The dearth of n -type sulfides with thermoelectric performance comparable to that of their p- type analogues presents a problem in the fabrication of all-sulfide devices. Chalcopyrite (CuFeS 2 ) offers a rare example of an n -type sulfide. Chemical substitution has been used to enhance the thermoelectric performance of chalcopyrite through preparation of Cu 1- x Sn x FeS 2 (0 ≤ x ≤ 0.1). Substitution induces a high level of mass and strain field fluctuation, leading to lattice softening and enhanced point-defect scattering. Together with dislocations and twinning identified by transmission electron microscopy, this provides a mechanism for scattering phonons with a wide range of mean free paths. Substituted materials retain a large density-of-states effective mass and, hence, a high Seebeck coefficient. Combined with a high charge-carrier mobility and, thus, high electrical conductivity, a 3-fold improvement in power factor is achieved. Density functional theory (DFT) calculations reveal that substitution leads to the creation of small polarons, involving localized Fe 2+ states, as confirmed by X-ray photoelectron spectroscopy. Small polaron formation limits the increase in carrier concentration to values that are lower than expected on electron-counting grounds. An improved power factor, coupled with substantial reductions (up to 40%) in lattice thermal conductivity, increases the maximum figure-of-merit by 300%, to zT ≈ 0.3 at 673 K for Cu 0.96 Sn 0.04 FeS 2 .

Fundamental understanding of the relationship between chemical bonding, lattice dynamics, and thermal transport is not only crucial for thermoelectrics but also essential in photovoltaics and optoelectronics. This leads to a widespread search for low thermally conductive crystalline metal halide perovskites with improved electrical transport and stability. Pb-free all-inorganic Sn-based halide perovskites are particularly compelling because of their degenerate hole doping capability, which generally results in p-type conduction. Herein, we demonstrate an n-type thermoelectric conduction in concurrence with an ultralow lattice thermal conductivity (κlat ∼0.29–0.22 W/m·K) in an air-stable vacancy-ordered double perovskite Cs2SnI6. Phonon dispersion calculated by density functional theory indicates the presence of low-frequency localized optical modes at 8 and 32 cm–1 due to the dynamical rotation of SnI6 octahedra and anharmonic rattling of Cs-atoms, respectively, which are experimentally verified by temperature-dependent Raman spectroscopy and low-temperature heat capacity measurement. Cs2SnI6 exhibits a soft elastic lattice with chemical bonding hierarchy that causes low bulk and shear moduli, which in turn results in a low measured sound velocity of ∼1158 m/s. Low-energy anharmonic optical modes strongly couple with heat-carrying acoustic phonons and, consequently, limit phonon group velocity and phonon lifetime to an ultrashort value, leading to an intrinsically ultralow κlat in n-type Cs2SnI6.

Earth-abundant, nontoxic crystalline compounds with intrinsically low lattice thermal conductivity (κlat) are centric to the development of thermoelectrics and thermal barrier coatings. Investigation of the fundamental origins of such low κlat and understanding its relationship with the chemical bonding and structure in solids thus stands paramount in order to furnish such low thermally conductive compounds. Herein, we synthesized earth-abundant, cost-effective, and nontoxic n-type ternary sulfide Cu1.6Bi4.8S8, which exhibits an intrinsically ultralow κlat of ∼0.71–0.44 W/m·K in the temperature range of 296–736 K. Structural analysis via atomic refinement unveiled large atomic displacement parameters (ADPs) for interstitial Cu clusters, demonstrating intrinsic rattling-like behavior. Electron localization function (ELF) analysis further shows that these rattling Cu atoms are weakly bonded and thus can generate low-energy Einstein vibrational modes. Low-temperature heat capacity (C p ) and temperature-dependent Raman spectra concord the presence of such low-energy optical modes. Density functional theory (DFT)-based phonon dispersions reveal that these low-lying optical phonons arise primarily due to the presence of chemical bonding hierarchy and simultaneous rattling of weakly bonded interstitial Cu atoms. These low-energy optical modes strongly scatter the heat-carrying acoustic phonons, thereby reducing the phonon lifetime to an ultrashort value (2–4.5 ps) and κlat to a very low value, which is lower than that of the many state-of-the-art metal sulfides.

High thermoelectric performance is generally achieved in solid-solution alloyed or heavily doped semiconductors. The consequent atomic disorder has a trade-off in the thermoelectric figure of merit, zT: lattice thermal conductivity...

Chalcopyrite, CuFeS2 is considered one of the promising n-type thermoelectric materials with high natural abundancy as a mineral. In this work, a partial substitution of germanium in CuFeS2 (CuFe1-xGexS2, 0.0...

The hyperbolic iso-frequency surface (dispersion) of photons in materials that arise from extreme dielectric anisotropy is the latest frontier in nanophotonics with potential applications in subwavelength imaging, coherent thermal emission, photonic density of state engineering, negative refraction, thermal hyperconductivity, etc. Most hyperbolic materials utilize nanoscale periodic metal/dielectric multilayers (superlattices) or metallic nanowires embedded inside the dielectric matrix that require expensive growth techniques and possess significant fabrication challenges. Naturally occurring bulk materials that exhibit tunable hyperbolic photonic dispersion in the visible-to-near-IR spectral ranges will, therefore, be highly beneficial for practical applications. Due to the layered structure and extreme anisotropy, a homologous series of (Bi2)m(Bi2Se3)n could serve as a unique class of natural hyperbolic material with tunable properties derived from different stoichiometry. In this Letter, we demonstrate hyperbolic photonic dispersion in a single crystal of weak topological insulator BiSe (m = 1 and n = 2), where a Bi2 layer is inserted between Bi2Se3 (m = 0 and n = 1) quintuple layers in the visible (525–710 nm) and near-UV (210–265 nm) spectral range. The origin of hyperbolic dispersion in homologous (Bi2)m(Bi2Se3)n topological quantum materials arises from their anisotropic epsilon-near-pole resonance corresponding to the interband transitions that lead to different signs of its dielectric permittivity. The tunability of hyperbolic dispersion is further demonstrated by alloying Bi2Se3 with Mn that alters the interband transition positions and expands their hyperbolic spectral regime from 500–1045 to 500–1185 nm.

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