Thermoelectric Enhancement in BaGa2Sb2 by Zn Doping - Chemistry

Subscriber access provided by McMaster University Library

Article

Thermoelectric Enhancement in BaGa2Sb2 by Zn-doping Umut Aydemir, Alex Zevalkink, Alim Hakan Ormeci, Zachary M. Gibbs, Sabah Bux, and G. Jeffrey Snyder Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5042937 • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Thermoelectric Enhancement in BaGa2Sb2 by Zn-doping Umut Aydemir* †, Alex Zevalkink‡, Alim Ormeci§, Zachary M. Gibbs†, Sabah Bux‡, and G. Jeffrey Snyder†

†

Department of Materials Science, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA ‡

Thermal Energy Conversion Technologies Group, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA §

Max Planck Institute for Chemical Physics of Solids. Nöthnitzer Straße 40, Dresden 01187, Germany

KEYWORDS. BaGa2Sb2, Thermoelectric, Zintl phase, Band Structure

ABSTRACT: The Zintl phase BaGa2Sb2 has a unique crystal structure in which large tunnels formed by ethane-like dimeric [Sb3Ga-GaSb3] units are filled by Ba atoms. BaGa2Sb2 was obtained in high purity from ball-milling followed by hot pressing. It shows semiconducting behavior, in agreement with the valence precise Zintl counting and band structure calculations, with a band gap ~0.4 eV. The thermal conductivity of BaGa2Sb2 is found to be relatively low (0.95 W/Km at 550 K), which is an inherent property of compounds with complex crystal structures. As BaGa2Sb2 has a low carrier concentration (~2 × 1018 h+/cm3) at room temperature, the charge carrier tuning was performed by substituting trivalent Ga with divalent Zn. Zndoped samples display heavily doped p-type semiconducting behavior with carrier concentrations in the range 5 – 8 × 1019 h+/cm3. Correspondingly, the zT values were increased by a factor 6 by doping compared to the undoped one, reaching a value of ~0.6 at 800 K. Zn-doped BaGa2Sb2 can thus be considered as a promising new thermoelectric material for intermediate temperature applications.

I. INTRODUCTION The thermoelectric effect refers to the direct conversion of heat to electricity, or conversely, the creation of a heat gradient by applying electric current. Hence, thermoelectric devices have great potential to contribute in solving the global energy dilemma by converting waste heat into electricity. The efficiency of a thermoelectric material is described by the dimensionless thermoelectric figure of merit, 𝑧𝑇 =

𝛼2𝑇 𝜌𝜅

, where ,  and κ stand for the Seebeck co-

efficient (thermopower), electrical resistivity and thermal conductivity, respectively.1 The electrical resistivity is low for metals, but the Seebeck coefficient is generally high in insulators. Hence, the power factor,

𝛼2 𝜌

, is typically maxim-

ized at a carrier concentration of around 1019 - 1020 cm–3, corresponding to heavily doped semiconductors.2 The thermal conductivity in solids comprises two primary contributions: the electronic part, e, due to electrons and holes and the lattice part, L, due to phonons. e is directly re𝐿𝑇 lated to  through the Wiedemann-Franz law, 𝜅𝑒 = , 𝜌

where L is the Lorenz number. In this sense, except for L,

these parameters depend on the charge carrier concentration. To optimize zT, the carrier concentration should be tuned and L should be lowered. The latter can be achieved by several methods such as point defect scattering, “rattling”, grain-boundary scattering, interface scattering, etc.3 Zintl phases are attractive thermoelectric materials as they are generally small band gap semiconductors with complex crystal structures.4-7 In Zintl compounds, each constituent atom achieves a closed valence shell by combining formal charge transfer with covalent bonds.8-12 The more electropositive atoms formally donate their valence electrons to the more electronegative ones, such that the latter complete their valence requirement, or octet rule, and form a covalently-bonded anionic structure. The carrier concentration in this family of compounds can, in many cases, be easily adjusted by substitutions or vacancy formation.13-16 Similarly found in other phases,17-19 Zintl phases display some of the lowest reported lattice thermal conductivities (0.4 WK–1 m–1 at room temperature for Yb14MnSb11) due mainly to large, complex unit cells reducing phonon velocities and inducing various phonon scattering mechanisms.20 So far, high zT values were reported mainly for Sb-

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

based Zintl phases e.g., Mg3Sb2,21,22 Yb9Mn4.2Sb9,24 Sr3GaSb3,25 or Ca5In2-xZnxSb6.26

Yb14AlSb11,23

BaGa2Sb2 crystallizes in the orthorhombic space group Pnma (No. 62) with Z = 8 formula units per primitive cell (Figure 1).27 It has a three dimensional anionic framework built by ethane-like dimeric [Sb3Ga-GaSb3] units, which are interconnected by common Sb atoms. The anionic units form large tunnels running along the b-axis, which accommodate Ba atoms. Similar anionic frameworks were also found in clathrates, but generally with large polyhedral cages encapsulating cations.28,29 BaGa2Sb2 is a charge-balanced Zintl compound with the following electron count [Ba2+][(4b)Ga1–]2[(3b)Sb0]2. Hence, it is expected to show semiconducting behavior, which has been previously confirmed by band structure calculations and low temperature electrical transport measurements.27 Due to its complex crystal structure and semiconducting nature, it can be considered as a potential thermoelectric material. In this study, we report the synthesis, high temperature thermoelectric properties, and the calculated electronic structure of BaGa2Sb2. Additionally, we demonstrate that substituting trivalent Ga with divalent Zn to increase the p-type carrier concentration leads to enhanced thermoelectric performance.

