Journal of Solid State Chemistry 203 (2013) 333–339

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Journal of Solid State Chemistry

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High figure of merit and thermoelectric properties of Bi-dopedMg2Si0.4Sn0.6 solid solutions

Wei Liu a,b, Qiang Zhang a, Kang Yin a, Hang Chi b, Xiaoyuan Zhou b,Xinfeng Tang a,n, Ctirad Uher b,n

a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Chinab Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA

a r t i c l e i n f o

Article history:Received 22 March 2013Received in revised form26 April 2013Accepted 28 April 2013Available online 7 May 2013

Keywords:Mg2.16(Si0.4Sn0.6)1−yBiyBi dopingThermoelectric propertiesPoint defects scattering

96/$ - see front matter & 2013 Elsevier Inc. Ax.doi.org/10.1016/j.jssc.2013.04.041

esponding author. Fax: +1 7347639694; +86ail addresses: tangxf@whut.edu.cn (X. Tang), c

a b s t r a c t

The study of Mg2Si1−xSnx-based thermoelectric materials has received widespread attention due to apotentially high thermoelectric performance, abundant raw materials, relatively low cost of modules, andnon-toxic character of compounds. In this research, Mg2.16(Si0.4Sn0.6)1−yBiy solid solutions with thenominal Bi content of 0≤y≤0.03 are prepared using a two-step solid state reaction followed by sparkplasma sintering consolidation. Within this range of Bi concentrations, no evidence of second phasesegregation was found. Bi is confirmed to occupy the Si/Sn sites in the crystal lattice and behaves as anefficient n-type dopant in Mg2Si0.4Sn0.6. Similar to the effect of Sb, Bi doping greatly increases theelectron density and the power factor, and reduces the lattice thermal conductivity of Mg2.16Si0.4Sn0.6

solid solutions. Overall, the thermoelectric figure of merit of Bi-doped Mg2.16Si0.4Sn0.6 solid solutions isimproved by about 10% in comparison to values obtained with Sb-doped materials of comparable dopantcontent. This improvement comes chiefly from a marginally higher Seebeck coefficient of Bi-doped solidsolutions. The highest ZT∼1.4 is achieved for the y¼0.03 composition at 800 K.

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1. Introduction

Propelled by the demand for clean and sustainable energysources, thermoelectricity has become an important part of aresearch portfolio seeking to identify novel energy materials forcooling and power generation applications. In order to enhancethe efficiency of thermoelectric materials and broaden applica-tions of thermoelectricity, one must identify and design novelstructures with the dimensional thermoelectric figure of meritmuch higher than ZT∼1, the current state-of-the-art. The figure ofmerit ZT comprises three key transport parameters (Seebeckcoefficient α, electrical conductivity s, and thermal conductivity κ)as well as the absolute temperature T in a form of ZT¼α2sT/κ [1].Thus, one seeks materials that conduct electrical current effi-ciently, have high Seebeck coefficients and are poor conductorsof heat. These deceptively simple requirements are very difficult torealize in practice on account of mutual interdependence oftransport parameters. Hence, a relatively few semiconductingcompounds have a good prospect as thermoelectric materials.One of them is solid solutions of Mg2Si1−xSnx. Apart from havinggood thermoelectric properties in the intermediate range oftemperatures between 500 K and 800 K, their distinct advantages

ll rights reserved.

2787860863uher@umich.edu (C. Uher).

are the abundance of constituting elements, low cost and theirnon-toxic character. Values of ZT better than unity have beenreported with these solid solutions in the past few years [2–7].

Nanostructuring and band structure engineering are twoapproaches that led recently to spectacular values of the figureof merit approaching or even exceeding the value of 2 in PbTe-based materials [8,9]. In Mg2Si1−xSnx solid solutions, and especiallyfor compositions near x¼0.6–0.7, the high ZT value of ∼1.3 wasachieved due to the precipitated Sn-rich nano-phase whichresulted in a very low lattice thermal conductivity, while electricalproperties were enhanced by the convergence of the heavy andlight conduction bands taking place in this range of compositions[6,7]. Optimal carrier concentration in these solid solutions wasmaintained by doping the structure with Sb.

