Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with α av = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced α av = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. A round Robin test of the uncertainty on the measurements o the thermoelectric dimensionless figure of merite of Co0.87Ni0.03Sb3 Eric Alleno, David Bérardan, Céline Byl, Christophe Candolfi, Ramzy Daou, Rodolphe Decourt, Emmanuel Guilmeau, Sylvie Hébert, Jiří Hejtmánek, Bertrand Lenoir, et al.

The preparation and characterization of pure fluorite-type phases allowed exploring the CeO2-ZrO2-PrOx (CZP) phase diagram. On the basis of magnetic susceptibility measurements, the Pr4+/Pr3+ molar ratio of several oxides annealed at T = 700 degrees C under air was determined; the higher the Zr content, the lower the Pr4+ concentration. Thermogravimetric analysis and temperature-programmed reduction measurements showed various Pr and Ce reduction steps. The Pr4+ reduction starts at T = 250 degrees C and is maximum around T = 400 degrees C. For the most reducible compositions, which exhibit the lowest Zr content and the highest Pr rate, the reduction phenomena strongly depend on the Pr/Ce molar ratio. As a remarkable result, Pr and Ce reductions can simultaneously take place at lower temperature (T > 430 degrees C) than for oxides of the CeO2-ZrO2 solid solution evidencing that the increase of Pr content also allows enhancing the reducibility of Ce4+ at low temperature. On the basis of a discussion taking into account the probability of oxygen surroundings in disordered fluorite networks and the rate of oxygen released in materials after the first reduction step performed at T < 500 degrees C, a mapping of the most probable labile oxygen sites in the CZP phase diagram is proposed. In particular, it is shown that for the oxides containing 10 atom % Zr, the most labile oxygen site should be systematically coordinated with one Zr atom, one Ce, and two Pr atoms. In the same series (10 atom % Zr), the electronic transport properties allowed showing semiconducting behavior with a strong increase of the total conductivity as the Pr content increases. On the basis of the thermal variation of the Seebeck coefficient, these phenomena are associated with hopping of electrons and holes, involving intra-atomic charge transfers, which depend on the reduction temperature of Pr4+ ions under air. Finally, the oxygen mobility strongly increases with the Pr content in this series. The oxygen tracer self-diffusion coefficient D* has been estimated by two independent measurements, and the best value is around 10(-8) cm(2)/s at T = 400 degrees C for the Ce0.45Zr0.1Pr0.45O2-x composition, which is quite high in this temperature range. These fundamental properties of CZP phases design very promising new materials like automotive exhaust catalysts, gas sensors, electrolytes, or oxygen electrodes for solid oxide fuel cells.

The phonon glass electron crystal concept is one of the key guiding principles for the development of efficient thermoelectric materials. Here, we confirm that SrTiO3 becomes a phonon glass for large numbers of A-site vacancies in the Sr1-xLa0.67x square 0.33xTiO3 series and show that its electron crystal properties are stymied by the presence of a core shell grain structure. Thermal conductivity, heat capacity, and neutron powder diffraction, complemented by representational analysis and phonon calculations, were used to investigate the thermal transport. This reveals that the heat carrying modes are dominated by Sr motions and that these become more localized upon the introduction of the A-site vacancies, consistent with the observed phonon glass state. Impedance spectroscopy and direct current electrical measurements were used to probe the electrical properties of insulating and conducting samples. This reveals the coring of grains due to oxidation on cooling from sintering temperatures. The resultant insulating shell limits the thermoelectric power factor to S-2/rho = 0.45 mW m(-1) K-2 and the figure-of merit to ZT = 0.15 at 900 K for Sr0.20La0.53 square 0.27Ti0.95Nb0.05O3-delta. The thermal properties of these materials are, therefore, controlled by an intrinsic feature of the microstructure (i.e., the A-site vacancies), whereas the electrical properties are grain boundary limited, which in principle can be controlled independently to raise S-2/rho and ZT.

Half-Heusler alloys based on TiNiSn are promising thermoelectric materials characterized by large power factors and good mechanical and thermal stabilities, but they are limited by large thermal conductivities. A variety of strategies have been used to disrupt their thermal transport, including alloying with heavy, generally expensive, elements and nanostructuring, enabling figures of merit, ZT ≥ 1 at elevated temperatures (>773 K). Here, we demonstrate an alternative strategy that is based around the partial segregation of excess Cu leading to grain-by-grain compositional variations, the formation of extruded Cu "wetting layers" between grains, and-most importantly-the presence of statistically distributed interstitials that reduce the thermal conductivity effectively through point-defect scattering. Our best TiNiCuySn (y ≤ 0.1) compositions have a temperature-averaged ZTdevice = 0.3-0.4 and estimated leg power outputs of 6-7 W cm-2 in the 323-773 K temperature range. This is a significant development as these materials were prepared using a straightforward processing method, do not contain any toxic, expensive, or scarce elements, and are therefore promising candidates for large-scale production.