Page 2 of 23

Fourier transform spectroscopy (DRIFTS) obtained with a Nicolet 6700 FTIR spectrophotometer (Thermo Scientific) equipped with a Praying Mantis Diffuse Reflection accessory (Harrick) as described in previous work.31 Kubelka Munk theory was used to relate the measured reflectance, R, to a value proportional to the absorption coefficient using: F(R) = (1-R)2/2R.

II.EXPERIMENTAL SECTION 1. Sample Preparation. As it is hard to handle elemental Ga in a steel vial during the ball milling, GaSb was used as a precursor for the synthesis of BaGa2Sb2. To prepare GaSb, stoichiometric amounts of Ga shot (Alfa Aesar, 99.99 %) and Sb shot (Alfa Aesar, 99.999 %) were loaded into a quartz ampoule in an Ar-filled glove box and sealed under vacuum (~ 10-6 mbar). Following that, the sample was heated in a vertical furnace up to 800 °C over 6 h, annealed for 12 h and then cooled to room temperature in 6 h. To synthesize BaGa2Sb2, crystalline dendritic Ba (Alfa Aesar, 99.9 %) were cut into small pieces, mixed with GaSb and loaded into a steel vial together with 2 steel balls with ½ inch diameter. Ball milling was then performed for 90 min using a SPEX Sample Prep 8000 Series Mixer/Mill. Zn-substituted samples were obtained in the same way by mixing stoichiometric amounts of Ba, Zn, GaSb and Sb. For consolidation, samples were placed in ½ inch diameter, highdensity graphite dies (POCO) and hot-pressed at 550 °C for 1 h under 40 MPa of pressure. 2. Sample Characterization. Powder X-ray diffraction (PXRD) data was collected using a Philips XPERT MPD diffractometer (Cu-K radiation) in reflection mode. The lattice parameter determination using -Si as internal standard and Rietveld refinements were performed using WinCSD program package.30 Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) were performed using a Zeiss 1550 VP SEM. Microprobe analysis with wavelength dispersive X-ray spectroscopy (WDXS, JEOL JXA - 8200 system) was carried out to determine the chemical compositions of the target phases. The electronic band gaps of the samples were determined at room temperature using diffuse reflectance infrared

Figure 1. a) A perspective view of the crystal structure of BaGa2Sb2 along the b-axis: Ba (green), Ga (blue) and Sb (red). b) Local arrangement of Ga and Sb atoms forming Ga-Ga dimers, distorted pentagons and ladder-like square chains. 3. Transport Properties Measurements. Electrical and thermal transport properties were measured from 300 K to 823 K. The electrical resistivity and Hall coefficient measurements were carried out using Van der Pauw technique under a reversible magnetic field of 1 T using pressure-assisted tungsten electrodes. The Seebeck coefficients of the samples were obtained using chromel–Nb thermocouples by applying a temperature gradient across the sample to oscillate between ±7.5 K. Thermal conductivity is obtained indirectly through measuring the thermal diffusivity, D, with a Netzsch LFA 457 laser flash apparatus. Thermalconductivity was then calculated by the relation:  = D × d × Cp, in which d is the density of material being investigated and Cp is the heat capacity at constant pressure. For Cp, the Dulong-Petit limit was employed. We note that the Dulong-Petit heat capacity may lead to overestimation of the thermal conductivity, particularly at high temperatures.


Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reproducibility of the transport data after several cycles was confirmed by measuring transport properties repeatedly (See Figure S1). Ultrasonic measurements were performed at room temperature to obtain the longitudinal and transverse sound velocities. 4. Electronic Structure Calculations. First-principles electronic structure calculations were carried out using the all-electron, full-potential local orbital (FPLO) method.32 The local density approximation (LDA) to the density functional theory was employed through the Perdew-Wang parametrization.33 The crystal structure optimizations were also performed by using the generalized gradient approximation (GGA).34 The basis set consisted of Ba: 5s; 5p + 6s, 7s; 5d; 6p, Sb: 4s; 4p; 4d + 5s, 6s; 5d; 5p, 6p, Ga(Zn): 3s; 3p; 3d + 4s, 5s; 4p, 5p; 4d, where the semi-core states are placed before the plus sign. The valence (core) states were treated in the scalar (fully-)relativistic approximation. In order to see the effect of spin-orbit interaction on the band gap region, BaGa2Sb2 was also calculated using a fully-relativistic Hamiltonian. Brillouin zone integrations were handled by the linear tetrahedron method with a mesh of 5 × 28 × 12. The disorder due to Zn substituting Ga was treated by applying the virtual crystal approximation (VCA). Since Zn and Ga are neighbors in the periodic table, this is a reasonable approach. In addition, Zn-doping levels achieved in the experiment are quite low which makes the use of supercells computationally very demanding. There is no quantitative information regarding the site preference of the Zn atoms, therefore a uniform distribution over all four Ga Wyckoff sites was assumed. III. RESULTS AND DISCUSSION 1. Phase Analysis. BaGa2Sb2 was obtained as a pure phase after ball milling (Figure 2). However after hot-pressing at 550 °C, a partial decomposition to GaSb (~ 3 wt.%) was observed (Figures 2, 3). Ba containing by-products could not be detected by X-ray diffraction or SEM analysis. This may indicate that a small amount of Ba is lost during the consolidation, leading to formation of GaSb as the secondary phase. The same behavior was observed for the Zn-substituted samples (Figure 2). Overall, ≥ 95 % compactness was achieved for disk-shaped samples. The chemical compositions determined from the WDXS analysis are tabulated in Table 1. The WDXS composition of the parent compound is found to be almost the same as the starting composition. With further Zn doping, the Ga content decreases without apparent change in the Ba and Sb content.