In this research, we adopted Bi rather than Sb as the dopingelement in Mg2Si0.4Sn0.6-based solid solutions and investigated itsinfluence on thermoelectric properties. Since Bi is nearly twice asheavy as Sb, we anticipated a further decrease in the latticethermal conductivity resulting from increased mass fluctuationsin the Bi-doped Mg2Si0.4Sn0.6 solid solutions. Moreover, while thedoping effect of Bi has been explored in binary Mg2Si compounds[10,11], reports on its influence in Mg2Si1−xSnx solid solutions and,specifically in the regime of band convergence, are sparse [12–14].In this paper we report on our findings and provide an assessmentof doping the structure with Bi versus doping with Sb.

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339334

2. Experimental Procedure

A series of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutionswere synthesized by a two-step solid state reaction (SSR) com-bined with spark plasma sintering (SPS) process [5,7]. High purityMg, Si, Sn and Bi in the form of fine powders were weightedaccording to their nominal compositions, while Mg was addedwith an excess of 8% over its stoichiometric amount. This was tocompensate for the evaporation loss of Mg during the synthesisprocess as well as to provide a suitable excess of Mg in order tooptimize the electron density and electrical properties, as wasdescribed in detail in Refs. [5,7]. The powders were mixed andhand grinded in an agate mortar, cold pressed into pellets, insertedinto p-BN crucibles and sealed in quartz tubes under vacuum. Thegrinding and mixing process were done in a glove box to minimizeoxidation. The solid state reaction was carried out at 873 K. Theresulting products were then taken out and the entire procedurerepeated in now the second step solid state reaction at 973 K topromote a homogeneous distribution of constituent elements andthe formation of solid solutions. After this two-step processing, weobtained nearly single phase Mg2.16(Si0.4Sn0.6)1−yBiy solid solutionsthat were grinded into fine powders and compacted into densebulk ingots by using Spark Plasma Sintering (SPS). All samples arevery dense with near 99% of the theoretical value. The ingots ofMg2.16(Si0.4Sn0.6)1−yBiy were then cut into appropriate sizes forthermal/electrical transport property measurements.

To carry out structural analysis, a part of each ingot was groundinto fine powders and XRD spectra were recorded using aPANalytical X’Pert Pro type X-ray diffractometer with CuKα radia-tion. The chemical composition and elemental distribution mapswere acquired from finely polished polycrystalline samples by aJXA-8230 SuperProbe Electron Probe Microanalyzer equipped withwavelength dispersive X-ray spectrometers (WDS). The electricalconductivity and the Seebeck coefficient were measured simulta-neously in the range of 300–773 K, using a commercial ZEM-1instrument (Ulvac Sinku-Riko Company) by a standard four-probedc method. The thermal diffusivity λ and heat capacity Cp (atconstant pressure) in the range of 300–773 K were obtained on aNetzsch LFA-457 apparatus and a Q20 differential scanning calori-meter (TA Instruments) respectively, while the room temperaturesample density was determined by the Archimedes’ method. Thethermal conductivity κ of samples was then calculated by theformula of κ¼λCpd. Low temperature Hall coefficient and resistiv-ity were obtained on a Quantum Design PPMS-9 system. HighTemperature Hall coefficient within 290–780 K was measured

Fig. 1. Powder X-ray diffraction patterns (a) and differential scanning calor

using a home-built system consisting of a large, 9 T Oxford air-bore superconducting magnet with an oven and a Hall probeinserted in the air-bore of the magnet. The data were recorded atfields of 71 T using a Linear Research AC Resistance Bridge (LR-700) operated with a 17 Hz excitation frequency. The overall errorin measurements of the electrical conductivity, the Seebeckcoefficient and the thermal conductivity were estimated to beabout 73%, 72% and 75%, respectively.

