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.

We report a complete study of the lattice dynamics, dielectric, elastic and piezoelectric properties of hexagonal semiconducting chromium disilicide (CrSi2).

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.

Metastable Al6Ge5 phase has been synthesized by mechanical alloying. Influence of milling time and stoichiometric ratio on the amount of Al6Ge5 were studied. In the better synthesis conditions, one finds purity of 58wt% from Rietveld refinement, the highest purity reported so far. For the first time, the crystal structure of Al6Ge5 has been fully solved by Rietveld refinement and confirmed by density functional theory (DFT) based calculation. We reported the experimental unpolarized Raman spectrum and the assignment of the main lines is performed using our DFT calculations. The existence of low energy optical modes able to scatter acoustical phonons and its semiconducting nature makes this compound promising for thermoelectric applications.

An inelastic neutron scattering study of the filled and partially-filled skutterudite compounds RFe4Sb12 and R0.5Fe2.75Ni1.25Sb12 (where R = Ce and La) was carried out to understand the nature of the spin dynamics. Strong magnetic scattering was observed in Ce0.5Fe2.75Ni1.25Sb12 at similar to 5 meV. The integrated intensity of this peak does not follow the Ce3+ form factor, but exhibits a maximum at a momentum transfer (vertical bar Q vertical bar) of 2 angstrom(-1). We attribute this feature to a Ce3+ crystal field excitation in the presence of magnetic exchange interactions. This picture is supported by thermodynamic and magnetic properties. Finally, we confirm the presence of a spin gap in CeFe4Sb12 suggested by our previous work.

We carried out a complete study (magnetic, electronic, dielectric, dynamic, and elastic properties) of the nickel hydroxide [Ni(OH)(2)] from first-principles calculations based on density functional theory. No theoretical investigations of these physical properties have been previously reported in literature. Our work supports that Ni(OH)(2) is an A-type antiferromagnetic material. In addition, it is negative uniaxial and semiconducting with a direct band gap at the Gamma point around 3 eV. By contrast to its electronic dielectric tensor, its static tensor is strongly anisotropic in the plane orthogonal to its optical axis. This anisotropy is mainly governed by a highly polar phonon centered around 510 cm(-1) and assigned as a rotational E(u) mode. Both Raman and infrared spectra have been computed to clarify the longstanding debate on the assignment of the Ni(OH)(2) phonon modes reported in literature. All these theoretical results are fruitfully compared to the experimental ones obtained on large Ni(OH)2 "pseudosingle" crystals when available.

The ternary intermetallic compound EuCu9Mg2 has been synthesized. The magnetic properties have been investigated by magnetization measurements and Eu-151 Mossbauer spectroscopy. Europium is divalent and a spin freezing is observed at 25K. However, this spin freezing is characterized by the onset of strong magnetic irreversibility, which suggests a cluster-glass freezing. The Mossbauer spectra can be analyzed as the superposition of two subspectra of Eu2+ ions, which is consistent with a cluster-glass freezing. A transition to the ferromagnetic state is induced by application of a magnetic field the order of 0.5T. This anomalous feature is attributed to quantum spin fluctuations that oppose the long-range ferromagnetic ordering, due to the two-dimensional topology of the geometric arrangement of the Eu2+ ions. This 2D-topology is also evidenced by a quadratic variation of the specific heat as a function of temperature down to 2.7 K. The electrical resistivity is also analyzed in the framework of intermetallic compounds dominated by ferromagnetic spin fluctuations above the spin-freezing temperature. (C) 2010 Elsevier B.V. All rights reserved.

AbstractThe title compound is characterized by magnetization measurements, 151Eu Moessbauer spectroscopy, and specific heat and electrical resistivity measurements.

