Thermoelectric clathrates hold significant promise for high temperature applications with zT values exceeding 1.3. The inorganic clathrates have been shown to be both chemically and thermally stable at high temperatures, and high performance can be obtained from both single crystals and processed powders. The clathrates also show excellent compatibility factors in segmented module applications. For a materials chemist it is furthermore of great importance that the clathrates exhibit a very rich chemistry with the ability for substitution of many different elements. This allows delicate tuning of both the crystal structure as well as the physical properties. With all these assets, it is not surprising that clathrates have been intensely investigated in the thermoelectric community during the past decade. The present perspective provides a review of the many studies concerned with the synthesis, crystal structure and thermoelectric properties of clathrates with emphasis on the type I clathrate.

For more than a decade strongly correlated semiconductors and Kondo insulators have been considered as potential thermoelectric materials. Such materials have large d-or f -character of the electronic band structure close to the Fermi level that theoretically leads to Seebeck coefficients (S) with large magnitudes. In this work it is shown for the first time that the strongly correlated semiconductor FeSb2 exhibits a colossal Seebeck coefficient of ∼ −45000 µVK −1 at 10 K. The thermoelectric power factor P F = S 2 • ρ −1 , where ρ is the electrical resistivity, reaches a record high value of ∼ 2300 µWK −2 cm −1 at 12 K and is 65 times larger than that of the state-of-the-art Bi2Te3-based thermoelectric materials. However, due to a large lattice thermal conductivity the dimensionless thermoelectric figure of merit is only 0.005 at 12 K. Nonetheless, the potential of FeSb2 as a future solid-state thermoelectric cooling device at cryogenic temperatures is underlined.

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.

FeSb(2) was recently identified as a narrow-gap semiconductor with indications of strong electron-electron correlations. In this manuscript, we report on systematic thermoelectric investigation of a number of FeSb(2) single crystals with varying carrier concentrations, together with two isoelectronically substituted FeSb(2-x)As(x) samples (x = 0.01 and 0.03) and two reference compounds FeAs(2) and RuSb(2). Typical behaviour associated with narrow bands and narrow gaps is only confirmed for the FeSb(2) and the FeSb(2-x)As(x) samples. The maximum absolute thermopower of FeSb(2) spans from 10 to 45 mV/K at around 10 K, greatly exceeding that of both FeAs(2) and RuSb(2). The relation between the carrier concentration and the maximum thermopower value is in approximate agreement with theoretical predictions of the electron-diffusion contribution which, however, requires an enhancement factor larger than 30. The isoelectronic substitution leads to a reduction of the thermal conductivity, but the charge-carrier mobility is also largely reduced due to doping-induced crystallographic defects or impurities. In combination with the high charge-carrier mobility and the enhanced thermoelectricity, FeSb(2) represents a promising candidate for thermoelectric cooling applications at cryogenic temperatures.

The thermoelectric power factor of the narrow-gap semiconductor FeSb 2 is greatly enhanced in comparison to the isostructural homologues FeAs 2 and RuSb 2 . Comparative studies of magnetic and thermodynamic properties provide evidence that the narrow and correlated bands as well as the associated enhanced thermoelectricity are only specific to FeSb 2 . Our results point to the potential of FeSb 2 for practical thermoelectric application at cryogenic temperatures and stimulate the search for new correlated semiconductors along the same lines.