Improvement in the thermoelectric properties of pressure-tuned β- K 2 Bi 8 Se 13J. F. Meng, N. V. Chandra Shekar, D.-Y. Chung, M. Kanatzidis, and J. V. Badding Citation: Journal of Applied Physics 94, 4485 (2003); doi: 10.1063/1.1599049 View online: http://dx.doi.org/10.1063/1.1599049 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/94/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Significant enhancement of thermoelectric properties and metallization of Al-doped Mg2Si under pressure J. Appl. Phys. 115, 213705 (2014); 10.1063/1.4881015 Thermoelectric properties of p-type Bi0.5Sb1.5Te2.7Se0.3 fabricated by high pressure sintering method J. Appl. Phys. 112, 073708 (2012); 10.1063/1.4754840 Thermoelectric properties of Bi 2 Sr 2 Co 2 O 9 whiskers under hydrostatic pressure Appl. Phys. Lett. 81, 2379 (2002); 10.1063/1.1510160 Highly anisotropic crystal growth and thermoelectric properties of K 2 Bi 8−x Sb x Se 13 solid solutions: Bandgap anomaly at low x J. Appl. Phys. 92, 965 (2002); 10.1063/1.1481967 Multifold enhancement of the thermoelectric figure of merit in p-type BaBiTe 3 by pressure tuning J. Appl. Phys. 90, 2836 (2001); 10.1063/1.1394905

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Improvement in the thermoelectric properties of pressure-tunedb-K2Bi8Se13

J. F. Menga) and N. V. Chandra Shekara)

Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802

D.-Y. Chung and M. KanatzidisDepartment of Chemistry and Center for Fundamental Materials Research, Michigan State University,East Lansing, Michigan 48824

J. V. Baddingb)

Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802

~Received 31 January 2003; accepted 18 June 2003!

The thermoelectric power, electrical conductivity, crystal structure, and Raman active phononmodes of the semiconductorb-K2Bi8Se13 have been studied under pressure. There is a largeincrease in the electrical conductivity in the pressure range 0–1 GPa and a sharp maximum in themagnitude of the thermoelectric power at;1.4 GPa. X-ray diffraction studies revealed no structuralchange upon compression. The Raman active vibrational modes did not shift substantially withpressure, suggesting that the pressure induced changes in the phonon thermal conductivity may notbe large. The thermoelectric figure of merit,ZT, likely increases by a factor of at least 2 from itsambient pressure value of 0.23. This is the firstn-doped material for which we have observed a peakin the thermoelectric power suggestive of an electronic topological transition uponcompression. ©2003 American Institute of Physics.@DOI: 10.1063/1.1599049#

INTRODUCTION

Several approaches have been adopted in the search forimproved thermoelectric materials, ranging from the synthe-sis of bulk materials1,2 and quantum-well structures3 thatmay exhibit improved thermoelectric figure of merit,ZT (Tis the temperature andZ5S2s/k, S is the thermoelectricpower,s is the electrical conductivity, andk is the thermalconductivity!, to combinatorial synthesis techniques that rap-idly screen materials for desirable thermoelectric properties.4

Pressure tuning may offer a means to guide the search forhigher ZT thermoelectric materials.5,6 Observation of im-proved properties under pressure can provide targets for syn-thesis at ambient pressure and a valuable ‘‘proof of prin-ciple’’ finding. Pressure can be changed more rapidly thannew materials can generally be synthesized, allowing thephase space of interaction parameters that determine materi-als properties such as thermoelectric power to be exploredmore rapidly.5 Thus far we have observed substantial in-creases in the thermoelectric power and thusZT upon pres-sure tuning semiconductors such as NdxCe32xPt3Sb4 ,6

Sb1.5Bi0.5Te3 ,7 BaBiTe3 ,8 and Sr8Ga16Ge30.9 Pressure tunedSb1.5Bi0.5Te3 , in particular, is observed to exhibitZT in ex-cess of 2, substantially higher than that observed for any bulkthermoelectric material at ambient pressure.

Chung et al. have reported synthesis ofb-K2Bi8Se13

which has a complex monoclinic structure~Fig. 1!.10 It isattractive as a thermoelectric material because of its low

thermal conductivity~1.28 W/m K! and high thermoelectricpower (2200mV/K). When doped appropriately, its electri-cal conductivity at ambient pressure can be as high as 250S/cm, resulting in aZT of 0.23. Here we report a substantialincrease in the thermoelectric power of pressure tunedn-typeK2Bi8Se13. This is the firstn-type material we have found toexhibit a large increase in thermoelectric power upon pres-sure tuning under nonhydrostatic conditions at pressures aslow as 2 GPa. We also have investigated the effect of pres-sure on the electrical conductivity, low frequency Ramanmodes, and crystal structure of K2Bi8Se13.

