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Institute of Chemical Sciences, Heriot-Wat
E-mail: [email protected]
† Electronic supplementary information (Erened parameters, selected distances anof selected hot-pressed samples. See DOI:
Cite this: J. Mater. Chem. A, 2013, 1,12270
Received 16th July 2013Accepted 23rd August 2013
DOI: 10.1039/c3ta12753b
www.rsc.org/MaterialsA
12270 | J. Mater. Chem. A, 2013, 1, 1
Synthesis, structural characterisation and thermoelectricproperties of Bi1�xPbxOCuSe†
Son D. N. Luu and Paz Vaqueiro*
The effect of Pb2+ doping on the structure and thermoelectric properties of BiOCuSe (also known as
BiCuSeO or BiCuOSe) is described. With increasing Pb2+ content, the expansion of the unit cell results in
a weakening of the bonding between the [Bi2(1�x)Pb2xO2]2(1�x)+ and the [Cu2Se2]
2(1�x)� layers. The
electrical resistivity and Seebeck coefficient decrease in a systematic way with growing Pb2+ levels. The
thermal conductivity rises due to the increase of the electronic contribution with doping. The power
factor of materials with a 4–5% Pb2+ content takes values of ca. 8 mW cm�1 K�2 over a wide
temperature range. ZT at 673 K is enhanced by ca. 50% when compared to values found for other
dopants, such as Sr2+ or Mg2+.
Introduction
Thermoelectric devices can convert waste heat directly intoelectricity, and offer the promise of a more efficient use ofexisting energy resources. The efficiency of thermoelectricpower generation is largely determined by the performance ofthe thermoelectric materials that constitute the device. Thisefficiency is given by a gure of merit, ZT, related to the Seebeckcoefficient (S), electrical conductivity (s) and thermal conduc-tivity (k) of the material by ZT ¼ S2sT/k.1 The need for thermo-electric devices with higher efficiencies has led to a tremendousgrowth of research into new thermoelectric materials.2
Lowering the thermal conductivity of a material withoutsignicantly reducing the power factor (S2s) would result in anincreased gure of merit. For this reason, recent researchstrategies for the design of new thermoelectric materials havefocused on reducing the thermal conductivity. For instance, inthe “phonon glass electron crystal” approach, materials containoversized cages in which “rattling” atoms produce a phonondamping effect which results in a reduced thermal conduc-tivity.3 Nanostructuring of thermoelectric materials can lead toreduced thermal conductivity through increased phonon scat-tering at the interfaces.4 “Natural superlattice” materials, inwhich layers with excellent electronic transport properties arecombined with a second type of layer which serves as a phononscatterer, can also exhibit low thermal conductivities.5 Exam-ples of “natural superlattice” materials include layered cobaltoxides, such as Ca3Co4O9 and NaCo2O4, which have a remark-able thermoelectric performance at elevated temperatures,6 or
t University, Edinburgh EH14 4AS, UK.
SI) available: Rietveld proles, tables ofd angles; powder X-ray diffraction data10.1039/c3ta12753b
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the mist layered compounds (SnS)1+x(TiS2), with a ZT of 0.37 at773 K.5 More recently, it has been reported that oxy-chalcogenides BiOCuQ (Q ¼ Se, Te) have promising thermo-electric properties. For instance, BiOCuSe exhibits a ZT of 0.76at 873 K, upon partial substitution of Bi3+ with Sr2+,7 whilstBiOCuTe has a ZT of 0.66 at the maximum temperature inves-tigated, 673 K.8
The BiOCuQ (Q ¼ S, Se, Te) phases are members of a familyof oxychalcogenides with the general formula AOBQ (A¼ La, Ce,Nd, Pr, Bi; B ¼ Cu, Ag and Q ¼ S, Se, Te).9 These materialscrystallise in the ZrSiCuAs structure, which consists of alter-nating [A2O2]
2+ and [B2Q2]2� layers stacked along the c-axis
(Fig. 1).The AOBQ phases, which are isostructural to the super-
conducting oxypnictides LnOFePn (Ln ¼ La, Pr, Ce, Sm; Pn ¼ Pand As), have been primarily investigated for their optoelec-tronic properties, as many of them are transparent p-typesemiconductors.10 However, whilst the rare-earth oxy-chalcogenides are wide-band semiconductors, it has beenfound that the BiOCuQ phases have smaller band gaps, due to
Fig. 1 View of the crystal structure of BiOCuSe along the [010] direction.
