, Cs), TlCu5Se3 with the smallest volume is selected to verify this strategy. Owing to the enhanced covalent character of Tl-Se interactions relative to nearly ionic K-Se and Cs-Se bonds, TlCu5Se3 exhibits an anomalously contracted Cu-Se framework despite the larger ionic radius of Tl+ than K+. This relative Tl-Se covalency contracts and moderately reinforces the Cu-Se framework, modifies Cu-Se orbital overlap and the valence-band-edge structure, and thereby provides a structural contribution favorable to carrier transport. Meanwhile, because the Tl-Se interaction remains weak and soft in absolute strength, heavy Tl+ cations with stereochemically active 6s2 lone pairs generate low-frequency localized vibrations and strong anharmonicity, efficiently scattering heat-carrying phonons. Consequently, TlCu5Se3 achieves an ultralow κL of 0.27 W m-1 K-1 and a PF of 9 μW cm-1 K-2 at 750 K, yielding ZT ≈ 1.9. These findings establish covalency-driven framework contraction combined with lone-pair-induced phonon scattering as an effective route for high-performance framework thermoelectrics.