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N-type Zintl phases with earth-abundant and non-toxic constituent elements have attracted intense research interest thanks to their high thermoelectric efficiencies in the mid-temperature range, exemplified by the recently discovered Mg3Sb2 material. In this study, the liquid phase is expelled from the microstructure of the optimized n-type phase Mg3+xSb1.5Bi0.49Te0.01 by applying a melt-centrifugation technique leading to the formation of lattice dislocations, grain boundary dislocations and increasing porosity. Additional phonon scattering mechanisms were introduced in the microstructure through this manufacturing method, resulting in a significant 50% reduction in the total thermal conductivity from ∼1 W m−1 K−1 to ∼0.5 W m−1 K−1 at 723 K. Combined with high power factors, this reduced heat transport leads to a dimensionless thermoelectric figure of merit, zT, value of ∼1.64 at 723 K, 43% higher than the value obtained in untreated Mg3+xSb1.5Bi0.49Te0.01 (zT ∼ 1.14 at 723 K). This peak zT value yields a predicted device ZT of 0.95, and a promising theoretical thermoelectric efficiency of about 12%. These results further underline the great potential of the lightweight Mg3Sb2 material for mid-temperature energy harvesting via thermoelectric effects.

Discovery of novel high-performance materials with earth-abundant and environmentally friendly elements is a key task for civil applications based on advanced thermoelectric technology. Advancements in this area are greatly limited by the traditional trial-and-error method, which is both time-consuming and expensive. The materials genome initiative can provide a powerful strategy to screen for potential novel materials using high-throughput calculations, materials characterization, and synthesis. In this study, we developed a modified diffusion-couple high-throughput synthesis method and an automated histogram analysis technique to quickly screen high-performance copper chalcogenide thermoelectric materials, which has been well demonstrated in the ternary Cu-Sn-S compounds. A new copper chalcogenide with the composition of Cu₇Sn₃S₁₀ was discovered. Studies on crystal structure, band gap, and electrical and thermal transport properties were performed to show that it is a promising thermoelectric material with ultralow lattice thermal conductivity, moderate band gap, and decent electrical conductivity. Via Cl doping, the thermoelectric dimensionless figure of merit zT reaches 0.8 at 750 K, being among the highest values reported in Cu-Sn-S ternary materials. The modified diffusion-couple high-throughput synthesis method and automated histogram analysis technique developed in this study also shed light on the development of other advanced thermoelectric and functional materials.

Deeper understanding of electrical and thermal transport is critical for further development of thermoelectric materials. Here we describe the thermoelectric performance of a group of rare-earth-bearing half-Heusler phases determined in a wide temperature range. Polycrystalline samples of ScNiSb, DyNiSb, ErNiSb, TmNiSb, and LuNiSb are synthesized by arc melting and densified by spark plasma sintering. They are characterized by powder x-ray diffraction and scanning electron microscopy. The physical properties are studied by means of heat-capacity and Hall-effect measurements performed in the temperature range from 2 to 300 K, as well as electrical-resistivity, Seebeck-coefficient, and thermal-conductivity measurements performed in the temperature range from 2 to 950 K. All the materials except TmNiSb are found to be narrow-gap intrinsic p-type semiconductors with rather light charge carriers. In TmNiSb, the presence of heavy holes with large weighted mobility is evidenced by the highest power factor among the series (17 mu W K-²cm(-¹) at 700 K). The experimental electronic relaxation time calculated with the parabolic band formalism is found to range from 0.8 x 10(-¹⁴) to 2.8 x 10(-¹⁴) s. In all the materials studied, the thermal conductivity is between 3 and 6 W m(-¹) K-¹ near room temperature (i.e., smaller than in other pristine d-electron half-Heusler phases reported in the literature). The experimental observation of the reduced thermal conductivity appears fully consistent with the estimated low sound velocity as well as strong point-defect scattering revealed by Debye-Callaway modeling. Furthermore, analysis of the bipolar contribution to the measured thermal conductivity yields abnormally large differences between the mobilities of n-type and p-type carriers. The latter feature makes the compounds examined excellent candidates for further optimization of their thermoelectric performance via electron doping.

