Tuning Thermoelectric Properties of Type I Clathrate K8âxBaxAl8+

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Tuning thermoelectric properties of type I clathrate K8-xBaxAl8+xSi38-x through Barium substitution Fan Sui, and Susan M. Kauzlarich Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00566 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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Chemistry of Materials

Tuning thermoelectric properties of type I clathrate K8-xBaxAl8+xSi38-x through Barium substitution Fan Sui and Susan M. Kauzlarich* Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616 Email: [email protected] Abstract The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 K to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K guest atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3n), confirmed by X-ray diffraction. The guest atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-doped samples were characterized from 2 K to 300 K. The K-Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the temperature range of 323 K to 873 K. Thermal conductivity from 323 K to 873 K was estimated from the Wiedemann-Franz relation and lattice thermal conductivity extrapolation from 300 K to 873 K. K8-xBaxAl8+xSi38-x (x = 1, 1.5) synthesized with Al deficiency showed enhanced electrical conductivity and the absolute Seebeck coefficients decrease with the increased carrier concentration. When x = 2, the Al content increases towards the electron balanced composition and the electrical resistivity increases with the decreasing charge carrier concentration. Overall, K6.5Ba1.5Al9Si37 achieves an enhanced zT of 0.4 at 873 K. _____________________________________________________________________ 1. Introduction Compounds with type I clathrate structure (cubic, Pm3n) possess an open framework crystal structure composed of 20 vertex dodecahedron cages and 24 vertex tetrakaidecahedron cages with guest atoms inside the cages (Figure 1).1 Type I clathrate compounds have drawn intense research interest for a variety of energy conversion and storage technologies. In particular, clathrate phases composed of silicon and other main group elements are highlighted because they contain light weight earth abundant elements. Type I silicon clathrate superconductors with alkali or barium metal were discovered and Ba8Si46 synthesized under high pressure possesses transition temperature of 8.0 K.2 Clathrate hydrates and silicon clathrates have been shown to store hydrogen, the clean energy carrier.3, 4 Lithium insertion into

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Ba-doped LixBayAl6Si40 (0 ≤ x ≤ 8; y = 6) type I clathrates has been recently studied.5 K8Al8Si38 has been identified as a promising system for solar energy conversion as it has an optimal band gap of 1.3 eV.6, 7 In the field of thermoelectric materials, clathrate compounds are considered as ideal phonon glass, electron crystal (PGEC) materials and have potential for thermoelectric applications.8 It has been found in many type I clathrate compounds that the guest atoms at the 6d site inside the 24 vertex cage are loosely bonded to the cage and possess a large anisotropic displacement parameter.9 The rattling motions can scatter phonons and contribute to glass-like low lattice thermal conductivity.10, 11 The clathrate frameworks are mainly composed of tetrahedral bonded group 14 elements. 10, 12, 13 The neighboring group 13 and group 15 elements can partially substitute the framework sites. Guest atoms can be from a variety of groups, including group 1, 2, 15 or rare earth elements with appropriate sizes, and play the role of charge donors.13 The well crystallized framework and wide combinations of elements with different valence electron numbers allows for tunable carrier concentration and therefore, crystal-like electrical conductivity can be achieved.14 As the thermoelectric figure of merit zT is defined by S2T/ρκ, where ρ is electrical resistivity and κ is thermal conductivity, PGEC materials are advantageous for achieving high thermoelectric efficiency, which is closely related with zT.8, 15 So far among the compounds with type I clathrate structure, large zT of type I clathrate compounds has been reported in n-type Ba8Ga16Ge30 single crystal, with a zT of 1.35 at 900 K,16 and n-type Ba8Ga16Ge30 polycrystalline sample, whose zT is 0.74 at 1000 K.17 Polycrystalline Sr8Ga16Ge30 and Eu8Ga16Ge30 exhibited glass-like lattice thermal conductivity at low temperature.18

Figure 1: A view of the structure of Type I Clathrate showing the two types of cages. The 20- and 24-vertex cages are shown in red and blue, respectively. Compounds of Si containing type I clathrate have been synthesized and


