Structure, Phase Composition, and Thermoelectric Properties of

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Structure, Phase Composition, and Thermoelectric Properties of YbxCo4Sb12 and Their Dependence on Synthesis Method

Britta Ryll,† Andreas Schmitz,‡ Johannes de Boor,‡ Alexandra Franz,† Pamela S. Whitfield,§,# Manfred Reehuis,† Andreas Hoser,† Eckhard Müller,‡,∥ Klaus Habicht,*,†,⊥ and Katharina Fritsch† †

Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, 14109 Berlin, Germany Institute of Materials Research, German Aerospace Center (DLR), Linder Höhe, 51147 Köln, Germany § Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Institute of Inorganic and Analytical Chemistry, Justus Liebig University Gießen, Heinrich-Buff-Ring 17, 35392 Gießen, Germany ⊥ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany ‡

S Supporting Information *

ABSTRACT: We present a combined microscopic and macroscopic study of YbxCo4Sb12 skutterudites for a range of nominal filling fractions, 0.15 < x < 0.75. The samples were synthesized using two different methods  a melt−quench−annealing route in evacuated quartz ampoules and a non-equilibrium ball-mill route  for which we directly compare the crystal structure and phase composition as well as the thermoelectric properties. Rietveld refinements of high-quality neutron powder diffraction data reveal about a 30−40% smaller Yb occupancy on the crystallographic 2a site than nominally expected for both synthesis routes. We observe a maximum filling fraction of at least 0.439(7) for a sample synthesized by the ball-mill routine, exceeding theoretical predictions of the filling fraction limit of 0.2−0.3. A single secondary phase of CoSb2 is observed in ball-mill-synthesized samples, while two secondary phases, CoSb2 and YbSb2, are detected for samples prepared by the ampoule route. A detrimental influence of the secondary phases on the thermoelectric properties is observed for secondary-phase fractions larger than 8 wt % regardless of the kind of secondary phase. The largest figure of merit of all samples with a ZT ∼ 1.0 at 723 K is observed for the sample with a refined Yb content of x2a = 0.159(3), synthesized by the ampoule route. KEYWORDS: thermoelectric materials, skutterudite, melt-quench-anneal, ball mill, neutron powder diffraction, thermoelectric properties, figure of merit

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with space group Im3̅ in which guest atoms can fill the voids on the 2a crystallographic site.6 These filling elements are often referred to as “rattlers” as they are weakly bound to the host structure and interact with the lattice vibrations lowering the lattice thermal conductivity,7,8 while at the same time preserving or even enhancing the electronic properties, a concept often referred to as the phonon−glass electron−crystal concept.9 CoSb3-based skutterudites are particularly promising thermoelectric materials due to their high carrier mobility and large effective mass, which leads to a large power factor PF = σS2.10 A variety of differently filled skutterudites RCo4Sb12 show maximum ZT values of up to ∼2 at temperatures between 600 and 850 K, where R is, e.g., In, Ga, Li, Ba, Sr, Tl, or a rare earth element such as Ce, Eu, La, Nd, or Yb.11−23 Additional strategies to increase ZT include multifilling and substitution on the CoSb3 framework and are discussed in several recent review articles.24,25

INTRODUCTION The ever increasing energy demand of today’s society and the need for alternative, sustainable energy resources as well as the need for an efficient use of any available form of energy has revived the interest in thermoelectric materials in the past couple of decades.1,2 Thermoelectric materials possess the ability to convert thermal into electrical energy and vice versa and, as such, are attractive for applications such as waste heat recovery from industrial processes or automotive exhausts, conceivably lowering fuel consumption.3,4 The potential of a thermoelectric material to be used in a thermoelectric device is characterized by its dimensionless figure of merit, ZT, which is given by ZT = σS2T/κ, wherein T denotes the temperature, S the Seebeck coefficient, σ the electrical conductivity, and κ the total thermal conductivity mainly consisting of lattice and electronic thermal conductivity, κlat + κe, respectively. One of the most efficient thermoelectric materials in the midtemperature regime (500−900 K) known to date are skutterudites,1,5 which derive from the aristotype CoSb3 with general formula AB3, where A is a transition metal atom, such as Fe, Ru, Os, Co, Ni, Ir, or Rh, and B a pnicogen atom, such as P, As, or Sb. Skutterudites crystallize in a cubic crystal structure © XXXX American Chemical Society

Received: October 10, 2017 Accepted: December 7, 2017 Published: December 7, 2017 A

DOI: 10.1021/acsaem.7b00015 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

microstructure and thermoelectric properties31 and such a study would constitute a detailed study in its own right, which is beyond the scope of this work. We determine the actual content of Yb on the 2a site of the skutterudite structure as well as the amount and the nature of secondary phases from Rietveld refinements of high-quality neutron diffraction data. Additional EDX measurements are carried out to relate the nominal to the overall Yb content in the samples and to compare the latter to the refined Yb content on the 2a site. Finally, we investigate the influence of the two different synthesis methods, filling fractions and secondary phases on the electrical and thermal conductivity as well as on the Seebeck coefficient in the temperature range from 300 to 765 K. The filling fraction for which the largest ZT value is achieved among all our samples is determined.