SSCI-VIDE+ECI2D+LPI; International audience; Intermetallic compounds exhibit a high degree of structural diversity, with numerousunique structure types. Among them, complex intermetallic compounds, also known ascomplex metallic alloys (CMAs), present large periodic structures based on building blocksconsisting in highly symmetric atomic polyhedra, often displaying motifs of simplercompounds, like hierarchical replacement of atomic positions in the more elementalstructures by larger atomic clusters or cages in the complex ones. Typical clusters/cagesfound in quasicrystals include Mackay, Bergman and Tsai clusters, and a large variety ofhighly symmetric polyhedra are generally found in CMAs.The structural complexity of CMAs extends to the surface. In this talk, the relationshipbetween the three dimensional cluster-based bulk structure and the two-dimensional surfacewill be questioned in two cases: the pseudo-10fold surfaces of Al13TM4 (TM = Fe, Co)quasicrystalline approximants [1] and the low-index surfaces of the Ba8Au5.25Ge40.75 cagecompounds [2], using a combination of experimental techniques under ultra-high vacuumand theoretical calculations based on the Density Functional Theory. The focus will be onthe different mechanisms by which stable surface terminations are selected, to highlight thefactors governing the atomic and electronic properties of the observed surface.The specific atomic arrangements at the surface induced by the intrinsic cluster-based bulkstructure of CMAs are expected to affect their surface properties. This will be discussed inthe case of the pseudo-10fold surfaces of Al13TM4 compounds, in terms of their catalyticproperties [3].[1] Ph. Scheid et al., Acta Cryst. A, 75 (2019) 314-324; É. Gaudry et al., Phys. Rev. B 94 (2016) 165406; J. Ledieu et al., Phys. Rev. Lett. 110 (2013) 076102.[2] K. Anand et al., J. Phys. Chem.. C 122 (2018) 2215 and 29298.[3] L. Piccolo et al., Sci. Technol. Adv. Mater. 20 (2019) 557.

We report on comprehensive de Haas–van Alphen (dHvA) and electronic band-structure studies of the superconducting skutterudites LaPt4Ge12 (Tc = 8.3 K) and PrPt4Ge12 (Tc = 7.9 K). Both materials show very rich spectra of dHvA oscillations with similar and only slightly varying angular-dependent frequencies. The spectral richness can partly be rationalized by the elaborated electronic band structures resulting in several Fermi surfaces built by six different bands. The effective cyclotron masses of both superconductors lie between about 0.5 and 1.1 times the free-electron mass. Although these values are small, we find moderate mass enhancements between about 2 and 4 when comparing to the calculated masses. Our results evidence the localized character of the 4f electrons in the Pr compound and are in line with an electron-phonon mediated multiband superconductivity, largely identical for both compounds.

Noncentrosymmetric superconductors have attracted tremendous research interest due to the possibility of mixed spin-singlet and spin-triplet pairing in these materials. In this work, physical properties of a noncentrosymmetric superconductor BeAu were investigated. It was established that BeAu undergoes a structural phase transition from a room-temperature noncentrosymmetric FeSi structure type to a high-temperature CsCl structure type at Ts = 860 K. The room-temperature modification exhibits a superconducting transition below Tc = 3.3 K. The values of lower (Hc1 = 32 Oe) and upper (Hc2 = 335 Oe) critical fields are rather small, confirming that this Type II (κG−L=2.3) weakly coupled (λe−p= 0. ∆ Ce/γnTc≈1.26) superconductor can be well understood within the Bardeen-Cooper-Schrieffer theory. The muon spin relaxation analysis indicates that the time-reversal symmetry is preserved when the superconducting state is entered, supporting conventionalsuperconductivity in BeAu. From the density functional calculations, a considerable contribution of the Be electrons to the superconducting state was established. Moreover, on average, a rather small mass renormalization was found, consistent with the experimental data.

The solid-state reaction of WO3 with W was studied in order to clarify the phase formation in the binary system W-O around the composition WOx (2.50 ⤠x ⤠3) during spark plasma sintering (SPS). A new phase âWO2.82â is observed in the range 2.72 ⤠x ⤠2.90 which might have the composition W12O34. The influence of the composition on the thermoelectric properties was investigated for 2.72 ⤠x ⤠3. The Seebeck coefficient, electrical conductivity and electronic thermal conductivity are continuously tunable with the oxygen-to-tungsten ratio. The phase formation mainly affects the lattice thermal conductivity κlat which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W18O49 and âWO2.82â. In single-phase WO2.90 and multi-phase WOx materials (2.90 ⤠x ⤠3), which contain crystallographic shear plane phases, a similar reduced κlat is observed only below 560 K and 550 K, respectively. Therefor e, the composition range x < 2.90 in which the pentagonal column structural motif is formed might be more suitable for decreasing the lattice thermal conductivity at high temperatures. © 2017 by the authors. Licensee MDPI, Basel, Switzerland.