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characterized.19 Si is a nontoxic element and more earth abundant compared to Ge, making compounds containing Si promising for large-scale industrial application. Si is a light element and stable up to 1680 K, which is advantageous for transportation and high temperature range applications.20 Ba8Al14Si31 was synthesized and has a zT reaching 0.34 at 1150 K.20 Polycrystalline samples of Ba8AlxSi46-x (8 ≤ x ≤ 15) with a wide phase range were investigated and reported with a maximum zT of 0.24 at 1000 K when x = 15.21 The highest reported zT for Ba8Al16Si30 to date is 0.4 at 900 K.22 The chemical composition of this sample was determined to be Ba8Al15Si31 and the solubility limit of 15 for Al in the Ba containing clathrate framework was confirmed.22 These results indicate that while the Ba8AlxSi46-x phases have favorable high temperature stability, an ideal stoichiometry for thermoelectric applications has not yet been achieved. Therefore, a path towards high zT in this system requires new approaches to control electronic transport properties. The reported lattice thermal conductivity of Ba8AlxSi46-x samples are about 1 W/mK at high temperature, relatively low compare to other silicon based thermoelectric materials, e.g., bulk Mg2Si and SiGe alloys.23, 24 The flexibility of the framework along with the guest atom is one feature that makes these structures promising for electronic tuning. In addition to the Ba guest atom clathrates containing Si and Al in the framework, alkali metal guest atom clathrates have also been reported. Only polycrystalline samples or small single crystal samples have been synthesized to date. The single crystal structures of A8Ga8Si38, (A = K, Rb) first reported in 1998 were synthesized from on-stoichiometry reactions of the pure elements.25, 26 Imai synthesized K8Ga8Si38 in the same manner in 2011 and reported on the semiconducting behavior.27 K8Ga8Si38 is n-type semiconducting material with high electrical resistivity.27 A8Ga8Si38 (A = K, Rb, Cs) were reported to be synthesized from pure elements and with excess alkali metal; high-purity samples were synthesized and their thermoelectric properties reported.7 Instead of starting from pure elements, a ball milling route of KH and Al/Si melted ingot was applied to achieve high-purity K8Al8Si38. In the case of A8Ga8Si38 samples, alkali metals have high solubility with gallium, whereas K does not dissolve in aluminum.7 Pure-phase polycrystalline samples A8Al8Si38 (A = K, Rb, Cs) have been synthesized from Al-halide fluxes, and their thermoelectric properties reported.28 Therefore the series A8E8Si38 (A = K, Rb, Cs; E = Al, Ga) have been synthesized and the thermoelectric properties show that all the phases are n-type semiconductors. Recently, Na8Al8Si38 was added to the series, prepared by spark plasma sintering and kinetically controlled thermal decomposition of NaSi and NaAlSi mixtures.29 Among the A8E8Si38 clathrate compounds, K8Al8Si38 is a promising semiconductor for both thermoelectric applications and solar energy conversion.6 The band structures of K8Al8Si38 with different aluminum occupancy models were calculated by first principles; the results revealed K8Al8Si38 to have a quasi-direct band gap of ~1.0 eV.6 UV-Vis. Surface photovoltage spectroscopy (SPV) confirmed the band gap to be in the range of 1.33 eV to 1.40 eV.1, 6 K8Al8Si38 possesses a high absolute Seebeck coefficient (~ − 90 µV/K) value and low thermal conductivity (~1.7 W/mK) at room temperature.7 The five-probe Hall measurement indicated K8Al8Si38



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had low carrier concentration (1.31x1018 cm-3) at room temperature (273 K) and as a result high electrical resistivity. In this paper, we report the K8Al8Si38 thermoelectric power factor for the high temperature range (323 K to 873 K) along with the synthesis and thermoelectric properties of the new K-Ba alloyed phases K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2). Ternary K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) phases were synthesized for the first time via metal hydrides instead of high-pressure synthesis to tune the thermoelectric properties. This system appears to be more amenable to optimization than Ba8AlxSi46-x. 2. Experimental Procedures 2.1 Synthesis The synthesis of polycrystalline K8Al8Si38 has been previously reported.7 K8-xBaxAl8+xSi38-x samples (x = 0, 1, 1.5, 2) were synthesized in a similar manner. Si lump (Alfa Aesar, 99.99999+ %) and Al shot (99.999%, Furuchi Chem. Co., Japan) were weighed in air and melted together in an argon-filled arc-welder. Potassium hydride (KH, Sigma-Aldrich, 30 wt % dispersion in mineral oil) was removed from the mineral oil, washed with hexane and dried before use. The dry potassium hydride and barium hydride (BaH2, Materion Advanced Chemicals, 99.7 %) powders, together with the arc-melted Al-Si mixture were transferred into a 5 ml tungsten carbide lined grinding vial with two 8 mm diameter tungsten carbide balls, sealed in an air tight plastic bag and ball milled by a SPEX Mixer/Mill 8000 for 45 min. The ratio of KH/BaH2/Al/Si for Ba 1 and 1.5 containing phases are 7.875/1.125/9/38 and 7.3125/1.6875/9/38 respectively, where 12.5 at % of the nominal composition in excess of KH and BaH2 were used to ensure that Si reacted completely. The Al/Si ratio loaded was consistent with the synthesis of K8Al8Si38. For K6Ba2Al10Si36, the ratio of KH/BaH2/Al/Si loaded is 7.5/2.5/10/36. Al and Si were used on stoichiometry in the Ba 2 sample, because if Al/Si ratio of 9/38 similar to the other Ba samples is used, the product contained excess Si impurity. The compositions are indicated throughout the following text as Ba 1, Ba 1.5, Ba 2 respectively according to content. The ball milled starting materials were sealed in a tantalum tube under argon and then sealed in a quartz ampule under vacuum to prevent oxidation. The ampules were heated to 950 ◦C, then cooled to 700 ◦C at the rate of 2 ◦C/minute and annealed at 700◦C for 40 hours in an electric box furnace. After annealing, the reaction vessel was furnace cooled to room temperature and transferred to a glove box. The reaction vessel was opened in a drybox and the contents inspected to ensure complete reaction of the starting reagents. Any small amount of K, Ba or Al phases were removed by washing the sample with ethanol and diluted hydrochloric acid as samples are stable towards both ethanol and dilute hydrochloric acid. The products are fine black powder. Caution: KH powder and BaH2 are reactive to oxygen and moisture and must be handled with care under an inert atmosphere.


