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Materials Science and Engineering B 149 (2008) 53–57

Transport and thermoelectric properties ofCrSb2−xTex at low temperatures

H.J. Li, X.Y. Qin ∗, D. LiKey Laboratory of Materials Physics, Institute of Solid State Physics Chinese Academy of Sciences, 230031 Hefei, PR China

Received 2 September 2007; received in revised form 4 December 2007; accepted 8 December 2007

bstract

Substitutional compounds CrSb2−xTex (0 ≤ x ≤ 0.05) were synthesized and the effects of Te substitution on transport and thermoelectric propertiesf CrSb2−xTex were investigated from 7 K to 310 K. It was established that the resistivity ρ, absolute value of thermopower |S| and thermalonductivity decrease with increasing Te content. The decrease of the resistivity ρ and absolute value of thermopower |S| could be mainlyxplained by an increase in electron concentration due to Te substitution for Sb, while the decrease of thermal conductivity of CrSb2−xTex could beaused by the enhancement of phonon scattering by the dopant. Besides, it was discovered that a transition from the semiconducting state (0.01 < x)o the metallic state (x > 0.01) occurred within the Te contents 0.01 < x < 0.02 owing to heavy doping. As a result, thermoelectric figure of merit, ZT,

f the properly doped CrSb2−xTex compounds (x = 0.005, 0.01, 0.03 and 0.05) was improved at T > ∼180 K. Specifically, at 310 K the ZT value ofrSb1.97Te0.03 is ∼27% larger than that of CrSb2, indicating that thermoelectric properties of CrSb2 can be improved by an appropriate substitutionf Te for Sb. 2008 Elsevier B.V. All rights reserved.

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eywords: CrSb2−xTex; Resistivity; Thermopower; Thermal conductivity

. Introduction

Thermoelectric materials have attracted a great deal ofttention in the past decade for their potential applications toefrigerators and electric-power generators [1–5]. The efficiencyf a thermoelectric material is determined by the dimension-ess figure of merit, ZT (ZT = S2T/ρκ, here S, ρ, κ and Tre the thermopower, electrical resistivity, thermal conductivitynd absolute temperature, respectively). Hence, a good ther-oelectric material should have high S, low κ and ρ. Several

lasses of materials are currently under investigation, whichnclude Bi2Te3-based materials [6], PbTe-based materials [7],omplex chalcogenides [8], skutterudites [9–11], half-Heuslerlloys [12,13], metal oxides [14–16], clathrates compounds

17], and pentatellurides [18]. Recently, it was reported thatgPb18SbTe20-based material achieved a ZT of 2.2 at ∼800 K

7]. Nevertheless, the performance of the state-of-the-art ther-

∗ Corresponding author at: Institute of Solid State Physics, Chinese Academyf Science, P.O. Box 1129, 230031 Hefei, Anhui, PR China. Tel.: +86 551592750; fax: +86 551 5591434.

E-mail address: xyqin@issp.ac.cn (X.Y. Qin).

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921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2007.12.004

oelectric materials cannot meet the requirements of large-scalendustrial applications, and explorations of new thermoelectric

aterials with better performance are of great importance touture applications.

CrSb2 has the orthorhombic marcasite structure (crystalroup Pnnm): each Cr atom has a distorted octahedral coor-ination of six nearest-neighbor Sb atoms; each Sb atom isetrahedrally coordinated by three Cr atoms and one Sb atom19–23]. CrSb2 is an antiferromagnetic compound with theeel temperature of TN = 273 ± 2 K [21,22,24,25]. The mag-etic properties of Cr1−xFexSb2 (0 ≤ x ≤ 1) were reported byjekshus et al. [24], which showed that the Neel tempera-

ure decreased with increasing Fe content and diminished toero at about x = 0.5. Takahashi et al. [26] have studied elec-rical resistivity ρ and magnetic properties of Cr1−xRuxSb2x = 0, 0.1). Their results showed that in the plot of ln ρ versus000/T for CrSb2 there was a plateau located in the tem-erature range from 50 K to 80 K; correspondingly a sharpeak appeared at ∼55 K in the plot of magnetic susceptibil-

ty χ versus temperature. Harada et al. [21] have investigatedhe resistivity and thermopower of the system Cr1−xRuxSb20 ≤ x ≤ 1) from ∼80 K to ∼650 K. Their result showed thathe absolute thermopower of Cr1−xRuxSb2 alloys increased

