Skutterudite Crystal Structure Research Articles

Induced nonstoichiometric effects of Sb in p-type skutterudite thermoelectrics

Sublimation of antimony (Sb) during synthetic processes can be fatal to the thermoelectric (TE) properties of MT4Sb12 (M: filler atoms; T = Fe, Co, Ni)-based skutterudite (SKD). In this study, the nonstoichiometric effects of Sb on the phase structure formation and, thereby, on the TE properties of p-type SKD with Nd0.9Fe3.5Co0.5Sb12+x compounds were systematically investigated. Research is carried out with x values varying from −0.8 to 1.0, and samples are prepared by combining induction melting and melt-spinning, followed by a spark plasma sintering (SPS) process. The analysis showed that the Sb-deficiency formed NdSb and FeSb2 secondary phases along with the SKD main phase, while the Sb-excess effectively suppressed the FeSb2 phase formation and a small amount of the Sb phase appeared at a high Sb-excess level. The appearance of the secondary phases induced by Sb nonstoichiometry directly affected the change in the filling fraction of Nd in the SKD crystal structure, which led to the carrier concentration change. Our results demonstrate that the TE performance of MT4Sb12-based compounds can be significantly changed by the Sb nonstoichiometry, which should be profoundly considered along with other strategies to improve the TE efficiency of Sb-based SKD materials.

Attrition-enhanced nanocomposite synthesis of indium-filled, iron-substituted skutterudite antimonides for improved performance thermoelectrics

ABSTRACTNanostructuring has been the foremost approach to the manufacture of high-performance thermoelectric materials for nearly a decade. This study explores a novel nanostructuring technique, attrition-enhanced nanocomposite synthesis, in maximum indium-filled, iron-substituted cobalt antimonide skutterudites. In0.3Fe0.8Co3.2Sb12 was synthesized and subjected to varying degrees of mechanical attrition (via ball milling). These samples exhibited increased indium precipitation coincident with the duration of mechanical attrition. Indium readily diffused through the skutterudite crystal structure and rapidly precipitated forming 20-50 nm-sized indium-rich inclusions during sintering.

Co 2p–3d resonant photoemission spectroscopy of CoSb 3

High-resolution synchrotron radiation photoemission spectroscopy (HRPES) has been performed in the Co 2p core excitation region on thermoelectric materials of CoSb 3 with the skutterudite crystal structure. At the Co 2p 3/2–3d peak, a photoionization cross-section of the Co 3p states becomes extremely larger than that at the Co 3p–3d peak. Co 2p3d3d, 2p3p3d, 2p3s3d, 2p3p3p and 2p3s3p Auger structures and plasmon satellites are recognized. It has been confirmed that the resonant photoemission process is less important in CoSb 3. The hν-dependence of the spectra suggests that the Co 3d electrons in CoSb 3 can be treated as the one-electron model rather than the many-body particle model, which is consistent with the covalency of CoSb 3.

Growth of NaFe4P12 Skutterudite Single Crystalline Nanosprings Synthesized through a Hydrothermal−Reduction−Alloying Method

NaFe4P12 nanosprings were synthesized using a hydrothermal method at 170 °C for 24 h. The nanosprings are 1000−5000 nm in length and 200−1000 nm in diameter and consist of coiled nanobelts of 80−150 nm in width and 20−50 nm in thickness. With an increase in the annealing time, the nanospring grows as a macrorod with a multilayered helical tubular structure. X-ray diffraction (XRD) patterns indicate that the nanosprings have the skutterudite crystal structure with cell parameter a = 7.782 Å. Electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) prove that the nanobelts grow along the (111) plane and that their growth direction is not parallel to the [110] direction in the growth plane. A possible growth process for NaFe4P12 nanosprings is suggested.