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the contribution of the Bi 6p orbitals to the conduction bandminima.9e Their lower band gap and higher electrical conduc-tivities may make these materials attractive for thermoelectricapplications. Although the lowest band gap is found for theoxytelluride, efforts to date have focused on the selenium-con-taining phase. Ohtani et al. showed that BiOCuSe can besuccessfully doped by aliovalent substitution at the bismuth siteor through copper deciency.11 Doped Bi1�xSrxOCuSe andBiOCu1�xSe were found to exhibit signicantly reduced elec-trical resistivities (with a metal-like temperature dependence)when compared to BiOCuSe.11 Subsequent studies of the ther-moelectric properties found that Bi1�xSrxOCuSe and BiO-Cu1�xSe exhibit reasonable values of the power factor atelevated temperatures, combined with a low thermal conduc-tivity.7,12 It has been also shown that ball milling of BiOCuSe13 orBi1�xBaxOCuSe to reduce grain sizes leads to improvements inperformance, with ZT reaching 1.1 at 923 K for Bi0.875Ba0.125O-CuSe.14 Here, a detailed account of the structural, electrical andthermal transport properties of Bi1�xPbxOCuSe samplesprepared by solid state reactions followed by hot pressing, ispresented.
ExperimentalSynthesis
Samples of polycrystalline Bi1�xPbxOCuSe (0 # x # 0.2) wereprepared by solid-state reaction between Bi2O3 (99.99%, SigmaAldrich), Bi (99.5%, Aldrich), PbO2 (99.5%, BHD Chemicals), Cu(99.5%, Johnson Matthey) and elementary Se (99.99%, Aldrich)using evacuated and sealed silica tubes (<10�4 Torr). Eachstoichiometric mixture was rst heated up to 623 K for 20 h andthen up to 773 K for 10 h with a 2 K min�1 ramp rate. A secondannealing process at 873 K for a further 7 h was carried out aerregrinding the obtained powders. For electrical and thermalproperty measurements, the as-prepared powders were hot-pressed into highly densied pellets ($95% of theoreticaldensity) at 853 K and a uniaxial pressure of 50 bars for 30minutes under a N2 ow.
Structural and microstructural characterisation
Samples were characterised by powder X-ray diffraction using aBruker D8 Advance Powder X-ray diffractometer, operating withgermanium monochromated CuKa1 radiation (l ¼ 1.54046 A)and tted with a LynxEye detector. Data were collected over arange of 5 # 2q/� # 120 for a period of 7 hours. Rietveldrenements were carried out using the GSAS soware.15 A FEIQuanta 650 FEG ESEM was used to study the morphology ofpristine and doped samples.
Thermogravimetric analysis
A DuPont 951 thermogravimetric analyzer was employed toinvestigate the thermal stability of BiOCuSe at elevatedtemperatures. The sample was loaded into a platinum crucibleand heated from room temperature to 973 K with a rate of 2 Kmin�1 under a N2 or an O2 ow.
This journal is ª The Royal Society of Chemistry 2013
Electrical transport measurements
The electrical resistivity of the sample over the temperaturerange 100 # T/K # 300 was measured using a 4-probe DCtechnique. For each sample, a rectangular bar (�6 � 3 � 1mm3) was cut from a hot-pressed pellet. Four 50 mm silver wireswere attached using silver paint and connections were made toa Keithley 2182 nanovoltmeter and a TTi QL564P power supply.The sample was mounted in an Oxford Instruments CF1200cryostat connected to an ITC502 temperature controller. Tomeasure the Seebeck coefficient over the same temperaturerange, the sample bar (�6 � 3 � 1 mm3) was mounted on acopper holder, which incorporates a small heater (120 U straingauge) located close to one end of the sample. The copperholder is attached to the hot stage of a closed-cycle refrigerator(DE-202, Advanced Research Systems), which is connected to aLakeshore LS-331 temperature controller. Two 50 mm copperwires were attached to the ends of the sample bar using silverpaint and connections made to a Keithley 2182A nanovoltmeter.Two Au: 0.07% Fe vs. chromel thermocouples were placed at thehot and cold ends of the sample, and connected to a secondLakeshore LS-331 temperature controller. The Seebeck coeffi-cient at a given temperature was determined by applying atemperature gradient, DT, across the sample and measuring thecorresponding thermal voltage, DV. The slope of the line,DV/DT, was used to determine the Seebeck coefficient.