In this study, we focus on single-element Yb-filled skutterudites YbxCo4Sb12. Yb filling is one of the most efficient ways to increase ZT since it reduces the lattice thermal conductivity very effectively due to the small radius and heavy mass of Yb.21,26 We address the filling fraction limit (FFL) for two different synthesis methods and investigate the resulting thermoelectric properties, as the FFL has been controversially discussed in the literature by theorists and experimentalists alike. Calculations using density functional theory predicted the filling limit at x ∼ 0.2 or x ∼ 0.3,27−29 above which the formation of secondary phases becomes energetically more favorable than that of the filled skutterudite phase. Consistent with these predictions, Nolas et al. have successfully prepared samples with x = 0.19 free of secondary phases by a melt− quench−annealing route by reacting stoichiometric amounts of Yb, Co, and Sb in quartz ampoules.21 The phase purity was confirmed by X-ray diffraction (XRD), metallographic analysis, and electron probe microanalysis (EPMA). Park et al. later synthesized skutterudite samples via encapsulated melting followed by an annealing process and deduced a FFL of 0.3 from the appearance of secondary phases in XRD patterns for samples with nominal Yb contents ≥ 0.3.30 Only recently, Tang et al. reported the possibility of synthesizing Yb0.49Co4Sb12 with a yet higher filling fraction of x = 0.49 determined by EPMA, using a similar synthesis routine but increasing the annealing temperature to 1073 K.31 These findings, however, were contested by Wang et al., who employed again a similar routine albeit with lower annealing temperature of 1023 K and who reported a FFL of only ∼0.29 based on energy-dispersive X-ray spectroscopy (EDX).32 Meanwhile, Yang et al. reported on a non-equilibrium synthesis method for YbxCo4Sb12 that allows to increase the filling fraction even further.33 They reported on samples with Yb contents of up to x = 0.5 synthesized by a ball-milling process of the pure elements Co, Sb, and Yb, followed by rapid hot pressing, for which the conditions were not specified. XRD patterns showed no evidence for the presence of secondary phases leading the authors to conclude that the nominal Yb content actually corresponds to the samples’ Yb content, presumably with Yb occupying the 2a site. A comparison of the room temperature thermoelectric properties of these samples with the samples studied by Nolas et al. revealed generally higher electric and lower lattice thermal conductivities leading to a maximum ZT of 1.2 at 880 K for Yb0.35Co4Sb12. This proposed synthesis method appears quite attractive, as it requires overall much shorter processing times when compared to the traditional encapsulating melting route involving longterm annealing and, hence, bears the possibility for commercialization. However, to our knowledge, the reported thermoelectric properties of ZT of 1.2 by Yang et al. have not been reproduced by any other group for any Yb content, possibly due to slightly different synthesis conditions, in particular annealing steps, and the formation of secondary phases. In this work, we present a direct comparison of YbxCo4Sb12 samples with different filling fractions x prepared by the two different synthesis methods, the traditional encapsulated melting route and the non-equilibrium ball-milling synthesis, with respect to actual Yb filling fraction, the presence of secondary phases, and their temperature-dependent thermoelectric properties. We note here that we investigate as-prepared ball-mill samples without studying the effects of annealing. In fact, annealing steps are known to significantly influence the

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EXPERIMENTAL DETAILS

YbxCo4Sb12 samples were synthesized by two different methods. For the equilibrium melt−quench−anneal (ampoule) method, stoichiometric quantities of high-purity elemental Yb, Co, and Sb were sealed in an evacuated quartz glass ampoule and melted in a furnace at a temperature of 1423 K for 2 h. Afterward, the sample was quenched in water to room temperature and then annealed in the ampoule at 923 K for 110 h. By subsequent grinding of the sample, a homogeneous powder with grain sizes less than 100 μm was obtained. This synthesis procedure is slightly different from that used by Tang et al., who employed a lower melting temperature of 1373 K as well as longer melting and annealing times of 12 and 168 h, respectively. For the non-equilibrium ball-mill method, pre-reacted CoSb3 powder and high-purity elemental Yb were milled in a planetary ball mill (PM 400, Retsch) under argon atmosphere for 23 h at 400 rpm with a 10 min wait−10 min milling procedure. Afterward, the powder was sintered by a current-assisted sintering method (direct sinter pressing) at 863 K and under 56 MPa and ground to obtain a powder with grain sizes of less than 100 μm, as with the ampoule method. In contrast to the synthesis route of Yang et al., we used pre-reacted CoSb3 skutterudite powder and one additional annealing step. For the macroscopic measurements, the samples from both synthesis methods were sintered again at 863 K and 56 MPa to obtain dense pellets with a diameter of 12.7 mm and thicknesses between 0.6 and 1.8 mm. All sintered samples exhibit relative densities between 93% and 98% of the theoretical densities, with an average 96(2)% and 95(2)% for samples prepared by the ampoule and the ball-mill methods, respectively. The temperature-dependent electrical conductivity and Seebeck coefficient were measured concurrently using a custom-built device at DLR, Cologne.34,35 The thermal diffusivity was measured by a laser flash device (LFA 427, Netzsch). In order to derive the thermal conductivity, κ, the thermal diffusivity, α, was multiplied by the sample density, ρ, determined via Archimedes’ method, and the theoretical heat capacity, Cp, in its high-temperature limit obtained by Dulong− Petit’s law, according to the relation κ = αρCp. The measurements of the Seebeck coefficient and the electrical conductivity as well as the LFA measurements were taken both during the heating and cooling phases of the measurement cycle to obtain information on the stability of the material properties. Hall measurements were performed at room temperature in a van der Pauw geometry using a custom-built setup at DLR.36 Measurement uncertainties are 5% for both the electrical conductivity, σ, and Seebeck coefficient, S, 8% for the thermal conductivity, κ, and 15% for both the charge carrier density, n, and mobility, μ. Hiqh-quality powder neutron diffraction data were obtained using the time-of-flight instrument POWGEN at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, Oak Ridge, TN, USA,37 using a 1 Å wavelength band centered at λ = 1.066 Å. Additional neutron diffraction measurements were carried out on the fine resolution powder diffractometer E9 at Helmholtz-Zentrum Berlin (HZB), Germany, using a monochromatic beam with a wavelength of λ = 1.798 Å.38 At both instruments, the powder was filled into B


