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

Abstract

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 (κ lat ) 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 CuBiI 4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κ lat 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 (E f ), 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 κ lat in CuBiI 4 . 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 κ lat in CuBiI 4 .

Metavalent Bonding-Mediated Dual 6s2 Lone Pair Expression Leads to Intrinsic Lattice Shearing in n-Type TlBiSe2

“…56−58 These antibonding states in the VBM (near-Fermi level) further weaken the chemical bonding and soften the lattice, which promotes structural distortion and creates intrinsic lattice shearing in TlBiSe 2 . 59 We analyzed the local coordination and chemical bonding environment in TlBiSe 2 using real-space descriptors such as charge density and electron localization function (ELF) calculated using DFT. The total charge density of TlBiSe 2 reveals covalent bonding between Bi and Se atoms, confirmed by the overlapping charge densities of Bi and Se (Figure 3b).…”

Low thermal conductivity in Bi8CsO8SeX7 (X = Cl, Br) by combining different structural motifs

“…Despite this, the relative uncertainties observed here are consistent with the relative uncertainties in other materials in the literature when measured by the same method, and it is clear that both materials retain the glass-like temperature dependence observed in the parallel measurements with no low-temperature peak. 31…”

Lattice Dynamics and Thermal Transport in Semiconductors with Anti-Bonding Valence Bands

“…In this work, we focus on a chemical bonding signature: highest-occupied valence bands with a strong anti-bonding character in a semiconductor. Recent studies have suggested that anti-bonding chemical bonds are closely related to ultralow thermal conductivities in a range of materials. − The advantage of using the anti-bonding character of the highest-occupied valence band as a descriptor is that it can be efficiently analyzed using the crystal orbital Hamilton populations (COHP) method, , which only requires ground-state density functional theory (DFT) calculations. This method can be applied to any inorganic crystal structure and requires only basic structural and compositional information as input and minimal time and computing resources.…”