EXPERIMENT

The experimental technique for the measurement of thethermoelectric power under pressure and an error analysis ofthis technique have been reported previously.11 Briefly,samples are compressed in a Mao–Bell diamond anvil cell atpressures between ambient and 10 GPa. A culet size of about1 mm and gasket hole of the same size are used. Typicalrectangular sample dimensions are 700mm350mm350mm. Monoclinic ZrO2 is used as the pressure transmit-ting medium. TypeK thermocouple junctions with 12.5mmleads are deeply embedded into the sample for the thermo-electric power measurements. A temperature gradient is in-duced by means of an infrared laser focused to a narrow line.During the thermoelectric power measurements, four lead-two probe resistance measurements are also performed bypassing current through two of the thermocouple leads whilemeasuring the voltage between the other two leads. This re-sistance measurement includes the contact resistances be-tween the sample and the thermocouple leads, allowing anupper bound to be placed this resistance, which was a few

a!On leave from Indira Gandhi Center for Atomic Research, Kalpa-kkam 603 102, Tamil Nadu, India.

b!Author to whom correspondence should be addressed; electronic mail:jbadding@chem.psu.edu

JOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 7 1 OCTOBER 2003

44850021-8979/2003/94(7)/4485/4/$20.00 © 2003 American Institute of Physics

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tenths of an ohm or less. We assume that if the contact re-sistance is low, the thermocouples and the sample are ingood thermal contact as well. The thermoelectric power as afunction of pressure measured by this technique for variousstandards such as bismuth~Fig. 2!, which undergoes severalphase transition up to 8 GPa,12 and several other materials isreproducible and in good agreement with previous reports.11

At ambient pressure with both diamonds in contact withthe ZrO2 media, the thermoelectric power measured forb-K2Bi8Se13 in the diamond cell is2196mV/K, in excel-lent agreement with the value of2200mV/K measured onbulk samples by standard techniques.10 Similar to the behav-ior observed for NdxCe32xPt3Sb4 ,6 Sb1.5Bi0Te3 , andBaBiTe3 ,8 the thermoelectric power versus pressure curvesare virtually identical in the increasing and decreasing pres-sure directions as long as the pressure is not increased above2–3 GPa. The reversibility of this measurement indicatesthat, at least up to these pressures, systematic errors associ-ated with changes in thermocouple configuration, etc., arenot significant. In contrast to the behavior observed forK2Bi8Se13, most of the semiconductors we compressed inthe diamond anvil cell did not exhibit sharp increases in thethermoelectric power. Separate electrical conductivity mea-surements are made in the standard configuration with fourindependent leads and four independent probes. Pressure ismeasured by ruby fluorescence method.13 Raman spectra are

collected using a DilorXY microfocus Raman spectrometerwith 514.5 nm excitation.

RESULTS AND DISCUSSION

Similar to several other small band gap semiconductorsthat exhibit increases in thermoelectric power upon pressuretuning, the electrical conductivity of K2Bi8Se13 increasesrapidly as a function of increasing pressure~Fig. 3! at modestpressures. The magnitude of the thermoelectric power in-creases rapidly with pressure~Fig. 4!, exhibiting a maxima at

FIG. 1. Crystal structure ofb-K2Bi8Se13 . Small dark atoms are potassium,small lightly shaded atoms are bismuth, and large unfilled atoms are sele-nium.

FIG. 2. Thermoelectric power of Bi standard vs pressure. Features in thethermoelectric power at 2.4, 2.7, and 7.2 GPa are associated with phasetransitions. The magnitude of the thermoelectric power at ambient pressureand between the phase transitions is in good agreement with a previousreport~see Ref. 12!. There are slight differences from the reported transitionpressures~2.5, 2.7, and 7.7 GPa! and the shape of curve near the transitionswhich are likely due to differences in nonhydrostatic stress.

FIG. 3. Electrical conductivity of K2Bi8Se13 vs pressure normalized by theambient pressure conductivity.