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ACS Applied Energy Materials standard vanadium cans and was measured at temperatures between 10 and 300 K. YbxCo4Sb12 samples with nominal filling fractions xnom = 0.15, 0.25, and 0.35 (ampoule method) as well as samples with filling fractions xnom = 0.15 and 0.35 (ball-mill method) were investigated using POWGEN. Neutron diffraction measurements on the instrument E9 were carried out for powders with xnom = 0.45 (ampoule method) and xnom = 0.55 and 0.75 (ball-mill method). The powder patterns obtained from POWGEN were analyzed with the programs GSAS and EXPGUI,39,40 while data obtained on E9 were analyzed with the FullProf program suite.41 Powder X-ray diffraction was performed on a Siemens Bruker D5000 device using Cu Kα radiation, and the data were refined with FullProf. EDX images were obtained on the same sintered specimens that were used for the measurements of the thermoelectric properties by using a SEM LEO GEMINI 1530 (Zeiss) equipped with an EDX system by Thermo Fisher. For all images, an acceleration voltage of 8 keV was used. The samples were polished prior to these measurements to ensure smooth and clean surfaces free of possible surface inhomogeneities originating from the sintering process.

increase much faster than those of Sb and Co, resulting in four times larger ADPs than for either Sb or Co at room temperature, consistent with the results of Mi et al.42 and in agreement with other studies on filled skutterudites45,46 that clearly identify the Yb atoms on the 2a site as rattler atoms. For a consistent data analysis over both instruments and all data sets, we have subsequently constrained the ADPs [2Uiso(Co) = 2Uiso(Sb) = Uiso(Yb) for the 10 K data] based on the experimental values of Uiso from Figure 2b. Using this constraint, the Yb(2a) occupancies were refined for the data of all samples (see Table 1, first two rows). Within this refinement scenario, Yb is considered soluble only on the 2a site, as expected intuitively based on the size and chemical differences between the Yb and Co or Sb atoms. Interestingly, and for the samples synthesized by the ball-mill routine only, additionally allowing the occupancy of Co on the 8c site to refine leads to an improvement of the overall quality of the refinements. Based on the resulting Co(8c) > 1 (1 = full occupancy), it is likely that an atom with larger scattering length such as Yb (5× larger coherent scattering length bc than that of Co47) partially occupies this site. The resulting modified site occupancies for Yb on the 2a and 8c sites, respectively, are given in Table 1 (third row). This result is somewhat surprising since an Yb occupation of the Co site seems unlikely due to the noncovalent bonding character of Yb and Co and their large difference in ionic radii. A similar improvement of the refinement quality can be achieved by assuming full occupancy of the Co(8c) site in combination with reduced Sb and Yb occupancies of the Sb(24g) and the Yb(2a) filler sites, respectively. This scenario leads to Sb vacancies, up to 20% smaller Yb occupancies of the 2a site and reduced weight fractions of the secondary phases (Table 1 (bottom row)). Indeed, the formation of Sb vacancies has been discussed in the literature previously and was observed experimentally by Hanus et al. in their samples using synchrotron X-ray diffraction,48 for which the smallest occupancy of the Sb 24g site was found as 0.958. In comparison, we refine a Sb(24g) site occupancy of 0.802(9) for our sample with largest nominal filling content, xnom = 0.75, synthesized by the ball-mill method. We speculate that this large deviation from full Sb occupancy and the concomitant observation of a secondary CoSb2 phase with a large ∼20 wt % phase contribution are most likely related to the nonequilibrium nature of the ball-mill synthesis procedure. A combination of both scenarios is also found compatible with the data, but the presence of Yb on the Co site with simultaneous Sb vacancies cannot be extracted from the data on a quantitative level using the Rietveld refinement procedure. Therefore, these two scenarios with the parameters listed in Table 1 define the extrema of a range of possible combinations. In contrast, for the samples synthesized by the ampoule route neither a significant amount of Yb on the 8c site nor Sb vacancies are found, suggesting once more that these defects originate from the non-equilibrium ball-mill synthesis procedure. Furthermore, Co vacancies can be excluded for any sample synthesized by either method. A finite Yb occupancy on the 24g site can also be excluded from the refinements. Further details of the refinement procedure, refinement results, and representative plots are provided with the Supporting Information. Regardless of the refinement scenario, our results show that an increase of the x2a occupancy leads to a linear increase of the lattice parameter, consistent with Vegard’s law (Figure 3a).49

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RESULTS AND DISCUSSION Powder Diffraction. An overview of the high-d-spacing region of the neutron powder diffraction data of all samples, taken at the lowest temperature of 10 K, is shown in Figure 1.

Figure 1. Neutron powder diffraction patterns for YbxCo4Sb12 at 10 K. The nominal Yb content is indicated on the right. The diffractograms for samples with xnom = 0.45, 0.55, and 0.75 were obtained on the instrument E9 while all others were obtained on POWGEN. The patterns are offset in intensity for clarity. Measured intensities for samples measured on E9 were normalized to the POWGEN data to allow for better visual comparison of the two data sets. Stars denote reflections associated with the sample environment on E9.

