Ultra-Low Thermal Conductivity in Mixed Anionic CuTe₂Cl: Strong Antibonding States and Lone Pair-Induced Te Ring...

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 CuTe₂Cl, a two-dimensional layered metal chalcohalide, which exhibits intrinsically ultralow lattice thermal conductivity (κ_lat) of ∼0.54-0.24 W m^-1K^-1 between 300 and 600 K along the layered growth direction. Strong p-d* antibonding states arising from Cu 3d-Te 5p and Cu 3d-Cl 3p 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 5s² 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. 5s² 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 CuTe₂Cl.