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2.2 Powder X-ray Diffraction (PXRD) Samples of K8-xBaxAl8+xSi38-x (x = 0, 1, 1.5, 2) were ground into a fine powder for X-ray diffraction characterization. Diffraction patterns were obtained with Cu Kα radiation (λ = 1.5405 Å) on a Bruker D8 Advance diﬀractometer operated at 40 kV and 40 mA. The whole profile fitting was performed with JANA 2006 package to obtain the lattice parameters.24 2.3 Synchrotron X-ray Diffraction K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) alloy phases indicated by Ba 1, 1.5, 2 where the number indicates x in the formula, were transferred into the Kapton tubes, sealed with clay, and inserted vertically into the sample base. Data collection was performed at 11-BM of the Advance Photon Source at the U.S. Department of Energy’s Argonne National Laboratory with radiation wavelength of 0.469659 Å at 100 K and ambient temperature 295 K. Rietveld refinement was performed with JANA 2006 package.30 2.4 Thermogravimetry and Differential Scanning Calorimetry (TG-DSC) About 20 mg of K8Al8Si38 powder was cold pressed into a 2 mm diameter pellet and used for TG-DSC analysis by Netzsch Thermal Analysis STA 409 PC. The measurement was performed in alumina crucible from room temperature to 1000 ˚C with heating rate of 10 K/min under argon flow, and then cooled to room temperature with the same parameters. 2.5 Spark Plasma Sintering (SPS) About 30 mg of each polycrystalline samples K8-xBaxAl8+xSi38-x (x = 0, 1, 1.5, 2) were loaded into φ 4 mm graphite die with a pair of tungsten carbide plungers plugged from the two ends. The loaded samples were sintered into dense sample bar with the length of about 8 mm by a Dr Sinter Junior Spark Plasma Sintering Systems (Fuji Electronic Industrial Co., Ltd.) at 650 ˚C for 10 minutes with 5 kN force (398 MPa on the cross section). 2.6 Low Temperature Thermoelectric Performance Slices of samples with the thickness of ~ 1 mm were cut from the sintered sample bar with a diamond saw for the characterization of thermoelectric properties. The electrical resistivity was measured using the AC transport option of Quantum Design physical property measurement system (PPMS) with platinum leads attached to the sample top surface with silver epoxy. Hall Effect measurement was performed with five probes at a scan magnetic field from 70000 Oe to –70000 Oe. Thermal conductivity was measured with the PPMS thermal transport option with two gold coated copper leads attached to the top and bottom sample surfaces. 2.7 Electron Microprobe Analysis Electron microprobe X-ray diffraction measurements were performed on sample slices after the thermoelectric properties measurements. The samples were mounted in



