Elsevier

Journal of Solid State Chemistry

Volume 193, September 2012, Pages 36-41
Journal of Solid State Chemistry

Synthesis, characterization and physical properties of the skutterudites YbxFe2Ni2Sb12 (0≤x≤0.4)

https://doi.org/10.1016/j.jssc.2012.03.041Get rights and content

Abstract

The skutterudites YbxFe2Ni2Sb12 (0≤x≤0.4) have been prepared by solid-state reaction and characterised by powder X-ray diffraction. The compounds crystallise in the cubic space group Im 3¯ (a≈9.1 Å) with Yb atoms partially filling the voids in the skutterudite framework. A neutron time-of-flight diffraction experiment for Fe2Ni2Sb12 confirms the disorder of Fe and Ni atoms on the transition-metal site. Electrical resistivity, Seebeck coefficient and thermal conductivity measurements indicate that the thermoelectric performance of the skutterudites shows a marked dependence on the Yb content. Magnetic measurements over the temperature range 2≤T/K≤300 show paramagnetic behaviour for all compounds. Decomposition studies under an oxidising atmosphere at elevated temperatures have also been carried out by thermogravimetric analysis.

Graphical abstract

The filled skutterudites YbxFe2Ni2Sb12 have been prepared by solid-state reaction and characterised by powder X-ray diffraction. The thermoelectric performance depends strongly on the Yb content. The physical properties and thermal stability of the compounds are further discussed in comparison with the current state-of-the art thermoelectric skutterudites.

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Highlights

► A new series of skutterudites has been prepared and characterised. ► Physical properties are affected by the degree of Yb filling. ► The highest thermoelectric performance is found for Yb0.15Fe2Ni2Sb12. ► The skutterudites decompose in air above 550 K.

Introduction

The ideal skutterudite structure [1] consists of an array of vertex-linked metal centred MX6 octahedra, which results in a framework of stoichiometry MX3 (Fig. 1). Octahedra are tilted according to the a+a+a+ tilt system [2], generating large voids within the structure. The anion sub-lattice forms four-membered rectangular rings, which are believed to play a key role in determining the electronic properties [3]. Binary skutterudites MX3 are known for M=Co, Rh, Ir; X=P, As, Sb. Moreover the large voids created within the framework may be occupied to varying degrees by filler atoms giving rise to filled skutterudites of general formula, AxM4X12, where A may be a rare-earth, alkali metal, alkaline earth or group 13 element [4], [5]. Filled skutterudites have been extensively studied as promising candidates for thermoelectric applications [6], [7]. It has been suggested that such materials are manifestations of the ‘phonon-glass and electron-crystal’ concept (PGEC) proposed by Slack [8]. The guest atoms within the void space exhibit localised vibrational modes [9], known as rattling vibrations. These localised vibrations perturb the propagation of phonons, thereby significantly reducing the phonon contribution to the thermal conductivity. The PGEC concept postulates that this may occur without a degradation of the electrical properties of the material, which are primarily determined by the characteristics of the framework. It has been established that the properties of filled skutterudites can be tuned by varying the degree of filling of the voids with electropositive elements and through chemical substitution within the framework. The latter can also lead to charge compensation. Filled skutterudites have some of the highest thermoelectric figures of merit (ZT=S2T/κρ where S is the Seebeck coefficient, ρ the electrical resistivity and κ the thermal conductivity) at elevated temperatures, for example n-type Ba0.30Ni0.05Co3.95Sb12 (ZT≈1.25 at 900 K) [10] and p-type Ce0.9Fe3CoSb12 (ZT≈1.1 at 700 K) [11].

The majority of studies of filled skutterudites have focused on the CoSb3 framework [5]. By contrast, more limited investigations have been carried out for the analogous Fe/Ni phases. Replacement of cobalt by equimolar amounts of iron and nickel, yields a ternary skutterudite that is isoelectronic with the binary cobalt antimonide. Fe2Ni2Sb12 exhibits lower thermoelectric performance than Co4Sb12. Rare-earth-filled Pr0.1Fe2Ni2Sb12 [12] and (Pr,Nd)0.08Fe2Ni2Sb12 [13] have been reported to exhibit n-type behaviour and a moderate thermoelectric response (e.g., ZT≈0.4 at 800 K for (Pr,Nd)0.08Fe2Ni2Sb12). A change to p-type behaviour may be induced by varying the Fe/Ni ratio as occurs in (Ln)x(Fe,Ni)4Sb12 (Ln=Ce and/or Yb, 0≤x≤0.95) [14]. Theoretical calculations on alkaline-earth-filled skutterudites Ae(Fe,Ni)4Sb12 (Ae=Ca, Sr, Ba) predict a high Seebeck coefficient in both n- and p- type variants [15]. However, this prediction is yet to be tested experimentally.

In the search for new thermoelectric materials, we have investigated filled skutterudites based on the isoelectronic ternary phase Fe2Ni2Sb12. In particular, we investigate the effect on thermoelectric and magnetic properties of varying degrees of filling through preparation of the new series of quaternary phases, YbxFe2Ni2Sb12 (0≤x≤0.4).

Section snippets

Experimental

The skutterudites YbxFe2Ni2Sb12 (0≤x≤0.4) were prepared by mixing stoichiometric quantities of the elements Fe (Alfa, 99.9%), Ni (Aldrich, 99.99%), Sb (Aldrich, 99.999%) and Yb (Aldrich, 99.9%). The reagents were loaded into glassy carbon crucibles in an Ar-filled glove box. The crucibles were loaded into fused silica tubes under an Ar atmosphere before transferring to a vacuum line. The tubes were then evacuated (<10−4 Torr) and sealed. The mixtures were heated for 12 h at 1173 K, quenched in

Results and discussion

Rietveld refinements for Fe2Ni2Sb12 were carried out using TOF powder neutron diffraction data. The initial structural model was that proposed by Kjekshus and Rakke [21]. Analysis of neutron diffraction data indicates that this material crystallises in the cubic space group Im3¯ No superstructure reflections were observed in the neutron diffraction data, consistent with complete disorder of the transition-metal cations over the octahedral sites. This is in accord with previous observations on

Conclusions

The Yb-filled compounds YbxFe2Ni2Sb12 (0≤x≤0.4) represent new members of the filled skutterudite family and behave as n-type, degenerate semiconductors. The Yb filling affects the thermal and electrical transport properties. The optimum filler content, with respect to the thermoelectric performance at 350 K, is found for Yb0.15Fe2Ni2Sb12. Although the title compounds contain less costly elements than the analogous filled cobalt antimonides and also exhibit a significantly lower thermal

Acknowledgments

The authors wish to thank the UK EPSRC for financial support (EP/H050396) and the STFC for access to neutron scattering facilities.

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