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Unification of experiment and computation show how Mn alters the crystal and electronic band structure of the Ge1−xMnxTe alloy space. As a result, the effective mass is dramatically increased and the thermoelectric performance is improved.

Polycrystalline samples of Er1-xHoxNiSb (x = 0, 0.2, 0.3, 0.5, 0.7, 0.8, 1) were characterized by means of x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and optical metallography. The results proved the formation of half-Heusler alloys in the entire composition range. Their electrical transport properties (resistivity, thermoelectric power) were studied in the temperature interval 350-1000 K. The measured electrical resistivity spanned between 5 and 25 μΩm. The maximum thermopower of 50-65 μV/K was observed at temperatures 500-650 K. Replacing Ho for Er resulted in a non-monotonous variation of the thermoelectric power factor (PF = S2/ρ). The largest PF of 4.6 μWcmK-2 was found at 660 K for Er0.5Ho0.5NiSb. This value is distinctly larger than PF determined for the terminal phases ErNiSb and HoNiSb

The electrical transport in the half-Heusler phases LuNiSb and YPdSb was measured in a temperature range 2--300 K. For both compounds, the electrical resistivity was found to decrease with increasing temperature, showing a linear-in-$T$ behavior over an extended temperature interval. In order to interpret the experimental data, a two-channel conductivity model was applied, which revealed that not only the semiconducting-like transport but also the metallic-like one exhibit negative temperature coefficients. The unusual behavior in the metallic channel was described within the Cote-Meisel formalism based on the diffraction model of strongly disordered metals. In addition, a weak localization scenario was considered including spin-orbit scattering and Coulomb interaction between conducting electrons. The electron-electron interaction was found most important at the lowest temperatures, where the semiconducting channel becomes ineffective, reminiscent of charge transport confined to a narrow yet finite-size metallic band located inside the semiconducting energy gap. The low-temperature resistivity of YPdSb appeared fully describable in terms of the Altshuler-Aronov quantum correction due to interacting electrons. In turn, the electronic transport in LuNiSb was found affected by the Kondo effect associated with a small amount of paramagnetic impurities present in the specimen investigated. The approach developed for LuNiSb and YPdSb can be applied to other half-Heusler compounds that exhibit atom disorder in their crystal structures.

Analysis of bipolar thermal conductivity might be very useful in preliminary stages of thermoelectric materials discovery. Using its product---mobility ratio between electrons and holes---it is possible to choose the most promising compound from the series and pave the correct direction of doping. Current work presents positive verification of this approach for $\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}$, which is anticipated to show superior mobility when tuned towards n-type behavior. In agreement with expectation, the mobility increases by an order of magnitude due to rising tellurium content in the ${\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ series. The effect is most likely driven by change of the dominant charge carriers' scattering mechanism from ionized impurity influence to point defect and acoustic phonon interaction. Simultaneously, due to a highly anisotropic conduction band, the effective mass of the carriers rises towards the n-type regime. These two effects lead to an improved thermoelectric power factor of electron-doped samples, up to $40\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{WK}}^{\ensuremath{-}2}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at 740 K for ${\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}}_{0.85}{\mathrm{Te}}_{0.15}$. Based on this result, we suggest n-type doping for other rare-earth-based half-Heusler compounds. Representatives of this group exhibit the smallest lattice thermal conductivity in the pristine form among any half-Heusler thermoelectrics, and are anticipated to show comparably good electrical properties to ScNiSb due to their high mobility ratio in favor of electrons.

Made available in DSpace on 2021-06-25T10:51:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure (P4/nmm, a = 4.09021(2) Å, c = 6.42027(4) Å). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ZnV0.91Sb0.96. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 Å known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 μB. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds. Physics Department Colorado School of Mines Institute of Low Temperature and Structure Research Polish Academy of Sciences University of Illinois at Urbana-Champaign Instituto de Física Teórica São Paulo State University (UNESP) University of California Santa Barbara Max Planck Institute for Chemical Physics of Solids Department of Chemistry and Biochemistry The Ohio State University National Synchrotron Radiation Research Center (NSRRC) Instituto de Física Teórica São Paulo State University (UNESP)

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 $\mathrm{Sc}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Dy}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Er}\mathrm{Ni}\mathrm{Sb}$, $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, and $\mathrm{Lu}\mathrm{Ni}\mathrm{Sb}$ 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 $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$ are found to be narrow-gap intrinsic $p$-type semiconductors with rather light charge carriers. In $\mathrm{Tm}\mathrm{Ni}\mathrm{Sb}$, the presence of heavy holes with large weighted mobility is evidenced by the highest power factor among the series ($17\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at 700 K). The experimental electronic relaxation time calculated with the parabolic band formalism is found to range from $0.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ to $2.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ s. In all the materials studied, the thermal conductivity is between 3 and $6\phantom{\rule{0.2em}{0ex}}{\mathrm{W m}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ 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.

Nonmagnetic germanides LaTE2Ge2 with TE = Fe, Co, Ni, Cu and Ru were studied by means of X-ray diffraction, magnetization and electrical resistivity measurements performed down to 2 K, supplemented by fully relativistic band structure and Fermi surface calculations. Results of the theoretical investigations of all the compounds are in line with the experimental findings, namely they show decreasing influence of 3d-electrons of the Fe, Co, Ni and Cu atoms on the electronic properties with increasing the atomic mass. In the case of LaFe2Ge2 the electrical resistivity measurements strongly suggest presence of spin fluctuations, which make that system very similar to the unconventional superconductor YFe2Ge2.