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epoxy and polished for analysis on a Cameca SX-100 electron microprobe equipped with five wavelength-dispersive spectrometers. The chemical composition was determined by wavelength-dispersive X-ray spectroscopy (WDXS) using the intensities of the X-ray lines of Kα of K from orthoclase standard, Lα of Ba from Benitoite standard, Al from aluminum standard and Si from silicon single crystal standard. 2.8 High Temperature Thermoelectric Power Factors The electrical resistivity (ρ) and Seebeck coefficient (S) of K8-xBaxAl8+xSi38-x (x = 0, 1, 1.5, 2) samples were characterized by a Linseis four probe measurement system heating from 50 ˚C to 550 ˚C and cooling 550 ˚C to 50 ˚C in helium atmosphere. The sample length was approximately 8 mm and the probes were set to 6 mm distance. Graphite foil was placed between the Pt electrodes and thermocouples to prevent reaction. The measurements were performed every 50 ˚C and 5 data readings were taken for each temperature point. The temperature difference between upper and lower electrodes was set to be 50 ˚C. 3. Results and Discussion 3.1 Phase characterization and crystal structures from Rietveld refinements: The whole profile fitting of collected laboratory XRD patterns at room temperature confirms the main phase of the samples to be type I clathrate structure. Lattice parameters were obtained from the whole profile fitting and they are K7BaAl9Si37 (10.4903(2) Å), K6.5Ba1.5Al9.5Si36.5 (10.5134(3) Å) and K6Ba2Al10Si36 (10.5489(6) Å). K8Al8Si38 has a unit cell parameter of 10.48071(2) Å.7 As Ba replaces K, the amount of Al also increases; Al is slightly larger than Si whereas K+ has a similar ionic radius (1.64 Å) to Ba2+ (1.61 Å) when the coordination number (CN) is 12.29 Overall, the unit cell of Ba-substituted samples expands with more Al content. The lattice parameters K8-xBaxAl8+xSi38-x phases are plotted in SFigure 1 in supporting information. Similar to what has been found in Ba8AlxSi46-x (6 ≤ x ≤ 15), the lattice parameters of K8-xBaxAl8+xSi38-x phases increase linearly with Al content, and are close to the lattice parameters of Ba8AlxSi46-x.15 The nominal stoichiometry of the samples is close to the electron balanced formula. The XRD pattern of K7BaAl9Si37 has additional small peaks from Si secondary phase in addition to the clathrate main phase, while K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 consistently were phase-pure samples. Synchrotron X-ray diffraction patterns were collected from polycrystalline samples of K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 at 100 K and 295 K. The X-ray wavelength of the synchrotron beamline was 0.414169 Å, therefore the X-ray diffraction data, with (sinθ)/λ > 1, provided high reliability for detailed atomic structure and mixed occupancy refinement. Rietveld refinement was performed on the collected patterns and the results are given in Table 1. The main phase of K7BaAl9Si37 was confirmed to possess Pm3n (No. 223) structure and the weight percentage of Si


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secondary phase was determined to be 0.44(2) % of the overall sample. Table 1: Rietveld Refinement Results of K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 at 295 K and 100 K Nominal

K7BaAl9Si37

formula

K6.5Ba1.5Al9.5Si36.5

Wavelength

K6Ba2Al10Si36

Synchrotron light source, 0.414169

(Å) Temperature (K)

295

100

100

295

100

P m𝟑n (No. 223)

Space group Unit cell

295

10.49300(10)

10.48237(10)

10.51821(8)

10.50696(8)

10.54933(10)

10.53781(9)

parameter (Å) Unit cell 1155.310(19)

1151.804(18)

1163.659(15)

1159.928(14)

1174.018(19)

1170.176(18)

Rall %

6.51

5.99

6.62

4.81

5.91

4.81

wRall %

7.78

7.45

9.51

7.00

7.32

6.55

GOF %

3.69

3.59

3.05

3.02

3.02

3.19

volume (Å3)

Ba s.o.f. at 2a

31.0(2) %

46.40(13) %

61.52(18) %

Ba s.o.f. at 6d

6.4(2) %

12.92(10) %

23.93(12) %

7:1

6.3 : 1.7

5.3 : 2.7

K/Ba atomic ratio ADP (Å2), 2a

2

ADP (Å ), 6d

0.0118(6)

0.0090(2)

0.04088(8)

0.0420(5)

0.0334(2)

0.0359(2)

U11 =0.0138(9)

U11 = 0.0131(5)

U11 = 0.0549(7)

U11 = 0.0536(7)

U11 = 0.0375(3)

U11 = 0.0379(5)

U22 = U33 =

U22 = U33 =

U22 = U33 =

U22 = U33=

U22 = U33 =

U22 = U33 =

0.0244(5)

0.0172(6)

0.0613(2)

0.0559(6)

0.0565(7)

0.0483(4)

The refined lattice parameters of synchrotron X-ray diffraction patterns at room temperature are consistent with the laboratory X-ray diffraction refinement results. The Ba/K mixed occupancy ratios at the 2a and 6d sites were refined and are listed in Table 1. Ba has higher occupancy at 2a compared with 6d in all three phases, indicating Ba site preference for the smaller E20 cages, consistent with its smaller ionic radius. This is consistent with the occupancy preference in type II clathrate (K,Ba)24(Ga,Sn,□)136 where the hexakaidecahedral cage (the larger cage in type II clathrate) and dodecahedral cage preferentially accommodate the K+ and Ba2+ ions, respectively.31 Calculations of the overall Ba content based on the refined occupancy ratios show that K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 have Ba content of 1, 1.7 and 2.68, respectively. The refined Ba content is consistent with the starting ratios for K7BaAl9Si37 and is higher than the nominal Ba content in K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 samples. Al and Si have similar electron density and their site occupancy was not determined from the XRD refinements. To better understand the



