Strong Antibonding I (p)–Cu (d) States Lead to Intrinsically Low Thermal Conductivity in CuBiI4

Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κlat) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κlat of ∼0.34-0.28 W m-1 K-1 in the temperature range 4-423 K with an Umklapp crystalline peak of 1.82 W m-1 K-1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (Ef), which arise from robust copper 3d and iodine 5p interactions, cause copper-iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κlat in CuBiI4. The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper-iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p-d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κlat in CuBiI4.