For measurements above room temperature (300 < T/K <673), the resistivity and Seebeck coefficient of each sample weremeasured simultaneously using Linseis LSR-3 instrument(Germany). Resistivity and Seebeck coefficient measurementswere carried perpendicular to the pressing direction of theingots, and in selected samples, parallel to the pressingdirection.
Thermal transport measurements
An Anter FL3000 system was employed for measuring thermaldiffusivity (a) and the heat capacity (Cp) of samples over atemperature range of 373 # T/K # 673 in 50 K steps. Sampleswere hot-pressed into highly densied pellets with an approxi-mate diameter of 13 mm and a thickness of 1–2 mm. A graphitecoating on surface of pellet was applied to maximize heatabsorption. The pellet was then loaded into the samplechamber which was purged with N2 during the measurement.The thermal conductivity (k) was calculated from the relation-ship k ¼ aCpr where r is the sample density. A referencematerial, PyroceramTM 9606, of known heat capacity was usedas a reference for the determination of the heat capacity ofsample. The details of procedure of determination of the heatcapacity are described in ref. 16. Thermal conductivitymeasurements were carried out parallel to the pressing direc-tion of the ingots.
Results and discussionStructural and microstructural characterisation
Powder X-ray diffraction data collected on as-preparedsamples of Bi1�xPbxOCuSe with compositions over the range
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Fig. 3 (a) Lattice parameters, (b) selected bond distances, and (c) selected bondangles in Bi1�xPbxOCuSe. The CuSe4 polyhedron is shown on the right.
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0 # x # 0.20 indicate that these materials crystallise in theZrSiCuAs structure (Fig. 1). Most samples are single phases,with the exception of x ¼ 0.15 for which a trace amount of Bi2O3
can be detected. In sharp contrast with a previous report whichsuggested that the maximum doping level was x ¼ 0.07,17 wehave found that lead can be incorporated up to a doping level ofx z 0.20. This is comparable to the solubility limit found forother divalent cations, such as Sr2+ and Ba2+, for which dopingcontents of x ¼ 0.15 have been reported.7,14 Values of x greaterthan 0.2 result in samples containing signicant amounts ofimpurities. The lower doping level reported in ref. 17 might berelated to the use of elemental lead for the synthesis of thesephases, given that as-purchased elemental lead oen containssome lead oxide.
Rietveld renements using powder X-ray diffraction datawere carried out for the as-prepared samples of Bi1�xPbxOCuSe(0 # x # 0.20). A representative example of nal observed,calculated and difference proles for the X-ray diffraction datais shown in Fig. 2, while remaining Rietveld proles, tables ofrened parameters, selected distances and angles have beenincluded as ESI.†
The lattice parameters of Bi1�xPbxOCuSe increase withincreasing x (Fig. 3(a)). This increase, which is more noticeablealong the c-axis, may be related to the larger ionic radius of Pb2+
(129 pm) when compared to that of Bi3+ (117 pm).18 Theexpansion of the unit cell along the c axis leads to an increase ofthe Bi–Se distances (Fig. 3(b)), which may indicate a weakeningof the bonding between the oxide and chalcogenide layers withincreasing lead content. The a and b Se–Cu–Se angles (Fig. 3(c))also change with increasing lead content, and this results in areduction of the distortion of the CuSe4 tetrahedra. As dis-cussed previously for the isostructural oxytelluride,8 the thermalparameter of the copper site is signicantly larger than those ofthe other three crystallographic sites, and this may be related tocopper deciency in the [Cu2Se2]
2� layers.Powder X-ray diffraction data collected on hot-pressed
samples indicates that the ZrSiCuAs structure is retained, andno decomposition of the samples is observed (ESI†). Diffractionmeasurements on hot-pressed ingots show no evidence ofpreferred orientation, and therefore the thermoelectric
Fig. 2 Rietveld refinement using powder X-ray diffraction data forBi0.825Pb0.175OCuSe (Rwp ¼ 11.5%).