samples’ chemical compositions and properties, electron microprobe analysis was performed on the sintered sample pellets and the results are shown in Table 2 in the following section. The guest atoms at the 6d site which is centered within the 24 vertex cages, possess large anisotropic atomic displacement parameters (ADPs) whereas the guest atoms at the 2a site show isotropic values for the ADPs. As shown in Figure 2, U22 and U33 at the 6d site, the directions parallel to the six-membered ring of the 24-vertex cage, possess larger values than U11, which is vertical to the six-membered ring. This is consistent with the previously reported A8Ga8Si38 (A = K, Rb, Cs) and Ba8Al15Si31 phases,7, 32 indicating the thermal ellipsoid of the guest atoms at the 6d site elongates along the 16i and 24k sites direction, parallel to the six-membered ring. The ADPs were refined from the diffractions at 295 K and 100 K, and they increase with temperature. The temperature dependence of guest atoms’ ADPs indicates high dynamic disorder, for example, A8Na16E136 (A = Cs or Rb and E = Ge or Si).33 The anisotropy of the ADPs for the 6d site show strong temperature dependence especially for the U22U33 direction, while the thermal displacement of the U11 direction and 2a sites’ show weaker temperature dependence.

0.06 0.05 0.04

U e q (A 2 )

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0.03 0.02

B a 1 B a 1.5 B a 2

0.01 100 K

295 K

T em p eratu re (K )

Figure 2. The 6d site Ueq values of K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 at 100 K and 295 K. The U11 change in temperature is indicated with solid lines and U22 U33 are shown in dash lines. 3.2 Thermal stability analysis, material sintering and chemical composition analysis on sintered pellets: Thermal behavior of K8Al8Si38 was tested under argon flow from room temperature to 1000 ˚C and cooling employing a TG-DSC instrument. The TG-DSC data are


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provided in Figure 3. K8Al8Si38 shows good thermal stability below 810 ˚C and no significant melting or phase transition processes can be identified in this temperature range. When temperature reaches above 810 ˚C, a large endothermic peak appears along with a weight loss of 7.63% (811 to 926 ˚C). During the cooling process, there is an exothermic peak indicating crystallization of decomposition products. Powder XRD of the sample after the TG-DSC meaurement showed that the sample decomposed to silicon, aluminum and other unidentified phases (Supporting Information, SFigure 4). The thermal stability of Ba-alloyed samples was tested in a sealed quartz tube in a box furnace instead of TG-DSC because of the negative consequences of the reaction of the samples with the Pt TG-DSC head. About 100 mg cold pressed powder were sealed in quartz tube under vacuum to simulate the SPS sintering vacuum environment. The samples were stable up to 650 ˚C. The quartz tube turned yellow color after heated to 700 ˚C for 10 mins, suggesting decomposition. XRD patterns were collected from the annealed powder, which confirmed the main phases remained type I clathrate; however, diffraction peaks due to Si were identified. Therefore, the Ba-containing samples partially decompose under vacuum between 650 ~ 700 ˚C.

Figure 3. TG-DSC measurement result of K8Al8Si38 in the temperature range of 25 ˚C to 1000 ˚C heating and cooling under flowing argon. The thermal gravity analysis data are shown in black and differential scanning calorimetry in red. To prevent sample decomposition and phase change during the materials sintering process by Spark Plasma Sintering (SPS), 650 ◦C was chosen as the sintering temperature for all three samples. The geometric density of K8Al8Si38, K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 samples were measured and are 89.4 %, 86.9 %, 90.7 % and 88.7 %, respectively.



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Wavelength dispersive spectroscopy (WDS) was performed on the polished surface of sintered pellet (Table 2). The chemical formula was determined from atomic percentage ratio of the intensity of the elements’ characteristic X-ray with the total element number set to be 54. The K7BaAl8Si38 and K6.5Ba1.5Al8Si38 samples were determined to be K6.62(29)Ba1.096(24)Al8.82(34)Si37.47(31) and K6.21(35)Ba1.83(21)Al9.21(25)Si36.74(40), close to the nominal chemical formula and consistent with the refinement of the synchrotron data. Assuming K donates 1 electron and Ba 2 electrons (8.812 electrons) for 8.82 Al atoms in K6.62(29)Ba1.096(24)Al8.82(34)Si37.47(31) and 10.02 electrons for 9.13 Al in K6.29(40)Ba1.76(27)Al9.13(32)Si36.81(42). The samples will be referenced by their nominal composition, x, according to Ba 1, Ba 1.5, and Ba 2. Table 2. Wavelength Dispersive Spectroscopy (WDS) Results for K7BaAl9Si37, K6.5Ba1.5Al9.5Si36.5 and K6Ba2Al10Si36 Atomic % x Determined Formula1 (Ba) K Ba Al Si 1 12.25(59) 2.02(57) 16.33(47) 69.39(56) K6.62(29)Ba1.096(24)Al8.82(34)Si37.47(31) 1.5 11.50(61) 3.39(41) 17.07(52) 68.04(89) K6.21(35)Ba1.83(21)Al9.21(25)Si36.74(40) 2 10.98(85) 4.06(32) 18.70(49) 66.29(49) K5.92(46)Ba2.18(17)Al10.10(26)Si35.79(17) 1 The determined chemical formula from electron microprobe analysis is shown with the total atoms set to be 54 which is consistent with fully occupied type I clathrate structure. 3.3 Low temperature transport properties:

Figure 4. Temperature dependent electrical resistivity of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 pellets, from 2 K to 300 K.


