Covalency-Driven Framework Contraction Weakens the Coupling Between Charge and Phonon Transport in Tunnel-Type TlCu5Se3

Abstract 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.