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High Temperature Thermoelectric Properties of the Solid-Solution Zintl Phase Eu11Cd6−xZnxSb12

Nasrin Kazem,† Antonio Hurtado,† Fan Sui,† Saneyuki Ohno,‡ Alexandra Zevalkink,‡ Jeffrey G. Snyder,‡ and Susan M. Kauzlarich*,† †

Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616, United States Materials Science, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States

‡

S Supporting Information *

ABSTRACT: Solid-solution Zintl compounds with the formula Eu11Cd6−xZnxSb12 have been synthesized from the elements as single crystals using a tin flux according to the stoichiometry Eu:Cd:Zn:Sb:Sn of 11:6−xp:xp:12:30 with xp = 0, 1, 2, 3, 4, 5, and 6, where xp is the preparative amount of Zn employed in the reaction. The crystal structures and the compositions were established by single-crystal as well as powder Xray diffraction and wavelength-dispersive X-ray analysis measurements. The title solidsolution Zintl compounds crystallize isostructurally in the centrosymmetric monoclinic space group C 2/m (No. 12, Z = 2) as the Sr11Cd6Sb12 structure type (Pearson symbol mC58). There is a miscibility gap at 3 ≤ xp ≤ 4 where the major product crystallizes in a disordered structure related to the Ca9Mn4Bi9 structure type; otherwise, for all other compositions, the Sr11Cd6Sb12 structure is the majority phase. Eu11Cd6Sb12 shows lower lattice thermal conductivity relative to Eu11Zn6Sb12 consistent with its higher mean atomic weight, and as anticipated, the solid-solution samples of Eu11Cd6−xZnxSb12 have effectively reduced lattice thermal conductivities relative to the end member compounds. Eu11.0(1)Cd4.5(2)Zn1.5(2)Sb12.0(1) exhibits the highest zT value of >0.5 at around 800 K which is twice as large as the end member compounds.

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interest in thermoelectric studies.2,6,7,9−14 Our previous study on the Eu11Cd6Sb12 Zintl compound showed exceptionally low lattice thermal conductivities from room temperature to 775 K, 0.78−0.49 W/(m·K), and high p-type Seebeck coefficient, from +118 to 153 μV/K, comparable to the state of the art thermoelectrics.4 However, high electrical resistivity of 6.8− 12.8 mΩ·cm in this compound resulted in deterioration of the zT value. The electrical resistivity (ρ) is inversely dependent on both charge carrier concentration, n, and the carrier mobility, μ, through the following equation: ρ = (neμ)−1. To reduce the electrical resistivity in this compound to improve thermoelectric efficiency, increasing and adjusting of either the charge carrier concentration or mobility are two possible strategies. In this work, we attempt to gain a higher power factor while maintaining the low lattice thermal conductivity by increasing the charge carrier mobility through substitution of Zn in Cd sites. Zn2+ substitution in larger Cd2+ sites should increase the constituent’s orbital overlaps, particularly A1-Pn4 which has the key role in transport properties of the A11TM6Pn12 family of compounds, resulting in the more covalent character of the A1−Pn4 bonds in the Zn analogue compared to that of the Cd analogue. As a result, higher mobility is expected, based on the assumption that in Zintl phases the lightest carriers (the lightest

INTRODUCTION Thermoelectric technology as a clean, scalable, and reliable approach to generate electricity from waste heat sources has inspired worldwide interest in searching for more efficient materials. A material needs to provide a combination of three antagonistic physical properties: large Seebeck coefficient, α, low electrical resistivity, ρ, and low thermal conductivity, κ, to maximize the zT equation, zT = α2T/κρ. This challenge mainly has its roots in the opposing relationship of α and ρ with respect to the charge carrier concentration, n, as α ∝ n−2/3 and ρ ∝ n−1. As a result, most good thermoelectric materials are heavily doped semiconductors with carrier concentrations between 1019 and 1021 carriers/cm3.1 Moreover, the electronic contribution to total thermal conductivity, κe, from the equation κtotal = κe + κlattice, is inversely proportional to ρ based on the Wiedemann−Franz law, κe = LT/ρ, where T is the absolute temperature and L is the Lorenz number. As high zT requires both low electrical resistivity and low thermal conductivity, the Wiedemann−Franz law shows the fundamental difficulty in achieving both goals simultaneously. Typically, improvement in zT is achieved through the reduction of lattice thermal conductivity, the only physical property component independent of the electronic structure of the materials. The complexity of the structure in Zintl phases2,3 characteristically results in low lattice thermal conductivities.4−8 In addition to their glass-like lattice thermal conductivity, their diverse chemistry along with flexibility in structure modification has aroused significant © XXXX American Chemical Society

