Journal ofMaterials Chemistry A

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Institute of Chemical Sciences and the C

Recovery, Heriot-Watt University, Edinburgh

ac.uk; Fax: +44 131 451 3180; Tel: +44 131

† Electronic supplementary informationprocedures, electrical property data,thermogravimetric analysis data. See DOI

‡ Present address: Department ofWhiteknights, Reading RG6 6AD, UK;+44 118 378 6585; Fax: +44 118 378 6331.

Cite this: J. Mater. Chem. A, 2013, 1,6553

Received 28th March 2013Accepted 19th April 2013

DOI: 10.1039/c3ta11264k

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Co3M2S2 (M ¼ Sn, In) shandites as tellurium-freethermoelectrics†

Jack Corps, Paz Vaqueiro and Anthony V. Powell‡*

Chemical substitution in Co3Sn2�xInxS2 (0# x # 2) enables tuning of

the Fermi level within narrow bands of Co d-states. This results in a

compositionally induced double metal–semiconductor–metal transi-

tion and manipulation of the thermoelectric power factor. The

maximum power factor (14 mW cm�1 K�2) is found for x ¼ 0.85,

which corresponds to ZT z 0.2 at 300 K.

Thermoelectric devices, consisting of an array of n- and p-typesemiconductors, are capable of effecting direct conversion ofwaste heat into electrical power. The efficiency of a device isdetermined by the physical properties of its constituent mate-rials, embodied in a gure-of-merit, ZT¼ S2sT/k, where S, s andk are the Seebeck coefficient, electrical and thermal conductiv-ities, respectively. Current commercial devices are constructedfrom alloys of Bi2Te3 (ZT z 1 at room temperature). The widerapplication of the technology is limited by relatively low effi-ciencies and increasingly, by the rising cost of the scarceelement, tellurium. The opportunities afforded by thermoelec-tric technology to make more efficient use of precious fossil fuelreserves have led to an upsurge in growth in the search for newmaterials. This effort has generated a wide range of candidatematerials, including LAST-m1 and related materials,2 skutter-udites3 and PbTe-based materials.4 More recently, focus hasbeen directed towards materials containing abundant elementssuch as sulphur5 and silicon.6 Whilst much of the recent efforthas focused on materials for high-temperature applications,there are signicant opportunities for energy harvesting from

entre for Advanced Energy Storage and

EH14 4AS, UK. E-mail: a.v.powell@hw.

451 8034

(ESI) available: Details of experimentalpowder X-ray diffraction data and: 10.1039/c3ta11264k

Chemistry, University of Reading,E-mail: a.v.powell@reading.ac.uk; Tel:

Chemistry 2013

low-grade (T # 350 �C) waste heat produced in the steel,ceramic, paper and petrochemical industries.7

A number of design strategies have been devised to introduceinto a material, the counter-intuitive combination of propertiesrequired for high thermoelectric performance. This includes thephonon-glass-electron crystal approach of Slack,8 band tuningand the introduction of resonant states,9 nanostructuring10 andmanipulation of interface scattering11 and low dimensionality.12

The last approach is derived from the theoretical work of HicksandDresselhaus13who demonstrated that quantum connementeffects in a low dimensional structure may signicantly enhanceS. In particular, the reduced length scale creates a more struc-tured density of states, N(E), potentially leading to sharp changesin N(E). Since S is related to the derivative of the electronicdensity of states, N(E), at the Fermi level (EF), through the Mottrelationship,14 S may be increased, provided that the position ofEF can be tuned to one of the discontinuities in N(E). Here weexploit electron lling effects, effected by chemical substitutionin a pseudo two-dimensional mixed-metal sulphide with theshandite structure, to enhance the electrical property contribu-tions to the thermoelectric performance. In this way we haveproduced a tellurium-free material with a high power factor (S2s)at temperatures close to ambient, demonstrating the promise ofthe approach in creating materials for energy recovery from low-grade waste heat.