Transport properties of germanium-filled CoSb3

We report the transport properties of dense polycrystalline Ge filled skutterudites Ge0.25Co4Sb12 and Ge0.05Co4Sb12 perepared by a high-pressure synthesis approach. Low temperature electrical resistivity, Seebeck coefficient, Hall coefficient, and thermal conductivity measurements were performed and compared with those of Co4Sb11Ge and CoSb3. The Ge atoms residing inside the interstitial voids of the skutterudite crystal structure act as electron donors. The lattice thermal conductivity of these compounds is lower than that of CoSb3 but higher than that of other filled skutterudites. The potential for thermoelectric applications is also discussed.

Enhancement of high temperature thermoelectric properties of intermetallic compounds based on a Skutterudite IrSb3 and a half-Heusler TiNiSb

Phonon glass and electron crystal (PGEC) thermoelectric materials have been expected to be a new class of thermoelectric materials for high temperature applications. Among the efforts to optimize the high temperature thermoelectric properties of various PGEC thermoelectric materials, recent experimental works on the Skutterudite IrSb3 and half-Heusler TiNiSb intermetallic compounds are presented herein by which the material design concept for high energy conversion efficiency, i.e. a high figure of merit, is suggested. It is revealed that the thermoelectric efficiency of IrSb3 can be increased by the decrease of lattice thermal conductivity due to the rattling effect of La atoms filled in the structural vacancies of the Skutterudite crystal structure. In the half-Heusler TiNiSn, high temperature thermoelectric properties are improved by Hf substitution to the Ti sites by reducing lattice thermal conductivity and also by Sb doping to increase power factor. It is concluded that the proper alloy designing for controlling crystal structure and carrier concentration could enable these intermetallic compounds to exhibit a high potential for elevated temperature thermoelectric applications.

Suppression of Binary Nucleation in Amorphous La−Fe−Sb Mixtures

The nucleation energy of a series of La(x)Fe(y)Sb(z) modulated elemental reactants was measured as a function of the Fe/Sb ratio over a large composition range while holding the La content constant. The nucleation energy of the ternary compound La(0.5)Fe(4)Sb(12) with the skutterudite crystal structure was found to depend very strongly on the Fe/Sb ratio in the modulated elemental reactant, with a higher nucleation energy as the Fe/Sb ratio is moved away from the 1:3 stoichiometric value. When the results of this study are compared with those from Fe(y)Sb(z) modulated reactants, the addition of lanthanum was found to suppress the nucleation of FeSb(2), thereby broadening the Fe/Sb composition range in which the ternary skutterudite compound La(x)Fe(4)Sb(12) nucleates. This suppression of nucleation of a binary phase on addition of a ternary component to an amorphous intermediate is in agreement with theoretical arguments. The observed suppression of nucleation also provides rational for the observed nucleation of metastable ternary and higher-order compounds from homogeneous amorphous reactants.

CoGe1.5Se1.5: Synthesis and characterization

ABSTRACTPolycrystalline CoGe1.5Se1.5 was synthesized and characterized. The compound is a member of the class of compounds with the skutterudite crystal structure. Synthesis assuming the above stoichiometry resulted in a n-type semiconductor with composition CoGe1.452Se1.379. The specimen exhibits a large room temperature Seebeck coefficient and resistivity. The electron mobility is very low and the thermal conductivity is lower than that of the binary skutterudite CoSb3. These properties can be attributed to the vacancies in the crystal lattice due to the non-stoichiometric nature of the specimen. The potential for thermoelectric applications of ternary skutterudites is also discussed.

Synthesis of the New Metastable Skutterudite Compound NiSb3 from Modulated Elemental Reactants.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of the new metastable skutterudite compound NiSb(3) from modulated elemental reactants.

A new metastable binary compound with the skutterudite crystal structure has been synthesized from modulated elemental reactants, through an amorphous intermediate, using a novel low-temperature synthesis technique. The amorphous reaction intermediate undergoes nucleation at 87 degrees C, an extremely low temperature for solid-state reactions. When heated above 350 degrees C, the metastable phase NiSb(3) disproportionates into the thermodynamically stable phases NiSb(2) and Sb. Also, if the sum of the individual elemental layer thicknesses is greater than 30 A, a mixture of different phases forms. Simulation of the high-angle powder X-ray diffraction spectrum confirms that NiSb(3) is isostructural with CoSb(3).