Received: April 8, 2015 Revised: May 14, 2015

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DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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wavelength-dispersive spectrometers using the standards of EuPO3, Cd (metal), Sb (metal), Zn (metal), and Sn (metal) for quantitative analysis of Eu, Cd, Sb, Zn, and Sn, respectively. The total weight percentage of each analyzed point is always slightly low (∼97%) for all samples (both single crystals and pellets). The low total can be attributed to possible mismatch between Eu standard and these samples due to the discrepancy of the chemical environment of Eu atoms in the analyzed samples and standard. If the Eu amount is adjusted so that the totals are 100%, the stoichiometries are consistent with 11-6-12 composition. Compositions are calculated by averaging (5−10 data points) adjusted atomic percentages for each crystal and pellet. Synchrotron Powder X-ray Diffraction. High resolution synchrotron powder diffraction data for Eu11Cd6−xZnxSb12 are collected at 100 K using beamline 11-BM at the Advanced Photon Source (APS),16 Argonne National Laboratory, using an average wavelength of 0.413723 Å produced by a bending magnet (BM) with 30 keV energy. Si(111) double crystals are used as the monochromator, and a sagittally bent Si(111) crystal focused the beam to the dimensions of 1.5 mm (horizontal) × 0.5 mm (vertical) on the sample. Twelve discrete detectors covering an angular range from −6° to 16° 2θ are scanned over a 34° 2θ range, with data points collected every 0.001° at a scan speed of 0.01°/s.17 All samples are diluted by mixing with pulverized fused silica with the mass ratio 1:1 amorphous SiO2 sample. The amorphous SiO2 dilutant is necessary to overcome the strong absorption issues due to the presence of high-Z elements in these solid solutions. The diluted samples are sealed in quartz capillaries of 0.3 mm in diameter to minimize oxidation. A broad peak at a low 2θ angle comes from the amorphous SiO2. The program JANA was used to perform profile matching with constant scale factor by employing pseudo-Voigt axial divergence asymmetry peak shape for the calculated patterns. Sample Preparation for Transport Properties. Crystals from the flux reactions are pulverized into fine powder. The powders are consolidated into dense disc shape pellets by hot press technique using 12 mm inner diameter hardened steel die sets. To get dense samples, polycrystalline powder samples are first cold-pressed at 415 MPa in hardened steel die sets; then the pressure is released to 206 MPa and immediately hot-pressed at this pressure for 30 min under an argon atmosphere at 500 °C to make 12 mm diameter pellets with approximately 1.5 mm thickness. Graphite foil is used between the plunger and the sample to prevent the stickiness of plunger and sample. The geometric densities for all samples are more than 85% of the theoretical densities from crystallographic data. The geometric densities (determined by weight and sample dimensions) are used for thermal conductivity calculations. Transport Properties Measurements. High temperature resistivity (ρ) data are measured to 773 K by using the van der Pauw technique. Hall effect measurements are taken in a 2 T magnetic field using pressed niobium contacts.18 Seebeck data are obtained using chromel−Nb thermocouples and by oscillating the temperature gradient ±10 K. For measurements at each temperature, the temperature gradient is oscillated about a fixed average temperature; the resulting voltage response to the temperature gradient reveals a linear relationship with the slope yielding the Seebeck coefficient (ΔV = α ΔT).19 Thermal diffusivity data are collected using a Netzsch LFA 457 instrument. Thermal conductivities are calculated from the equation, κtotal = CpdD, in which Cp is the Dulong−Petit heat capacity (Cp = 3RN/M; R is the gas constant; N is number of atoms per formula unit; M is the molar mass, d is the geometric density; and D is the measured thermal diffusivity from flash diffusivity measurements given in the Supporting Information.

band mass and as a result highest electronic mobility) are typically found in the direction of the covalent network.

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EXPERIMENTAL SECTION

Synthesis. To attain enhanced diffusion of reactants, zinc-doped Eu11Cd6Sb12 compounds are prepared by tin flux reaction similarly to the literature procedure7 for arsenic substitution in antimony sites; details of flux-growth synthetic procedures can be found elsewhere.15 All operations are carried out in argon- or nitrogen-filled gloveboxes or under vacuum. For all syntheses, 5 cm3 alumina crucibles are loaded by Eu:Cd:Zn:Sb:Sn in molar ratios of 11:6−xp:xp:12:30 (xp = 0, 1, 2, 3, 4, 5, and 6, where xp indicates the preparative x, to a total weight of ∼10 g; (Eu, Ames Lab, 99.999%; Cd pieces, Alfa, 99.98%; Zn shot, Alpha Aesar, 99.9%; and Sn shot, Alpha Aesar, 99.99%). The crucibles are placed into fused silica tubes with SiO2 wool placed on top, and the quartz tubes are sealed under pressure of less than 200 mTorr. The sealed quartz tube is placed in a box furnace and heated at 50 °C/h to 500 °C, allowed to dwell 6 h, and then heated at 100 °C/h to 950 °C. Subsequently, the reaction containers are slowly cooled at 5 °C/h to 650 °C to grow large crystals of the solid-solution products. Sn-flux is removed by inverting and placing the quartz tube into a centrifuge, and spinning for 2−3 min at 6500 rpm. Finally, the reaction containers are opened in a N2-filled glovebox equipped with an optical microscope and at moisture levels below 1 ppm. Reflective needle-like crystals of Eu11Cd6−xZnxSb12 are obtained as the product. Single-Crystal X-ray Diffraction. Unit cell determination is performed for more than 10 crystals of different shapes of each reaction to determine the phase width and purity of each reaction product. The structure determination is performed for only one of the crystals belonging to Eu11Cd6−xZnxSb12 solid solution for each reaction. To minimize the oxidation, single crystals of Eu11Cd6−xZnxSb12 are selected in Paratone N oil under a microscope for data collection. Glass fibers or MiTeGen microloops are used to mount the selected crystals on the goniometer under the nitrogen stream. Diffraction data for Eu11Cd6−xZnxSb12 are collected at 90 K on a Bruker Apex II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) with CCD area detector. Several sets of ωscans (0.3°/frame) at different Φ settings are collected while in a nitrogen stream. Determinations of unit cell parameters, refinements, and raw frame data integrations are completed using the APEX II v2011.4-1 software. The space group is determined on the basis of systematic absences using XPREP, and the structure is solved using direct methods from the SHELXTL Version 6.14 package. The mixed occupancies involving Zn and Cd are refined by examining three different crystallographic sites of the transitional metal in Eu11Cd6Sb12 until a reasonable model with optimized R-factors, U-values, and peak/ hole values is finalized. Occupancies of the shared Zn/Cd sites are fixed to fully fill each crystallographic site, and they are assigned the same coordinates and atomic displacement parameters. CIFs are provided in the Supporting Information. The occupancy for all of the Sb sites is examined individually for any possible contribution from Sn which shows no evidence of Sn inclusion in the structure. For example for xp = 1, site occupancy factors (SOFs) for Sb1, Sb2, Sb3, Sb4, Sb5, and Sb6 are 1.00428, 0.99918, 0.99785, 1.00003, 0.99681, and 1.00052, respectively. The mixed occupancies involving Sn and Sb makes the refinement unstable for both restrained and unrestrained coordinate and atomic displacement parameters refinements. The refinement of the occupancies for all of the Sb sites as Sn resulted in SOF > 1 implying that a heavier element is required to fill those crystallographic sites. For example, for x = 1, the SOF for Sb1, Sb2, Sb3, Sb4, Sb5, and Sb6 as Sn resulted in 1.02539, 1.02088, 1.01967, 1.02144, 1.01825, and 1.02192, respectively. The multiplication of 1.02 (the average of SOF for all of the Sb sites) by 50 (atomic number of Sn) results in the value of 51, the atomic number of Sb. So, the singlecrystal X-ray diffraction shows no indication of Sn inclusion in the structure. Electron Microprobe Analysis. Single-crystal and pellet samples are mounted in epoxy and polished for analysis. The samples are placed in a Cameca SX-100 electron microprobe equipped with five