Nanoparticles (NPs) of Ba8Ga16Ge30 clathrate-I were synthetized via sol-gel-calcination/chemical-reduction route. A solution of the metallic cations was used to prepare an acryl-amid-based gel. The gel is dried and calcined to obtain nanocrystalline powders of precursor oxides. The oxides are reduced by reacting with CaH2 to produce the clathrate, which is embedded in a CaO matrix. CaO is removed by a washing step to obtain the clathrate NPs. The shape and size of the precursor oxide NPs can be modified by addition of complexing agents, surfactants or by varying the pH or the metal and surfactant concentration in the gel. Powder X-ray diffraction and SAED patterns confirm the clathrate-I-type crystal structure of the products. SEM/TEM investigations show that the size and morphology of the oxides are retained in the clathrate NPs after the reduction. The clathrate NPs exhibit morphology of thin plates similar to 300 x similar to 300 nm(2) and thickness of similar to 50 nm, or sphere-like morphology with similar to 200 nm diameter, depending on the sol gel synthesis conditions. The clathrate NPs were compacted via spark plasma sintering (SPS) to pellets with 53-93 % of crystallographic density. The total thermal conductivity (kappa) of the pellet with 93 % density shows a reduction of 25 % in comparison to the reported K in bulk clathrate. Preliminary characterization of the Seebeck (S) and electrical resistivity (R) of the low density sample (53 %) indicates n-type conduction and semiconductor behavior of the Ba8Ga16Ge30 clathrate-I. The transport properties of Ba8Ga16Ge30 clathrate-I with 3-, 4- or 5-layer slabs and [100] surface termination as well as of the bulk material were calculated by using the semi-classical Boltzmann transport theory within the constant scattering approximation. Our results show an increase in S for the geometries with reduced dimensions in agreement with the experimental observations.

The Ba - containing type - I clathrates show promising thermoelectric (TE) properties [1]. Therefore, Ba - Au - Ge type - I clathrates are thus of interest for TE applications as well [2]. Both p - and n - type conductivity were found in the Ba - Au - Ge system, depending on Au content. Here we present the measurements of the electrical and thermal transport on the p - type single crystal Ba 8 Au 5.25 Ge 40.3 □ 0.45 along [001], [110] and [111] directions. The sample shows a semiconducting - like behavior. The Hall coefficient R H exhi bits a positive sign, pointing on the dominance of electron - like conduction. The Hall mobility μ H below ~ 100 K follows approximately T - 3/2 law, while above 100 K the μ H is temperature independent, which denotes on neutral - impurity scattering. Thermal conductivity κ , measured by steady state method and 3ω method, is rather low with κ ~ 1 Wm - 1 K - 1 at room temperature. It is interesting to point out that the electrical resis tivity, R H and κ along [110] direction revealed a different magnitude compared to [001] and [111] directions. It seem reasonable to assume that this observation is due to different electron - phonon coupling for different directions, as it will be shown by l ow - temperature κ data. [1] A. Saramat, G. Svensson, A. Palmqvist, C. Stiewe, E. Mueller, D. Platzek, S. Williams, D. Rowe, J. Bryan, and G. Stucky, J. Appl. Phys. 99 , 023708 (2006). [2] H. Zhang, H. Borrmann, N. Oeschler, C. Candolfi, W. Schnelle, M. Schmidt, U. Burkhardt, M. Baitinger, J. - T. Zhao, Y. Grin, Inorg. Chem. 50 , 1250 (1250). We acknowledge financial support from the Austrian Science Fund (FWF project TRP 176 - N22) and from the German Research Foundation (DFG project SPP 1386 - nanOcla)

International audience; We have investigated the structure of the Al13Fe4(010) surface using both experimental and ab initio computational methods. The results indicate that the topmost surface layers correspond to incomplete puckered (P) planes present in the bulk crystal structure. The main building block of the corrugated termination consists of two adjacent pentagons of Al atoms, each centered by a protruding Fe atom. These motifs are interconnected via additional Al atoms referred to as "glue" atoms which partially desorb above 873 K. The surface structure of lower atomic density compared to the bulk P plane is explained by a strong Fe-Al-Fe covalent polar interaction that preserves intact clusters at the surface. The proposed surface model with identified Fe-containing atomic ensembles could explain the Al13Fe4 catalytic properties recently reported in line with the site-isolation concept [M. Armbrüster et al., Nat. Mater. 11, 690 (2012)].