3. Results and Discussions

3.1. Phase and microstructures

Fig. 1(a) shows the XRD patterns of Mg2.16(Si0.4Sn0.6)1−yBiy(0≤y≤0.03) samples after compaction. Results indicate that, exceptfor one slight peak positioned at 2θ≈431 which represents a tinyamount of MgO in the matrix, all diffraction peaks can be indexedand belong to the Mg2Si0.4Sn0.6 solid solution with the JCPDS filenumber 01-089-4254. This confirms that we successfully synthe-sized nearly phase pure Bi-doped Mg2Si0.4Sn0.6 solid solutions.Differential scanning calorimetry analysis, displayed in Fig. 1(b), isadopted to detect any abnormal thermal effects from roomtemperature to 823 K which could be symptomatic of the presenceof a free segregation of Mg and excess doping of Sb [5,7]. Noextraneous exothermic or endothermic peaks are detected andonly very smooth temperature dependent traces are found for allsamples with the content of Bi between 0.005 and 0.03. Based onfirst principles calculations [15] as well as the chemical propertiesand comparable electronegativity of Sb and Bi, several researcheshave postulated that Bi was most likely to occupy the Si/Sn sites inthe crystal lattice of Mg2Si1−xSnx-based materials [10–14]. Thus,according to the XRD and DSC results, we can conclude that thenominal y¼0.03 content of Bi (the actual content of y¼0.025)used in this work did not exceed the solubility limit in theMg2Si0.4Sn0.6 solid solution (see Table 1).

The WDS elemental distribution maps of the finely polishedMg2.16(Si0.4Sn0.6)0.97Bi0.03 sample are displayed in Fig. 2. The darkerregion in Fig. 2(a), the back scattered image of Mg2.16(Si0.4Sn0.6)0.97Bi0.03, reveals phase segregation of micron-sized Si-rich Mg2Si1−xSnxwith a higher content of Si compared to the amount of Si residing inthe Mg2Si0.4Sn0.6 matrix. Elemental maps shown in Fig. 2(b)–(d) confirm a much higher Si content in the above mentioned darkerareas. The precipitation of the Si-rich second phase in Bi-dopedMg2Si0.4Sn0.6 is similar to the case of Sb-doped samples (see Ref. [7]).

imeter analysis (b) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions.

Fig. 2. Back scattered image (a) and elemental mapping distribution (b)–(e) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions.

Table 1Some room temperature physical parameters of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions.

Nominalcomposition

Actual composition Carrier concentration, Ćn(cm−3)

Carrier mobility, Ćμ (cm2/Vs)

Reduced Fermi Ćenergy, η¼EF/(kBT)

Lorenz number, ĆL0(10−8 V2 K−−2)

y¼0 Mg2.15Si0.44Sn0.56 2.28�1018 69.02 −2.76 1.50y¼0.005 Mg2.10Si0.39Sn0.60Bi0.003 8.4�1019 57.7 0.48 1.66y¼0.01 Mg2.13Si0.41Sn0.59Bi0.006 1.45�1020 54 0.91 1.71y¼0.015 Mg2.12Si0.40Sn0.59Bi0.009 2.02�1020 51 1.4 1.78y¼0.02 Mg2.13Si0.39Sn0.60Bi0.013 2.37�1020 47.23 1.70 1.82y¼0.03 Mg2.12Si0.37Sn0.60Bi0.025 2.36�1020 51.68 1.78 1.83

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339 335

It has its origin in the features of the pseudo-binary phase diagram ofMg2Si–Mg2Sn and the existence of the miscibility gap [16]. Fig. 2(e) indicates the dispersion of Bi in the sample matrix and demon-strates its even distribution in the samples.

3.2. Electronic transport properties

Temperature dependence of the electron density (Fig. 3(a)) andcarrier mobility (Fig. 3(b)) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03)solid solutions is shown in Fig. 3. As seen in Fig. 3(a) and Table 1,the electron concentration of Mg2.16(Si0.4Sn0.6)1−yBiy is enhancedby 1–2 orders of magnitude through Bi doping, which confirmsthat Bi is just as effective n-type dopant as Sb [7]. The Bi-dopedsamples exhibit a nearly constant carrier density across the wholetemperature range. An exception is a very weakly doped y¼0.005sample which shows an increasing carrier density with tempera-ture above T∼573 K. This is a consequence of intrinsic excitationstaking place at elevated temperatures in this small band gapsemiconductor with a comparatively low electron density.It should be pointed out that, in this case, the extraction of carrierdensity calculated from the Hall effect measurements assuming asingle band is no longer valid. The carrier mobility μ wascalculated from the electrical conductivity s and the Hall coeffi-cient RH by utilizing the equation of μ¼sRH. One notes a distinctly