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RESULTS AND DISCUSSION Crystal Structures and Compositions. Single-crystal and powder X-ray diffraction (PXRD) along with electron microprobe analysis (EMPA) were used to investigate the products of each reaction to synthesize Eu11Cd6−xZnxSb12 (xp = 0, 1, 2, 3, 4, 5, 6) solid solutions, where xp is the preparative amount of Zn B

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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Table 1. Selected Single-Crystal Data Collection and Refinement Parameters for Eu11Cd6−xZnxSb12 (xp = 1, 2, and 5) Solid Solutions empirical formula fw temp, K wavelength, Å cryst syst space group unit cell dimens a, Å b, Å c, Å vol, Å3 Z density (calcd), g·cm−3 absorption coeff, mm−1 cryst size, mm3 θ range for data collecn, deg index ranges independent reflcns data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R1 wR2 R indices (all data) R1 wR2 extinction coeff largest diff. peak and hole, e·Å−3

Eu11Cd5.06Zn0.94Sb12

Eu11Cd4.74Zn1.25 Sb12

Eu11Cd1.65 Zn4.35Sb12

3762.52 89(2) Mo Kα, 0.71073 monoclinic C2/m

3747.94 89(2) Mo Kα, 0.71073 monoclinic C2/m

3602.61 89(2) Mo Kα, 0.71073 monoclinic C2/m

32.4248(15) 4.7079(2) 12.4035(6) 1780.18(14) 2 7.019 31.521 0.02 × 0.03 × 0.04 2.53−31.00 −46 ≤ h ≤ 46, −6 ≤ k ≤ 6, −17 ≤ l ≤ 17 3147 [R(int) = 0.0217] 3147/0/93 1.243

32.3927(14) 4.7025(2) 12.3961(6) 1775.61(14) 2 7.010 31.626 0.02 × 0.04 × 0.08 2.54−31.65 −47 ≤ h ≤ 47, −6 ≤ k ≤ 6, −18 ≤ l ≤ 18 3306 [R(int) = 0.0235] 3306/0/93 1.16

32.116(2) 4.6371(3) 12.3564(8) 1731.15(19) 2 6.911 32.680 0.02 × 0.03 × 0.05 2.55−33.12 −48 ≤ h ≤ 48, −7 ≤ k ≤ 7, −18 ≤ l ≤ 18 3581 [R(int) = 0.0872] 3581/0/93 0.991

0.0176 0.0361

0.0189 0.0361

0.0329 0.0571

0.0186 0.0364 0.000383(9) 1.569 and −1.604

0.0208 0.0367 0.000250(8) 1.423 and −1.723

0.0569 0.0631 0.000189(11) 2.840 and −3.057

Table 2. Compositions Obtained from Elemental Analysis by EMPA on Single Crystals and Pressed Pellets of Eu11Cd6−xZnxSb12 comp from EMPA on pellets xp

comp from EMPA on single crystals

major phase 11-6-2

1.0 2.0 5.0 6.0

Eu10.98(7)Cd5.1(1)Zn0.8(1)Sb12.06(5) Eu10.99(5)Cd4.4(2)Zn1.5(2)Sb12.04(4) Eu11.03(5)Cd1.65(6)Zn4.25(9)Sb12.07(4) Eu10.98(6)Zn6.06(8)Sb11.96(4)

Eu11.02(6)Cd5.0(1)Zn1.0(1)Sb12.02(7) Eu11.0(1)Cd4.5(2)Zn1.5(2)Sb12.0(1) Eu11.01(5)Cd1.5(1)Zn4.4(1)Sb12.05(5) Eu11.04(7)Zn5.9(1)Sb12.09(5)

minor phase related related related related

to to to to

9-4-9 9-4-9 9-4-9 9-4-9

composition composition composition composition

space group C 2/m (No. 12) with 15 crystallography unique atomic positions in the asymmetric unit comprised of 6 Eu, 6 Sb, and 3 transition metal (TM) sites; see Figure 1a. All of the atoms reside on the special positions of the 4i Wyckoff position (m, mirror plane symmetry element) except Eu6 that resides on the 2a Wyckoff position (2/m symmetry element). The covalent framework sublattice in Eu11Cd6−xZnxSb12 structures can be described as the one-dimensional (1D) double pentagon canals formed by the corner sharing of the TM2 and TM3 tetrahedra to make a cavity in which further strong Sb5−Sb5 interactions (d(Sb−Sb) ≈ 2.8 Å) divides the moiety into two pentagons; see Figure 1b. The [TM1Sb4] unit is linked to the double pentagon by sharing the corners of the TM1 and TM2 tetrahedra. These [TM1Sb4] tetrahedral units are themselves corner shared in the b-direction. The Eu2+ cations provide the electrons needed for the covalent bonds in the polyanionic network to satisfy the octet rule. The structure can be described based on the Zintl formalism as the valence precise compound of (Eu2+)11[(4b-Cd2−)6(1b-Sb2−)2(2b-Sb1−)6(3b-Sb0)4] by assigning three-bonded Sb atoms (3b-Sb), two-bonded Sb atoms (2b-Sb), and one-bonded Sb atoms (1b-Sb) as Sb0, Sb1−, and