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime which contradicts the usual assumption of strong anharmonic phonon--phonon scattering processes. Within the framework of ab-intio density functional theory calculations we interpret these hybridizations as a series of an ti-crossings which act as a low pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba$_8$Ge$_{40}$Ni$_{6}$ and compare it to those of pure Ge and a Ge$_{46}$ empty-cage model system., 10 pages, 10 figures

The vibrational dynamics of the ternary Ca1-xFe 4Sb12, Ba1-xFe4Sb12, Sr1-xFe4Sb12, and Yb1-xFe 4Sb12compounds is studied for its temperature dependence, dispersion relations, and material properties by experiments and ab initio powder-averaged lattice-dynamics (PALD) calculations. The experimental techniques used are high-resolution inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). Vibrational properties of polycrystalline material have been mapped out in an extensive energy-momentum phase space facilitating the study of powder-averaged phonon-dispersion relations within the first and second Brillouin zones. Qualitatively, the experimental dynamic structure factor S(Q,?) matches, to a high degree, PALD calculations allowing different schemes of collective vibrational eigenmodes to be evaluated. From the PALD calculations, quantitative values of velocity of sound, Debye temperatures, and elastic, bulk, and Young's moduli are computed. The variation of the generalized density of states G(?) with temperature is studied experimentally in the range from 2 to 600 K. No particular anomalies in the collective dynamics are observed apart from a global softening of G(?) upon heating.

The corrosion of thermoelectric clathrates alpha- and beta-Eu8Ga16-xGe30+x is studied under static air (DSC/TG) up to 650 °C. Oxidation product layers with thicknesses of 10.5 µm (alpha-Eu8Ga16-xGe30+x) and 4.5 µm (beta-Eu8Ga16-xGe30+x) are formed. The oxidation begins at 360 and 380 °C, respectively. XRD pattern of the beta-Eu8Ga16-xGe30+x oxidized layer shows Ge and four Eu2O3 reflections; while alpha-Eu8Ga16-xGe30+x data (sample crushed and grinded) shows alpha-Eu8Ga16-xGe30+x reflections (majority phase), two EuGa2±xGe4minus-or-plus signx strongest reflections and Ge (minority phases). These results indicate the amount of oxides is very small or they are amorphous. High temperature powder XRD measurements on alpha-Eu8Ga16-xGe30+x under air confirm the oxidation onset temperature determined in DSC studies.

International audience; In a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study of the surface of Al13Co4(100), all techniques have found that after annealing to 1165 K, the surface structure is consistent with a dense Al-rich plane with surface Co atom depletion. Various structure models were considered, and in the LEED study, the best agreement was found with a model that consists of Al-rich terminating planes with no Co atoms, and otherwise a structure similar to the bulk puckered layers. This structure was also found to be stable in the DFT study. The best-fit structural parameters are presented for the two domains of this structure, which contain bipentagons that can be related to the pentagonal bipyramidal structures in the bulk, plus additional glue atoms between them. These domains are not strictly related to each other by symmetry, as they have different surface relaxations. The STM study found significant differences in the surfaces of samples grown by different methods and is able to explain a different interpretation made in an earlier study.