12272 | J. Mater. Chem. A, 2013, 1, 12270–12275
properties of samples prepared by hot pressing can be assumedto be isotropic. However, hot pressing leads to a markedbroadening of all of the diffraction peaks, which may beindicative of microstrain. Examination of selected as-preparedand hot-pressed samples by electron microscopy (Fig. 4) indi-cates that the as-prepared samples contain plate-like grains,with a thickness of ca. 0.2 mm and lengths of �2 mm. The grainsobserved on fractured surfaces of hot-pressed ingots havesimilar plate-like shapes, although grain growth has clearlyoccurred.
Thermal stability
Thermogravimetric analysis, collected under a N2 atmosphereindicates that BiOCuSe is stable up to the maximum tempera-ture investigated, 973 K. However, when measurements arecarried out in an O2 atmosphere, the sample weight starts toincrease at ca. 653 K, indicating that oxidation is occurring(Fig. 5). Taking this into account, we have limited our study ofthe thermoelectric properties of these materials to the temper-ature range 100 # T/K # 673, although incorporation of thesematerials in thermoelectric devices for operation at highertemperatures would be feasible under a protective atmosphere.
Fig. 4 SEM micrographs for (a) as-synthesised BiOCuSe; (b) hot-pressedBi0.8Pb0.2OCuSe.
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Fig. 5 Thermogravimetric data for BiOCuSe collected under a N2 (red line) andan O2 atmosphere (black line).
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This is the approach already adopted for PbTe-containingthermoelectric devices.19
Electrical and thermal transport properties
The electrical resistivity of undoped BiOCuSe is rather low overthe whole temperature range (Fig. 6). In contrast to a previousreport on BiOCuSe prepared by cold pressing and sintering,9e
which shows semiconducting behaviour, the electrical resis-tivity of our undoped BiOCuSe over the temperature range100 # T/K # 550 increases with increasing temperature, atemperature dependence which is consistent with degeneratesemiconducting behaviour. The difference between the cold-pressed and the hot-pressed materials may be related to thelower density of the former, which will lead to a signicantcontribution of grain boundaries to the overall sample resis-tance. For samples prepared by spark plasma sintering (SPS),metal-like behaviour is observed at low temperatures, althoughat temperatures above 400 K the temperature dependencebecomes characteristic of semiconducting behaviour.7 Amaximum in the resistivity is also found for our hot-pressed
Fig. 6 Electrical resistivity and Seebeck coefficient for Bi1�xPbxOCuSe as afunction of temperature. High-temperature resistivity data are shown on a log-arithmic scale for clarity.
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BiOCuSe, but in this case themaximum occurs at approximately550 K. It is well known that hot pressing or SPS can causechanges in stoichiometry (e.g. oxygen deciency) or reduction,as the graphite in the die oen acts as a reducing agent at hightemperatures.20 Although powder X-ray diffraction datacollected on hot-pressed samples indicate that they have thesame purity as the starting powders, small changes in stochi-ometry caused by the pressing process cannot be excluded, andthis could account for the difference between hot-pressed andSPS samples. Measurements on ingots cut perpendicularand parallel to the pressing direction show no signicantchanges in the Seebeck coefficient or electrical resistivity. Thisis consistent with the lack of preferred orientation observed inour X-ray diffraction measurements.
The Seebeck coefficient of the undoped sample is positive,indicating that the electrical transport properties are domi-nated by holes, and is consistent with p-type semiconductingbehaviour. For this family of oxychalcogenides, p-type conduc-tion is normally observed,9 and is attributed to the formation ofa covalent hybridised band between Cu and Se atoms at the topof the valence band.10 However, the origin of this p-typebehaviour may also be related to copper deciency, which cangenerate mobile holes.