employed in the reaction. Although samples of end members of Eu11Cd6−xZnxSb12 (xp = x = 0 and 6, where xp = preparative stoichiometry) both possess mainly the Sr11Cd6Sb12 structure type, single-crystal and powder X-ray diffraction studies show that the solid solutions of Eu11Cd6−xZnxSb12 with 3 ≤ xp ≤ 4 prefer a highly disordered phase related to the Ca9Mn4+xSb9 structure type that was published recently by Liu et al.20 So, because the Sr11Cd6Sb12 structure type is the focus of this work, only Eu11Cd6−xZnxSb12 (xp = 0, 1, 2, 5, and 6) samples that keep the Sr11Cd6Sb12 structure type are considered. All of the crystal structure refinements resulted in satisfactory R1 and wR2 values (R1 < 3.3% and wR2 < 5.8%), see Table 1; CIFs can be found in Supporting Information. There was no indication for Sn substitution from the crystal structure refinements, in good agreement with EMPA results tabulated in Table 2. Compositions of crystals from single-crystal refinement for xp = 1, 2, and 5 results in composition in close agreement with x = 1, 1.25, and 4.35 in the formula for Eu11Cd6−xZnxSb12 solid solutions. Eu11Cd6−xZnxSb12 (xcryst = 0, 1, 1.25, 4.35, and 6, where x cryst = crystallographic stoichiometry) solid solutions crystallize with the monoclinic C

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. (a) View of the crystal structure of Eu11Cd6Sb12 down the b-axis (along the 2-fold axis and perpendicular to the mirror plane). The covalent network formed by corner sharing of the [CdSb4] tetrahedral units and Sb−Sb bond that divides the cavity formed by the tetrahedral assembly into double pentagons are highlighted (sky blue spheres = Cd; orange spheres = Sb; purple spheres = Eu). (b) View of the double pentagonal moiety that makes tubes by connecting CdSb4 tetrahedral units along the crystallographic b-axis. Six crystallographic Sb sites and three crystallographic Cd sites are labeled; Eu atoms are removed for clarity. (c) Lattice parameters for Eu11Cd6−xZnxSb12 obtained from single-crystal Xray diffraction plotted as a function of crystallographic values x, xcryst = 0.00, 0.98, 1.00, 1.25, 4.35, and 6.00. a-axis, b-axis, and c-axis are presented in the top, middle, and bottom plots, respectively; lattice parameters for x = 0 and 6 are from the literature.22

Table 3. Comparison of Selected Bond Distances and the Zn:Cd Ratio at the Transitional Metal Sites in Single Crystals of Eu11Cd6−xZnxSb12 Solid Solutionsa Sb5−Sb5 Eu1−Sb4 TM3-Sb5 Zn:Cd at TM1 Zn:Cd at TM2 Zn:Cd at TM3 a

x=0

x = 0.94

x = 1.25

x = 4.35

x=6

2.8121(12) 3.3588(5) 3.1861(9) 0 0 0

2.8100(7) 3.3447(3) 3.1919(6) 0.15:0.85 0.19:0.81 0.13:0.87

2.8081(7) 3.3396(3) 3.1921(6) 0.20:0.80 0.25:0.75 0.17:0.83

2.8018(13) 3.2996(6) 3.2074(12) 0.79:0.21 0.79:0.21 0.58:0.42

2.795(2) 3.279(1) 3.249(2) 1 1 1

TM = transition metal.

Sb2−, respectively. The lattice parameters decrease with increasing Zn content x (determined from single-crystal X-ray diffraction), a result which is in agreement with the covalent radii of Cd (1.40 Å) and Zn (1.20 Å); see Figure 1c.21 The double pentagonal moiety highlighted in Figure 1b indicates the three and five crystallographic positions of Cd and Sb atoms, respectively. Refined site occupancies obtained from single-crystal diffraction data show that Zn substitution on the Cd sites results in similar occupation factors for Cd1 and Cd2 sites. However, the Cd3 site systematically shows lower occupancy factors by Zn occupation, and the ratios of Zn3/ Zn1(2) in all solid solutions are around 0.7. Selective crystallographic data comparing relevant structural properties in the solid-solution Eu11Cd6−xZnxSb12 (xp = 1, 2, and 5) are listed in Table 3. In Eu11Cd6Sb12 there is a long interaction between TM3 and Sb5 which represents a weakly bonding interaction, ∼3.1861(9) Å. This contact is elongated by substituting Zn in Cd sites (∼3.249(2) Å in Eu11Zn6Sb12) and cannot be assigned as a usual two-center two-electron bond. The elongation trend is counterintuitive since a reverse trend is expected as Zn is smaller than Cd. However, at the same time, the Sb5−Sb5 interaction becomes stronger as evidenced by shorter bond lengths in the more Zn containing phases; see Table 3. These results are consistent with the

hypothesis that the stronger Sb5−Sb5 interaction may require fewer electrons as a result of the possible multiple bonding interactions.23 The preference of Cd over Zn at the TM3 site suggests that the larger transition metal can provide better orbital overlap with Sb5 and, as a result, better stabilize the structure. Crystals from the Sn flux reactions by using elemental reagents were powdered and followed by hot-pressing to make pellet samples (resulting in densities in excess of 85%) for transport properties measurements. Synchrotron powder XRD patterns of each powdered product were used to evaluate the quality of the sample. Representative patterns of the samples of Eu11Cd6−xZnxSb12 are shown in Supporting Information (SI, Figure S1). The crystal structures solved for crystals from each reaction were used for the Le Bail method whole profile matching within JANA 2006. The excellent profile matching of most of the peaks with the parent structure Eu11Cd6Sb12 is indicative of good quality samples for transport properties measurements. The average compositions with standard deviations in parentheses from EMPA on single crystals and pressed pellets are provided in Table 2 along with the compositions from single-crystal X-ray diffraction. Their corresponding Zn Lα X-ray maps by EMPA techniques on both single crystals and pressed pellets are provided in SI, D