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Electrical resistivity measurements on K8-xBaxAl8+xSi38-x, Ba 1, 1.5, 2 pellets are shown in Figure 4. In the temperature range of 2 K to 300 K, K7BaAl9Si37 and K6.5Ba1.5Al9.5Si36.5 samples showed metal-like behavior, with their electrical resistivity increasing with increasing temperature. Compare to the electrical resistivity of K8Al8Si38 which was ~ 1.2x10-3 µΩm at 300 K, reported previously7, the electrical resistivity for the Ba 1 sample is significantly lower at the same temperature. For the sample Ba 1.5, the electrical resistivity is further lowered to 12.1µΩm at 300 K. This decreased electrical resistivity is consistent with the Al deficiency in the Ba1.5 sample, giving rise to an enhanced electrical conductivity. K6Ba2Al10Si36 sample showed higher electrical resistivity than Ba 1.5 and Ba 1 samples. The electrical resistivity first decreased with increasing temperature (2 K to about 100 K) and then slowly increased with temperature, reaching about 38 µΩm at 300 K. As the Al content changes with Ba substitution and therefore the electrons contributed from guest atoms are balanced with the Al atoms, the electrical resistivity does not simply follow the Ba content. Instead it is the relative ratio of guest atoms vs Al that determines the electron carrier concentration. However, compared to the electrical resistivity of K8Al8Si38, the Ba 2 sample is significantly more conducting. Applying the chemical composition obtained from WDS to count the electrons, as the electron number from one unit cell would be 2 x Ba + K−Al, Ba 1.5 sample possesses the largest number of electrons (0.66) among the three samples with low electrical resistivity. Ba 1 and Ba 2 samples have 0.008 and 0.18, which is closer to electron balanced formula. The electron carrier concentrations were measured at room temperature with a magnetic field of 70,000 Oe and are provided in Table 3. The mobility is calculated from the measured carrier concentration and electrical resistivity at 273 K, with the equation µ = 1/neρ. The result reveals a large n-type carrier concentration and confirms the increases in carrier concentrations of samples Ba 1, Ba 1.5 and Ba 2, and mobility is decreased because of the acoustic phonon scattering mechanism.34 Table 3. Carrier concentration (cm-3) and Mobility (cm2/Vs) from PPMS Hall measurement at 273 K and 70000 Oe magnetic field. Sample K8Al8Si38 Ba 1 Ba 1.5 Ba 2

Carrier concentration (cm-3) {ref} 1.31 x 1018{6} 20 2.91 x 10 21 4.49 x 10 1.25 x 1020


Mobility (cm2/Vs){ref} ~39{6} 6.8 1.2 13.2


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Figure 5. (a) Temperature dependence of the thermal conductivity of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2, from 2 K to 300 K. (b) Electrical thermal conductivity of Ba 1, Ba 1.5, Ba 2 samples in the temperature range of 2 K to 300 K, calculated from Wiedemann-Franz equation κe = LT/ρ, where L is Lorentz number, 2.44x10-8 WΩK-2. (c) Lattice thermal conductivity of Ba 1, Ba 1.5 and Ba 2 samples, calculated from subtracting κe from measured κtotal. K8Al8Si38 was calculated from data in reference.7 The temperature dependence of total thermal conductivity of Ba 1, Ba 1.5 and Ba 2 was measured from 2 K to 300 K (shown in Figure 5). Thermal conductivity data measured from the TTO option on the PPMS were corrected for heat loss due to sample radiation and heat leak from the shoe assembly. The raw data and corrections are provided in supporting informations. (SFigure 5) The thermal conductivity increases from almost 0 at 2 K sharply with temperature, and then slowly after reaching 100 K. The thermal conductivity of Ba 1 is about 1.22 W/Km at 300 K,