The most prominent reflections are attributed to YbxCo4Sb12.42 Additional reflections at d-spacings of 2.71, 2.62, and 2.77 Å, can be attributed to the secondary phases YbSb2 (orthorhombic) and CoSb2 (monoclinic), respectively.43,44 Rietveld refinements of the neutron data were performed based on the filled cubic skutterudite structure (see Figure 2a) with the nominal Yb content used as the initial parameter for the refinement of the actual Yb occupancy at the crystallographic 2a site. The atomic displacement parameters (ADPs) were refined simultaneously with the Yb(2a) occupancy from the POWGEN data. We present the refinement results in Table 1. The temperature evolution of the isotropically refined ADPs (Uiso) is shown in Figure 2b. At the lowest temperature of 10 K, the ADPs of Yb are approximately a factor of 2 larger than those of the framework atoms Sb and Co. With increasing temperature, the Yb ADPs C


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Figure 2. (a) Crystal structure of YbxCo4Sb12, here shown with hypothetical full filling (x = 1). The Yb “filler” atoms are located at the 2a sites of the cubic unit cell (a ∼ 9.04 Å; space group, Im3̅) and are 12-fold-coordinated to Sb atoms located at 24g sites forming the corners of an icosahedral cage (shown in blue). Co atoms occupy the 8c sites and are 6-fold-coordinated to Sb atoms, forming distorted CoSb6 octahedra (shown in red). (b) Isotropic atomic displacement parameters Uiso versus temperature for two exemplary samples synthesized by the ampoule route and investigated at POWGEN, SNS.

Table 1. Results of the Rietveld Refinement of Neutron Powder Diffraction Data at 10 K, Tabulated for Ampoule Samples and Three Different Scenarios for Samples Synthesized by the Ball-Mill Routinea method

xnom

Yb(2a)

ampoule ampoule ampoule ampoule

0.15 0.25 0.35 0.45

0.099(3) 0.159(3) 0.201(4) 0.290(6)

0 0 0 0

1 1 1 1

ball ball ball ball

mill mill mill mill

0.15 0.35 0.55 0.75

0.002(3) 0.050(3) 0.362(6) 0.493(8)

0 0 0 0

1 1 1 1

ball ball ball ball

mill mill mill mill

0.15 0.35 0.55 0.75

0.006(3) 0.057(3) 0.384(7) 0.548(9)

0.008(1) 0.011(1) 0.029(3) 0.063(4)

1 1 1 1

ball ball ball ball

mill mill mill mill

0.15 0.35 0.55 0.75

0.006(3) 0.054(3) 0.345(6) 0.439(7)

0 0 0 0

0.971(5) 0.959(5) 0.900(10) 0.802(9)

Yb(8c)

Sb(24g)

Rwp, R|F|2 or RF (%)

secondary phase (wt %)

sample composition

0.66(9) YbSb2 3.3(3) CoSb2, 1.83(12) YbSb2 5.7(3) CoSb2, 2.61(10) YbSb2

Yb0.099(3)Co4Sb12 Yb0.187(5)Co4Sb12.055(9) Yb0.268(6)Co4Sb11.96(6) Yb0.376(8)Co4Sb11.89(16)

5.08, 4.62, 5.90, 4.08,

5.03 6.36 5.52 4.37

9.2(3) CoSb2, 0.55(5) Yb2O3 22.4(4) CoSb2

Yb0.002(3)Co4Sb12 Yb0.050(3)Co4Sb12 Yb0.362(8)Co4Sb11.48(15) Yb0.346(8)Co4Sb10.81(14)

4.20, 4.09, 3.75, 4.20,

3.54 4.88 3.99 5.45

9.3(3) CoSb2, 0.45(5) Yb2O3 22.3(4) CoSb2

Yb0.038(5)Co4Sb12.097(9) Yb0.102(5)Co4Sb12.133(9) Yb0.484(15)Co4Sb11.77(15) Yb0.583(15)Co4Sb11.29(15)

4.18, 4.06, 3.66, 3.86,

3.56 4.82 3.67 3.90

8.3(3) CoSb2, 0.40(4) Yb2O3 18.4(4) CoSb2

Yb0.006(3)Co4Sb11.65(9) Yb0.054(3)Co4Sb11.51(9) Yb0.340(15)Co4Sb10.5(4) Yb0.344(14)Co4Sb9.3(3)

4.18, 4.06, 3.66, 3.86,

3.56 4.82 3.66 3.88

The results for ampoule samples are given in the first data set. Yb only occupies the 2a site while the Co and Sb sites are fully occupied by Co and Sb, respectively (second data set), Yb occupies the 2a site and partially the 8c site (third data set) and Yb only occupies the 2a site and Sb vacancies on the 24g site exist (fourth data set). For clarity, we denote the occupancy of Yb on the 2a site with Yb(2a), the partial occupancy of Yb on the 8c site by Yb(8c), and that of Sb on the 24g site by Sb(24g). For all occupancies, a value of 1 corresponds to a fully occupied site. Values without uncertainties were not refined. Rwp and global R-factors in percent are given in the last column. For further refinement parameters and other agreement factors as well as parameter definitions, please refer to the Supporting Information. a

The refinements of the 2a site occupancy reveal that for both methods only ∼60−70% of the nominal Yb atoms occupy the 2a site (see Table 1 and Figure 3b). However, for samples with xnom = 0.15 and xnom = 0.35 (ball-mill method), the difference between the actual, refined Yb content on the 2a site (x2a) and the nominal Yb content is much larger, indicating that much less Yb enters the skutterudite phase than one would expect for these relatively low nominal filling contents. A partial occupancy of Yb on the 8c site can only explain the overall Yb deficiency in the skutterudite phase to a certain extent, and a final conclusion remains an open issue to address in the future. The refinement results shown in Table 1 also reveal an increasing fraction of secondary phases with increasing nominal Yb content. A remarkable difference between the two synthesis routes is the formation of YbSb2. While the samples synthesized