different behavior of the carrier mobility in the un-doped solidsolution in comparison to all Bi-doped samples. In general, ifacoustic phonon scattering dominates, one expects the mobility tofollow the T−3/2 dependence. In the case that ionized impurityscattering dominates the transport behavior, the mobility shouldfollow the T3/2 dependence. As is evident from Fig. 3(b), below200 K the carrier scattering in the un-doped sample is dominatedby the ionized impurity mechanism leading to a rapid increase ofmobility with temperature. Above 200 K, the mobility of theun-doped sample reaches saturation and then decreases, mimickingthe trend in Bi-doped samples. In contrast, Bi-doped Mg2.16(Si0.4Sn0.6)1−yBiy solid solutions at low temperatures (T<100 K) show analmost temperature independent mobility. It is only above 200 K thatone discerns a distinct decrease in the mobility with increasingtemperature which, well above room temperature, approaches theacoustic phonon limited T−3/2 power law.

Table 1 lists room temperature physical parameters forMg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions. Results indicatethat all Bi-doped samples are n-type semiconductors. The dopingefficiency of Bi in Mg2.16Si0.4Sn0.6 is comparable to that of Sb.At room temperature, the presence of Bi leads to only about 15–35% degradation of the mobility compared to the un-dopedMg2.16Si0.4Sn0.6. A small decrease in the mobility upon Bi dopingis more than compensated for by a vastly enhanced carrier density

Fig. 3. Temperature dependence of electron density (a) and carrier mobility (b) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions in the range of 10–773 K.

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339336

which leads to a much enhanced electrical conductivity (see Fig. 4(a)).All Bi-doped Mg2.16(Si0.4Sn0.6)1−yBiy solids solutions show charac-teristics of a heavily-doped degenerate semiconductor with theelectrical conductivity decreasing monotonously with tempera-ture. Fig. 4(b) displays the temperature dependence of the Seebeckcoefficient. While the Seebeck coefficient of pure Mg2.16Si0.4Sn0.6

initially increases and peaks near 400 K with a very large value ofabout −500 μV/K and then rapidly decreases, the Seebeck coeffi-cient of Bi-doped solid solutions shows a monotonically risingtrend with temperature except at the highest temperatures whereintrinsic excitations cause bending of the curves. As expected, thedoping dependence of the Seebeck coefficient is opposite to that ofthe dependence of the electrical conductivity, i.e., the Seebeckcoefficient decreases with the increasing content of Bi (increasingelectron density). From the temperature Tmax where the Seebeckcoefficient reaches its maximum value αmax, we estimate the bandgap according to the formula [17] Eg¼2eαmaxTmax which yields thevalues of 0.36, 0.37, and 0.37 for samples with y¼0, 0.005 and 0.01,respectively. This indicates that, in this concentration range, thepresence of Bi does not alter the band gap of solid solutions. Thepower factor, calculated using the electrical conductivity and theSeebeck coefficient data, is plotted in Fig. 4(c). Compared with Sb-doped samples of similar concentration, Bi-doped Mg2Si0.4Sn0.6

solid solutions show higher power factors by as much as 15%, see asolid line in Fig. 4(c) that represents the power factor [7] of Sb-doped Mg2.16Si0.4Sn0.6 with the Sb content of 0.015. Enhancementsin the power factor were found for all Mg2.16(Si0.4Sn0.6)1−yBiy solidsolutions with the content of Bi y>0.01. Fig. 4(d) illustrates therelationship between electrical conductivity and power factor ofSb [7] and Bi doped Mg2.16Si0.4Sn0.6 samples. It is clear that theimprovement in the power factor of Bi doped samples is caused bythe increase in the Seebeck coefficient because the two solidsolutions have nearly the same electrical conductivity in the entiretemperature range. Since the bottom of the conduction band ofMg2Si1−xSnx solid solutions is dominated by contributions from Siand Sn states, and the increasing ratio of Sn/Si results in theconvergence of the two nearby lying conduction bands [6,18], theslight increase in the Seebeck coefficient through Bi doping, ascompared to Sb-doped solid solutions, is likely caused by someminor modification of the conduction band structure that, in turn,leads to a possible enhancement in the density-of-states effectivemass. Fig. 4(e) presents a Pisarenko-like plot of the Seebeckcoefficient as a function of electron concentration. Regardingdegenerate semiconductors, the Seebeck coefficient and carrierconcentration should satisfy Eq. (1), assuming the carrier mean-

free path is independent of energy [19]