Calorimetric study of the nucleation of La xCo 4Sb 12 prepared from modulated elemental reactants

A series of new compounds La x Co 4Sb 12 with the skutterudite crystal structure, where 0< x<1, was prepared using modulated elemental reactants. The evolution of the reaction was followed using calorimetry and X-ray diffraction. The modulated elemental reactants were found to inter-diffuse at temperatures below 150 °C forming an amorphous reaction intermediate. The crystallization temperature and nucleation energies for the transformation of the amorphous intermediate into the skutterudite crystal structure were both found to vary linearly with lanthanum content. The lattice parameter of the resulting skutterudite structures increased as the lanthanum filling was increased. The relative intensities of the Bragg reflections indicate that the lanthanum content in the skutterudite structure increases as the lanthanum content of the modulated elemental reactant increases.

Structural and magnetic properties of RFe 4P 12 (R=Pr, Nd) studied by neutron diffraction

In this work we present a neutron powder diffraction study of the ternary rare-earth phosphides PrFe 4P 12 and NdFe 4P 12. High-resolution measurements of the structural temperature dependence reveal large-amplitude thermal vibrations of the rare-earth ions, consistent with speculations that the ion is weakly bound in an oversized atomic ‘cage’ formed in the skutterudite crystal structure. In addition, low temperature neutron diffraction measurements of RFe 4P 12 (R=Pr, Nd) were performed. Whereas the proposed long-range antiferromagnetic ordering in PrFe 4P 12 could not be confirmed, NdFe 4P 12 orders ferromagnetically below T C=1.94 K with an ordered magnetic Nd 3+ moment of 1.61(7) μ B at 1.5 K.

Thermoelectric and optical properties of the filled skutteruditeYbFe4Sb12

Transport measurements, including magnetoresistivity, Hall resistivity, thermal conductivity, and thermopower, and an assessment of the dimensionless thermoelectric figure of merit $\mathrm{ZT}$ below room temperature are reported on a polycrystalline sample of the strongly correlated electron material ${\mathrm{YbFe}}_{4}{\mathrm{Sb}}_{12}.$ Neutron diffraction measurements are presented, which reveal large amplitude thermal vibrations of the Yb ion, consistent with speculations that the ion is weakly bound in an oversized atomic ``cage'' formed in the skutterudite crystal structure. Infrared spectroscopy measurements reveal a resonance in the dissipative part of the optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega}),$ which develops below ${T}^{*}\ensuremath{\approx}70 \mathrm{K},$ the characteristic temperature for the Yb ion valence fluctuations. The frequency dependence of the optical constants is suggestive of a hybridization band gap in the quasiparticle density of states at low temperatures.

Effect of partial void filling on the lattice thermal conductivity of skutterudites

Polycrystalline samples of antimonides with the skutterudite crystal structure with La partially filling the voids have been prepared in an effort to quantify the impact of partial void filling on the lattice thermal conductivity of these compounds. It is observed that a relatively small concentration of La in the voids results in a relatively large decrease in the lattice thermal conductivity. In addition, the largest decrease in the lattice thermal conductivity, compared to ‘‘unfilled’’ CoSb 3 is not observed near 100% filling of the voids with La, as was previously believed. This suggests a point-defect-type phonon scattering effect due to the partial, random distribution of La in the voids as well as the ‘‘rattling’’ effect of the La ions, resulting in the scattering of a larger spectrum of phonons than in the case of 100% filling. An additional benefit of partial filling in thermoelectric materials is that it may be one way of adjusting the electronic properties of these compounds. Seebeck, resistivity, Hall effect and structural data for these skutterudite compounds are also presented. @S0163-1829~98!02926-9#

