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NbFeSb-based half-Heusler alloys have been recently identified as promising high-temperature thermoelectric materials with a figure of merit zT > 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta-alloyed (Nb1− xTa x)0.8Ti0.2FeSb ( x = 0 - 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single-phase Ta-alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying. [ABSTRACT FROM AUTHOR]

We developed a facile microwave-assisted solvothermal method to produce large-scale SnSe1−xTex nanoplates with tens of microns in length and several hundred nanometers in thickness. Enhancements in both peak figure-of-merit (1.1) at 800 K and average figure-of-merit (0.56) from 300 to 800 K were achieved in the p-type SnSe0.9Te0.1 pellet. In addition to the decreased thermal conductivity, the enhancement in figure-of-merit was mainly due to the increase in the power-factor over the mid-temperature range. The enhanced power-factor is caused by the high preferential orientation, large carrier concentration, and the band convergence of multiplevalences. The as-synthesized two-dimensional SnSe1−xTex structures with a large size ratio between the lateral and axial directions secure high preferential orientation in the correspondingly sintered pellet, and the produced Sn vacancies increase the carrier concentration. Based on the optical properties and density functional calculations, we examined the band structure evolution of SnSe1−xTex with increasing Te ratio to confirm the band convergence. This study of alloying with Te provides an alternative approach to enhance the thermoelectric performance of SnSe. [ABSTRACT FROM AUTHOR]

EuIr4In2Ge4 is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal ${I\bar 42m}$ space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap Eg=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin-orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu2+ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K. [ABSTRACT FROM AUTHOR]

EuIr4In2Ge4 is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal I̅ 42m space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap Eg=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin-orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu2+ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K. [ABSTRACT FROM AUTHOR]

Non-invasive local probes are needed to characterize bulk defects in binary and ternary chalcogenides. These defects contribute to the non-ideal behavior of topological insulators. The bulk electronic properties are studied via 125Te NMR in Bi2Te3, Sb2Te3, Bi0.5Sb1.5Te3, Bi2Te2Se, and Bi2Te2S. A distribution of defects gives rise to asymmetry in the powder lineshapes. The Knight shift, line shape, and spin-lattice relaxation are investigated in terms of how they affect carrier density, spin-orbit coupling, and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi2Te2Se is the best topological insulator candidate material at the present time. These results, which are in good agreement with transport and angle-resolved photoemission spectroscopy studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials. [ABSTRACT FROM AUTHOR]

Lead chalcogenide thermoelectric systems have been shown to reach record high figure of merit values via modification of the band structure to increase the power factor or via nanostructuring to reduce the thermal conductivity. Recently, (PbTe)1-2x(PbSe)x(PbS)x was reported to reach high power factors via a delayed onset of interband crossing. Conversely, the (PbTe)1-2x(PbSe)x(PbS)x was reported to achieve low thermal conductivities arising from extensive nanostructuring. Here we report the thermoelectric properties of the pseudoternary 2% Na-doped (PbTe)1-2x(PbSe)x(PbS)x system. The (PbTe)1-2x(PbSe)x(PbS)x system is an excellent platform to study phase competition between entropically driven atomic mixing (solid solution behavior) and enthalpy-driven phase separation. We observe that the thermoelectric properties of the PbTe-PbSe-PbS 2% Na doped are superior to those of 2% Na-doped PbTe-PbSe and PbTe-PbS, respectively, achieving a ZT ≈2.0 at 800 K. The material exhibits an increased the power factor by virtue of valence band modification combined with a very reduced lattice thermal conductivity deriving from alloy scattering and point defects. The presence of sulfide ions in the rock-salt structure alters the band structure and creates a plateau in the electrical conductivity and thermopower from 600 to 800 K giving a power factor of 27 μW/cmK2. The very low total thermal conductivity values of 1.1 W/m∙K of the x = 0.07 composition is accounted for essentially by phonon scattering from solid solution defects rather than the assistance of endotaxial nanostructures. [ABSTRACT FROM AUTHOR]

Phase immiscibility in PbTe-based thermoelectric materials is an effective means of top-down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe1- x S x thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post-annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase-separated) samples of PbTe-PbS are analyzed by in situ high-resolution synchrotron powder X-ray diffraction, solid-state 125Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state-of-the-art synchrotron X-ray diffraction, providing an example of a PbTe thermoelectric 'alloy' that is in fact phase inhomogeneous. Thermally-induced PbS phase separation in PbTe-PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites. [ABSTRACT FROM AUTHOR]

An inside cover of the August 2015 issue of the journal "Angewandte Chemie" is presented with reference of the article "Frontispiz: Hybridization Gap and Dresselhaus Spin Splitting in EuIr4In2Ge4" by Nicholas P. Calta and colleagues.

EuIr4In2Ge4 is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal ${I\bar 42m}$ space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap Eg=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin-orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu2+ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K. [ABSTRACT FROM AUTHOR]

EuIr4In2Ge4 is a new intermetallic semiconductor that adopts a non-centrosymmetric structure in the tetragonal I̅ 42m space group with unit cell parameters a=6.9016(5) Å and c=8.7153(9) Å. The compound features an indirect optical band gap Eg=0.26(2) eV, and electronic-structure calculations show that the energy gap originates primarily from hybridization of the Ir 5d orbitals, with small contributions from the Ge 4p and In 5p orbitals. The strong spin-orbit coupling arising from the Ir atoms, and the lack of inversion symmetry leads to significant spin splitting, which is described by the Dresselhaus term, at both the conduction- and valence-band edges. The magnetic Eu2+ ions present in the structure, which do not play a role in gap formation, order antiferromagnetically at 2.5 K. [ABSTRACT FROM AUTHOR]

In this Rapid Communication we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect. [ABSTRACT FROM AUTHOR]