The shandite structure (Fig. 1) of general formula A3M2S2(A ¼ Ni, Co, Rh, Pd; M ¼ Pb, In, Sn, Tl)15 consists of sheets ofmetal atoms with the kagome-type topology stacked in an ABCfashion. Triangular arrays of A atoms in adjacent layersgenerate trigonal anti-prismatic inter-layer sites. Each of the A3

triangles is capped above or below the kagome sheets by asulphur atom. M atoms are distributed over M(2) sites in thekagome layers and M(1) inter-layer sites. Band structure calcu-lations16 of Co3Sn2�xInxS2 (x ¼ 0, 1, 2) reveal that states in thevicinity of EF are of predominantly Co d-character, giving rise tosharp, narrow bands. Electron populations indicate metallicbehaviour for phases with x¼ 0 and 2. A pseudo gap is predictedin the former, whilst spin-polarised band structure calculations

J. Mater. Chem. A, 2013, 1, 6553–6557 | 6553

Fig. 1 The shandite structure of A3M2S2 phases (a), consisting of layers of A andM(2) cations with a kagome topology (b), capped by sulfide anions. Powder X-raydiffraction data for end-member phases and selected mixed-metal compositionsare presented in (c). A, M and S atoms are represented by blue, red and yellowcircles, respectively. M(1) cations are located in trigonal antiprismatic inter-layersites, shown as red polyhedra in (a).

Fig. 2 Compositional variation of the lattice parameters (R�3m: hexagonal setting)determined by powder X-ray diffraction for the series Co3Sn2�xInxS2 (0 # x # 2).

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indicate a loss of spin degeneracy. Majority and minority spinstates respectively show metallic and semiconducting charac-teristics, leading to the description of Co3Sn2S2 as a type IA halfmetal. Intriguingly, the calculations predict a transition tosemiconducting behaviour in Co3SnInS2 (x ¼ 1) when cationordering is complete. The relationship between the In/Sn orderand the electrical properties in this phase has recently beeninvestigated.17 The ferromagnetic (TC ¼ 175 K) end-memberphase, Co3Sn2S2, despite its metallic properties, exhibits anunusually large Seebeck coefficient.18 The combination of a lowelectrical resistance, high Seebeck coefficient and the possibilityto optimize the electron-transport properties by chemicalsubstitution led us to investigate the thermoelectric propertiesof the series Co3Sn2�xInxS2 over the range 0 # x # 2, whichcorresponds to a span of two in the electron count.

Samples of Co3Sn2�xInxS2 (0# x# 2) were prepared by solid-state synthesis from the powdered elements in evacuated sealedfused-silica tubes at temperatures in the range 773 # T/K #

1073 (ESI†). Powders were characterized by X-ray diffraction(Bruker, D8 Advance, Cu-Ka1 ¼ 1.5405 A). Samples for electricaland thermal transport property measurements were hot pressedat 993 K and 60 bar for 30 min under an atmosphere of oxygen-free N2. This procedure yielded pellets with densities in excessof 90% of the theoretical density. The thermal stability ofCo3SnInS2 was investigated by thermogravimetric analysis(DuPont Instruments 951 Thermal Analyser) at temperatures to1273 K in both nitrogen and oxygen atmospheres.

Powder X-ray diffraction data (Fig. 1(c)) for all compositionscan be indexed on the basis of the rhombohedral (R�3m) unit cell

6554 | J. Mater. Chem. A, 2013, 1, 6553–6557

of the end-member Co3Sn2S2 phase. Rietveld analysis using theatomic parameters for Co3Sn2S2 for the initial structural model,with a statistical distribution of Sn and In atoms over the M(1)and M(2) sites, reveals (Fig. 2) that whereas the c parameterincreases smoothly with increasing indium content, the aparameter initially decreases before remaining essentiallyconstant above a composition of x ¼ 1.2. The substitution of tinby the larger indium cation therefore leads to an increase in theseparation between the kagome layers, effectively increasing thetwo-dimensional character of the structure. Thermogravimetricanalysis of Co3SnInS2 (ESI†) demonstrates that this phase,which may be considered to be representative of all members ofthe series gained <1% weight at temperatures to 1050 K innitrogen and 673 K in oxygen ows.