Similar relations between the Yb content of the skutterudite phase and the observed lattice parameter were reported by both Tang et al. and Wang et al., which relied on a combination of XRD/EPMA or XRD/EDX, respectively.31,32 The maximum Yb occupancy of the 2a site observed in our study is significantly larger than the theoretically proposed FFL of 0.3. For the sample xnom = 0.75 (ball mill), the 2a site occupancy was found to be 0.548(9) assuming an additional partial occupancy by Yb on the 8c site or 0.439(7) assuming the presence of Sb vacancies. In fact, even the refinement with an exclusive Yb 2a occupancy results in a value of 0.493(8). This is in agreement with the maximum filling fractions published by Tang et al. for samples synthesized by an ampoule route.31 Here, we show that such large filling fractions can be obtained by the ball-mill routine as well. D


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Figure 3. Results of the refinement: (a) Lattice parameters versus x2a determined by XRD and neutron diffraction (ND) at room temperature. A linear behavior consistent with Vegard’s law is observed (solid blue line). Similar linear relationships were observed by Tang et al.31 and Wang et al.32 (dashed lines). (b) Actual Yb content x2a versus nominal content xnom as determined from the ND refinement. The black line indicates the case of equal x2a and xnom Yb contents, thus showing that a significant fraction of approximately onethird of the nominal Yb content does not enter the 2a site of the skutterudite phase.

Figure 4. Representative SEM/EDX images of the samples with nominal compositions Yb0.35Co4Sb12 synthesized by the ampoule route (panels a and b) and by the ball-mill route (panels c and d), both for a scan area of ∼0.3 mm2. Panels a and c (top row) show the EDX signal in the energy range between 1.446 and 1.600 keV, which contains the Mα and Mβ lines of Yb at 1.5214 and 1.5675 keV as tabulated by Bearden,50 respectively. Panels b and d (bottom row) show the SEM image of the corresponding EDX area.

by the ampoule method contain both orthorhombic YbSb2 and monoclinic CoSb2 as secondary phases, samples synthesized by the ball-mill method contain only monoclinic CoSb2. This places the samples obtained for the two different synthesis methods in different regions of the Co-rich side of the isothermal sections of the Yb−Co−Sb phase diagram proposed by Tang et al.31 In fact, it appears that the samples synthesized by the equilibrium ampoule method are found in the threephase region whereas the samples synthesized from the nonequilibrium ball-mill routine are located in the two-phase region of the phase diagram. The Rietveld refinements for samples synthesized by the ampoule route show that a significant fraction of Yb atoms enter the secondary YbSb2 phase. Due to the resulting Sb deficiency, the secondary CoSb2 phase is subsequently formed for an increasing nominal Yb content. The remaining amount of Yb may lead to the precipitation of other Yb-containing secondary phases such as Yb2O3 and Yb with weight fractions less than 0.4 wt %. A quantitative confirmation of these possible scenarios is however beyond the sensitivity limit of the Rietveld refinements on the present measurements. In contrast, for samples synthesized by the ball-mill method, no significant Yb-containing secondary phases were observed. This is surprising since this implies that less than 60% of the nominal Yb content is soluble in the skutterudite structure for this synthesis method. Even if a significant fraction of the Yb atoms were to occupy the 8c site, a large fraction of the nominal Yb content remains unaccounted for. In order to investigate this apparent inconsistency, we performed EDX measurements on the pressed pellet samples. EDX Measurements. Scanning electron microscope (SEM) EDX measurements were carried out on two selected areas with a size between 0.1 and 0.6 mm2 for each sintered sample to ensure representative results. The SEM images demonstrate that all sintered samples are very dense based on the observation of only a small number of sizable pores (Figure 4, bottom row). For samples synthesized by the ampoule route, a considerable number of Yb-rich regions with a size of up to 30 μm in diameter are observed (Figure 4a). These regions are oxidized at the surface. Similar observations were reported by Wang et al., which they attributed to the air and water sensitivity of YbSb2 leading to oxidation of the surface.32

Samples synthesized by the ball-mill route (Figure 4c) show only small Yb-rich regions (1−5 μm), with only a few of them reaching sizes up to 20 μm in diameter. This is perhaps not surprising, given that this synthesis route involves ball milling of a powder which favors a fine dispersion of Yb in the material and impedes grain growth of an Yb-rich phase during the sinter process. In order to relate the actual Yb content of the whole sample, xsample, to the nominal content xnom, the ratio of the intensities of the signals of Yb and Co (IYb/ICo) was determined and is plotted versus xnom (Figure 5). Assuming a constant Co content

Figure 5. EDX results: Intensity ratios (IYb/ICo) versus nominal Yb content for several averaged measurements on each sample. The solid line is a linear fit to samples from both methods ampoule (red squares) and ball mill (blue triangles).

for all samples, IYb/ICo can be considered directly proportional to the actual Yb content of the samples.51 The data shown in Figure 5 suggest that the nominal Yb content indeed determines the actual Yb content of the whole sample, xsample, as IYb/ICo follows xnom linearly. Interestingly, these data indicate that sample xnom = 0.15 (ball mill), which does not contain any Yb on the 2a site based on the refinement, does contain the same amount of Yb as sample xnom = 0.15 prepared by the E


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Figure 6. Measurements of the Seebeck coefficient, S (a and b); electrical conductivity, σ (c and d); total thermal conductivity, κ (e and f) (lines are guides to the eye); and thermal conductivity subtracted by the extrinsic electronic part of the conductivity, κe (g and h). Closed symbols represent the data of the heating process while the open symbols represent the data of the cooling process. The dashed orange and the dashed-dotted dark blue lines represent the data of the Yb0.36Co4Sb12 sample taken from Tang et al.31 and of the sample Yb0.35Co4Sb12 taken from Yang et al.,33 respectively.