The electrical resistivity of the doped samples decreases in asystematic way with increasing level of doping, and this isconsistent with a rise in charge carrier concentration. Pb2+ is avery effective dopant, as shown by the large enhancement of theelectrical conductivity at relatively low levels of doping: at roomtemperature the electrical conductivity increases from 63 Scm�1 for BiOCuSe to �500 S cm�1 for Bi0.95Pb0.05OCuSe. Bycomparison, doping with 5% of Ba2+ results in a conductivity of250 S cm�1,14 and with 5% of Sr2+ in only 100 S cm�1.7 TheSeebeck coefficient, which remains positive, also decreases withincreasing doping levels.
Whilst there is excellent agreement between the low- andhigh-temperature values of the Seebeck coefficient for eachsample, the low-temperature resistivity data is affected bygreater uncertainties in sample dimensions, as well as by thePeltier effect arising from the use of a constant dc currentsource.3b For this reason, low-temperature resistivity datahave not been used in a quantitative way, and the powerfactor (Fig. 7) has been calculated only for the temperature
Fig. 7 Power factor for Bi1�xPbxOCuSe as a function of temperature.
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range 300 < T/K < 673. For samples with x # 0.05, the powerfactor decreases slightly with increasing temperature, whilst forhigher doping levels, the power factor shows a marked increasewith increasing temperature. The highest values of the powerfactor have been found for samples with a Pb2+ content between4 and 10%, at the maximum temperature investigated. Our hot-pressed materials exhibit higher power factors than thoseprepared by SPS.17 Given that the processing conditions ofthermoelectric materials can have a signicant effect on theirnal performance, further optimisation of the consolidationprocess may lead to additional improvements in thermoelectricperformance.
Although the power factors of these materials are moderatein comparison to that of Bi2Te3 (�40 mW cm�1 K�2),21 theirmagnitude is similar to those of other thermoelectric materialssuch as Zn4Sb3 (�13 mW cm�1 K�2 at 673 K).22 Calculations ofthe band structure of BiOCuQ (Q ¼ S, Se, Te) indicate that thetop of the valence band consist of a mixture of light- and heavy-mass bands.23 This has been shown to be a desirable feature forgood thermoelectric performance,24 as a light-mass bandpromotes good electrical conduction, whilst a heavy-mass bandcan lead to high Seebeck coefficients. However, the holemobility in BiOCuSe, which is 20 cm2 V�1 K�1, is rather small,and decreases markedly with doping due to point-defect scat-tering.25 As the thermoelectric gure of merit is proportional tothe mobility, according to the expression Z f (m*)3/2m (wherem* is the effective mass and m the mobility),26 this can explainthe moderate power factors observed for these materials.
The thermal conductivity as a function of temperature forthe Bi1�xPbxOCuSe series is shown in Fig. 8. The total thermalconductivity we found for BiOCuSe (0.97 Wm�1 K�1 at 373 K) is
Fig. 8 Thermal conductivity and figure of merit for Bi1�xPbxOCuSe as a functionof temperature.
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comparable to previously reported values,7 and signicantlylower than that of Bi2Te3 (�2 W m�1 K�1).21 The electronic andlattice contributions were estimated using the electricalconductivity data in conjunction with the Wiedemann–Franzlaw, with a Lorenz constant of 2.45 � 10�8 W U K�2. ForBiOCuSe, the electronic contribution is only ca. 4% of the totalthermal conductivity. The total thermal conductivity rises withincreasing lead content up to 1.58 W m�1 K�1 at 373 K forBi0.8Pb0.2OCuSe. The electronic thermal conductivity for dopedsamples increases markedly with x. This indicates that the risein the total thermal conductivity with doping originates fromthe increase in electronic thermal conductivity caused bygrowing charge carrier concentrations. It has been suggestedthat the remarkably low thermal conductivity of BiOCuSe arisesfrom the two-dimensional nature of the structure of thismaterial, which leads to scattering of phonons at the interfacesbetween the [Cu2Se2]