Electrical resistivity data (Fig. 3) for the end-member phaseCo3Sn2S2 are consistent with metallic behaviour, which isretained to a composition corresponding to x ¼ 0.95. At thepoint at which the stoichiometric phase Co3SnInS2 (x ¼ 1) isreached, the absolute value of the resistivity increases markedlyand a semiconducting temperature dependence is observed.Further substitution of tin by indium above the 50% leveldecreases the resistivity, although the semiconducting behav-iour persists over a narrow range of composition extending to x¼ 1.05, before metallic behaviour is restored for x $ 1.1. Thevariation in electrical properties and the double metal tosemiconductor to metal transition is perhaps most clearlyillustrated by examination of the resistivity at a given temper-ature as a function of composition (Fig. 4).

The Seebeck coefficient (Fig. 3) of the end-member Co3Sn2S2phase reveals that the dominant charge carriers are electronswhile the linear S(T) dependence is consistent with the metallicbehaviour indicated by the resistivity measurements. Withincreasing indium content, |S| increases signicantly, attaininga value of�185 mV K�1 at 267 K, for x¼ 0.9. Amarkedly differentS(T) behaviour is observed on entering the semiconductingregion (1 # x # 1.05) and |S| decreases towards zero. In thesecond metallic region the Seebeck coefficient is positive, andthe material with a composition corresponding to x ¼ 1.1 maybe described as a p-type metal. At higher levels of indiumincorporation, the Seebeck coefficient changes from positive tonegative values on cooling, suggesting that these are mixed

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 Electrical property data and power factors (P.F.) for selected compositionsin the series Co3Sn2�xInxS2 (0 # x # 2). Open points denote semiconductingmaterials.

Fig. 4 Compositional variation of the electrical resistivity (lower plot) and See-beck coefficient (upper plot) of Co3Sn2�xInxS2 (0 # x # 2) at 300 K.

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conductors, the overall sign of the Seebeck coefficient beingdetermined by the conductivity-weighted sum of electron andhole conduction at a given temperature. The compositionalvariation in S is illustrated in Fig. 4.

The compositional variation in the electronic properties ofthe series Co3Sn2�xInxS2 may be understood in terms of the

This journal is ª The Royal Society of Chemistry 2013

changing electron population induced by substitution. The end-member phase, Co3Sn2S2, has 47 valence electrons. Bandstructure calculations (both spin-polarised and non-polarised)16

locate EF in the 24th (of 31) valence bands, with the result thatthis narrow band of predominantly Co d-character is half-occupied. Replacement of tin by indium progressively removesvalence electrons: decreasing by one electron for each unitincrement in indium composition. Initially, this reduction inthe electron count does not adversely affect the electricalresistivity which remains low and of metallic character.However, as EF is lowered through the Co d-band, the magni-tude of the Seebeck coefficient increases markedly, suggestingthat EF may be lowered to energies where the curvature inN(E) ismore pronounced. At the mid-point in the series, Co3SnInS2 (x¼ 1), the valence electron count falls to 46 electrons. As thiscomposition is approached, the resistivity increases and asemiconducting region is entered. Band structure calculationsfor the x ¼ 1 composition reveal that the highest occupied stateis a lled band (band 23), accounting for the semiconductingnature of this material. Further indium substitution will intro-duce holes into this band, which is consistent with the changeto p-type behaviour indicated by the Seebeck coefficientmeasurements. Initially, it appears that the concentration ofholes is insufficient to allow metallic behaviour and the mate-rial with a composition x ¼ 1.05 retains the semiconductingproperties of the stoichiometric mid-series phase. However, athigher levels of indium substitution, metallic behaviour isrestored as the valence electron count decreases progressively to45 electrons in the end-member phase, Co3In2S2, although thetransport properties appear to have contributions from bothelectrons and holes.