Thermoelectric Properties. The thermoelectric properties of our YbxCo4Sb12 samples are influenced mainly by three effects: the actual Yb content x2a in the skutterudite phase, the volume fraction of secondary phases and the existence of an amorphous Yb-rich phase. A large Yb content x2a is expected to lower the lattice thermal conductivity according to the “rattler scenario”. At the same time, an increase of the charge carrier density n is expected as a consequence of doping, which increases the electrical conductivity but lowers the Seebeck coefficient. Therefore, there exists an optimal value of n as a compromise between σ and S. Composite materials have been shown to exhibit a smaller overall ZT in classical, i.e., macroscopically sized, phase mixtures, than their largest ZT of the individual components.52 Thus, smaller ZT values are expected for samples with a large volume fraction of secondary phases. However, this does not hold for nanostructured systems. Zhao et al. showed that the presence of Yb2O3 particles with sizes ranging from micro- to nanometers lowers the lattice thermal conductivity of YbxCo4Sb12 due to phonon scattering on nanoscale inclusions and thus increases ZT.53 Consistent with both findings, Rogl et al. pointed out that YbSb2, Sb, and Yb2O3 occurring simultaneously as secondary phases lead to a decrease ZT in multiply filled skutterudites while a homogeneous and nanoscale distribution of Yb2O3 increases ZT.20 Therefore, we expect an increase of ZT due to a reduced lattice thermal conductivity for samples containing an amorphous Yb-rich phase and a decrease of ZT for samples with large volume fractions of YbSb2 and CoSb2. A summary of the temperature-dependent electrical and thermal conductivity as well as the Seebeck coefficient of all samples is displayed in Figure 6. In order to compare our thermoelectric data to the data obtained previously by other groups, the data points of Tang et al.31 and Yang et al.33 have

ampoule route. Measurements on two different areas of the cross-section of the sample confirmed that the detection of Yb in this sample is not a surface effect which might result from an inhomogeneous Yb distribution inside the sample. Further, we investigated the Yb content in the skutterudite phase of the samples prepared by the ampoule route, for which considerably sized precipitates of YbSb2 and CoSb2 were observed, by probing homogeneous regions of the sample surface devoid of any secondary phases. For these regions, a much lower Yb content than the total Yb content is obtained, which is in agreement with the lower x2a content of the skutterudite phase determined from the Rietveld refinement, as expected. For each sample, three such homogeneous regions were analyzed and the results added as black circles to Figure 5. In comparison to the samples synthesized by the ampoule method, the samples synthesized by the ball-mill routine show a more homogeneous distribution of the elements. Regions with much lower Yb content could not be identified. In conclusion, EDX measurements reveal that the samples with xnom = 0.15 and 0.35 (ball mill) contain an amount of Yb similar to those samples synthesized by the ampoule route. It is clear from the Rietveld refinements that for these samples much less Yb is soluble in the crystalline skutterudite phase. One might therefore speculate that the remaining Yb instead forms an Yb-rich amorphous phase precipitating at grain boundaries that is only detected in EDX measurements. While the neutron diffraction technique is in principle also sensitive to amorphousphase content, we did not observe any signatures of the presence of an Yb-rich amorphous phase, likely because of the limited amount present in the sample. Obviously, EDX is sensitive to Yb both in crystalline and amorphous phases and provides thus useful complementary information, especially on such composite systems. F


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this result to the larger percentage of secondary phases present in this sample, as a ∼10 wt % secondary phase observed in the xnom = 0.55 sample does not seem to affect the linear behavior. Likewise, the influence of an Yb occupancy of the 8c site or Sb vacancies on the charge carrier density appears not to be significant. The mobility, μ, decreases with x2a as a result of an increase of carrier−carrier scattering and an increased amount of defects. The heating curves of the electrical conductivity, σ, for samples synthesized by the ampoule routine shift to higher values for increasing x2a content, consistent with an increase of charge carrier density (Figure 6c). Similar behavior is observed for the samples synthesized by the ball-mill routine (Figure 6d). An obvious difference between the two series is the temperature evolution of σ. While the σ of samples synthesized by the ampoule route decreases with increasing temperature in agreement with the behavior of heavily doped semiconductors, the conductivity of samples synthesized by the ball-mill routine slightly increases or stays constant due to structural disorder such as point defects and grain boundaries, which are introduced by the ball-milling procedure. This structural disorder also leads to lower values of σ for samples synthesized by the ball-mill routine as compared to the values of samples synthesized by the ampoule routine. The samples xnom = 0.35 and 0.45 (ampoule) show different curves for heating and cooling measurement cycles. A decrease of σ was observed after the heating process indicating a possible phase segregation at the grain boundaries during the heating cycle. The samples of Yang et al. and Tang et al. both show a larger electrical conductivity than our samples. Especially the results for σ of Tang et al.’s ampoule sample are larger than those of any of our samples. A possible explanation for this could be different grain sizes due to a different hot press procedure or an even higher density of their investigated pellet samples. The temperature dependence of the thermal conductivity, κ, is displayed in Figure 6e,f. All samples show relatively small thermal conductivities in the range of 2−4 W/(m·K), with the ball-mill samples generally exhibiting lower values than their ampoule counterparts. The samples of Tang et al. and Yang et al. both exhibit higher thermal conductivities, which we attribute to a larger contribution of the electronic part to the thermal conductivity. In order to subtract the electronic part of the conductivity, κe, a variable Lorenz number corresponding to the single parabolic band model was used. The resulting κ − κe is plotted in Figure 6g,h. A relation between x2a and κ − κe is observed: the higher x2a, the lower is κ − κe, in agreement with the expectations of the rattling scenario. However, large amounts of secondary phases as observed for xnom = 0.45 (ampoule), 0.55 (ball mill), and 0.75 (ball mill) increase the resulting lattice thermal conductivity. As the sample xnom = 0.35 (ball mill) with only x2a = 0.05 displays lower lattice thermal conductivities than that with xnom = 0.15 (ampoule) with x2a = 0.10 and as both are mainly free of crystalline secondary phases, we attribute the decrease of κ − κe to the presence of an amorphous Yb-rich phase. Thus, it is the amorphous Yb-rich phase that lowers the lattice thermal conductivity. In general, the κ(T) curves of samples synthesized by the ball-mill routine are flatter at lower temperatures than the curves of samples synthesized by the ampoule routine and show a pronounced upturn at higher temperatures. We attribute this behavior to being a consequence of the ball-milling process. While structural disorder leads to a flatter curve of the lattice