We report on molecular simulations of zinc oxide nanostructures obtained within silica nanopores of diameter D = 1.6 nm and D = 3.2 nm. Both the effects of confinement (by varying the pore size) and degree of pore filling on the structure of the nanomaterial are addressed. Two complementary approaches are adopted: 1) the stability of the three crystalline phases of ZnO (wurtzite, rocksalt, and blende) in the silica nanopores is studied, and 2) ZnO nanostructures are obtained by slowly cooling down a homogeneous liquid phase confined in the silica pores. None of the ideal nanostructures (wurtzite, rocksalt, blende) retains the ideal structure of the initial crystal when confined within the silica pores. Only the structure starting from the ideal wurtzite nanocrystal remains significantly crystalline after relaxation, as revealed by the marked peaks in the pair correlation functions for this system. The morphology and degree of cristallinity of the structures are found to depend on the parameters involved in the synthesis (pore size, filling density). Nanograin boundaries are observed between domains of different crystal structures. Reminiscent features of the bulk behavior, such as faceting of the nanostructures, are also observed when the system size becomes large. We show that the use of nanopores as a template imposes that the confined particles exhibit neutral (basal) surfaces. These predictions provide a guide to experiments on semiconductor nanoparticles.

The ternary solid solution relating CoSb and Fe1+delta Sb has been synthesized and the Fe spin dilution effects on the magnetic properties has been investigated. The Fe magnetic moments are localized despite the fact that the metallic character increases with the Fe concentration. The covalent bonding along the c direction that is responsible for the metallic character is also responsible for a smaller magnetic moment on Fe sites, which depends on the Fe concentration. On the Fe-rich side, the antiferromagnetic (AF) ordering is observed down to a Fe concentration that is comparable to the site percolation threshold for the basal plane. At lower temperatures, re-entrant spin-glass phases are observed, which become spin-glass phases below the percolation threshold for AF ordering. The dynamic scaling shows the same behavior as that of the conventional spin glasses in which the Ising anisotropy is small.

The ternary solid solution relating CoSb and ${\mathrm{Fe}}_{1+\ensuremath{\delta}}\mathrm{Sb}$ has been synthesized and the Fe spin dilution effects on the magnetic properties has been investigated. The Fe magnetic moments are localized despite the fact that the metallic character increases with the Fe concentration. The covalent bonding along the $c$ direction that is responsible for the metallic character is also responsible for a smaller magnetic moment on Fe sites, which depends on the Fe concentration. On the Fe-rich side, the antiferromagnetic (AF) ordering is observed down to a Fe concentration that is comparable to the site percolation threshold for the basal plane. At lower temperatures, re-entrant spin-glass phases are observed, which become spin-glass phases below the percolation threshold for AF ordering. The dynamic scaling shows the same behavior as that of the conventional spin glasses in which the Ising anisotropy is small.

Highly crystalline zinc oxide (ZnO) nanomaterials are synthesized using a seeded growth sol-gel method. In order to control the morphology and the dimensionality of ZnO crystals, a double hydrophilic block copolymer, the Polyacrylic acid-Polyacrylamide (PAA-PAM), was introduced during the growth step. It was shown, first, that depending on the amount of PAA-PAM copolymer, different morphologies of ZnO particles precipitated from homogeneous solutions were obtained, such as ZnO nanostructured spheres or flat hexagonal crystals. Secondly, the dimensionality of ZnO objects grown from seeds on a substrate could be tuned by varying the amount of copolymer leading to ZnO films with variable characteristics. Thus, systematic studies were done in order to investigate the influence of copolymer addition on ZnO nanomaterial morphologies and then explain the mechanisms of the morphological and dimensional modifications. (C)2008 The Ceramic Society of Japan. All rights reserved.

The magnetic properties of the skutterudite compound $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ have been investigated by heat capacity and inelastic neutron scattering measurements. Heat capacity measurements reveal a broad peak centered at $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whose magnitude is much larger than that expected from a Schottky anomaly due to a cubic crystal electric field. At $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, inelastic neutron scattering experiments clearly show the existence of a broad magnetic peak at $40(3)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The absence of quasielastic scattering at this temperature, together with the almost total account of the magnetic signal in the inelastic peak, shows that the excitation has a different origin than a splitting of the electronic levels due to crystal field potential. Instead, we propose a model in which the signal originates from inelastic excitations across two hybridization bands near the Fermi energy, usually referred to as a spin gap. A simple phenomenological two-level model can account for the peak in the specific heat, with a spin-gap energy of $36(2)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which is in very good agreement with the inelastic scattering data. Further, at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the inelastic response becomes purely quasielastic, which is in agreement with the theoretical calculations. Interestingly, the spin-gap energy in $\mathrm{Ce}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ exhibits a universal scaling behavior with the Kondo temperature ${T}_{K}$. The relation between the spin-gap energy and the associated anomalies in the heat capacity or thermal expansion is discussed for a series of Ce- and Yb-based compounds.