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higher than the thermal conductivity of Ba 1.5 which is 1.15 W/Km. Ba 2 sample shows the lowest thermal conductivity which is about 1 W/Km. Compare to the thermal conductivity of K8Al8Si38, which was reported to be 1.77 W/Km at 300 K previously, the thermal conductivities were significantly lowered with Ba dopant.7 The total thermal conductivity is composed of the charge carrier transport part, i.e. electrical thermal conductivity (κe) and the phonon transport part, i.e., lattice thermal conductivity (κL). κe can be estimated with the Wiedemann-Franz equation κe = LT/ρ, where L is Lorentz number, T is the temperature and ρ is the measured electrical resistivity at the corresponding temperature. In this case, 2.44x10-8 WΩK-2, the Lorentz number of free electrons, was applied to calculate the electrical thermal conductivity. The calculated results, shown in Figure 5(b), indicate an almost linear increase. The magnitude of samples’ electrical thermal conductivities are consistent with samples’ electrical resistivity trend. Ba 1.5 possesses higher κe than Ba 1, as it has a higher carrier concentration, and Ba 2 sample has low electrical thermal conductivity because of the low charge carrier concentration. Lattice thermal conductivity is the subtraction of κe from the total thermal conductivity, which are shown in Figure 5(c). The samples show sharp increase at low temperature, and reach a plateau at 300 K. Sample K8Al8Si38 has a thermal conductivity of 1.77 W/Km at 300 K.1 With Ba substitution, the lattice thermal conductivity dropped from 1.77 W/Km to ~1 W/Km in the case of Ba 1, and below 0.6 W/Km in the case of Ba 1.5 and Ba 2 samples. Compared to the lattice thermal conductivity of Ba8AlxSi46-x samples which are in the range of 1 W/Km ~ 1.5 W/Km14, the samples with K/Ba mixed occupancies possess lower lattice thermal conductivity than either the all-Ba or all-K containing clathrate samples.

3.4 High temperature thermoelectric performance:



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Figure 6. (a) Temperature dependence of the electrical resistivity for K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 samples from 323 K to 873 K. (b) Temperature dependence of the absolute Seebeck coefficient measurements for K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 samples from 323 K to 873 K. Electrical resistivity and Seebeck coefficient measurement results of K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 samples were obtained from an off-axis measurement set up from 322 K to 873 K.35 The results are plotted in Figure 6. The clathrate samples all showed increasing electrical resistivity with increasing temperature in the measured range. For Ba-doped samples, they exhibit metallic behavior, which is consistent with the low temperature behavior obtained from the PPMS measurement. This sample of K8Al8Si38 has higher resistivity and Seebeck coefficient at room temperature than what was reported previously,7 which is attributed to a more stoichiometric composition. The lattice parameters of this sample (10.4828(7) Å) were also slightly larger than previously reported prepared via KH (10.48071(2) Å) and is more similar to results for a sample prepared via salt flux synthesis which shows much larger resistivity and Seebeck coefficient.7, 28 Therefore,


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the electrical resistivity of K8Al8Si38 is higher than that reported at low temperatures7 but slightly lower than that reported from the flux route28. With Ba substitution in the Ba 1 sample, K7BaAl8Si38, the electrical resistivity is decreased from ~ 640 µΩm at 323 K for K8Al8Si38 to 60 µΩm for K7BaAl8Si38, more than ten times lower. When introducing more Ba substitution in sample Ba 1.5, electrical resistivity was further decreased to 30 µΩm. Ba 2 sample has high electrical resistivity of 225 µΩm at 323 K with its low carrier concentration. The samples all showed negative Seebeck coefficient, indicating they are n-type semiconductor. The absolute Seebeck coefficient values increases with temperature linearly. As the Seebeck coefficient follows the Pisarenko relation: 𝑺=

𝟖𝝅𝟐 𝒌𝟐𝑩 ∗ 𝝅 𝟐/𝟑 𝒎 𝑻( ) 𝟑𝒆𝒉𝟐 𝟑𝒏

Seebeck coefficient values decrease with increasing carrier concentration level. The K8Al8Si38 sample showed very high absolute Seebeck value in the range of 200 µV/K to 300 µV/K, consistent with its very low carrier concentration. Ba substitution of K7BaAl8Si38 and K6.5Ba1.5Al8Si38 decreased the absolute Seebeck values of the samples, which is not beneficial for thermoelectric performance. The Seebeck coefficient and electrical resistivity are interrelated by carrier concentration and Ba substitution introduced conflicting influence on the electrical resistivity and the Seebeck coefficient. The thermoelectric power factors (α2/ρ) of the samples are therefore calculated to determine the optimal carrier concentration level. The calculated power factors of sample K8Al8Si38, Ba 1, Ba 1.5 and Ba 2 are plotted in Figure 8(a). The power factor of K8Al8Si38 was around 70 µW/mK2 at 373 K. The Ba substitution samples showed great enhancement in power factor at around 373 K, which was ~ 150µW/mK2 at 373 K. The power factors increase with increasing temperature for all samples. At the higher temperature range, Ba 1.5 has a larger thermoelectric powder factor than sample Ba 1 than Ba 2, mainly because of its lower electrical resistivity. The high temperature thermal conductivity was not measured experimentally because dense pellets of sufficient size were not obtained. However, thermal conductivity can be estimated from the equation κ = κe + κl. Here assuming that κl stays constant in the temperature range of 300 K to 873 K, and that κe can be calculated from the measured electrical resistivity with Wiedemann-Franz equation κe = LT/ρ, where L is Lorentz number, 2.44x10-8 WΩK-2, then the thermal conductivity can be estimated for temperatures above 300 K. It is well-known that κl decreases with increasing temperature and this has been shown experimentally for many type I clathrate compounds.14, 36 Therefore, this manner of estimating κ with a constant κl results in an upper bound of the experimental value. A further correction using a Lorentz number calculated from the experimental Seebeck coefficient can also be applied. The Lorentz number can be corrected according to the absolute Seebeck coefficients at the corresponding temperature, following the equation of L = 1.5 + exp[-│S│/116] (L is in 10−8WΩK−2).37 The corrected Lorentz number is applied to



