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 .

GeTe is among the most fascinating inorganic compounds for thermoelectric (TE) conversion of waste heat into electricity. However, TE performance in its ambient rhombohedral phase is strongly impeded by natural...

The interplay between charges and spins may influence the dynamics of the carriers and determine their thermoelectric properties. In that respect, magneto-thermoelectric power MTEP, i.e. the measurements of the Seebeck coefficient S under the application of an external magnetic field, is a powerful technique to reveal the role of magnetic moments on S . This is illustrated by different transition metal chalcogenides: CuCrTiS 4 and CuMnTiS 4 magnetic thiospinels, which are compared with magnetic oxides, Curie-Weiss (CW) paramagnetic misfit cobaltites, ruthenates, either ferromagnetic perovskite or Pauli paramagnet quadruple perovskites, and CuGa 1- x Mn x Te 2 chalcopyrite telluride and Bi 1.99 Cr 0.01 Te 3 in which diluted magnetism is induced by 3%-Mn and 1%-Cr substitution, respectively. In the case of a ferromagnet (below T C ) and CW paramagnetic materials, the increase of magnetization at low T when a magnetic field is applied is accompanied by a decrease of the entropy of the carriers and hence decreases. This is consistent with the lack of MTEP in the Pauli paramagnetic quadruple perovskites. Also, no significant MTEP is observed in CuGa 1- x Mn x Te 2 and Bi 1.99 Cr 0.01 Te 3 , for which Kondo-type interaction between magnetic moments and carriers prevails. In contrast, spin glass CuCrTiS 4 exhibits negative MTEP like in ferromagnetic ruthenates and paramagnetic misfit cobaltites. This investigation of some chalcogenides and oxides provides key ingredients to select magnetic materials for which S benefits from spin entropy.

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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...

Improvement of thermoelectric parameters is reported with graphite incorporation in n-type Bi2Te3/graphite nanocomposite system. In-depth thermoelectric properties of nanostructured Bi2Te3/graphite composites are probed both microscopically and macroscopically using X-ray diffraction, Raman spectroscopy, inelastic neutron scattering and measurement of the temperature dependence of thermal conductivity , Seebeck coefficient S, resistivity ρ, and carrier concentration nH. Raman spectroscopic analysis confirms that graphite introduces defects and disorder in the system. Graphite addition induces a large (17%) decrease of , originating from a strong phonon scattering effect. A low lattice thermal conductivities L, value of 0.77 Wm -1 K -1 , approaching the min value, estimated using the Cahill-Pohl model, is reported for Bi2Te3+1 wt% graphite sample.Graphite dispersion alters the low energy inelastic neutron scattering spectrum providing evidence for modification of the Bi2Te3 Phonon Density of States (PDOS). Improvement of the other thermoelectric parameters, viz., Seebeck Coefficient and resistivity, is also reported. Theoretical modeling of electrical and thermal transport parameters is carried out and a plausible explanation of the underlying transport mechanism is provided assuming a simple model of ballistic electron transport in 1D contact channels with two different energies.

Te-impurity-incorporated Sb 2 Te 3 , i.e., Sb 2 Te 3 + x mol % Te (x = 0, 4, 6, and 9) composites were synthesized by solid-state reaction technique. Analysis of x-ray diffraction indicates not only Te impurity as a second phase but also doping of Te via suppression of inherent Te vacancies in the Sb 2 Te 3 matrix. As a result of this doping and of the change in formation energy of different types of native defects in Sb 2 Te 3 due to synthesis in a Te-rich condition, carrier concentration (n H ) lower than the pristine sample was observed. Low n H along with gradual convergence of valence bands due to progressive suppression of Te vacancies increases the Seebeck coefficient (S) in Te-incorporated samples. Even though Te impurities increase electrical resistivity (ρ), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor = 17 μW cm −1 K −2 at T = 480 K for x = 6 has been achieved. In addition, Te addition strengthens phonon scattering via an increase of phonon-phonon Umklapp scattering and point-defect-induced scattering of phonons. Due to such a strong phonon scattering, thermal conductivity (κ) decreases, and a reduced lattice thermal conductivity (κ L ), as low as 0.28 W m −1 K −1 at 500 K for x = 6, has been achieved. As a result of simultaneous increase of S and decrease of κ, a high ZT ∼0.87 at 480 K, almost 33% higher than that of the host material, has been achieved.

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