5 and Engineering B 149 (2008) 53–57

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sCtibslap∼lwhile the lattice parameter a decreases; nevertheless, the volumeV of unit cell of CrSb2−xTex becomes larger after doping. Theseresults indicate that Te has been successfully substituted for Sb,leading to the formation of CrSb2−xTex compounds.

4 H.J. Li et al. / Materials Science

rom |−62| �V K−1 to |−252| �V K−1 at 300 K by the sub-titution of Ru for Cr, and the room-temperature resistivityf Cr1−xRuxSb2 increased with increasing Ru content. On thether hand, the electrical resistance measurements conductedy Adachi et al. [19] and Harada et al. [21] showed thatrSb2 is a narrow gap semiconductor with the energy gap of.07 eV, which suggests that CrSb2 system would be a potentialandidate for thermoelectric applications. We have studied sub-titutional compounds Cr1−xMnxSb2 (0 ≤ x ≤ 0.05) [27], whichhowed that resistivity and low-temperature thermal conductiv-ty decrease with increasing Mn content, and ZT was improvedfter proper Mn substitution. In the present work, CrSb2 andts Te-doped compounds CrSb2−xTex were prepared, and theirlectrical resistivity (ρ), thermopower (S), and thermal con-uctivity (κ) were investigated in the temperature range of–310 K. Our results show that the thermoelectric propertiesf CrSb2−xTex system can be enhanced by proper Te dop-ng.

. Experimental methods

Polycrystalline samples of CrSb2−xTex (0 ≤ x ≤ 0.05) wereynthesized by using the following procedures. The mixturesf constituent elements Cr (purity: 99.9 at.%), Sb (purity:9.9 at.%), and Te (purity: 99.9 at.%) in stoichiometric pro-ortions were sealed into evacuated quartz tubes at a pressuref ∼2 × 10−2 Pa. Then they were heated slowly to 650 ◦Cnd isothermally kept for 7 days to form the compounds ofrSb2−xTex. The phase structure of the obtained samples washecked using XRD (Philips-X’PERT PRO diffractometer) withu K� irradiation. The accurate lattice parameters were deter-ined from the d-values of the XRD peaks using a standard

east-squares refinement method with a Si standard for calibra-ion.

To measure their transport properties, the synthesizedrSb2−xTex powders were compacted by hot-pressing (under

he pressure of 300 MPa) in vacuum at 400 ◦C to obtainulk samples. Then bar-shaped specimens with the size of13 mm × ∼3 mm × ∼1.5 mm were cut from the bulk samples

or the measurements. All the transport properties (resistivity,hermopower and thermal conductivity) were measured simulta-eously using a physical property measurement system (PPMS,uantum Design, USA) in the temperature range from 7 K to10 K.

. Results and discussions

.1. Phase determination and measurements of latticearameters

The XRD patterns of CrSb2−xTex are shown in Fig. 1. It can beeen from curve (a) that all the main diffraction peaks correspond

o those of standard JCPDS card of CrSb2 with orthorhombic

arcasite structure (crystal group Pnnm). As compared withhat of pristine CrSb2, no obvious changes are observed in XRDatterns for the doped samples, suggesting that all the doped

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ig. 1. XRD patterns (Cu K� irradiation) for CrSb2−xTex (0 ≤ x ≤ 0.05) at roomemperature.

pecimens have the same crystallographic structure as that ofrSb2. The peak around 2θ = 30◦ in XRD (Fig. 1) is (2 0 0) reflec-

ion for CrSb2, not a peak from an impurity. The fact that the peakntensity decreases with increasing doping content of Te woulde caused by the distortion of CrSb2−xTex lattice after doping, aseen in Fig. 2. The lattice parameters of CrSb2−xTex were calcu-ated by using XRD data. The values of lattice parameters a, b, cnd the volume of unit cell V are presented in Fig. 2. The latticearameters a, b and c of our synthesized CrSb2 are ∼6.01 A,6.86 A and ∼3.27 A, respectively. As plotted in Fig. 2, the

attice parameters b and c increase with increasing Te content,

ig. 2. Composition (x) dependence of lattice parameters a, b, c and volume ofnit cell V for CrSb2−xTex (0 ≤ x ≤ 0.05) at room temperature.