α¼ 8π2kB2

3eh2mnT

π

3n

� �2=3ð1Þ

Here, kB, e, h, m(n) and n represent, in turn, the Boltzmannconstant, the charge of an electron, Planck's constant, the carriereffective mass and the carrier density. Dashed lines in Fig. 4(e) wereplotted by assuming a single band model and the effective mass ofcarriers uncorrelated with the carrier density, which yields α∝n−2/3.Both experimental curves, at 300 K and 573 K, deviate from thecalculated lines, suggesting that the carrier effective mass hasincreased with the carrier concentration. Based on the features ofthe two-conduction-band structure for Mg2Si0.4Sn0.6 [6,7] (see thesketch image of the band structure of Mg2Si0.4Sn0.6 in Fig. 4(f)), theincrease in the carrier effective mass of Mg2.16(Si0.4Sn0.6)1−yBiyupon Bi doping likely arises from the Fermi level moving deeperinto the conduction band as the Bi content increases.

3.3. Thermal transport properties and dimensionless figure Ćof meritZT

Fig. 5(a) shows the temperature dependence of thermalconductivity of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions.Similar to the other doped Mg2Si1−xSnx solid solutions, the thermalconductivity of Mg2.16(Si0.4Sn0.6)1−yBiy increases with the amountof Bi doped into the structure. This reflects a considerably largerelectronic contribution to the total thermal conductivity associatedwith a dramatically enhanced electrical conductivity upon doping.This enhancement is easily quantified using the Wiedemann–Franz law, κe¼L0sT, where L0, s, and T are the Lorenz number,electrical conductivity and absolute temperature, respectively. TheLorenz number L0 can be estimated on the basis of the Fermi–Diracstatistics from Eq. (2) [20,21]:

L0 ¼kBe

� �2 3F0ðηÞF2ðηÞ−4F21 ðηÞF0

2ðηÞ

" #ð2Þ

where FiðηÞ is the Fermi integral

FiðηÞ ¼Z ∞

0

xidx1þ expðx−ηÞ ð3Þ

and η¼ ðEF=kBTÞ is the reduced Fermi energy.The calculated L0 are listed in Table 1. Subtracting the electronic

term, the remaining thermal conductivity comprises the latticeand the bipolar contributions and is shown in Fig. 5(b). The

Fig. 4. Temperature dependent electrical conductivity (a), Seebeck coefficient (b), power factor (c), relationship between electrical conductivity and power factor (d), andcorrelation between Seebeck coefficient and electron density (e) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions. (f) Sketch of the band structure of Mg2Si0.4Sn0.6, whichshows the relative position of the light conduction band (CL, in blue) and the heavy conduction band (CH, in red) [3,6].

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339 337

un-doped Mg2.16Si0.4Sn0.6 shows a rapid upturn around 500 K thatindicates an early onset of intrinsic excitations and associated withthe strongly rising bipolar thermal conductivity as T>500 K. TheBi-doped solid solutions also show notable upturns but at highertemperatures. As expected, with the increasing amount of dopant,the lattice thermal conductivity decreases and the lowest latticethermal conductivity is achieved in a solid solution with y¼0.03.In this case, the lattice thermal conductivity at 300 K is suppressedby about 6% with respect to the un-doped sample and progres-sively much more at higher temperatures. Compared with latticethermal conductivities of Sb-substituted samples shown in Figs. 5

(b) for y¼0.015 (the black cubic symbol) and y¼0.025 (the redcircles icon), Bi did not show the hoped for advantage (largermass) in suppressing the lattice thermal conductivity for similarlydoped solid solutions.