Figure 2. PXRD patterns of BaGa2-xZnxSb2 samples (bm = ball-milled, Cu-K radiation). The upper and lower ticks mark the calculated reflection positions of GaSb and BaGa2Sb2, respectively. Hot-pressed samples contain a small amount of GaSb, for which the most intense peak is marked by asterisks.

Figure 3. SEM images of BaGa2-xZnxSb2 samples in backscattered electron mode showing the large grains of the target phases and the formation of GaSb as the secondary phase within the grains of the BaGa2Sb2 phase.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1: Nominal and WDXS compositions of BaGa2xZnxSb2 samples. Nominal Comp.

WDXS Comp.

BaGa2Sb2

Ba1.014(3)Ga1.99(1)Sb1.999(8)

BaGa1.975Zn0.025Sb2

Ba1.037(2)Ga1.95(1)Zn0.018(4)Sb1.989(5)

BaGa1.95Zn0.05Sb2

Ba1.031(2)Ga1.922(9)Zn0.049(4)Sb1.996(5)

BaGa1.9Zn0.1Sb2

Ba1.022(3)Ga1.89(1)Zn0.085(6)Sb2.001(8)

2. Crystal Structure. As discussed above, BaGa2Sb2 displays a unique structure with large tunnels running along the b-axis (see Figure 1a). The tunnels are formed by 26membered rings of Ga and Sb atoms, which are filled by the Ba atoms. In the crystal structure, [Ga2Sb6/3]2– units constitute the main building blocks of the framework, in which six Sb atoms are connected to Ga–Ga dimers (each Ga is tetrahedrally coordinated to 1 × Ga and 3 × Sb atoms) forming an ethane-like staggered conformation (see Figure 1b). In this way, square nets and five-membered rings of Ga and Sb atoms are created, forming narrow tubes running along the b-axis. Similar Ga-Ga bonds are found in isoelectronic compounds such as GaS, Na2[Ga3Sb3] or La13Ga8Sb21.35-37 According to the Zintl charge counting, in which 4-bonded Ga can be considered as 1– and 3-bonded Sb as 0, the following charge-balance can be considered: [Ba2+][(4b)Ga1– ]2[(3b)Sb0]2. However, the combined electron density – electron localizability indicator (ED-ELI) analysis indicate that the Sb atoms rather than the expected Ga atoms are the recipients of the electrons transferred from the Ba atoms implying a charge balance of [Ba1.2+][Ga0]2[Sb0.6–]2.38 Overall, based on these two counting schemes, it is expected that the compound will show semiconducting behavior. Based on the PXRD analysis, all reflections of BaGa2Sb2 were indexed with the orthorhombic space group Pnma (No: 62), with a = 25.442(3) Å, b = 4.4375(6) Å, c = 10.251(1) Å. Compared to the previously reported data (a = 25.454(5) Å, b = 4.4421(9) Å, c = 10.273(6) Å),27 all three lattice parameters were found to be slightly shorter. It is difficult to differentiate Zn and Ga from PXRD data due to low X-ray scattering contrast (having only 1 e– difference). The lattice parameters of Zn-doped samples are shown in Figure 4. Compared to BaGa2Sb2, a, c and V slightly increase and b very slightly decreases for sample with x = 0.025. This might indicate that incorporation of Zn (slightly larger in radius than Ga) into the framework slightly increases the framework size and overall volume. The lattice parameters and the volume of x = 0.05 and 0.1 samples are very similar, hence, the maximum Zn-substitution based on PXRD analysis could be close to BaGa1.95Zn0.05Sb2 and further Zn may fill some interstitial sites or lead to an undetectable amount of secondary phases. Based on WDXS analysis (see

Page 4 of 23

Table 1), Zn substitutes mostly for Ga atoms, but incorporation of small amounts of Zn in the other atomic or interstitial sites cannot be ruled out.