Synthesis of New Thermoelectric Materials Using Modulated Elementary Reactants

AbstractModulated elemental reactants have been used to synthesize metastable ternary compounds with the skutterudite crystal structure. The initial reactants are made up of multiple repeats of a unit containing elemental layers of a ternary metal, iron or cobalt (or a combination of these two) and antimony. The elemental layers interdiffuse upon low temperature annealing and form amorphous reaction intermediates. In this paper we target the compounds PbxFe4−yCoySb12. On annealing at temperatures between 12° and 150° C (depending on the composition) crystallization of the skutterudite structure occurs. The compounds are only kinetically stable, decomposing into a mixture of binary compounds upon annealing past a temperature of about 600°C. Preliminary data for the Seebeck coefficient and the electrical conductivity was collected The thermoelectric properties of the lead cobalt antimony skutterudite - films were measured as a function of lead occupancy.

Rational Synthesis of Metastable Skutterudite Compounds Using Multilayer Precursors

A new metastable binary compound and a series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been prepared through controlled crystallization of amorphous reaction intermediates formed by low-temperature interdiffusion of modulated elemental reactants. Discussed in this paper are La{sub 1-x}Fe{sub 4}Sb{sub 12}, FeSb{sub 3}, Hf{sub 1-x}Fe{sub 4}Sb{sub 12}, and Y{sub 1-x}Fe{sub 4}Sb{sub 12}. The amorphous reaction intermediate for each system crystallizes exothermically near 200 {degree}C forming the desired skutterudite structure. At temperatures above 500 {degree}C, they decompose exothermically forming a thermodynamically more stable mixture of binary compounds and elemental components. We propose that the desired compounds nucleate from the amorphous precursor because slow solid state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. This general synthetic approach to prepare new metastable compounds give control of both the composition and structure of the product compounds. 15 refs., 6 figs., 2 tabs.

Synthesis of New Thermoelectrics Using Modulated Elemental Reactants

AbstractA series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been synthesized using modulated elemental reactants. The initial reactants are made up of multiple repeats of a ˜25Å thick unit containing elemental layers of the desired ternary metal, iron and antimony. Low temperature annealing (150°C) results in interdiffusion of the elemental layers to form amorphous reaction intermediates. Annealing these intermediates at temperatures between 200°C and 250°C results in exothermic crystallization of the desired skutterudite crystal structure. Most of the new compounds prepared are only kinetically stable, decomposing exothermically to form thermodynamically more stable mixtures of binary compounds and elements. Low angle x-ray diffraction studies show that the resulting films are exceedingly smooth. These films have an ideal geometry for measuring properties of importance for thermoelectric devices—the Seebeck coefficient and the electrical conductivity. Thermal conductivity can be measured using a modification of the 3ω technique of Cahill. Samples can be produced rapidly, allowing for systematic screening and subsequent optimization as a function of composition and doping levels.

Low Thermal Conductivity Skutterudites

AbstractRecent experimental results on semiconductors with the skutterudite crystal structure show that these materials possess attractive transport properties and have a good potential for achieving ZT values substantially larger than for state-of-the-art thermoelectric materials. Both n-type and p-type conductivity samples have been obtained, using several preparation techniques. Associated with a low hole effective mass, very high carrier mobilities, low electrical resistivities and moderate Seebeck coefficients are obtained in p-type skutterudites. For a comparable doping level, the carrier mobilities of n-type samples are about an order of magnitude lower than the values achieved on p-type samples. However, the much larger electron effective masses and Seebeck coefficients make n-type skutterudites promising candidates as well. Unfortunately, the thermal conductivities of the binary skutterudite compounds are too large, particularly at low temperatures, to be useful for thermoelectric applications.Several approaches to the reduction of the lattice thermal conductivity in skutterudites are being pursued: heavy doping, formation of solid solutions and alloys, study of novel ternary and filled skutterudite compounds. All those approaches have already resulted in skutterudite compositions with substantially lower thermal conductivity values in these materials. Recently, superior thermoelectric properties in the moderate to high temperature range were achieved for compositions combining alloying and “filling” of the skutterudite structure. Experimental results and mechanisms responsible for low thermal conductivity in skutterudites are discussed.