H.J. Li et al. / Materials Science an

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ig. 3. Temperature dependence of resistivity ρ for CrSb2−xTex (0 ≤ x ≤ 0.05).he inset (plotted as ln ρ vs. T) shows temperature behavior of resistivity forrSb2 below 145 K.

.2. The electrical resistivity and thermopower

Fig. 3 shows the temperature dependence of the electri-al resistivity for CrSb2−xTex (0 ≤ x ≤ 0.05) in the temperatureange from 7 K to 310 K (the inset, plotted as ln ρ versus T,hows the temperature behavior of the resistivity for CrSb2elow 145 K). It can be seen from Fig. 3 that the resistiv-ty ρ of CrSb2−xTex decreases with increasing Te content x,specially at low temperatures. For instance, the resistivityecreases from 2.5 × 10−5 �m to 1.5 × 10−5 �m at 280 K androm 1.3 × 10−3 �m to 0.6 × 10−5 �m at 25 K as x increasesrom 0 to 0.05. Moreover, the temperature dependence ofhe electrical resistivity was found to be affected strongly bye substitution. As x ≤ 0.005, CrSb2−xTex shows semiconduc-

or behavior (i.e., dρ/dT < 0), the observed plateau locating at= ∼50–80 K on the curve of ln ρ versus T for CrSb2 (the insetf Fig. 3) was related to an electronic change of the electron-spinystem in the antiferromagnetic semiconductor [26]. By com-arison, one can find from Fig. 3 that the plateau only exists onhe curves of ln ρ versus T for the CrSb2−xTex samples with< 0.02. This result is in contrast to Mn substitution for Cr

27] where the plateau disappeared after doping. In addition,n agreement with pervious result reported by other authors21] an anomaly appears on the ρ–T curve at about 273 Konset temperature) for CrSb2, which was proved to originaterom antiferromagnetic transition [22,24,25]. One can see fromig. 3 that the position of the anomaly on the ρ–T curves forrSb2−xTex does not shift obviously with increase doping con-

ent of Te, suggesting that Te substitution for Sb hardly affectseel temperature of CrSb2. This phenomenon is reasonable,

or Te atoms possess non-magnetic feature with fully filled d-rbits.

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d Engineering B 149 (2008) 53–57 55

On the other hand, one notices that as doping content> 0.01 metallic behavior (i.e., dρ/dT > 0) was observed for

he CrSb2−xTex samples. In other words, a transition fromemiconductor to metallic state occurs for CrSb2−xTex at.01 < x < 0.02. Furthermore, with increasing doping content,he metallic behavior of CrSb2−xTex enhances. For instance,he slope of the linear part of the ρ–T plot increases from.3 × 10−8 � m K−1 to 4.7 × 10−8 � m K−1 with increasing Teontent from 0.02 to 0.05 (Fig. 3), indicating that the metal-ic behavior enhances in the case of heavier doping. Accordingo previous work [19–23], the bonding nature of CrSb2 has theovalent character: each Cr atom has a distorted octahedral coor-ination of six nearest-neighbor Sb atoms, and each Sb atom isetrahedrally coordinated by three Cr atoms and one Sb atom,hich means that it needs 14 electrons to form the covalentond per CrSb2. Therefore, the left two unpaired electrons perr do not contribute to the bond. Thus CrSb2 has a d-stateanifold per Cr atom 3d2 in CrSb6-octahedra [19,24,28,29],