Figure 4. The volumes and lattice parameters of BaGa2xZnxSb2 samples with standard deviations. The behavior of V, a, and c is non-monotonic with Zn content, x. In the crystal structure of BaGa2Sb2, there are 2 × Ba, 4 × Ga and 4 × Sb positions all with 4c: x, ¼, z. From the Rietveld refinements (Figure 5), all the Ba and Sb positions were found to be fully occupied for each sample. All Ga positions are also fully occupied in BaGa2Sb2. In the case of Zn-substituted samples, due to lower X-ray scattering contrast, only Ga is refined for possible mixed-occupancy positions. In BaGa2Sb2, Ga–Ga dimers are formed by both Ga1–Ga3 and Ga2–Ga4 atoms with bond lengths of 2.55 Å and 2.51 Å, respectively (see Figure 1b). These bond lengths are close to the sum of the covalent radii of 2.52 Å and comparable to those found in Na2[Ga3Sb3] and La13Ga8Sb21 (2.541 Å).35,36 Similarly, Ga-Sb bond lengths vary between 2.68 and 2.81 Å, which are slightly longer than that of 2.64 Å in GaSb, but are in the range of 2.62 - 2.81 Å found for Na2[Ga3Sb3]. The angles around Ga atoms were found to be 79 – 138° which strongly deviate from regular tetrahedral angles. The Ba atoms have 5 × Ga and 6 × Sb atoms as nearest neighbors with interatomic distances in the range of 3.34 – 3.89 Å.


Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. PXRD pattern of BaGa1.95Zn0.05Sb2 (Cu-Ka1 radiation). Upper and lower ticks mark the calculated reflection positions of the target phase and GaSb, respectively, and the baseline corresponds to the residuals of a Rietveld refinement based on the reported structure model.27 3. Electronic Band Structure. The use of LDA in the firstprinciples calculations of semiconducting compounds is known to give smaller band gaps than inferred from experimental measurements. Our calculations for the undoped BaGa2Sb2 using the experimentally determined crystal structure data yielded a band gap of only 0.02 eV which should be compared to experimental values of ~0.4 eV (see the following section). Usually the LDA describes the occupied states well, while the energies of the unoccupied part are predicted to be too close to the valence band. The crystal structure of BaGa2Sb2 was fully optimized using both LDA and GGA with a maximum force criterion of 10 meVÅ-1. The equilibrium volume obtained by LDA (GGA) is 2.8% smaller (5% larger) than the experimental one.27 The theoretical lattice parameters are 25.205, 4,421, 10.134 Å (LDA) and 25.840, 4.528, 10.429 Å (GGA) for a, b and c, respectively. The band gaps computed at the optimized structures are larger than that computed at the experimental structure: 0.035 (LDA) and 0.030 eV (GGA). A fullyrelativistic calculation using the experimental structure yielded a smaller band gap, 0.015 eV. The band dispersions along the high symmetry directions of the BaGa2Sb2 Brillouin zone are shown in Figure 6a. The valence band maximum occurs close to the Γ point along the Γ -Y line, and the conduction band minimum is slightly offset from U in the Z direction. This implies that BaGa2Sb2 is expected to have an indirect band gap, in contradiction to the report in Ref. 27 based on a less accurate non-self-consistent electronic structure method.

level moves downward as the amount of Zn doping increases. Using the results of the VCA calculations, which amount to a rigid band approximation,39 we can relate the Zn concentration x to the value of the corresponding Fermi energy as measured with respect to the valence band maximum of BaGa2Sb2. The computed Fermi energy values are -0.17, -0.25, -0.33 and -0.37 eV for x equal to 0.025, 0.05, 0.075 and 0.1, respectively. The estimations based on these values can be stated as follows. As x increases from zero to 0.025, the Fermi energy cuts the highest-lying band first between Gamma-Y and then between Gamma-Z also giving rise to two hole pockets. Along these directions this band is singly degenerate. When x reaches 0.025, the Fermi energy starts to cut the highest-lying band between X and S, along which it is doubly degenerate. Taking into account the orthorhombic symmetry, we expect, for this concentration, that two hole pockets each with total degeneracy of 2 (N = 2) and one hole pocket with total degeneracy of 4 (N = 4) will contribute p-type carriers. The already high overall total band degeneracy (N = 8) at x = 0.025, increases to 11 when x equals 0.05, at which doping level two additional hole pockets (one around Gamma other between Gamma and Z) appear. However at this energy (E ~ -0.25 eV) the areas of these additional pockets are expected to be relatively small, suggesting that the total hole carrier concentration may not increase significantly as x increases to 0.05 from 0.025 (see Fig. 7 (b)). The VCA results reveal that for x = 0.075 and 0.1 the number of energy bands crossing the Fermi level becomes four implying even more complex Fermi surfaces. Accordingly, very good p-type thermoelectric properties can be obtained in BaGa2Sb2 provided that optimal doping levels are achieved. The conduction band edge is likewise made up of multiple carrier pockets, although they are less dispersive than the valence bands, suggesting that n-type carriers will have a larger effective mass. Finally, a large degree of anisotropy in the band structure seems to indicate that a single crystal sample would show higher mobility along the Ba-containing tunnels (Gamma-Y and X-S directions) than in the perpendicular directions.