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Figure S2. There is good agreement between the compositions obtained from single-crystal X-ray diffraction and from EMPA. The formula obtained from EMPA for both single crystals and pressed pellet is also consistent. The experimental EMPA compositions x ∼ 1, 1.5, 4.4, and 6 will be employed for the discussion on the transport properties. The observed minor phase with the generalized stoichiometry of Eu9TM4+xSb9 (TM = transition metal) by EMPA in the pressed pellets have contributions less than ∼10% based on the powder patterns obtained from synchrotron powder XRD, see SI, Figure S1. Electronic Transport Properties. Eu11Cd6Sb12 behaves as a p-type degenerate semiconducting material with a possible band gap (Eg) of ∼0.1 eV as determined from the maximum Seebeck value of ∼150 μV/K at about 550 K, Eg = 2αmaxTmax.7,22 The relatively high Seebeck coefficient coincided with high electrical resistivity of ∼12 mΩ·cm, leading to low thermoelectric efficiencies with zT of ∼0.2 at 800 K. Charge carrier concentration and mobility are two possible variables that can be modified to decrease the resistivity of Eu11Cd6Sb12. Here, Zn2+ is substituted for Cd2+ to reduce the electrical resistivity as it was recently shown to be an effective strategy for YbCd2Sb2.11 In the case of YbCd2Sb2, Zn2+ substitution for Cd2+ led to increases in both mobility and positive charge carrier concentration, a desired occurrence leading to lower resistivity. Figure 2a shows the temperature-dependent electrical resistivity (ρ) of Eu11Cd6−xZnxSb12 compounds from room temperature to ∼800 K. Resistivity for all samples increases approximately linearly with increasing temperature up to a maximum temperature consistent with degenerate semiconducting electrical conductivity properties (metallic behavior with transport dominated by extrinsic carriers). This maximum temperature shifts to the higher temperatures by having Zn in the substructure. Above this maximum temperature, the resistivity decreases with increasing temperature for all samples, as minority carrier activation leads to a transition from extrinsic semiconducting behavior to intrinsic, two-carrier-type behavior. Eu11Zn6Sb12 shows a large rise in electrical resistivity at ∼500 K that is consistent with the melting point of Sn (TM(Sn) ≈ 500 K), indicating the presence of residual tin from the flux synthesis of this sample. The transition from extrinsic semiconducting behavior to intrinsic in all samples coincides with the melting point of the residual tin flux and contributes to the change in the electrical resistivity. EMPA results showed the presence of the residual Sn flux in all samples in differing amounts. Because of the presence of Sn in the samples, it is not possible to confidently assign the maximum of the electrical resistivity and Seebeck curves (discussed later) as the point of extrinsic-to-intrinsic transition, two-carrier-type behavior. To deconvolute the effects of these two factors, an alternative nonflux method for synthesis is required to exclude Sn melting; however, these measurements are important for gaining some insight into the promise of this new system. Parts b and c of Figure 2 show the measured Hall carrier concentration and mobility as a function of temperature, respectively. The positive Hall coefficients of all of the samples reveal that holes are the dominant carriers at room temperature (nH = 1/eRH) consistent with the positive Seebeck coefficients observed, discussed later. All samples show constant charge carrier concentrations up to ∼600 K which can be considered as the extrinsic regime; then a rise in charge carriers is observed due to the thermally excited carriers across the band gap, defining the bipolar regime, where the hole carrier concentration is

Figure 2. (a) Temperature dependence of the electrical resistivity for Eu 1 1 Cd 6 − x Zn x Sb 1 2 . (b) Hall carrier concentration for Eu11Cd6−xZnxSb12 illustrating the transition from extrinsic regime to intrinsic regime around 550 K (where, for example, 2E+20 represents 2 × 1020). (c) Hall mobility, indicative of acoustic phonon scattering.

overestimated by single-carrier-type equation. Through isovalent substitution of Cd2+ sites by Zn2+, no large change in carrier concentration is expected,7 and as a result end members hold similar charge carrier concentration. However, Hall measurements show that all of the solid solutions have higher carrier concentration compared to the end members. This disparity might be attributed to the size mismatch of Zn2+ and Cd2+ (Cd and Zn covalent radii of 1.40 and 1.20 Å, respectively)21 that alters the equilibrium defect concentration as observed in AZn2Sb2.24,25 The compound of x ∼ 1 shows the highest charge carrier concentration while end members show the same low charge carrier concentration consistent with the same valence states of Zn2+ and Cd2+. Despite roughly similar carrier concentrations across the Eu11Cd6−xZnxSb12, the Hall mobility varies significantly within Eu11Cd6−xZnxSb12 compounds, with the x = 1.5 and 6 compounds possessing the highest while the Cd analogue (x = 0) has the lowest; see Figure 2c. The mobility in x = 1.5 and 6 compounds (μ ≈ 55 cm2/(V·s)) is more than two times higher than in x = 0 compound (μ ≈ 25 cm2/(V·s)) at room temperature. The mobility in all samples decreases with a T−v dependence with increasing temperature where v ranges from E