aking a localized high-spin configuration [20,22,30]. Accordingo this bonding model, the substitution of Te([Kr] 4d105s25p4)or Sb([Kr] 4d105s25p3) in CrSb2 will introduce donor level (ormpurity states) into the energy gap of the host, which shouldead to an increase in the carrier (electron) concentration, lead-ng to a decrease of the resistivity. With increasing Te content,he donor levels (or impurity states) would broaden and shiftowards the edge of the conduction band. Specially, in the casef heavy doping, overlapping of the donor levels (or impu-ity band) with the conduction band would occur, which couldainly be responsible for the transition from semiconductor toetallic state for CrSb2−xTex at x > 0.01 (Fig. 3). Nevertheless,

ne cannot fully exclude possible occurrence of local ferroictates in CrSb2−xTex due to anharmonic electron–phonon inter-ctions and its possible influence on the band structure of theompounds (especially at low temperatures), for ferroelectric-ty was experimentally observed below 25 K by Gruhn et al.31] in compounds SexTe1−x (x = 0.08–0.13) with similar con-tituents to those of CrSb2−xTex. However, according to the workerformed by Gruhn et al. [31] the influences of this local fer-oic state on the band structure of CrSb2−xTex is very limitedspecially at relative higher temperatures (30–310 K)) since ouramples CrSb2−xTex are coarse-grained bulk material and theoping contents of Te in all the samples are very low (x ≤ 0.05),ll of which mean that the possibility of the occurrence of localerroic states (ferroelectricity) in CrSb2−xTex is very low and itscale (if any) is very limited. In other words, the effect of possi-le occurrence of local ferroic states in our CrSb2−xTex samplesn band structure is not a main factor, especially at relativelyigher temperatures (30–310 K).

The temperature dependence of thermopower for CrSb2−xTex

as shown in Fig. 4. The negative values of the thermopoweror all the samples in the whole temperature range investigatedean that the contribution of electrons to the thermopower

s dominant. As presented in Fig. 4, the absolute values of

he thermopower |S| for CrSb2 increase from |−42| �V K−1

o |−431| �V K−1 with increasing temperature, and then |S|ecreases to |−75| �V K−1 with further increasing temperature,eaving a large peak at ∼60 K. By comparing Fig. 3 with Fig. 4,

56 H.J. Li et al. / Materials Science and Engineering B 149 (2008) 53–57

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ig. 4. Variation of the thermopower S with temperature T for CrSb2−xTex

0 ≤ x ≤ 0.05).

ne notices that the observed large peak in the plot of S versus Tor CrSb2 corresponds to the plateau in the ln ρ versus T curve,uggesting strongly that they are correlated to each other. Onean see from Fig. 4 that the absolute values of the thermopowerS| became smaller after doping, and decrease monotonicallyith increasing Te content. For instance, the peak value of |S|ecreases from |−431| �V K−1 for x = 0 to |−177| �V K−1 for= 0.005, and |−117| �V K−1 for x = 0.01. Notably, no such anbvious peak could be detected in the S–T plot for CrSb2−xTex

ith x > 0.01. The decrease of |S| in the whole temperature rangeith increasing Te content would be caused by an increase in

he electron concentration due to Te doping, as is deduced in thehange behavior of the resistivity after Te substitution for Sb;hile the disappearance of the peak in the S–T plot (curves (d–f)f Fig. 4) could originate from the suppression of the electronichange of the electron-spin system due to the transition to theetallic state, which is in agreement with the elimination of the

lateau in the plot of ρ versus T as x > 0.01 (Fig. 3).

.3. Thermal conductivity and figure of merit

The thermal conductivity of CrSb2−xTex (0 ≤ x ≤ 0.05) waseasured in the temperature range from 7 K to 310 K (Fig. 5).

ig. 5. Plot of thermal conductivity κ vs. temperature T for CrSb2−xTex

0 ≤ x ≤ 0.05).