We report magnetic properties of iron in Co1-xFexSb3 for x in the range 0 < x < 0.2, since x = 0.2 is found to be the limit of solubility of iron in the skutterudite lattice. The magnetic ions diluted in the matrix carry a small magnetic moment reduced to that of the spin-only S = 1/2 value of the Fe3+ in the low spin d(5) configuration in presence of a strong crystal field that screens the orbital momentum. The magnetic properties give evidence that a small fraction of iron is spin-frozen in magnetite ferrimagnetic clusters, and antiferromagnetic FeO clusters. Because both types of clusters represent only very minor phases, their detection by the usual analytical means such as X-rays is not possible. The remaining part is diluted in the matrix to form a semimagnetic semiconductor characterized by a Fe-Fe nearest-neighbor exchange interaction J that is antiferromagnetic, with vertical bar J vertical bar/k(B) approximate to 19.6K. (c) 2006 Elsevier B.V. All rights reserved.

Highly crystalline zinc oxide (ZnO) nanomaterials are synthesized using a seeded growth sol-gel method. In order to control the morphology and the organization of the ZnO nanomaterials, a double hydrophilic block copolymer has been introduced during the seeded growth synthesis: the Polyacrylic acid-Polyacrylamide (PAA-PAM). Depending on the amount of PAA-PAM copolymers, different morphologies were obtained, such as ZnO nanostructured spheres or flat hexagonal crystals. Thus, systematic studies have been done to investigate the influence of the copolymer addition on ZnO nanomaterial morphologies and explain the mechanisms of the morphological modifications.

Transport and magnetic properties of CezLa(1-z)Fe(4)Sb(12) have been investigated in the whole range of substitution 0 < z < 1. The cerium contribution rho(Ce) to the resistivity goes through a maximum at temperature similar or equal to 140 K, close to the spin fluctuation temperature T-sf for CeFe4Sb12. T-sf is the single magnetic energy scale which accounts for the magnetic properties of the Ce Kondo lattice. At low temperature, the resistivity shows a T-2-deviation from saturation. At z <= 0.7 this is the behaviour expected for Ce impurities in metals in the Kondo limit. At z = 1, however, Hall effect measurements show that this behaviour is due to a decrease of the free carrier concentration which we can attribute to the formation of a hybridization pseudogap E-g similar or equal to 25 meV. Upon dilution of the cerium, Eg decreases and this pseudogap is smeared out, replaced by the Kondo resonance at the Fermi level characteristic of Kondo impurities in the small z limit.

We have investigated the thermoelectric properties of the Ce0.9−yYbyFe4Sb12 series as well as those of a few compounds in the series Cey/2Yby/2Fe4−xCoxSb12. In the former series, increasing the Yb fraction decreases the resistivity and the Seebeck coefficient. These effects are related to the fact that Yb is not trivalent in this series. In Ce0.40Yb0.53Fe4Sb12, the phonon part of the thermal conductivity is strongly reduced compared to Ce0.85Fe4Sb12, leading to a larger figure of merit. We think that the increase of the mass and bonding fluctuations on the rare-earth site would be the main reason for this reduction. In the latter series, an extrapolated value of ZT=0.95 at 800K was obtained in not yet optimized Ce0.44Yb0.32Fe3.02Co0.98Sb12.

The structural properties of the Ce delta Fe4-xNixSb12 series with 0.06 <= delta <= 0.72 have been determined by neutron powder diffraction in the temperature range 10-300 K. Large isotropic atomic displacement parameters are observed for the cerium atom throughout the series, however with two distinct origins. The first term, purely dynamic, is well described as an Einstein oscillator and has essentially no dependence with the filling fraction 8 and the lattice parameter. A vibrational frequency of 55(2) cm(-1) fitted the experimental data in the entire phase diagram. The second contribution is essentially static and well evidenced by a large residual displacement parameter extrapolated to 0 K. This latter term does depend strongly on 3 and is maximum at around delta = 0.5, representing almost 50% of the room temperature value. A number of structural changes of the Sb sublattice are correlated with the disorder of Ce as well as disorder on the Fe/Ni sublattice.

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.