Layered vanadium oxides have been extensively explored due to their interesting metal-insulator transitions and energy conversion/storage applications. In the present study, we have successfully synthesized VO2 (A) polymorph powder samples by a single-step hydrothermal synthesis process and consolidated them using spark plasma sintering. The structural and electronic properties of VO2 (A) are measured over a large temperature range from liquid helium, across the structural transition (400-440 K) and up to 500 K. The structural analysis around this transition reveals an antiferrodistorsive to partially ferrodistorsive ordering upon cooling. It is followed by a progressive antiferromagnetic spin pairing which fully settles at about 150 K. The transport measurements show that, in contrast to the rutile archetype VO2 (R/M1), the structural transition comes with a transition from semiconductor to band-type insulator. Under these circumstances, we propose a scenario with a high temperature antiferrodistorsive paramagnetic semiconducting phase, followed by an intermediate regime with a partially ferrodistorsive paramagnetic semiconducting phase, and finally a low temperature partially ferrodistorsive antiferromagnetic band insulator phase with a possible V-V Peierls-type pairing.

SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures θD1 = 345(9) K and θD2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal properties. Comparison with phonon calculations reveals that the soft substructure is associated with the low energy phonon modes that are responsible for the thermal transport. This suggests that searching for materials containing highly divergent bond distances should be a fruitful route for discovering low thermal conductivity materials.

Large thermoelectric power factors and low thermal conductivities linked to changes in texturing have been observed in consolidated polycrystalline SnSe ingots.

Despite the relatively simple composition and numerous applications of layered VO2(B), several issues such as effect of electron correlations and the nature of its metal–insulator transition remain unresolved due to the low irreversible phase transition temperature. We have overcome this challenge by using spark plasma sintering and reporting its reliable electronic transport properties. All our transport, magnetic and thermal data, converge in favor of an interesting multiphase nature of VO2(B): low temperature phase (insulating and magnetically ordered), an intermediate temperature phase (insulating), and a high temperature phase (presumably metallic with strong electron correlations) coexist. The coexistence domain for these three phases is broad and extends over about 60 K around 235 K. The low temperature phase with spin singlets is always associated with, at least one another phase and becomes dominant below 200 K. The high temperature phase is present over the full temperature range and exists alone above room temperature. We believe that our results will give considerable insight into understand this complex VO2(B) material and widen up its future applications.

A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.

Two series of Fe and Al double substituted MnSiγ chimney ladders with a nominal valence electron count, VEC=14 per transition metal were prepared (γ=1.75). Simultaneous replacement of Mn with Fe and Si with Al yielded the Mn1−xFexSi1.75−xAlx series while the second Mn1−xFexSi1.75–1.75xAl2x series follows the pseudo-binary between MnSi1.75 and FeAl2. Scanning electron microscopy and elemental mapping revealed that ~60% of the nominal Al content ends up in the product with the remainder lost to sublimation, and that up to 7% Al can be substituted in the main group sublattice. Profile analysis of X-ray powder diffraction data revealed gradual changes in the cell metrics, consistent with the simultaneous substitution of Fe and Al in a fixed ratio. All samples are p-type with VEC≈13.95 from the structural data and ~1×1021 holes cm−3 from variable temperature Seebeck measurements. The substituted samples have lower electrical resistivities (ρ300K=2–5mΩcm) due to an improved microstructure. This leads to increased thermoelectric power factors (largest S2/ρ=1.95mWm−1 K−2) compared to MnSiγ. The thermal conductivity for the Mn0.95Fe0.05Si1.66Al0.1 sample is 2.7Wm−1K−1 between 300 and 800K, and is comparable to literature data for the parent material.

PrBaCo2−xFexO5+δ solid solution is investigated in order to understand the Oxygen Reduction Reaction (ORR) and water formation occurring at the H+-SOFC cathode. Careful attention is paid to the study of the physical properties as a function of composition by Thermogravimetry Analysis (TGA), Mossbauer spectroscopy, electrical conductivity and Seebeck coefficient measurements, with the aim to establish the correlation existing with the ORR activity for these Mixed Ionic Electronic Conductors (MIEC). The oxygen diffusion coefficients are determined by Electrical Conductivity Relaxation (ECR) and Isotopic Exchange Depth Profile (IEDP) coupled with Secondary Ion Mass Spectroscopy (SIMS) methods. An electrochemical study is then carried out and shows that the amount of oxygen vacancies is the most influential parameter. Indeed, it allows some hydration of PrBaCo2O5+δ oxide and the formation of protonic defects that can induce protonic diffusivity in these MIEC oxides.

