Ultra‐Low Thermal Conductivity in Mixed Anionic CuTe 2 Cl: Strong Antibonding States and Lone Pair‐Induced Te Ring Puckering

Metal chalcohalides, an emerging class of mixed‐anion compounds, have recently garnered increasing interest as they unite the exceptional stability of metal chalcogenides with the versatile electronic tunability of metal halides. Here, we synthesize single‐ and polycrystalline CuTe 2 Cl, a two‐dimensional layered metal chalcohalide, which exhibits intrinsically ultralow lattice thermal conductivity ( κ lat ) of ∼0.54–0.24 W m −1 K −1 between 300 and 600 K along the layered growth direction. Strong p‐d* antibonding states arising from Cu 3 d ‐Te 5 p and Cu 3 d ‐Cl 3 p interactions in the valence band just below the Fermi level weaken the Cu‐Te and Cu‐Cl bonds to render the lattice elastically soft. Moreover, heteroleptic polyhedral coordination enforces chemical bonding hierarchy, which, together with the stereochemically active 5 s 2 lone pair of Te, generates pronounced lattice anharmonicity. These effects give rise to localized low‐energy optical phonon modes, which suppress the acoustic phonons, as confirmed by phonon dispersion calculations. 5 s 2 lone pair driven Te‐ring puckering and lattice shearing between layers provide low‐frequency optical modes, which are experimentally identified by low‐temperature Raman spectroscopy and heat‐capacity measurements. Collectively, strong p‐d* antibonding interaction induced lattice softening and lone pair driven instability produce strong anharmonicity, which markedly suppresses the heat‐carrying acoustic phonons, leading to intrinsically ultralow κ lat in mixed anionic CuTe 2 Cl.