been added to the plots. The sample of Tang et al. with nominal composition of Yb0.36Co4Sb12 was synthesized by an ampoule route similar to our melt−quench−annealing method, while the sample of Yang et al. with the nominal composition Yb0.35Co4Sb12 was synthesized by a ball-mill routine similar to that employed in this work. The temperature-dependent Seebeck coefficients of all samples are displayed in Figure 6a,b. The more Yb atoms fill the 2a site, the lower is the Seebeck coefficient suggesting that the charge carrier density is increasing with x2a. Only samples xnom = 0.15 and 0.35 (ball mill) behave differently. Sample xnom = 0.15 (ball mill) shows, in contrast to all other samples, p-type behavior. This can be explained based on the negligible amount of Yb on the 2a site, basically leaving the skutterudite structure unfilled and leading to a behavior similar to that of the unfilled, p-type CoSb3 system. In fact, for unfilled CoSb3, the carrier type is very sensitive to Co and Sb deficiencies10,54 and unintentionally doped samples are usually p-type semiconductors.55 Furthermore, it cannot be excluded that minute amounts of Fe entered the sample during the ball-milling process which would then also lead to p-type semiconducting behavior. Sample xnom = 0.35 (ball mill) shows a maximum of the Seebeck coefficient between 500 and 550 K while the other samples exhibit a smoothly increasing S over the whole temperature range measured. This maximum is typical for samples with only small Yb content indicating the onset of intrinsic conduction.21,30,53,56 Due to the proximity to the intrinsic conduction regime, lower absolute values of the Seebeck coefficient are observed as expected for sample xnom = 0.35 (ball mill) containing both types of free charge carriers electrons and holes. The results of Tang et al. for xnom = 0.36 agree well with our results, placing the Seebeck coefficients intermediate between that of our sample xnom = 0.35 (ampoule) and xnom = 0.45 (ampoule). The data points of Yang et al. are in the same range as the sample Yb0.35Co4Sb12 (ball mill), but, overall, Yang’s sample exhibits a larger Seebeck coefficient. Hall effect data yield a direct relation between charge carrier densities n and the filling fraction x2a. The results of these measurements are shown in Figure 7. The higher x2a, the higher the charge carrier density in agreement with the Seebeck coefficient data. A linear behavior of n as a function of x2a is observed for all samples except sample xnom = 0.75 (ball mill), where the data point deviates from linear behavior. We attribute

Figure 7. Charge carrier density, n (a), and mobility, μ (b), at room temperature. The black line in panel a is a linear fit of the first four data points. The data points are labeled with the nominal Yb content. G


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the best compromise between high S, relatively low κ, and high σ is seen for sample xnom = 0.25 (ampoule). A decrease of the lattice thermal conductivity is seen for samples synthesized by the ball-milling process. For these samples, structural disorder influences the temperature dependence of the electrical conductivity and the thermal conductivity κ − κe in such a way that both curves lack any pronounced variation with temperature. Moreover, a very low carrier density for some of the ball-milled samples but also nanogranularity induce a strong bipolar contribution to the thermal conductivity, visible as an upturn at temperatures larger than 600 K.

thermal conductivity in general, nanogranularity, which is supposed to induce metallic surface states,57 leads to a stronger bipolar contribution and therefore to the pronounced upturn of the curves at high temperatures. In order to deduce the figure of merit, ZT, the cooling curves of the Seebeck coefficient, the thermal and the electrical conductivities were used. The resulting ZT data are plotted in Figure 8. The largest figure of merit, ZT ∼ 1.0, is observed for

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CONCLUSIONS This work directly compares the thermoelectric properties, the Yb occupancy of the 2a site in the skutterudite structure, and the presence of secondary phases for Yb-filled CoSb 3 synthesized by two different routines, emphazising the importance of combining both microscopic and macroscopic measurements to understand the underlying mechanisms responsible for specific thermoelectric behaviors. In all samples investigated, regardless of preparation method, the Yb content on the 2a site is smaller than the nominal Yb content. In general, for the samples prepared by the melt− quench−anneal or ampoule method, a large fraction of the Yb atoms not soluble in the skutterudite phase form an YbSb2 secondary phase, in addition to a CoSb2 secondary phase. This is in stark contrast to the samples synthesized by the nonequilibrium ball-mill synthesis method for which no or only a very small amount of Yb-containing crystalline secondary phases (e.g., Yb2O3) is formed and for which large weight fractions of secondary phase CoSb2 are observed. From our refinements, we find that an additional partial occupancy of Yb atoms on the 8c site is consistent with our data, which would explain the discrepancy between xnom and x2a for the ball-mill synthesized samples to some extent. Nevertheless, for the ballmilled samples with xnom = 0.15 and 0.35, more than two-thirds of the nominal Yb atoms were not found in the refinement. On the other hand, EDX measurements confirm the presence of Yb in the sample xnom = 0.15 (ball mill), even though no significant amount of Yb was found on the 2a position from the neutron diffraction data. We therefore conclude that the remaining amount of Yb is present in an amorphous phase, possibly as an oxide or antimonide, in these samples. This result motivates studies of annealing strategies and extended X-ray absorption fine structure measurements as part of future experimental investigations. In contrast to the theoretical predictions of a filling limit of 0.2 or 0.3, a sample with an Yb content with x2a = 0.439(7) and 0.548(9) for the case of Yb occupancy on both 2a and 8c sites and the case of Sb vacancies, respectively, was synthesized by using the ball-mill routine similar to the samples produced by Tang et al. using an ampoule technique. However, we identified a large fraction of the secondary phase CoSb2 in this sample, which leads to a reduced thermoelectric figure of merit ZT. The largest ZT of ∼1.0 at 723 K is observed for the sample with nominal xnom = 0.25 (ampoule method) for which the filling fraction is determined as x2a = 0.159(3) and for which a carrier concentration of n = 2.3 × 1020 cm−3 was determined. For larger Yb contents, the thermal conductivity κ − κe is seen to decrease while the electrical conductivity increases. Simultaneously, the Seebeck coefficient decreases, resulting in a decrease of ZT. The samples synthesized by the ball-mill

