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Low thermal conductive semiconductors have attracted huge attention for heat management and heat harvesting applications. Although the weak chemical bonding in the Cu/Ag-based chalcogenides is promising in suppressing heat transport, their quaternary analogs remain less explored. Here, we report on a comparative study of the crystal structure and thermal conductivity of various Ag-containing variants of Cu4TiSe4, i.e., Ag1–xCu3+xTiSe4(x= 0–0.8) samples. Analysis of the crystal structure, phase transition, and temperature-dependent lattice thermal conductivity (κL) of pristine AgCu3TiSe4and Ag1–xCu3+xTiSe4have been carried out both experimentally and theoretically. The cubic crystal structure (space group P4̅3m) of these Ag variants is identical to that of Cu4TiSe4or Cu4TiTe4, where a positionally disordered Cu sublattice is either replaced by Ag (for AgCu3TiSe4) or by Ag/Cu substructure (for Ag1–xCu3+xTiSe4). Upon cooling, the symmetry reduction to a rhombohedral (space group R3m) structure is attributed to the partial ordering of the positionally disordered Ag. The proposed structural models at different temperatures have been further analyzed using the Maximum Entropy Method (MEM). X-ray photoelectron spectroscopy measurement suggests that the parent compound forms a charge-precise (Ag+)(Cu+)3(Ti4+)(Se2–)4chemical formula. Interestingly, the lattice thermal conductivity of the Ag1–xCu3+xTiSe4samples remains very low, with values varying in the range of ∼0.65–0.24 W m–1K–1between 293 and 623 K. Density Functional Theory (DFT) calculation shows the presence of antibonding states of Cu(3d)/Ag(4d)–Se(4p) below the Fermi level (EF), providing softness to the lattice of AgCu3TiSe4. In addition, the positional disordered site plays a crucial role in further softening the framework and provides large lattice anharmonicity. The calculated phonon dispersions evidence the presence of several soft optical phonon modes at ca.25 cm–1, originating from the atomic vibrations of Ag, Cu, and Se. Further confirmation of these phonon modes is obtained from the low-temperature heat capacity study. The low-lying optical phonon modes in AgCu3TiSe4are mainly caused by the presence of a soft lattice framework, positional disorder and associated rattling-like vibrations of Ag+/Cu+ions. Their strong interaction with the heat-carrying acoustic phonon modes is key ingredient that explains the very low κL.

Mixed-anion compounds, which incorporate multiple types of anions into materials, display tailored crystal structures and physical/chemical properties, garnering immense interest in various applications such as batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed studies regarding correlations among crystal structure, chemical bonding, and thermal/vibrational properties are rare for these compounds, which limits the exploration of mixed-anion compounds for associated thermal applications. In this work, we investigate the lattice dynamics and thermal transport properties of the metal chalcohalide, CuBiSCl2. A high-purity polycrystalline CuBiSCl2sample exhibits a low lattice thermal conductivity (κL) of 0.9–0.6 W/(m·K) from 300 to 573 K. By combining various experimental techniques, including three-dimensional (3D) electron diffraction, with theoretical calculations, we elucidate the origin of low κLin CuBiSCl2. The stereochemical activity of the 6s2lone pair of Bi3+favors an asymmetric environment with neighboring anions involving both short and long bond lengths. This particularity often implies weak bonding, low structure dimensionality, and strong anharmonicity, leading to a low κL. In addition, the strong 2-fold linear S–Cu–S coordination with weak Cu···Cl interactions induces a large anisotropic vibration of Cu, which enables strong phonon–phonon scattering and decreases κL. The investigations into lattice dynamics and thermal transport properties of CuBiSCl2broaden the scope of the existing mixed-anion compounds suitable for the associated thermal applications, offering a new avenue for the search for low thermal conductivity materials in low-cost mixed-anion compounds.

Cu2SnS3is the parent compound of a series of phases in the Cu2+xSn1–xS3section (0 ≤ x≤ 0.15) of the ternary Cu–Sn–S phase diagram, the crystal structure of which can be controlled by varying the synthesis process and/or through a fine-tuning of the chemical composition. Despite being structurally close to the sphalerite structure, the thermal transport of these compounds is strongly dependent on the exact lattice symmetry and degree of atomic disorder. Here, we investigate the lattice dynamics of the monoclinic ordered Cu5Sn2S7(space group C2) and cubic disordered Cu5Sn2S6.65Cl0.35(space group F4̅3m) compounds by temperature-dependent powder inelastic neutron scattering (INS). In both cases, the INS spectra feature low-energy optical modes mostly weighed by the thermal motion of Cu atoms. The response of the INS spectra of Cu5Sn2S6.65Cl0.35to temperature variations is indicative of quasi-harmonic behavior. Combined with analyses of the low-temperature specific heat, these findings show that the significant lowering of the lattice thermal conductivity in cubic Cu5Sn2S6.65Cl0.35is due to a reduced phonon mean free path tied to the increased level of disorder in the unit cell. These results highlight how the stabilization of highly symmetric, yet strongly disordered crystal structures akin to those observed in high-entropy alloys, can lead to a drastic reduction in the heat transport, offering an effective approach to design high-performance thermoelectric sulfides.

Comprehending the relationship between crystal structures and transport properties is crucial to develop materials with improved electrical and thermal properties for thermoelectric applications. In this article, we report on the complex crystal structure and physical properties of Cr2Sn3S7, a n-type magnetic semiconductor with a low energy band gap and low thermal conductivity. Importantly, we demonstrate that the high level of structural complexity is related to the coexistence of two sublattices: a host lattice, [Cr4Sn2S11]2–, characterized by a mixed Sn4+/Cr3+occupancy of its cationic sites, and a guest lattice characterized by [Sn4S3]2+chains, containing Sn2+cations only, closely related to the ones encountered in the orthorhombic SnS compound. By combining experiments including X-ray diffraction with ab initio calculations of electrons and phonons, we succeeded to elucidate the origin of the low thermal conductivity in Cr2Sn3S7.We demonstrate that the low dimensionality of the [Sn4S3]2+chains, which generates weak Sn···S interactions with the 3D host lattice, is induced by the lone pair stereochemical activity of Sn2+. This lattice softening favors strong anisotropic vibrations at low frequencies, highlighting the primordial role played by Sn2+cations in both crystal structure dimensionality and low thermal conductivity in tin-based sulfides.

Understanding the mechanisms that connect heat transport with crystal structures is fundamental to develop materials with optimized electrical and thermal properties for thermoelectric applications. In this work, we synthesized a series of bulk Cl-doped PbBi2S4by mechanical alloying combined with spark plasma sintering. A detailed structural analysis of PbBi2S4(m= 1 member of the series PbmBi2S3+m) and of the compounds Bi2S3(m= 0) and Pb3Bi2S6(m= 3) shows that the low dimensionality of their frameworks is induced by the stereochemical activity of Bi3+and Pb2+6s2lone pairs (L) and is mainly governed by the presence of BiS3L chains of tetrahedrons. By combining experiments with the ab initio band structure and phonon calculations, we discuss the structure-thermoelectric property relationships and clarify the interesting crystal chemistry in this system. We demonstrate that the ultralow thermal conductivity of these sulfides originates from the prominent 1D character induced by the bismuth chains in these frameworks, leading to weak interchain interactions compared to their strong intrachain bonds.