In general, indirect semiconductors are desirable for thermoelectric applications. This stems from the fact that at least one of the band extrema is offset from a high symmetry point in the Brillouin zone, which may increase the number of carrier pockets (i.e, band degeneracy). In BaGa2Sb2, the valence band edge is complex, and consists of several distinct hole pockets. Since the substitution of Ga by Zn decreases the number of electrons, the Fermi



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

4. Electronic Transport Properties. BaGa2Sb2 is expected to show semiconducting behavior, predicted by both Zintl counting and electronic band structure calculations. The temperature dependences of the electrical resistivity at high temperatures are shown in Figure 7a. At room temperature, BaGa2Sb2 displays high electrical resistivity, which decreases with increasing temperature due to thermal activation of intrinsic carriers. The band gap, Eg, of a semiconducting material can be estimated from the Arrhenius plot of resistivity at high temperatures according to   𝑒 (𝐸𝑔/2𝑘𝐵𝑇) . From this plot, Eg ~ 0.4 eV was obtained, consistent with the value (0.35 eV) reported previously from infrared absorption spectroscopy.27 It was shown previously that Zn is an effective p-type dopant for replacing Al, Ga, or In in Sb-based Zintl phases.13,25,26,40 In this case, each Zn substitution is predicted to provide a free hole according to valence counting. This is illustrated in Figures 7a-b. The resistivity values of Zn-doped samples (0.025 ≤ x ≤ 0.1 per formula unit) are more than an order of magnitude less at room temperature compared to BaGa2Sb2, signifying a substantial increase in carrier concentration. The resistivity of the Zn-doped samples show almost T-independent resistivity until 600 K and decreasing resistivity above this temperature due to thermal activation of intrinsic carriers.

Figure 6. a) Dispersion curves along high symmetry directions near the Fermi level of BaGa2Sb2. The Brillouin zone is shown with the high symmetry points labeled. b) Partial density of states of BaGa2Sb2. The vertical dashed line stands for the Fermi level. The projected densities of states (pDOS) for the most relevant contributions are depicted in Figure 6b. The Fermi energy, set to 0 eV, is located at the top of the valence band. Although atoms of the same element at different Wyckoff positions have somewhat differing contributions, these differences are not significant, therefore for each element average pDOS are plotted. The s-state block between -11 and -9 eV is dominated by Sb. The states between -7.5 and -4 eV have mostly Ga 4s and Sb 5p contributions. The highest-lying manifold of the valence band contains contributions mainly from the Sb 5p, Ga 4p and Ba 5d orbitals. In particular we note that the Ba 5d occupation computed from the pDOS is 0.80 and 0.75 for the Ba1 and Ba2 symmetry types, respectively. This implies that in addition to the ionic bonding due to charge transfer, Ba atoms participate in covalent bonding through the hybridization of their 5d orbitals with the valence orbitals of the Sb and Ga atoms.

Figure 7b shows the Hall carrier concentrations (nH) of BaGa2-xZnxSb2 samples as a function of temperature. At room temperature, undoped BaGa2Sb2 already contains a low concentration of extrinsic p-type carriers (n ≈ 2 × 1018 h+cm–3), which is due probably to intrinsic defects in the crystal structure. Assuming that each Ga3+ substituted by Zn2+ leads to 1 free hole, then we estimate an increase in carrier concentration to 1.7 × 1020 h+cm–3 for the most lightly-doped sample, x = 0.025. Although carrier concentrations increased drastically with Zn substitution, they were found to reach a maximum of less than 1.0 x 1020 h+cm– 3 at room temperature, even for the Zn = 0.1 sample. This indicates that very little Zn is soluble on the intended crystallographic Ga sites, suggesting that alternative doping strategies should be considered. The remaining Zn may partially substitute the other atomic sites, fill the interstitial sites, go to the grain boundaries or even form secondary phases, which are below the detection limits of PXRD. The carrier concentration (nH) increases exponentially with temperature for BaGa2Sb2, whereas for Zn substituted samples, nH is almost constant up to 600 K, signifying extrinsic transport and starts to increase again above this temperature due to activation of intrinsic carriers across the band gap. This explains the decreasing resistivity behavior of the same samples above 600 K. To verify that the increase in carrier concentration is not due to Ga deficiency in the sample, we prepared a sample with composition BaGa1.95Sb2. The carrier concentration and the resistivity values of that sample was found to be very similar to the BaGa2Sb2 phase, which verifies the successful doping of samples with Zn (See Figure S2).


Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials The temperature dependence of the Hall mobility (𝜇𝐻 = 1 ) is shown in Figure 7c. The mobility of BaGa2Sb2 ex𝑛𝐻 𝑒

hibits a negative temperature dependence, indicating acoustic phonon scattering as the prominent scattering mechanism at play. In parallel to the resistivity, the mobility of Zn-doped samples is temperature independent up to 550 K and decreasing above this temperature. This is most likely indicative of boundary scattering by microstructural features in the samples. The reduced mobility of the undoped material when compared with Zn-substituted samples may be due to the failure of single-carrier approximation to explain the conduction behavior at high temperatures. The temperature dependence of the Seebeck coefficient is depicted in Figure 8a. The positive values for each sample in the whole range indicate holes as the dominant charge carriers. BaGa2Sb2 displays high Seebeck values that decrease with increasing temperature, characteristic of a non-degenerate semiconductor. This is in contrast with a previous report of a room temperature Seebeck coefficient of only 65 V/K in a BaGa2Sb2 ingot.41 However, the ingotsample also displayed metallic conductivity, which suggests that the sample had a large p-type carrier concentration. This can potentially arise from intrinsic point defects, such as Ba vacancies, which can be influenced by synthesis method. For Zn-doped samples, the values decrease substantially as expected from the increased carrier concentrations. The Seebeck coefficients of these samples increase with temperature, and then display a broad maximum due to minority carrier activation. For semiconducting materials, the band gap energy can be estimated from Eg 2emaxTmax where max is the maximum Seebeck value and the Tmax is temperature at which this maximum occurs. This yields ~0.33 eV, which is lower than the value given by the resistivity data. Optical absorption edge measurements were also performed, yielding a more accurate estimate for the band gap of 0.39 eV (Figure 9). The optical band gap result was extrapolated using a Tauc-plot assuming direct transitions for the undoped sample. Zn-substituted samples showed an increase in the energy of the absorption edge, and a strong free-carrier absorption along with a reflectivity minimum (or F(R) maximum) corresponding to the plasma frequency. The increase in absorption edge energy is likely a result of the Burstein-Moss shift due to the doping of the samples.31 Figure 7. The temperature dependence of a) the electrical resistivity (inset: Arrhenius plot with linear fit to determine Eg from   𝑒 𝐸g/2𝑘B𝑇 ), b) Hall carrier concentration, and c) Hall mobility of BaGa2-xZnxSb2 samples shows a transition from nondegenerate to degenerate semiconducting behavior when Zn is substituted for Ga.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

The dependence of the Seebeck coefficients on carrier concentration is illustrated by the Pisarenko relation in Figure 8b. The dashed and solid curves (350 K and 550 K, respectively) were generated using a single parabolic band (SPB) model with acoustic phonon scattering as the dominant scattering event.42,43 A valence band effective mass of 1.8 me was used to fit the data at both temperatures. At 350 K, the undoped and Zn-doped samples are found to be well-described by the SPB model and the effective mass remains fairly constant at this range of carrier concentration. At 550 K, the doped samples can still be described by a band mass of approximately 1.8 me. The Seebeck coefficient of the undoped sample at 550 K cannot be described within a single band model due to the strong contribution from minority carriers.

Figure 8. a) The temperature dependence of Seebeck coefficients of BaGa2-xZnxSb2 samples. b) The experimental Seebeck coefficients decrease with increasing carrier concentration in agreement with an SPB model (square: BaGa2Sb2, circle: Zn-doped samples).

5. Thermal Transport Properties. The total thermal conductivities, , of BaGa2-xZnxSb2 samples are shown in Figure 10a. As expected for this class of compounds with large unit cells and complex crystal structures, the values are relatively low. The total thermal conductivity values decrease until 550 K and then start to increase above this temperature. e was calculated using the Wiedemann-Franz relation (e = LT/). Here, the Lorenz number, L, was estimated as a function of temperature from the experimental Seebeck coefficients using an SPB model and assuming that acoustic phonon scattering limits the mobility.43 After subtracting the e from the total thermal conductivities, one can find the contributions of lattice (L) and bipolar (B) effects. The lattice contribution is dominant for each sample in the entire temperature range and decreasing with the 1/T temperature dependence in accordance with the Umklapp processes. Compared to Zn-doped samples, the bipolar contribution is very significant above 550 K for undoped BaGa2Sb2 sample. The minimum lattice thermal conductivity min can be calculated by considering a minimum scattering distance, l, of /2 where is the phonon wavelength. min, can be approximated by the following equation at high temperatures:44,45 1

2 1  3 ( ) 𝑘𝐵 𝑉 −3 (2𝑣𝑇 + 𝑣𝐿 ) 2 6 in which V is the average volume per atom and vT (2125 m/s) and vL (3475 m/s) are the experimental transverse and longitudinal sound velocities, respectively. The dashed line in Figure 10b shows the estimated min for BaGa2Sb2. For all samples, the L values are higher than the min signifying that further reductions to L might be possible, which might be achieved e.g., by reducing the grain size in order to enhance the boundary scattering of heat carrying phonons.

min =

Figure 9. Normalized optical absorption spectrum for a series of BaGa2-xZnxSb2 samples with the doped and undoped samples labeled.


Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials could be achieved through the use of more effective dopants e.g., Mn or alkali metals.