High temperature thermoelectric properties of bulk and ball-milled cold-pressed (DyNiSn)1−x(DyNiSb)x composite materials have been studied. For bulk pure DyNiSn and DyNiSb samples the Seebeck coefficient reaches − 5.5 μV/K at 480 K and 120 μV/K at 540 K, respectively. Composite materials show metallic-like electrical resistivity and positive sign of Seebeck coefficient with values up to 50 times higher than in pure DyNiSn compound at 1000 K. Only for the sample with x = 0.47, the ball-milling drives to increase of Seebeck coefficient of about 37% at 650 K.

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F 4 &macr, 3m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2&ndash, 950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 &mu, V K&minus, 1 near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 &mu, &Omega, m at 350 K), resulting in a rather small magnitude of the power factor (less than 1 &times, 10&minus, 3 W m&minus, 1 K&minus, 2) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m&minus, 1 occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.

We report on the high-temperature (350 K, Comment: 5 pages, 3 figures. Subbmited to Materials Today: Proceedings, 15th European Conference on Thermoelectrics (ECT2017)

Half-Heusler phase HoPtSb was studied by means of powder X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectrometry, high-resolution transmission electron microscopy (HRTEM), heat capacity, magnetization, electrical resistivity and magnetoresistance. Measurements were performed in temperature range 2–300 K and magnetic fields up to 14 T. The results revealed significant atomic disorder in the crystal structure of this compound, and indicated that the electrical transport in HoPtSb is noticeably affected by scattering conduction electrons on structural inhomogeneities. Due to large value of the resistivity accompanied by small overall change of resistivity with changing temperature the high-temperature behavior was attributed to disorder-induced violation of the Matthiessen’s rule. In turn, at low temperatures, strong magnetic field of 14 T disclosed resistivity upturn reminiscent of Altshuler-Aronov quantum correction observed in disordered metallic systems.

Study on half-Heusler compounds RENiSb, RE = Sc, Dy, Ho, Er, Tm, Lu, was performed in perspective of high-temperature thermoelectric application. Relatively small electrical resistivity (4–60 μ Ω m in 300–1000 K) combined with fairly high positive Seebeck coefficient (40–160 μV/K) yielded for the RENiSb ternaries rather large thermoelectric power factors at 750 K, namely 14 μW K−2cm−1 for ScNiSb and 11 μW K−2cm−1 for TmNiSb. At room temperature the thermal conductivity measured for the two materials was 8.1 W K−1m−1 and 4.4 W K−1m−1, respectively. The value obtained for the latter compound is smaller than those reported for unoptimized half-Heusler phases based on d-electron elements. This finding may be explained by strong point defect scattering occurring due to intrinsic disorder in crystal structure of TmNiSb, or to smaller extent by enhanced scattering on incoherent grain boundaries with the impurity phases. Nevertheless, achieving good thermoelectric performance of the RENiSb phases at high temperatures requires proper tuning of carrier concentration, as well as further reduction in their thermal conductance by means of alloying and/or nanostructurization.

Structural, electronic, and transport properties of ScNiSb, ScPdSb, and ScPtSb were investigated from first principles. Electronic band structures derived within the fully relativistic MBJLDA approach were compared with those obtained from the standard GGA calculations. All the compounds studied exhibit indirect narrow band gaps (0.24-0.63 eV). The effective masses of hole-like carriers are relatively small (0.27-0.36), and decrease with an increasing atomic number of the transition metal component. The carrier relaxation time, required for realistic calculations of the electrical conductivity, was approximated within the deformation potential theory. The GGA approach yielded overestimated transport characteristics with respect to those derived within the MBJLDA analysis. The largest power factor of 4-6 mWK^-2m^-1 ) at high temperatures was obtained for ScPtSb. This value is comparable with those observed experimentally for Fe-Nb-Sb half-Heusler alloys, and hence makes ScPtSb a very good candidate material for thermoelectric applications., Comment: 14 pages, 5 figures

Electrical transport studies of half-Heusler (HH)-based composites (TmNiSb)1−x(TmNiSn)x were carried out in a wide temperature range aimed at searching for possible enhancement in the thermoelectric power factor (PF) over that observed in the parent compound TmNiSb. The best thermoelectric performance was found in the sample with x = 0.25, which showed PF = 1.3 × 10−3 W/mK at 1000 K, i.e., about 70% larger than PF of TmNiSb at the same temperature. The PF improvement was obtained due to the formation in the composite system of a microstructure in which semiconducting-like particles of TmNiSb were covered with a metallic layer of TmNiSn. The largest Seebeck coefficient S = 137 μV/K was observed for HH alloy TmNiSb at 560 K. In turn, TmNiSn showed a metallic behavior with small negative thermoelectric power (S = –2.6 μV/K).Electrical transport studies of half-Heusler (HH)-based composites (TmNiSb)1−x(TmNiSn)x were carried out in a wide temperature range aimed at searching for possible enhancement in the thermoelectric power factor (PF) over that observed in the parent compound TmNiSb. The best thermoelectric performance was found in the sample with x = 0.25, which showed PF = 1.3 × 10−3 W/mK at 1000 K, i.e., about 70% larger than PF of TmNiSb at the same temperature. The PF improvement was obtained due to the formation in the composite system of a microstructure in which semiconducting-like particles of TmNiSb were covered with a metallic layer of TmNiSn. The largest Seebeck coefficient S = 137 μV/K was observed for HH alloy TmNiSb at 560 K. In turn, TmNiSn showed a metallic behavior with small negative thermoelectric power (S = –2.6 μV/K).