4486 J. Appl. Phys., Vol. 94, No. 7, 1 October 2003 Meng et al.

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1.38 GPa, whereS52324mV/K. At higher pressures themagnitude of the thermoelectric power decreases. Three in-dependent measurements on different samples confirmed thereproducibility of the maximum in the thermoelectric powerat 1.38 GPa. The behavior observed for K2Bi8Se13 is quali-tatively similar to that of p-type NdxCe32xPt3Sb4 ,6

Sb1.5Bi0.5Te3 ,7 and BaBiTe3 .8

It is important to ascertain whether the large change inthe thermoelectric power is associated with a structural tran-sition. High pressure x-ray diffraction studies in a Mao–Bellcell14 under the same compression conditions used for thethermoelectric power measurements revealed no structuralchanges in the pressure range of interest.

Pressure and stress can in general introduce significantchanges in electronic structure that have a major impact ontransport properties.15–21Sharp maxima in the thermoelectricpower as a function of pressure have been observed for semi-conductors undergoing an electronic topological or Lifshitztransition ~ETT!.22,23 Similar to the behavior observed forpressure tuned Sb1.5Bi0.5Te3 ,7,22 the thermoelectric powerversus pressure curve~Fig. 4! for K2Bi8Se13 is suggestive ofthe presence of an ETT. Such transitions occur when thetopology of the Fermi surface changes due to a perturbationsuch as compression or alloying.24 A band extremum, whichis associated with a Van Hove singularity in the density ofstates, crosses the Fermi energy, resulting in a change in thetopology of the Fermi surface and a strong energy depen-dence to the electronic density of states near the Fermi en-ergy. K2Bi8Se13 is the firstn-type semiconductor for whichwe have observed a maximum in the thermoelectric powersuggestive of an ETT. We note that because an ETT can beinduced by alloying as well as pressure, it may be possible toreproduce the improved behavior observed at high pressureat ambient pressure.

To determine the thermoelectric figure of merit,ZT, it isimportant to have the thermal conductivity and the electricalconductivity in addition to the thermoelectric power. We didnot measure the thermal conductivity under pressure, al-

though it is generally possible to place bounds it.6,7 Becausethere were variations in the electrical conductivity of variousK2Bi8Se13 samples, presumably due to adventitious varia-tions in doping level, and it is technically difficult to measurethe electrical conductivity and thermoelectric power simulta-neously under pressure in a diamond cell, there are uncer-tainties in estimatingZT at high pressure. Both the thermo-electric power and the electrical conductivity vary with thedoping level. K2Bi8Se13 samples which exhibit a thermoelec-tric power of ;2200mV/K, such as those compressed inthe diamond cell, typically have an electrical conductivitynear 250 S/cm and aZT of 0.23. Because the thermoelectricpower is squared in the expression forZT, increases in ittend to have a large effect onZT. For K2Bi8Se13 the ratiosquared of the maximum in the thermoelectric power at 1.38GPa to the thermoelectric power at ambient pressure is(2324/2196)2 or 2.7.

The electronic contribution to the thermal conductivity(ke) for K2Bi8Se13 is only at most 10% of the total thermalconductivity (k5ke1kp).10 Thus the phonon contribution(kp) to the thermal conductivity dominates at ambient pres-sure.kp typically increases with pressure, but not likely bymore than a factor of 2 for semiconductors at pressures lessthan 2 GPa.6,7 If the electrical conductivity of the samplescompressed in the diamond cell for thermoelectric powermeasurements increases by the factor of 4~at 1.38 GPa!observed for those samples compressed for electrical con-ductivity measurements, it is likely that the decrease inZTdue to increases in the thermal conductivity will be at leastcounterbalanced by the increase due to the electrical conduc-tivity ~the electronic contribution to the thermal conductivitywill also increase with the increase in the electrical conduc-tivity according to the Wiedemann–Franz law,25,26 but willstill not dominate!. Thus although it is difficult to quantita-tively estimateZT at high pressure, it appears that it willincrease by a factor of 2 or more due to the increase in thethermoelectric power, but may not approach values in excessof 1 observed for materials such as pressure tunedNdxCe32xPt3Sb4 ,6 and Sb1.5Bi0.5Te3 .7

Finally, we note that there is a slight softening of one ofthe Raman modes of K2Bi8Se13 under pressure~Fig. 5!. Onemode at 221 cm21 at 0.68 GPa, softens by 11 cm21 at 2.1GPa. Near an electronic topological transition phonon modesoften soften.27 The crystal structure of K2Bi8Se13 does notchange up to 2.1 GPa, so the shift in intensities and peakpositions upon compression is not due to altered crystal sym-metry. The change in the Raman spectrum observed between0.08 and 0.68 GPa is reversible. None of the mode positionschanges very much up to 2.1 GPa, suggesting that thechanges in phonon frequencies and phonon thermal conduc-tivity in this pressure range may not be very large.