Figure 10. a) The total thermal conductivity of BaGa2xZnxSb2 samples. b) The lattice and bipolar contributions to the thermal conductivity, the latter of which is most significant for the undoped sample. 6. Thermoelectric Figure of Merit. The thermoelectric figure of merit, zT, is calculated by polynomial fitting of the experimental data (Figure 11a). The undoped BaGa2Sb2 sample has a peak zT of around 0.1 at 600 K. A significantly improved peak zT of around 0.6 was obtained at 800 K for all Zn-doped samples. To provide a guideline for the optimum carrier concentration level, the dependence of mobility, Lorenz number and Seebeck coefficients on n was calculated based on the SPB model. From these parameters, zT was calculated as a function of carrier concentration at 550 K, assuming that L is independent of n (Figure 11b). Figure 11b shows that the optimum carrier concentration at 550 K within a SPB model was achieved by the Znsubstitution levels applied in this study. However, based on the calculated band structure (see Figure 6a), parabolic, single band behavior is not expected. Instead, with increasing carrier concentration, additional bands are expected to contribute to transport, leading to increased density of states and increased Seebeck coefficients. Further, the experimental zT peaks at 800 K, at which, higher carrier concentrations will be necessary to optimize the zT. For these reasons, we expect that significantly increased zT values

Figure 11. a) The temperature dependence of thermoelectric figure of merit of the BaGa2-xZnxSb2 samples. b) Experimental zT values at 550 K show that the carrier concentration is nearly optimized within an SPB model at this temperature (circles: Zn-doped samples). IV. CONCLUSION In this study, BaGa2Sb2 was successfully synthesized and the high temperature thermoelectric properties of undoped and Zn-doped samples were investigated. BaGa2Sb2 is an indirect band gap semiconductor (Eg = ~0.4 eV), displaying relatively low thermal conductivity inherent to Zintl compounds. As the carrier concentration of the undoped sample (~1018h+/cm3) was found to be two orders of magnitude below the desired value (~1020h+/cm3), samples were doped with Zn to tune the carrier concentration. In this way, the carrier concentration was considerably increased and a peak zT of ~0.6 was achieved at 800 K, revealing a new thermoelectric material for medium temperature applications. The presence of multiple additional hole pockets in the valence band, revealed by DFT calculations, suggest that increasing the carrier concentration may lead to further zT improvements.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ASSOCIATED CONTENT Supporting Information The data for repeated electronic transport measurements of BaGa2-xZnxSb2 for x = 0.1 sample and a comparison of transport properties of BaGa2Sb2 and Ga deficient sample BaGa1.95Sb2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-Mail: [email protected]. Phone: +1 626 395 4814 Fax: +1 626 395 8868

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT U. A. acknowledges the financial assistance of The Scientific and Technological Research Council of Turkey. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and was supported by the NASA Science Missions Directorate’s Radioisotope Power Systems Technology Advancement Program. We would like to acknowledge the Molecular Materials Research Center (MMRC) at Caltech for allowing use of their instruments for the optical measurements obtained in this work. A. O. thanks Ulrike Nitzsche from IFW Dresden, Germany for technical help in computational work.

REFERENCES (1) Rowe, D. M. Thermoelectrics Handbook : Macro to Nano; CRC ; London : Taylor & Francis [distributor]: Boca Raton, Fla., 2006. (2) Snyder, G. J.; Toberer, E. S. Nat Mater 2008, 7, 105. (3) Tritt, T. M.; Subramanian, M. A. Mrs Bull 2006, 31, 188. (4) Kauzlarich, S. M.; Brown, S. R.; Snyder, G. J. Dalton Trans. 2007, 2099. (5) Khatun, M.; Stoyko, S. S.; Mar, A. Inorg Chem 2014, 53, 7756. (6) Mills, A. M.; Mar, A. J. Am. Chem. Soc. 2001, 123, 1151. (7) Singh, N.; Pöttgen, R.; Schwingenschlögl, U. J. Appl. Phys. 2012, 112, 103714. (8) Schäfer, H.; Eisenman, B.; Müller, W. Angew Chem Int Edit 1973, 12, 694. (9) von Schnering, H. G. Angew Chem Int Edit 1981, 20, 33. (10) Kauzlarich, S. M. Chemistry, Structure, and Bonding of Zintl Phases and Ions; VCH: New York ; Cambridge, 1996. (11) Zintl, E.; Dullenkopf, W. Z Phys Chem B-Chem E 1932, 16, 183. (12) Klemm, W. P Chem Soc London 1958, 329. (13) Johnson, S. I.; Zevalkink, A.; Snyder, G. J. J Mater Chem A 2013, 1, 4244. (14) Aydemir, U.; Candolfi, C.; Borrmann, H.; Baitinger, M.; Ormeci, A.; Carrillo-Cabrera, W.; Chubilleau, C.; Lenoir, B.; Dauscher, A.; Oeschler, N.; Steglich, F.; Grin, Y. Dalton Trans. 2010, 39, 1078. (15) Aydemir, U.; Candolfi, C.; Ormeci, A.; Borrmann, H.; Burkhardt, U.; Oztan, Y.; Oeschler, N.; Baitinger, M.; Steglich, F.; Grin, Y. Inorg Chem 2012, 51, 4730.