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sample Ba 1.5 which possesses the highest thermoelectric power factor; these results are labeled with * in Figures 7 and 8. The calculated thermal conductivities are shown in Figure 7; consistent with expectations, K6.5Ba1.5Al9.5Si36.5 provides the lowest thermal conductivity. The thermoelectric figures of merit (zT) were calculated from the experimental thermoelectric power factor and the estimated thermal conductivity; these are plotted in Figure 8(b). Ba-substituted samples show enhanced thermoelectric performance over either the all-K or all-Ba samples with the sample Ba 1.5* providing a zT close to 0.4 at 873 K with thermal conductivity calculated from corrected Lorentz number with Seebeck coefficient. The increase in zT is attributed to the enhanced electrical conductivity and the decreased lattice thermal conductivity with the mixed occupied guest atom sites.

Figure 7. Calculated thermal conductivity for K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5. Ba 2 samples in the temperature range of 373 K to 873 K based on the equation κ = κe + κl with κe calculated from Lorentz number of 2.44x10-8 WΩK-2 and κl obtained at 300 K (Figure 5). The thermal conductivity of Ba 1.5* was calculated with Lorentz numbers derived from the high temperature Seebeck coefficient.


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Figure 8. (a) Temperature dependence of the thermoelectric power factors (S2σ) for K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 samples from 323 K to 873 K. (b) Temperature dependence of zT for K8Al8Si38, K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2 samples from 373 K to 873 K. Ba 1.5* zT used thermal conductivity calculated with Lorentz numbers derived from the high temperature Seebeck coefficient. 4. Conclusions: K8Al8Si38 and K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) with type I clathrate structure were synthesized. The Ba-substituted samples were investigated by Rietveld refinement of synchrotron powder XRD patterns. The refinement reveals Ba has occupancy preference at the 2a site of the smaller 20-vertex cage, and the 6d site shows anisotropic ADPs, dependent on temperature. Both low (2 – 300 K) and high (373 – 873 K) transport properties were measured. Compared to K8Al8Si38, K8-xBaxAl8+xSi38-x showed decreased electrical resistivity and semi-metallic trend with increasing temperature. K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), Ba 1, Ba 1.5 and Ba 2



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samples possess lower lattice thermal conductivity. The thermoelectric power factors of K8Al8Si38 and K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), were calculated from measured electrical resistivity and Seebeck coefficient in the temperature range of 373 K to 873 K. The zT of K8Al8Si38 and K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2), were estimated with calculated thermal conductivity based on extrapolation of the room temperature lattice thermal conductivity plus calculated electronic thermal conductivity. Since lattice thermal conductivity decreases with increasing temperature, the maximum zT values are conservative estimates for the high temperature range. With Ba substitution, K8-xBaxAl8+xSi38-x samples showed improved thermoelectric power factor and lower lattice thermal conductivity. These results show that earth abundant clathrate phases are promising materials for high temperature thermoelectric applications. 5. Acknowledgements: NSF grant DMR-1405973 is acknowledged for funding. The authors gratefully acknowledge Nicholas Botto for the microprobe analysis. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

6. Supporting information available: Lattice parameters (Å) of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) phases obtained from Rietveld refinement of Synchrotron XRD at 295 K are plotted in comparison with lattice parameters (Å) of Ba8AlxSi38-x (x = 8, 10, 12, 14, 15)15 in SFigure 1. Laboratory XRD patterns of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) with whole profile fitting are shown in SFigure 2. Rietveld refined synchrotron XRD patterns of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) collected at 100 K are shown in SFigure 3. Powder XRD pattern collected from the decomposed K8Al8Si38 sample after TG-DSC measurement up to 1000 ◦C is shown in SFigure 4. SFigure 5 shows the raw thermal conductivities and corrected data of K8-xBaxAl8+xSi38-x (x = 1, 1.5, 2) measured from PPMS. 7. References: 1.

Kasper, J. S.; Hagenmul.P; Pouchard, M.; Cros, C., Clathrate Structure of Silicon Na8Si46 and

NaxSi136 (x

Tuning Thermoelectric Properties of Type I Clathrate K8âxBaxAl8+

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