SUMMARY

In summary,b-K2Bi8Se13 is an n-type thermoelectricmaterial for which we have found a substantial maximum inthe magnitude of the thermoelectric power upon compres-sion. The peak in the thermoelectric power is suggestive ofan electronic topological transition.ZT likely increases by a

FIG. 4. Thermoelectric power of K2Bi8Se13 vs pressure up to 10 GPa.

4487J. Appl. Phys., Vol. 94, No. 7, 1 October 2003 Meng et al.

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factor of at least 2 from its ambient pressure value of 0.24.These results suggest that there is further potential for opti-mizing the ambient pressure thermoelectric properties ofb-K2Bi8Se13.

1B. C. Sales, D. Mandrus, and R. K. Williams, Science272, 1325~1996!.2D. Y. Chung, T. Hogan, P. Brazis, M. Rocci-Lane, C. Kannewurf, M.

Bastea, C. Uher, and M. G. Kanatzidis, Science287, 1024~2000!.3L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B47, 12727~1993!.4X. D. Xiang, X. D. Sun, G. Briceno, Y. L. Lou, K. A. Wang, H. Y. Chang,W. G. Wallacefreedman, S. W. Chen, and P. G. Schultz, Science268, 1738~1995!.

5J. V. Badding, J. F. Meng, and D. A. Polvani, Chem. Mater.10, 2889~1998!.

6J. F. Meng, D. A. Polvani, C. D. W. Jones, F. J. DiSalvo, Y. Fei, and J. V.Badding, Chem. Mater.12, 197 ~2000!.

7D. A. Polvani, J. F. Meng, N. V. C. Shekar, J. Sharp, and J. V. Badding,Chem. Mater.13, 2068~2001!.

8J. F. Meng, N. V. C. Shekar, J. V. Badding, D. Y. Chung, and M. G.Kanatzidis, J. Appl. Phys.90, 2836~2001!.

9J. F. Meng, N. V. C. Shekar, J. V. Badding, and G. S. Nolas, J. Appl. Phys.89, 1730~2001!.

10D. Y. Chunget al., Chem. Mater.9, 3060~1997!.11D. A. Polvani, J. F. Meng, M. Hasegawa, and J. V. Badding, Rev. Sci.

Instrum.70, 3586~1999!.12B. T. Ferdin, N. V. Jaya, K. Anbukumaran, and S. Natarajan, Rev. Sci.

Instrum.66, 5636~1995!.13G. J. Piermarini, S. Block, J. Barnett, and R. A. Foreman, J. Appl. Phys.

46, 2774~1975!.14T. Atou and J. V. Badding, Rev. Sci. Instrum.66, 4496~1995!.15N. B. Brandt, M. Yu Lavrenyuk, N. Ya Minina, and A. M. Savin, Zh. Eksp.

Teor. Fiz.94, 235 ~1988!.16N. B. Brandt, M. Yu Lavrenyuk, N. Ya Minina, A. M. Savin, W. Kraak,

and R. Herrmann, Phys. Status Solidi B143, 601 ~1987!.17X. F. Chen, G. X. Tessema, and M. J. Skove, Phys. Rev. B48, 13141

~1993!.18T. A. Davis, W. Schaffer, M. J. Skove, and E. P. Stillwell, Phys. Rev. B39,

10094~1989!.19T. M. Tritt, E. P. Stillwell, and M. J. Skove, Phys. Rev. B34, 6799~1986!.20Y. T. Tseng, G. X. Tessema, and M. J. Skove, Solid State Commun.82,

841 ~1992!.21Y. T. Tseng, G. X. Tessema, and M. J. Skove, Solid State Commun.94,

867 ~1995!.22E. S. Itskevich, L. M. Kashirskay, and V. F. Kraidenov, Semiconductors

31, 276 ~1996!.23I. M. Lifshitz, Sov. Phys. JETP11, 1130~1960!.24Ya. M. Blanter, M. I. Kaganov, A. V. Pantsulaya, and A. A. Varlamov,

Phys. Rep.245, 159 ~1994!.25H. J. Goldsmid,Electronic Refrigeration. ~Pion Ltd., London, 1986!.26N. W. Ashcroft and N. D. Mermin,Solid State Physics~Holt, Rinehart,

and Winston, 1976!.27L. Dagens, J. Phys. F: Met. Phys.8, 2093~1978!.

FIG. 5. Raman spectra of K2Bi8Se13 at 0, 0.08, 0.68, 1.3, and 2.1 GPa.

4488 J. Appl. Phys., Vol. 94, No. 7, 1 October 2003 Meng et al.

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