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials ∼1 to 2. In most bulk thermoelectric materials, above room temperature, v of ∼1 and 1.5 are expected for degenerate and nondegenerate semiconductors, respectively, when mobility is limited only by acoustic phonon scattering. So, in all samples, the temperature dependence of μ is indicative of the dominant acoustic phonon scattering phenomena. When acoustic phonon scattering controls the mobility, mobility is inversely related to (mDOS * )5/2; mDOS * is the density of states effective masses. So, * becomes smaller by based on the Hall mobilities, mDOS incorporating more Zn into the Eu11Cd6Sb12 structure. The values of m*DOS can be calculated by using the Seebeck coefficients and the Hall charge carrier concentrations that will be discussed in the following part. The higher mobility observed in Zn substituted compounds of Eu11Cd6−xZnxSb12 is similar to the trend observed for YbCd2−xZnxSb2 solid solutions.11 It is traditionally believed that the charge transport occurs within the covalent network in Zintl compounds.8,25 Moreover, previous studies on Eu11Cd6Sb12−xAsx showed that As substitution at the Sb4 site creates a definite band gap whereas As substitution at the other five antimony sites leaves a pseudogap.7 This result is consistent with other calculations done on Sr11Cd6Sb12 and Ba11Cd6Sb12 by Xia et al. showing that the bottom of the conduction bands and the top of the valence bands originate predominantly from dz2 orbitals of Sr1/ Ba1 and py orbitals of Sb4.23 By substituting Zn at the Cd sites, the unit cell parameters become smaller because of the smaller atomic radii of Zn than Cd. The crystallographic b-direction is only dependent on the distance between the double pentagons. Since Eu1 is located between the two successive double pentagons, where it overlapping with the Sb1 atom, the value of the b-axis determines how closely the Eu1 and Sb4 can reside with respect to each other. Thus, the smaller the distance of the double pentagons (or b parameter), a higher orbital overlap of Eu1−Sb4 is expected which is also reflected in the smaller Eu1−Sb4 bond length (Table 3) and, as a result, causes higher mobility. In Eu11Cd6Sb12−xAsx solid solutions, although the double pentagon distances become smaller, the significantly smaller As (compared to Sb here) cannot provide effective orbital overlap, and as a result these samples have lower mobility. Moreover, smaller b parameters can be achieved by Zn substitution at Cd sites, b = 4.6371(3) Å for Eu11Zn6Sb12 and b = 4.6788(2) Å for Eu11Cd6Sb9.59As2.41 (higher arsenic contents could not be produced as the major phase). So, double pentagon distances are more noticeably reduced by adding Zn compared to As substitution at Sb sites. This observation suggests that as long as the Eu1−Sb4 orbital overlap is maintained, high mobility for charge carriers can be guaranteed. By using smaller TM atoms, this overlap can be assured; by selecting the right atom types in the TM site, the charge carrier concentration can be finely tuned to increase the power factor. These postulations should be assessed by performing detailed theoretical calculations. Figure 3 shows the Seebeck coefficient of Eu11Cd6−xZnxSb12 as a function of temperature over the range of 300−750 K. The Seebeck coefficients of all samples are positive in the measured temperature range suggestive of holes as the dominant carrier concentrations. For x = 0, 1.5, and 6, the Seebeck coefficient increases with temperature reaching a maximum and then slowly decreasing suggestive of a transition from an extrinsic regime to intrinsic regime. The magnitude of the maximum Seebeck coefficient and its corresponding temperature (αmax and Tmax) can be used to roughly estimate the band gap using Eg = 2eαmaxTmax.26 This estimate suggests Eg = 0.2−0.3 eV. For

Figure 3. (a) Temperature dependence of Seebeck coefficient for Eu11Cd6−xZnxSb12 compounds. (b) Total density-of-state effective masses of the charge carriers from the parabolic-band model at 330 K.

x = 1 and 4.4, the Seebeck coefficient increases linearly with increasing temperature all over 300−750 K showing a metallic behavior. No definite trend is diagnosed in the magnitude of Seebeck coefficients, and x values as x = 1 and 1.5 show the lowest and highest values, respectively. The chemical potentials from the experimental Seebeck coefficients at 330 K are calculated within a single parabolic band (SPB) model. By using the Hall carrier concentration, the density of state effective masses (m*DOS) of the charge carriers can be calculated.27 The graph of the calculated mDOS * at ∼330 K is provided in Figure 3. This calculation shows that all of the compounds hold effective masses less than the free electron mass (m*/me = 1 is shown in black dotted line) and the ones containing Zn hold smaller values. Cd and Zn end members show the heaviest and lightest mDOS * values of 0.71 and 0.50, respectively, which is consistent with their lowest and highest Hall mobilities discussed earlier. Thermal Transport Properties. The total thermal conductivities (κtotal) of Eu11Cd6−xZnxSb12 compounds shown in Figure 4a were calculated from the measured thermal diffusivity (D) using k = DdCp, where d is geometric density and Cp is Dulong−Petit heat capacity (Cp values ranging 0.187−0.200 J·g1−·K−1). All of the samples show low thermal conductivities ranging from ∼1.2 to 0.8 W·m−1·K−1 at room temperature to ∼0.9−0.6 W·m−1·K−1 at 800 K. The total thermal conductivities for Eu11Cd6−xZnxSb12 samples indicate that thermal conductivity values of the solid solutions tend to be between the end members where Cd and the Zn analogues bear the lowest and highest values, respectively. The electronic contribution to κtotal can be estimated from κe = LT/ρ, where T is temperature, ρ is electrical resistivity, and L is the Lorenz number. Lorenz numbers are estimated from the L = 1.5 + exp[−|S|/116] equation26 rather than given as a typical constant value to improve the estimate of lattice thermal conductivity. These L values range from 1.9 × 108 to 2.1 × 108 W·Ω·K−2 (inset of Figure 4b) which is between the L = 1.49 × F

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

K) is suggestive of contribution from the bipolar involvement for temperatures above 500 K. κlattice decreases simply by alloying within the substructures;28 see Figure 4c. The environment of the atomic size and mass fluctuations throughout the crystal lattice provide phonon scattering that generally results in low thermal conductivity. Consequently the mixed compounds exhibit low κlattice of around 0.9 to 0.6 W· m−1·K−1 at around room temperature. Consistent with the Keyes’ expression29 that shows that phonon−phonon scattering is related to A−7/6 where A is the mean atomic weight, the Cd analogue shows lower lattice thermal conductivity compared to the Zn analogue, κlattice ∼ 0.76 W·m−1·K−1 at room temperature. The larger the mean atomic weight (A) in the structure typically results in the weaker bonding that decreases the speed of sound and consequently low lattice thermal conductivities.30 κtotal − κe decreases with temperature approximately as 1/T, characteristic of lattice thermal conductivity limited by Umklapp phonon− phonon scattering. Figure of Merit. The figures of merit for Eu11Cd6−xZnxSb12 compounds calculated from the polynomial fits to the thermal conductivity, Seebeck coefficient, and electrical resistivity data, are shown in Figure 5. For x = 1, the drop in Seebeck

Figure 5. Color coded curves of figure of merit for Eu11Cd6−xZnxSb12 solid solutions. Figure 4. (a) Total thermal conductivity, κtotal (color coded markers) and (b) lattice thermal conductivity and bipolar contributions, κlattice + κB + κtotal + κe (color coded markers), in samples of Eu11Cd6−xZnxSb12. The slight bipolar contribution can be seen at temperatures higher than 500 K where κtotal − κe begins to increase instead of continuing its reduction by raising temperature. L values were approximately calculated from the L = 1.5 + exp[−|S|/116] equation and inserted in the graph.26 (c) Room temperature κlattice values as a function of xp from elemental analysis by EMPA. The dashed line is a polynomial fit.