2� and [Bi2O2]2+ layers.7,13 This is supported
by the low sound speed and a low Young's modulus of BiOCuSe,which may be indicative of weak bonding between layers.13
Moreover, band structure calculations on these phasesindicate that the bonding is highly anisotropic,27 and that thenature of the bonding between the oxide and chalcogenidelayers is mainly ionic and weaker than the intralayer bonding.9e
The thermoelectric gure of merit as a function oftemperature is shown in Fig. 8. For all compositions investi-gated, ZT increases with increasing temperature up to themaximum temperature investigated. Given the trend shown bythese data, we would expect that even higher values of ZTwould be achieved at higher temperatures. However, as shownby our thermogravimetric data (Fig. 5), use of these materialsat higher temperatures would require the use of a protectiveatmosphere to prevent oxidation. The values found for theundoped material are comparable to those previously reportedby Zhao et al.7 The maximum value of ZT, which is 0.65 at 673K, is found for doping levels of 4–5% lead. This represents animprovement of ca. 50% when compared with values found forother dopants at the same temperature. For example, at 673 K,a ZT z 0.35 is found for x ¼ 0.15 in Bi1�xSrxOCuSe,7 ZT rea-ches ca. 0.3 for x ¼ 0.985 in BiOCu1�xSe,12 and a ZT z 0.3 hasbeen determined for x ¼ 0.125 in Bi1�xMgxOCuSe.28 Theperformance of the materials reported here is comparable tothat of ball-milled Bi1�xBaxOCuSe, for which reduction of grainsizes to 200–400 nm decreases the thermal conductivity by ca.40%, resulting in an improved ZT z 0.6 at 673 K for x ¼ 0.10.14
The enhanced performance of Pb-doped samples may berelated to their slightly higher hole mobility when compared tothose doped with Ba2+ or Sr2+,17 as evidenced by the signi-cantly higher electrical conductivities found for the same levelsof doping. This may be related to the similarity in atomicmasses of Pb2+ and Bi3+, which may lessen point-defect scat-tering. Furthermore, for the materials described here, thedistortion of the CuSe4 tetrahedra (Fig. 3(c)), which could bedetrimental to the Cu 3d – Se 4p orbital mixing and hence tothe hole mobility,25 decreases with increasing levels of doping.This may also be a contributory factor to the higher holemobility of the Pb-doped samples when compared to thosecontaining other dopants.
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Conclusions
We have demonstrated that Pb2+ doping in BiOCuSe results in amarked enhancement of the thermoelectric performance whencompared to other dopants. Further improvements in theperformance of these materials may be achieved by ball milling,given that it has been shown that the thermal conductivity ofBiOCuSe can be reduced to 0.54 W m�1 K�1 at 300 K14 throughthis process. This could result in materials with ZT > 1 at rela-tively low temperatures, which would be well suited for ther-moelectric waste heat recovery.
Acknowledgements
We thank the Energy Technology Partnership and EuropeanThermodynamics Ltd. for nancial support for this project. DrJ.W.G. Bos is thanked for access to electrical transportmeasurement equipment.
Notes and references
1 D. M. Rowe, Thermoelectrics Handbook: Macro to Nano, ed. D.M. Rowe, CRC Press, Boca Raton, FL, 2006, ch. 1.
2 J. R. Sootsman, D. Y. Chung and M. G. Kanatzidis, Angew.Chem., Int. Ed., 2009, 48, 8616.
3 (a) G. A. Slack, CRC Handbook of Thermoelectrics, ed. D. M.Rowe, Press, Boca Raton, FL, 1995, p. 407; (b) G. S. Nolas,J. Sharp and H. J. Goldsmid, Thermoelectrics: BasicPrinciples and New Materials Developments, Springer, BerlinHeidelberg, 2001.
4 P. Vaqueiro and A. V. Powell, J. Mater. Chem., 2010, 20, 9577.5 C. Wan, Y. Wang, N. Wang, W. Norimatsu, M. Kusunoki andK. Koumoto, Sci. Technol. Adv. Mater., 2010, 11, 044306.