The introduction of A-site vacancies in SrTiO3 results in a glass-like thermal conductivity while Nb substituted samples maintains good electrical conductivity. This unexpected result brings SrTiO3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.

Well crystallized VO2 (A) microrods were grown via a single step hydrothermal reaction in the presence of V2O5 and oxalic acid. With the advantage of high crystalline samples, we propose P4/ncc as an appropriate space group at room temperature. From morphological studies, we found that the oriented attachment and layer by layer growth mechanisms are responsible for the formation of VO2 (A) micro rods. The structural and electronic transitions in VO2 (A) are strongly first order in nature, and a marked difference between the structural transition temperatures and electronic transitions temperature was evidenced. The reversible intra- (LTP-A to HTP-A) and irreversible inter- (HTP-A to VO2 (M1)) structural phase transformations were studied by in-situ powder X-ray diffraction. Attempts to increase the size of the VO2 (A) microrods are presented and the possible formation steps for the flower-like morphologies of VO2 (M1) are described.

The present study provides a rapid way to obtain VO2 (B) under economical and environmentally friendly conditions. VO2 (B) is one of the well-known polymorphs of vanadium dioxide and is a promising cathode material for aqueous lithium ion batteries. VO2 (B) was successfully synthesized by rapid single-step hydrothermal process using V2O5 and citric acid as precursors. The present study shows that phase-pure VO2 (B) polytype can be easily obtained at 180 °C for 2 h and 220 °C for 1 h, that is, the lowest combination of temperature and duration reported so far. The obtained VO2 (B) is characterized by X-ray powder diffraction, high-resolution scanning electron microscopy, and Fourier transform infrared spectroscopy. In addition, we present an indirect way to obtain VO2 (M1) by annealing VO2 (B) under vacuum for 1 h.

The possible existence of an intermediate spin state is now a few decades fascinating solid state chemists and physicists, experimentalists, and theoreticians. In this article, we revise some recent results on the stability diagrams of spin states, low, high, or intermediate, in distorted environments and extend their approach to redraw more realistic diagrams for d(4), d(5), and d(6) ions in a tetragonally distorted 6-fold oxygen environment (D4h, D2d, and C4ν). The model relies on a point charge model and further uses effective parameters to account for the cubic field drift on spin state change and for suitable values for solid state of the expectation values of the 3d-radial wave functions; additionally the model uses rational parameters to characterize the distortion; finally, we also consider the possible existence of states' combinations to propose reliable stability diagrams. Whatever the representation involved in the distortion, the existence domain of the intermediate spin state appears very small and more likely replaced with mixtures of cations in low and high spin states; the opportunity of induced distortive ordering is discussed.

The monoclinic layered (B) polymorph of VO2, a promising cathode material for aqueous lithium ion batteries, is hydrothermally synthesized from aqueous mixtures of V2O5 and citric acid (autoclave, 180 °C for 2 h or 220 °C for 1 h).

Lithium cobalt double oxide LiCoO2 was synthesized at 220°C by soft hydrothermal method using Co(OH)2 and LiOH as precursors, LiOH/NaOH as mineralizers and H2O2 as oxidant. The soft hydrothermal synthesis method offers the dual advantage of a much lower synthesis time and a higher purity in comparison with other synthesis methods. The compound was identified by X-ray diffraction and its purity was checked by magnetic and electron magnetic resonance measurements. The grain morphology was studied by Scanning Electron Microscopy and an exponential growth of particle size with synthesis time was observed.

The composition and synthesis conditions of the (Li/Na)CoO(2) phase with an ordered 1:1 Li/Na stacking alternating with CoO(2) slabs were determined from a careful study of the P2-Na(∼0.7)CoO(2)-O3-LiCoO(2) system. An in situ X-ray diffraction (XRD) thermal study emphasizes the metastable character of this phase that can be stabilized only by very fast quenching. Its composition, (Li(0.42)Na(0.37))CoO(2), is significantly different from the ideally expected one, (Li(0.50)Na(0.35))CoO(2), and its structure, confirmed by Rietveld refinement of the XRD pattern, presents an ideal alternate ordering of lithium, cobalt, and sodium layers within OP4-type oxygen packing. The presence of vacancies in both alkali-ion layers was confirmed by electrochemical intercalation of lithium and sodium. For the first time, a new type of layered oxide exhibiting OPP9-type oxygen packing was evidenced. Between the CoO(2) slabs, alkali ions are intercalated in the following order: Li(octa)-Na(prism)-Na(prism). This material crystallizes in the R3m space group with a(hex) = 2.828 Å and c(hex) = 46.85 Å cell parameters.