Recently, the new skutterudite superconductor PrPt4Ge12 with the comparably high transition temperature of Tc = 7.9 K was discovered [1]. First experiments gave evidence for strong coupling and hint at point-like nodes of the order parameter [2]. The Sommerfeld coefficient is rather low and indicates that PrPt4Ge12 is not a heavy-fermion superconductor. Here, we present results of a de Haas-van Alphen (dHvA) study performed on a high-quality single crystal. Several dHvA frequencies could be resolved over a broad angular range. The small band-resolved effective masses (less than half the free-electron mass) confirm the non-heavy-fermion character of the electron system. To get insight into the nature of the mass renormalization, the experimental data are compared with state-of-the-art band-structure calculations

We have investigated magnetic susceptibility, electrical resistivity, specific heat, thermoelectric power, Hall coefficient and thermal conductivity of the Al13Fe4 and Al13(Fe, Ni)4 monoclinic approximants to the decagonal quasicrystal. While the Al13Fe4 crystals are structurally well ordered, the ternary extension Al13(Fe, Ni)4 contains quenched disorder and can be viewed as a disordered version of Al13Fe4 . The crystallographic-direction-dependent measurements were performed along the a*, b and c directions of the monoclinic unit cell, where (a*, c) atomic planes are stacked along the perpendicular b direction. The electronic transport and magnetic properties exhibit significant anisotropy. The stacking b direction is the most conducting direction for the electricity and heat. The effect of quenched disorder in Al13(Fe, Ni)4 is manifested in the large residual resistivity and significantly reduced thermal conductivity of this compound, as compared to the ordered Al13Fe4. Specific heat measurements reveal that the electronic density of states at the Fermi level of both compounds is metallic-high. The anisotropic Hall coefficient reflects complicated structure of the anisotropic Fermi surface that contains electron-like and hole-like contributions. Depending on the combination of directions of the current and magnetic field, electron-like ( Rh0 ) contributions may dominate, or the two contributions compensate each other (RH=0). Similar complicated anisotropic behavior was observed also in the thermopower. The anisotropic Fermi surface was calculated ab-initio using the recently refined Al13Fe4 structural model that is also presented in this work.

International audience; We present a detailed study of the (100) surface of the orthorhombic Al13Co4 crystal using both experimental and ab initio computational methods. This complex metallic alloy is an approximant of the decagonal Al-Ni-Co quasicrystalline phase. After sputter-annealing preparation of the surface at 1073 K, the low-energy electron diffraction pattern recorded exhibits a pseudotenfold symmetry with lattice parameters consistent with those of the bulk model. At this stage, scanning tunneling microscopy (STM) measurements reveal two different types of surface terminations. A comparison between these two surface structures and the bulk planes indicate that the terminations correspond either to an incomplete puckered layer (P) or to an incomplete flat layer (F). At 1173 K, the majority of the surface consists of P layer terminations. STM images calculated from our proposed surface model are in good agreement with experimental images. X-ray photoelectron diffraction patterns and single scattering cluster calculations further confirm that the local atomic arrangements present in the bulk model are preserved within the near-surface region.

The intermetallic compounds PdGa and Pd3Ga7 are introduced as selective catalysts for the hydrogenation of acetylene. Single phase PdGa and Pd3Ga7 can readily be prepared by the appropriate thermal treatment of the stoichiometric mixtures of the corresponding elements. The initial low surface areas of the as-prepared materials can be increased by careful mechanical treatment without decomposition. Detailed investigations of PdGa and Pd3Ga7 by DSC/TG, in situ powder X-ray diffraction and in situ X-ray absorption spectroscopy during thermal treatment under various inert or reactive gas atmospheres showed a high thermal stability. The long-range and short-range order in the crystal structures remained intact up to temperatures of about 600 K. Neither phase transitions nor decomposition were detectable. In addition to high thermal stability—preserving the active-site isolation under reaction conditions—no incorporation of hydrogen or carbon in the intermetallic compounds under reducing conditions was observed. Besides being interesting model systems, palladium–gallium intermetallic compounds are promising candidates for the application as highly selective hydrogenation catalysts.

A new concept for the development of catalysts is introduced. Utilizing ordered Pd-Ga intermetallic compounds high selectivity and long-term stability in the acetylene hydrogenation can be achieved. The high selectivity observed for all compounds (Pd3Ga7, PdGa, Pd2Ga) indicates that not only the surface geometric site isolation but also the suppression of hydride formation through a covalent bonding interaction absent in conventional Pd alloys are important for achieving superior catalytic properties. Our results demonstrate that structurally well-defined intermetallic compounds exhibit a high potential in heterogeneous catalysis and are promising candidates for industrial applications.

Physical review / B 72(2), 020402 (2005). doi:10.1103/PhysRevB.72.020402, Published by Inst., Woodbury, NY