6 I. Terasaki, Y. Sasago and K. Uchinokura, Phys. Rev. B:Condens. Matter Mater. Phys., 1997, 56, R12685.
7 L. D. Zhao, D. Berardan, Y. L. Pei, C. Byl, L. Pinsard-Gaudartand N. Dragoe, Appl. Phys. Lett., 2010, 97, 092118.
8 P. Vaqueiro, G. Guelou, M. Stec, E. Guilmeau andA. V. Powell, J. Mater. Chem. A, 2013, 1, 520.
9 (a) M. Palazzi, C. Carcaly and J. Flahaut, J. Solid State Chem.,1980, 35, 150; (b) A. M. Kusainova, P. S. Berdonosov,L. G. Akselrud, L. N. Kholodkovskaya, V. A. Dolgikh andB. A. Popovkin, J. Solid State Chem., 1994, 112, 189; (c)G. H. Chan, B. Den, M. Bertoni, J. R. Ireland,M. C. Hersam, T. O. Mason, R. P. Van Duyne andJ. A. Ibers, Inorg. Chem., 2006, 45, 8264; (d) M. L. Liu,
This journal is ª The Royal Society of Chemistry 2013
L. B. Wu, F. Q. Huang, L. D. Chen and J. A. Ibers, J. SolidState Chem., 2007, 180, 62; (e) H. Hiramatsu, H. Yanagi,T. Kamiya, K. Ueda, M. Hirano and H. Hosono, Chem.Mater., 2008, 20, 326.
10 K. Ueda, H. Hiramatsu, M. Hirano, T. Kamiya andH. Hosono, Thin Solid Films, 2006, 496, 8.
11 T. Ohtani, Y. Tachibana and Y. Fujii, J. Alloys Compd., 1997,262, 175.
12 Y. Liu, L.-D. Zhao, Y. Liu, J. Lan, W. Xu, F. Li, B.-P. Zhang,D. Berardan, N. Dragoe, Y.-H. Lin, C.-W. Nan, J.-F. Li andH. Zhu, J. Am. Chem. Soc., 2011, 133, 20112.
13 F. Li, J.-F. Li, L.-D. Zhao, K. Xiang, Y. Liu, B. P. Zhang,Y.-H. Lin, C.-W. Nan and H.-M. Zhu, Energy Environ. Sci.,2012, 5, 7188.
14 J. Li, J. Sui, Y. Pei, C. Barreteau, D. Berardan, N. Dragoe,W. Cai, J. He and L.-D. Zhao, Energy Environ. Sci., 2012, 5,8543.
15 A. C. Larson and R. B. von Dreele, General Structure AnalysisSystem, Los Alamos Laboratory, 1994 [Report LAUR 85–748].
16 P. S. Gaal and S. P. Apostolescu, US Patent No. US 6,375,349,2002.
17 L. Pan, D. Berardan, L. Zhao, C. Barreteau and N. Dragoe,Appl. Phys. Lett., 2013, 102, 023902.
18 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,Theor. Gen. Crystallogr., 1976, 32, 751.
19 R. D. Abelson, Thermoelectrics Handbook: Macro to Nano,ed. D.M. Rowe, CRC Press, Boca Raton, FL, 2006, ch. 56.
20 R. W. Rice, Ceramic Fabrication Technology, Marcel Dekker,New York Basel, 2003.
21 H. Scherrer and S. Scherrer, CRC Handbook ofThermoelectrics, ed. D. M. Rowe, CRC Press, Boca Raton,FL, 1995, ch. 19.
22 T. Caillat, J.-P. Fleurial and A. Borshchevsky, J. Phys. Chem.Solids, 1997, 58, 1119.
23 D. Zou, S. Xie, Y. Liu, J. Lin and J. Li, J. Mater. Chem. A, 2013,1, 8888.
24 D. J. Singh and I. I. Mazin, Phys. Rev. B: Condens. MatterMater. Phys., 1997, 56, R1650.
25 C. Barreteau, D. Berardan, E. Amzallag, L. D. Zhao andN. Dragoe, Chem. Mater., 2012, 24, 3168.
26 C. Wood, Rep. Prog. Phys., 1988, 51, 459.27 V. V. Bannikov, I. R. Shein and A. L. Ivanovskii, Solid State
Sci., 2012, 14, 89.28 J. Li, J. Sui, C. Barreteau, D. Berardan, N. Dragoe, W. Cai and
Y. Pei, J. Alloys Compd., 2013, 551, 649.
J. Mater. Chem. A, 2013, 1, 12270–12275 | 12275