coefficient due to high extrinsic charge carrier concentration from the possible defects in the structure degrades the zT values. Both end members yield similar zT values up to ∼600 K. However, the Zn analogue yields higher zT at higher temperatures (zT(Zn) ∼ 0.3 compared with zT(Cd) ∼ 0.2 at 800 K) due to its higher Seebeck coefficient. The maximum Seebeck coefficient in Eu11Zn6Sb12 shifts to higher temperature with respect to Eu11Cd6Sb12 causing zT value growing at temperatures > 800 K while Eu11Cd6Sb12 reaches to its maximum zT of 0.22 at 700 K. Over the 300−800 K temperature range, the effect of the increased mobility and Seebeck coefficient of x = 1.5 leads to an apparent improvement in zT value compared to the end member compounds. Note that the zT curve of x = 1.5 still is rising at 800 K and does not reach to its maximum in the 300−800 K temperature range since the drop in resistivity outweighs the drop in Seebeck coefficient at high temperatures (>600 K).

108 and 2.45 × 108 W·Ω·K−2 values for nondegenerate semiconductors and free electron model, respectively. These are appropriate L values since Hall measurements indicate that Eu11Cd6−xZnxSb12 compounds are heavily doped semiconductors (metallic behavior). Subtraction of the electronic contribution from the total thermal conductivity, κtotal − κe, leaves the collective effect of the lattice (κlattice) and bipolar (κB) contributions to the thermal conductivities, since κtotal = κe + κB + κlattice, Figure 4b. At room temperature, the bipolar contribution is expected to be insignificant; κtotal − κe at room temperatures can be considered as the intrinsic κlattice. Note that the rise in the κtotal − κe at high temperatures (>500

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SUMMARY The effects of metal-site substitution on transport properties of Eu11Cd6Sb12 by isoelectronic substitution of Zn at Cd sites are G

DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX

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(7) Kazem, N.; Xie, W.; Ohno, S.; Zevalkink, A.; Miller, G. J.; Snyder, G. J.; Kauzlarich, S. M. High-Temperature Thermoelectric Properties of the Solid−Solution Zintl Phase Eu11Cd6Sb12−xAsx (x < 3). Chem. Mater. 2014, 26 (3), 1393−1403. (8) Toberer, E. S.; Zevalkink, A.; Crisosto, N.; Snyder, G. J. The Zintl Compound Ca5Al2Sb6 for Low-Cost Thermoelectric Power Generation. Adv. Funct. Mater. 2010, 20 (24), 4375−4380. (9) Cox, C. A.; Toberer, E. S.; Levchenko, A. A.; Brown, S. R.; Snyder, G. J.; Navrotsky, A.; Kauzlarich, S. M. Structure, Heat Capacity, and High-Temperature Thermal Properties of Yb14Mn1−xAlxSb11. Chem. Mater. 2009, 21 (7), 1354−1360. (10) Roudebush, J. H.; Grebenkemper, J.; Hu, Y.; Kazem, N.; Abdusalyamova, M. N.; Kauzlarich, S. M. Yb14−xTmxMnSb11 (0 < x < 0.5): Structure and Magnetic Properties. J. Solid State Chem. 2014, 211, 206−211. (11) Wang, X.-J.; Tang, M.-B.; Chen, H.-H.; Yang, X.-X.; Zhao, J.-T.; Burkhardt, U.; Grin, Y. Synthesis and High Thermoelectric Efficiency of Zintl Phase YbCd2−xZnxSb2. Appl. Phys. Lett. 2009, 94 (9), No. 092106. (12) Yi, T.; Cox, C. A.; Toberer, E. S.; Snyder, G. J.; Kauzlarich, S. M. High-Temperature Transport Properties of the Zintl Phases Yb11GaSb9 and Yb11InSb9. Chem. Mater. 2010, 22 (3), 935−941. (13) Yi, T.; Zhang, G.; Tsujii, N.; Fleurial, J.-P.; Zevalkink, A.; Snyder, G. J.; Grønbech-Jensen, N.; Kauzlarich, S. M. Phase Characterization, Thermal Stability, High-Temperature Transport Properties, and Electronic Structure of Rare-Earth Zintl Phosphides Eu3M2P4 (M = Ga, In). Inorg. Chem. 2013, 52 (7), 3787−3794. (14) Yu, C.; Zhu, T. J.; Zhang, S. N.; Zhao, X. B.; He, J.; Su, Z.; Tritt, T. M. Improved Thermoelectric Performance in the Zintl Phase Compounds YbZn2−xMnxSb2 Via Isoelectronic Substitution in the Anionic Framework. J. Appl. Phys. 2008, 104 (1), No. 013705. (15) Ribeiro, R. A.; Avila, M. A. Single Crystal Flux Growths of Thermoelectric Materials. Philos. Mag. 2012, 92 (19−21), 2492−2507. (16) Wang, J.; Toby, B. H.; Lee, P. L.; Ribaud, L.; Antao, S. M.; Kurtz, C.; Ramanathan, M.; Von Dreele, R. B.; Beno, M. A. A Dedicated Powder Diffraction Beamline at the Advanced Photon Source: Commissioning and Early Operational Results. Rev. Sci. Instrum. 2008, 79 (8), No. 085105. (17) Lee, P. L.; Shu, D.; Ramanathan, M.; Preissner, C.; Wang, J.; Beno, M. A.; Von Dreele, R. B.; Ribaud, L.; Kurtz, C.; Antao, S. M.; Jiao, X.; Toby, B. H. A Twelve-Analyzer Detector System for HighResolution Powder Diffraction. J. Synchrotron Radiat. 2008, 15 (5), 427−432. (18) Borup, K. A.; Toberer, E. S.; Zoltan, L. D.; Nakatsukasa, G.; Errico, M.; Fleurial, J.-P.; Iversen, B. B.; Snyder, G. J. Measurement of the Electrical Resistivity and Hall Coefficient at High Temperatures. Rev. Sci. Instrum. 2012, 83 (12), No. 123902. (19) Iwanaga, S.; Toberer, E. S.; Lalonde, A.; Snyder, G. J. A High Temperature Apparatus for Measurement of the Seebeck Coefficient. Rev. Sci. Instrum. 2011, 82 (6), No. 063905. (20) Liu, X.-C.; Wu, Z.; Xia, S.-Q.; Tao, X.-T.; Bobev, S. Structural Variability Versus Structural Flexibility. A Case Study of Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2). Inorg. Chem. 2015, 54 (3), 947−955. (21) Mantina, M.; Valero, R.; Cramer, C. J.; Truhlar, D. G. Atomic Radii of the Elements. In CRC Handbook of Chemistry and Physics, 94th ed.; Haynes, W. M., Ed.; CRC Press: London, 2013. (22) Saparov, B.; Bobev, S.; Ozbay, A.; Nowak, E. R. Synthesis, Structure and Physical Properties of the New Zintl Phases Eu11Zn6Sb12 and Eu11Cd6Sb12. J. Solid State Chem. 2008, 181 (10), 2690−2696. (23) Xia, S.-Q.; Bobev, S. Are Ba11Cd6Sb12 and Sr11Cd6Sb12 Zintl Phases or Not? A Density-Functional Theory Study. J. Comput. Chem. 2008, 29 (13), 2125−2133. (24) Pomrehn, G. S.; Zevalkink, A.; Zeier, W. G.; Van De Walle, A.; Snyder, G. J. Defect-Controlled Electronic Properties in AZn2Sb2 Zintl Phases. Angew. Chem. 2014, 126 (13), 3490−3494. (25) Toberer, E. S.; May, A. F.; Melot, B. C.; Flage-Larsen, E.; Snyder, G. J. Electronic Structure and Transport in Thermoelectric Compounds AZn2Sb2 (a = Sr, Ca, Yb, Eu). Dalton Trans. 2010, 39 (4), 1046−1054.