The marked increase in |S| with increasing indium contentoutweighs the slight rise in resistivity as the semiconducting(x ¼ 1) composition is reached, with the result that the ther-moelectric power factor attains maximum values for composi-tions in the range 0.85–0.9, decreasing sharply as the metal–semiconductor boundary is approached. The power factor rea-ches values of 12–14 mW cm�1 K�2, at temperatures in theregion of ambient, which compare favourably with those ofcommercial Bi2Te3-based materials (36 mW cm�1 K�2). Themaximum thermoelectric performance occurs at a compositionjust prior to the onset of semiconducting behaviour. In thesemiconducting region, the power factors are low, due to bothan increased resistivity and a marked reduction in |S|, whichmay be attributed to a mixed conduction in which both holesand electrons act as charge carriers. Beyond the 50% level ofindium incorporation, the power factors remain low, principallydue to the low Seebeck coefficient, which may indicate that theDOS of the lower sub-band exhibits less curvature than that ofthe upper band which is populated at x < 1.0. Preliminarymeasurements of the thermal conductivity for samples in thecompositional range 0.8 # x # 0.9, where the highest powerfactors are observed at temperatures close to ambient, indicatevalues of the order of 2.1 W m�1 K�1 at 300 K, signicantlyreduced from that of the stoichiometric (x ¼ 0) phase (4.6 Wm�1 K�1). Therefore we can estimate the maximum ZT value forCo3Sn1.15In0.85S2 to be 0.2 at 300 K. An energy gap of 0.3 eV

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between the fully occupied (23rd) and half-occupied (24th) bandshas been calculated for Co3Sn2S2.19 This value is comparable tothat of PbTe, for which the maximum ZT occurs at ca. 700 K.20

This suggests that the optimal thermoelectric performancein substituted shandites, in which EF is tuned to the regionof the band gap, may occur at temperatures above 300 K:the temperature regime required for waste-heat recoveryapplications.

There have been a number of recent investigations onsulphide thermoelectrics motivated by the search for a tellu-rium-free replacement for Bi2Te3. Amongst conventional bulkmaterials, the Chevrel phase Cu4Mo6S8,21 LaGd1.92S3 (ref. 22)and CuxTiS2 (ref. 23) have been reported to exhibit ZT $ 0.4 atelevated temperatures. Materials derived from PbS showpromising high-temperature performance.5a In particular, theintroduction of low levels of dopants such as Bi2S3 SrS, Sb2S3and CaS into a PbS matrix5b produces substantial reductions inthermal conductivity, leading to n-type materials with ZT valuesas high as 0.8 at 723 K. The origin of this performanceenhancement appears to lie in the formation of nanoscaleprecipitates which result in pronounced phonon scattering.Similar improvements in performance have been realized bynanostructuring sodium-doped p-type PbS,5c through theintroduction of SrS and CaS, yielding ZT values as high as 1.11at 923 K. Investigations5d of Bi2S3, the sulphide analogue ofBi2Te3, albeit with a different structure, have demonstrated thatlow mol% levels of BiCl3 lead to maximum ZT values of theorder of 0.6 at 760 K. Common to all of these materials is thatthe highest ZT values are exhibited at elevated temperatures,suggesting that they may be viable alternatives to PbTe for high-temperature waste heat recovery. However, examination of thetemperature dependence of the ZT values for these materialsreveals ZT < 0.2 at temperatures below 350 K. The materialsreported here may exceed this performance at temperaturesslightly above ambient and show promise for low-grade waste-heat recovery.

Conclusions

In conclusion, we have demonstrated that by selecting a low-dimensional material whose band structure contains sharpnarrow bands in the vicinity of the Fermi level, it is possible touse chemical substitution to tune the electrical transportproperties to maximise the thermoelectric power factor byexploiting the increase in the Seebeck coefficient as the positionof the Fermi level within the band is varied. This result is inaccordance with the predictions of Hicks and Dresselhaus andproduces a 3-fold increase in the gure of merit over that of theternary end-member material. Chemical control of the electronpopulation produces an unusual double metal to semi-conductor to metal electronic transition as the indium contentis increased across the series. A maximum ZT¼ 0.2 is estimatedfor Co3Sn1.15In0.85S2. Further optimisation through for examplenanostructuring may render such materials viable for applica-tions in low-grade waste heat recovery. Key to the promisingperformance of these materials is a highly structured DOS,leading to narrow bands in the region of the Fermi level. This is

6556 | J. Mater. Chem. A, 2013, 1, 6553–6557

a characteristic feature of half metals, of which Co3Sn2S2 isdesignated type IA in the classication scheme of Coey et al.24 inwhich the gap is between crystal-eld split d-based bands. Thissuggests that other type I half-metallic ferromagnets should bescreened as prospective thermoelectric materials.

Notes and references

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