Fig. 6(a) displays the temperature dependence of the dimen-sionless figure of merit ZT for Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03)solid solutions. Results indicate that Bi doping significantlyincreases the ZT value of the parent Mg2.16Si0.4Sn0.6 due to muchenhanced electron carrier density which results in high values ofthe power factor and due to stronger mass defect phonon scatter-ing that decreases the lattice thermal conductivity. The highest ZT

Fig. 5. Temperature dependent thermal conductivity (a) and the combination of lattice and bipolar term thermal conductivity (b) of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solidsolutions. The trend lines added in (a) are a guide for eyes. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 6. (a) Temperature dependent dimensionless figure of merit ZT of Mg2.16(Si0.4Sn0.6)1−yBiy (0≤y≤0.03) solid solutions. (b) Repeated measurement of electrical conductivity(square symbols in black), Seebeck coefficient (triangular symbols in blue), thermal conductivity (circular symbols in deep yellow) and ZT values (rhombus symbols in red) ofMg2.16(Si0.4Sn0.6)0.97Bi0.03; hollow symbols indicate the repeated measurement results. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339338

value of ∼1.4 was observed in Mg2.16(Si0.4Sn0.6)0.97Bi0.03 withy¼0.03. This is to be compared with the highest value of ZT∼1.3for the comparably Sb-doped samples [7]. To verify the result, were-measured the thermoelectric properties of theMg2.16(Si0.4Sn0.6)0.97Bi0.03 solid solution and they are shown ashollow symbols in Fig. 6(b). The repeated measurement substan-tially confirmed the original data and verified the value of ZT at alevel of 1.4. In relation to Sb-doped solid solutions, the comparableBi-doped solid solutions yield marginally higher ZT, attributed to asmall increase in their Seebeck coefficients. Since we have not seensignificant phase segregation even at Bi doping levels of y¼0.03,perhaps there is a possibility to further increase the dopingamount and thus enhance point defect scattering.

4. Conclusion

In this work we are reporting on the influence of Bi doping ontransport coefficients in Mg2Si0.4Sn0.6 solid solutions. Resultsindicate that the crystal structure of Mg2Si0.4Sn0.6 can accommo-date Bi at a level of at least y¼0.025 without detectable phasesegregation, and Bi occupies the Si/Sn sites in the crystal lattice.As an n-type dopant, the doping of Bi into the structure significantlyincreases the density of electrons which, in turn, leads to 1–2orders of magnitude higher electrical conductivity. The power

factor of Bi-doped Mg2.16(Si0.4Sn0.6)1−yBiy solid solutions is margin-ally higher than that of comparably Sb-doped solid solutions. Thehighest power factor of 4.2 mW m−1 K−2 was obtained near 800 Kfor the solid solution with y¼0.03. Meanwhile, the band gap ofMg2.16(Si0.4Sn0.6)1−yBiy does not seem to change through Bi doping.The lattice thermal conductivity of Mg2.16(Si0.4Sn0.6)1−yBiy isreduced by doping with Bi but the expected further decreasedue to heavier Bi in comparison to Sb is not realized. The highestZT of ∼1.4 is achieved at 800 K with Mg2.16(Si0.4Sn0.6)0.97Bi0.03. Thethermoelectric properties of Mg2Si1−xSnx solid solutions could beenhanced further if one were able to increase the doping amountof Bi so as to increase point defect scattering.

Acknowledgments

We wish to acknowledge supports of the International Science& Technology Cooperation Program of China (Grant no.2011DFB60150), the Natural Science Foundation of China (Grantnos. 51172174 and 51002112), the 111 Project (Grant no. B07040).High temperature Hall effect measurements at University ofMichigan were carried out with the support of CERC-CVC, the U.S.–China program supported by the U.S. Department of Energyunder Award no. DE-PI0000012. The authors also would like toacknowledge Doctor Meijun Yang (from the Center for Materials

W. Liu et al. / Journal of Solid State Chemistry 203 (2013) 333–339 339

Research and Analysis of Wuhan University of Technology) for theassistance on the EPMA measurements.

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