Page 10 of 23

(16) Pomrehn, G. S.; Zevalkink, A.; Zeier, W. G.; van de Walle, A.; Snyder, G. J. Angew. Chem. 2014, 126, 3490. (17) Schmitt, D. C.; Haldolaarachchige, N.; Xiong, Y. M.; Young, D. P.; Jin, R. Y.; Chan, J. Y. J. Am. Chem. Soc. 2012, 134, 5965. (18) Lu, X.; Morelli, D. T.; Xia, Y.; Ozolins, V. Chem. Mater. 2015, 27, 408. (19) Al Rahal Al Orabi, R.; Gougeon, P.; Gall, P.; Fontaine, B.; Gautier, R.; Colin, M.; Candolfi, C.; Dauscher, A.; Hejtmanek, J.; Malaman, B.; Lenoir, B. Inorg Chem 2014, 53, 11699. (20) Brown, S. R.; Kauzlarich, S. M.; Gascoin, F.; Snyder, G. J. Chem. Mater. 2006, 18, 1873. (21) Bhardwaj, A.; Rajput, A.; Shukla, A. K.; Pulikkotil, J. J.; Srivastava, A. K.; Dhar, A.; Gupta, G.; Auluck, S.; Misra, D. K.; Budhani, R. C. Rsc Adv 2013, 3, 8504. (22) Bhardwaj, A.; Misra, D. K. Rsc Adv 2014, 4, 34552. (23) Toberer, E. S.; Cox, C. A.; Brown, S. R.; Ikeda, T.; May, A. F.; Kauzlarich, S. M.; Snyder, G. J. Adv. Funct. Mater. 2008, 18, 2795. (24) Bux, S. K.; Zevalkink, A.; Janka, O.; Uhl, D.; Kauzlarich, S.; Snyder, J. G.; Fleurial, J. P. J Mater Chem A 2014, 2, 215. (25) Zevalkink, A.; Zeier, W. G.; Pomrehn, G.; Schechtel, E.; Tremel, W.; Snyder, G. J. Energ. Environ. Sci. 2012, 5, 9121. (26) Zevalkink, A.; Swallow, J.; Snyder, G. J. Dalton Trans. 2013, 42, 9713. (27) Kim, S. J.; Kanatzidis, M. G. Inorg Chem 2001, 40, 3781. (28) Aydemir, U.; Candolfi, C.; Ormeci, A.; Oztan, Y.; Baitinger, M.; Oeschler, N.; Steglich, F.; Grin, Y. Phys Rev B 2011, 84, 195137. (29) Aydemir, U.; Akselrud, L.; Carrillo-Cabrera, W.; Candolfi, C.; Oeschler, N.; Baitinger, M.; Steglich, F.; Grin, Y. J. Am. Chem. Soc. 2010, 132, 10984. (30) Akselrud, L.; Grin, Y. J Appl Crystallogr 2014, 47, 803. (31) Gibbs, Z. M.; LaLonde, A.; Snyder, G. J. New J Phys 2013, 15, 075020. (32) K. Koepernik, H. E. Phys Rev B 1999, 59, 1743. (33) Perdew, J. P.; Wang, Y. Phys Rev B 1992, 45, 13244. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (35) Mills, A. M.; Mar, A. Inorg Chem 2000, 39, 4599. (36) Cordier, G.; Ochmann, H.; Schäfer, H. Materials Research Bulletin 1986, 21, 331. (37) Kuhn, A.; Chevy, A.; Chevalier, R. Acta Crystallographica Section B 1976, 32, 983. (38) Aydemir, U.; Zevalkink, A.; Ormeci, A.; Bux, S.; Snyder, G. J. in preperation. (39) Takagiwa, Y.; Pei, Y. Z.; Pomrehn, G.; Snyder, G. J. Apl Mater 2013, 1, 011101. (40) Brown, S. R.; Toberer, E. S.; Ikeda, T.; Cox, C. A.; Gascoin, F.; Kauzlarich, S. M.; Snyder, G. J. Chem. Mater. 2008, 20, 3412. (41) Zevalkink, A.; Zeier, W. G.; Cheng, E.; Snyder, J.; Fleurial, J. P.; Bux, S. Chem. Mater. 2014, 26, 5710. (42) Zevalkink, A.; Toberer, E. S.; Zeier, W. G.; Flage-Larsen, E.; Snyder, G. J. Energ Environ Sci 2011, 4, 510. (43) May, A. F.; Toberer, E. S.; Saramat, A.; Snyder, G. J. Phys Rev B 2009, 80, 125205. (44) Cahill, D. G.; Pohl, R. O. Annu Rev Phys Chem 1988, 39, 93. (45) Cahill, D. G.; Watson, S. K.; Pohl, R. O. Phys Rev B 1992, 46, 6131.


Page 11 of 23


SYNOPSIS TOC.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

127x194mm (300 x 300 DPI)


Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


153x275mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

52x32mm (300 x 300 DPI)


Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


77x70mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

59x41mm (300 x 300 DPI)


Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


115x157mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

190x427mm (300 x 300 DPI)


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


131x204mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

67x53mm (300 x 300 DPI)


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


128x194mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

131x203mm (300 x 300 DPI)


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35x29mm (300 x 300 DPI)


Thermoelectric Enhancement in BaGa2Sb2 by Zn Doping - Chemistry

Recommend Documents