investigated. These substitutions have no effect on the small extrinsic carrier concentration that results from defects as both metals can accommodate the same number of electrons, except for x = 1 that shows an almost 2-fold number of charge carriers compared to the rest of the members. This larger charge carrier concentration is attributed to the addition of defects as a result of the size incompatibility of Zn and Cd atoms. This size mismatch also results in the limited range of homogeneity of the solid solutions of Eu11Cd6−xZnxSb12 compounds for x ≤ 2 and x ≥ 5. Electrical resistivity is improved for ∼25% Zn substitution at Cd sites by simultaneous improvements in both carrier concentration and mobility resulting in ∼250% improvement in the zT value, a change from ∼0.2 to 0.5. The higher mobility offered from Zn substitution is attributed to the delocalization of electrons through the greater degree of Eu−Sb bonding and detailed theoretical investigation is necessary to confirm.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information (.cif) on Eu11Cd6−xZnxSb12 (xp = 1, 2, and 5) solid solutions, figures showing synchrotron powder XRD patterns and Zn Lα X-ray maps of single crystals and pellets of Eu11Cd6−xZnxSb12 (x = 1, 1.25, 4.35, and 6), and a table listing thermal diffusivities, heat capacities, and density data for the studied solid solutions. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01301.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from NASA/JPL. We thank Dr. Sarah Roeske and Nick Botto for assistance with microprobe analysis. This research was funded by a GAANN fellowship (N.K.) and NSF Grant DMR-1405973. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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REFERENCES

(1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7 (2), 105−114. (2) Janka, O.; Kauzlarich, S. M. Zintl Compounds. Encyclopedia of Inorganic and Bioinorganic Chemistry; John Wiley & Sons: Hoboken, NJ, USA, 2011. (3) Kauzlarich, S. M. Chemistry, Structure, and Bonding of Zintl Phases and Ions; VCH Publishers: NewYork, NY, 1996. (4) Brown, S. R.; Kauzlarich, S. M.; Gascoin, F.; Snyder, G. J. Yb14MnSb11: New High Efficiency Thermoelectric Material for Power Generation. Chem. Mater. 2006, 18 (7), 1873−1877. (5) Bux, S. K.; Zevalkink, A.; Janka, O.; Uhl, D.; Kauzlarich, S.; Snyder, J. G.; Fleurial, J.-P. Glass-Like Lattice Thermal Conductivity and High Thermoelectric Efficiency in Yb9Mn4.2Sb9. J. Mater. Chem. A 2014, 2 (1), 215−220. (6) Kazem, N.; Hurtado, A.; Klobes, B.; Hermann, R. P.; Kauzlarich, S. M. Eu9Cd4−xCM2+x−y□ySb9: Ca9Mn4Bi9-Type Structure Stuffed with Coinage Metals (Cu, Ag, and Au) and the Challenges with Classical Valence Theory in Describing These Possible Zintl Phases. Inorg. Chem. 2014, 54 (3), 850−859. H

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Chemistry of Materials (26) Kim, H.-S.; Gibbs, Z. M.; Tang, Y.; Wang, H.; Snyder, G. J. Characterization of Lorenz Number with Seebeck Coefficient Measurement. APL Mater. 2015, 3 (4), No. 041506. (27) Ohno, S.; Zevalkink, A.; Takagiwa, Y.; Bux, S. K.; Snyder, G. J. Thermoelectric Properties of the Yb9Mn4.2−xZnxSb9 Solid Solutions. J. Mater. Chem. A 2014, 2 (20), 7478−7483. (28) Zevalkink, A.; Swallow, J.; Ohno, S.; Aydemir, U.; Bux, S.; Snyder, G. J. Thermoelectric Properties of the Ca5Al2−xInxSb6 Solid Solution. Dalton Trans. 2014, 43 (42), 15872−15878. (29) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and Old Concepts in Thermoelectric Materials. Angew. Chem., Int. Ed. 2009, 48 (46), 8616−8639. (30) Toberer, E. S.; Zevalkink, A.; Snyder, G. J. Phonon Engineering through Crystal Chemistry. J. Mater. Chem. 2011, 21 (40), 15843− 15852.

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DOI: 10.1021/acs.chemmater.5b01301 Chem. Mater. XXXX, XXX, XXX−XXX