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ig. 6. Plot of lattice thermal conductivity κL vs. temperature T for CrSb2−xTex

0 ≤ x ≤ 0.05). The inset shows mobile charge carriers thermal conductivity κC

s. temperature T for CrSb2−xTex (0 ≤ x ≤ 0.05).

s shown in Fig. 5, the thermal conductivity for all the sam-les increases with increasing temperature, reaching a maximumalue at a certain temperature and then decreasing with furtherncreasing temperature. The maximum values of the ther-

al conductivity may occur when phonon–phonon Umklappcattering becomes important [32]. Total thermal conductiv-ty of a solid can be expressed by the sum of a latticeomponent (κL) and a component of mobile charge carriersκC) as � = κL + κC. The κC values can be estimated from

iedemann–Franz’s law as κC = L0T/ρ (here, L0 is the Lorentzumber and L0 = 2.44 × 10−8 V2 K−2 for free electrons, ρ islectrical resistivity). Consequently, lattice thermal conductivityL can be obtained from κ and κC (inset of Fig. 6), as shown inig. 6. By comparing Fig. 5 with Fig. 6, one finds that the thermalonductivity of all compounds comes mainly from their latticehermal conductivity. The lattice thermal conductivity of theoped samples decreases greatly as compared with that of CrSb2n the temperature range of ∼30–100 K. For instance, the latticehermal conductivity decreases from 29.7 W m−1 K−1 for x = 0

o 12.9 W m−1 K−1 for x = 0.05 at ∼50 K. The more the dopede content, the lower the lattice thermal conductivity is. This

arge and continuous decrease of low-temperature lattice thermal

ig. 7. Variation of ZT with temperature T for CrSb2−xTex (0 ≤ x ≤ 0.05).

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onductivity with increasing Te content could be ascribed to thenhancement of the phonon scattering by impurity (Te) atoms.

Fig. 7 gives the temperature dependences of the dimension-ess figure of merit ZT for CrSb2−xTex. As shown in Fig. 7,he ZT values for all the samples increase with increasingemperature. At T < ∼180 K, the ZT values of the doped sam-le CrSb1.995Te0.005 are larger than those of other compounds.bove ∼180 K, however, ZT of heavily doped compoundsrSb2−xTex with x = 0.03 is obviously the largest. Specifically,T value for CrSb1.97Te0.03 is 0.014 at 310 K, which is about7% larger than that of CrSb2, indicating that thermoelectricroperties of CrSb2 can be improved by proper substitution ofe for Sb.

. Conclusions

Substitutional compounds CrSb2−xTex (0 ≤ x ≤ 0.05) wereynthesized and the effect of Te substitution on transport andhermoelectric properties of CrSb2−xTex was investigated atemperatures below 310 K. The results indicate that electricalesistivity of CrSb2−xTex decreases substantially with increasinge content, which could be ascribed to the increase in elec-

ron concentration caused by Te substitution for Sb. Besides, theemperature behavior of the resistivity of CrSb2−xTex changedensitively with doped Te content, and CrSb2−xTex was foundo transit from semiconductor to the metallic state at Te con-ent 0.01 < x < 0.02. In addition, accompanying the transitiono metallic state, the plateau appearing in the plot of ρ versus

and the large peak in the S–T plots for CrSb2−xTex disap-ears. The observed decrease of |S| with Te content could bexplained by an increase in carrier concentration, which is con-istent with the decrease in electrical resistivity. Moreover, thexperiments show that the low-temperature lattice thermal con-uctivity of CrSb2−xTex decreases substantially with increasinge content presumably due to the enhancement of phononcattering by the dopant. At T > ∼180 K, ZT of doped sam-le CrSb2−xTex (x = 0.005, 0.01, 0.03 and 0.05) was improved,nd specifically the ZT of CrSb1.97Te0.03 is ∼27% greater thanhat of CrSb2 at 310 K, indicating that thermoelectric proper-ies of CrSb2 can be enhanced by proper substitution of Te forb.

cknowledgement

Financial support from National Natural Science Foundationf China (nos. 10774145, 50472097 and 10504034) is gratefullycknowledged.

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d Engineering B 149 (2008) 53–57 57

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