Figure 8. Figure of merit ZT obtained from the cooling process (a) for the samples synthesized by the ampoule routine and (b) for the samples synthesized by the ball-mill routine. The dashed orange and dotted dark blue lines represent the data of the sample Yb0.36Co4Sb12 taken from Tang et al.31 and Yb0.35Co4Sb12 taken from Yang et al.,33 respectively.

sample xnom = 0.25 (ampoule) with an actual Yb content on the 2a site of x2a = 0.159, and this is observed at the maximum measured temperature of 723 K. The ZT values of Tang et al. (ZT ∼ 1.1 at T = 725 K) and Yang et al. (ZT ∼ 1.2 at T = 725 K) are similar to the value of our xnom = 0.25 (ampoule) sample. All ZT values of the samples synthesized by the ballmill routine are smaller than the values obtained for the samples synthesized by the ampoule routine, except those for sample xnom = 0.55 (ball mill), which shows values comparable with the ones of samples xnom = 0.45 (ampoule). Our ZT values are also smaller than those obtained by Yang et al. for their nominal x = 0.35. We attribute this discrepancy mainly to the large amount of secondary phases present in our samples with large nominal Yb content and speculate that Yang’s x = 0.35 sample may have been more phase-pure, had a higher density than our xnom = 0.55 sample, or both. The temperature evolution of the thermoelectric properties of samples xnom = 0.15, 0.25, and 0.35 prepared by the ampoule routine looks similar. In perfect agreement with the expectation for Yb doping on the rattler position, the curves of the Seebeck coefficient and the lattice thermal conductivity κ − κe shift to smaller values with increasing x2a while the curve of the electrical conductivity shifts to larger values. Therefore, we conclude that, for these samples, the presence of secondary phases has only little influence on the overall thermoelectric properties. As the power factor σS2 is maximal for the sample xnom = 0.25 (ampoule), the optimal doping is found for a 2asite occupancy of x2a = 0.159 with a measured charge carrier density of 2.3 × 1020 cm−3. Theoretical studies predict an Yb content of x2a = 0.25 for a maximal power factor,58 which is slightly larger than our findings. Increasing Yb doping reduces the lattice thermal conductivity but cannot compensate for the negative effect of the doping on the power factor. Therefore, H


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SPP1386. The structure drawing in Figure 2,, was created using the VESTA software.59

method show in general smaller ZT values due to a less optimized charge carrier density. A maximum ZT of 0.75 at 760 K was observed for xnom = 0.55 and a x2a of 0.345(6) and 0.384(7) for the two scenarios assuming an additional partial occupancy of Yb on the 8c site or the presence of Sb vacancies, respectively. Evidence of a decrease of the lattice thermal conductivity due to the presence of an amorphous Yb-rich phase in the samples synthesized by the ball-milling route is found. Finally, the non-equilibrium ball-milling process is seen to induce structural disorder which leads to flattened σ(T) and κ − κe(T) curves. Secondary phases start to strongly influence the thermoelectric properties for weight fractions larger than 8 wt %, resulting in an increase of the lattice thermal conductivity and result in a decrease in ZT. These results show that while FFL larger than 0.2 or 0.3 can be attained, maximizing the Yb filling at the 2a site is not required to obtain the highest ZT values. Moreover, without introducing additional annealing steps, the traditional melt−quench−anneal or ampoule synthesis method is favorable over the non-equilibrium ball-mill method which results in the formation of large amounts of secondary phases for moderate Yb filling detrimental to thermoelectric performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00015. Experimental procedures for neutron powder diffraction measurements and results of Rietveld refinements (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Klaus Habicht: 0000-0002-9915-7221 Present Address #

Excelsus Structural Solutions PARK InnovAARE, 5234 Villigen, Switzerland. Author Contributions

B.R., K.F., and K.H. wrote the manuscript. A.S. synthesized the samples and carried out X-ray diffraction measurements and analysis. K.F., B.R., A.F., P.S.W., and A.H. performed the neutron powder diffraction measurements, and B.R., A.F., M.R., and K.F. analyzed the neutron diffraction data. B.R. performed the SEM/EDX experiments and analysis. A.S. and J.d.B. carried out the thermoelectric characterization and analysis. K.F., K.H., A.S., J.d.B., and E.M. designed the experiments and analyzed the results. All co-authors discussed the results and have edited the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by Oak Ridge National Laboratory. We thank Helmholtz-Zentrum Berlin (HZB) for the allocation of neutron beamtime. We acknowledge access to the experimental facilities of the electron microscopy group at HZB and the generous support by Daniel Abou-Ras. A.S., J.d.B., and E.M. gratefully acknowledge the support by the DFG Schwerpunktprogramm I


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