Ca_9Zn_(4.5-δ)Sb_9(0<δ<0.5)化合物Sr/Eu阳离子掺杂与热电性质
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摘要
作为一种非常有前景的热电材料, Ca_9Zn_(4.5-δ)Sb_9(0<δ<0.5)相较于其他材料有着成本低、环保高效等优势.以往研究表明间隙金属元素在该体系中起着关键作用,然而由于其狭窄的域值范围制约了Ca_9Zn_(4.5-δ)Sb_9的性质调控.本文用原子半径更大的阳离子Sr/Eu来取代少量Ca,使其Seebeck系数和电学性质得以同步增强,得到了间接调控间隙原子的结果.载流子测试表明,载流子浓度有轻微降低,而载流子迁移率有着明显提升,这与增大的晶体热导性质都表明晶体结构发生了明显变化.对Ca_(9-x)Sr_(x)Zn_(4.5-δ)Sb_9系列进行了单晶X射线衍射测试,结果表明随Sr含量增大晶格参数有增大趋势,间隙Zn原子浓度上升,材料热电性能得到协同优化.最终掺杂样品中, Ca_(8.2)Eu_(0.8)Zn_(4.5-δ)Sb_9在873 K得到最大热电优值(ZT)约为0.81.
Previous studies suggested that Zintl phases related to the Ca_9 Zn_(4.5-δ)Sb_9(0<δ<0.5) structure may be very promising thermoelectric candidates because of the complex crystal structure and extensively existing transition metal defects.Studies on compounds Yb_9 Mn_(4.2) Sb_9, Eu_9 Cd_4 Sb_9 and Ca_9 Zn_(4.5-x)Cu_xSb_9 indicated that they could achieve an optimized zT around 0.7. Especially for Ca_9 Zn_(4.5-δ)Sb_9, a very recent report suggested that a high zT of 1.1 at 875 K could be obtained in combination of various secondary phases introduced by the variation of Zn contents, which provided a chemical potential to change the composition. However, to change the composition of Ca_9 Zn_(4.5-δ)Sb_9 is actually very difficult to implement owing to the narrow homogeneity range. In this work, with Ca substituted by big cations such as Eu~(2+)and Sr~(2+), the unit cell of Ca_9 Zn_(4.5-δ)Sb_9 was expanded, and similar significant enhancement on the thermoelectric performance was resulted.All syntheses were handled inside a glovebox, then the reactants were loaded in the Nb tube and arc-welded, which were enclosed in a fused silica tube. The reactions were firstly heated to 1173 K in 6 h and then maintained at this temperature for24 h. After the homogeneity process, the furnace was slowly cooled down to 873 K at a rate of 6 K/h, followed by another dwelling for 6 hours and finally cooled down to 573 K at a rate of 10 K/h. At last, the furnace was shut down and the reactions were opened in the glovebox. The compound were fully characterized by single-crystal X-ray diffraction(SXRD)and powder X-ray diffraction(PXRD), Energy dispersive spectrometer(EDS) and Hall effect measurement, etc. Singlecrystal X-ray diffraction(SXRD) proves that with the increase of Sr component, the crystal parameters are slightly increasing. Hall effect measurement show that the substitution of Ca by Eu or Sr can both lead to an obvious decreasing on the carrier(holes) concentration, and the resultant mobility for Sr/Eu-doped materials is very similar if compared by the same doping level. It is also noted that neither the carrier concentration nor the mobility seems to vary linearly with the Eu/Sr-doping contents, which means a saturated state will be eventually approached. This phenomenon is consistent with the speculation above that the changes on the properties should predominantly originate from the change of interstitial Zn content, which the size effect dominated. While the decreasing carrier concentrations as well as the increasing mobility and lattice thermal conductivity all point to a structure modification, the crystal structures of these doped materials were systematically verified through the single crystal X-ray diffraction.In conclusion, although a direct measurement on such a small composition variation is very difficult(the X-ray diffraction results prove it), the significantly reduced carrier concentration and increased mobility as well as lattice thermal conductivity can still provide some useful hints on understanding the property optimization of these materials. For material Ca_(8.2) Eu_(0.8) Zn_(4.5-δ)Sb_9, high figure of merit with ZT～0.81 has been achieved at 873 K.
Previous studies suggested that Zintl phases related to the Ca_9 Zn_(4.5-δ)Sb_9(0<δ<0.5) structure may be very promising thermoelectric candidates because of the complex crystal structure and extensively existing transition metal defects.Studies on compounds Yb_9 Mn_(4.2) Sb_9, Eu_9 Cd_4 Sb_9 and Ca_9 Zn_(4.5-x)Cu_xSb_9 indicated that they could achieve an optimized zT around 0.7. Especially for Ca_9 Zn_(4.5-δ)Sb_9, a very recent report suggested that a high zT of 1.1 at 875 K could be obtained in combination of various secondary phases introduced by the variation of Zn contents, which provided a chemical potential to change the composition. However, to change the composition of Ca_9 Zn_(4.5-δ)Sb_9 is actually very difficult to implement owing to the narrow homogeneity range. In this work, with Ca substituted by big cations such as Eu~(2+)and Sr~(2+), the unit cell of Ca_9 Zn_(4.5-δ)Sb_9 was expanded, and similar significant enhancement on the thermoelectric performance was resulted.All syntheses were handled inside a glovebox, then the reactants were loaded in the Nb tube and arc-welded, which were enclosed in a fused silica tube. The reactions were firstly heated to 1173 K in 6 h and then maintained at this temperature for24 h. After the homogeneity process, the furnace was slowly cooled down to 873 K at a rate of 6 K/h, followed by another dwelling for 6 hours and finally cooled down to 573 K at a rate of 10 K/h. At last, the furnace was shut down and the reactions were opened in the glovebox. The compound were fully characterized by single-crystal X-ray diffraction(SXRD)and powder X-ray diffraction(PXRD), Energy dispersive spectrometer(EDS) and Hall effect measurement, etc. Singlecrystal X-ray diffraction(SXRD) proves that with the increase of Sr component, the crystal parameters are slightly increasing. Hall effect measurement show that the substitution of Ca by Eu or Sr can both lead to an obvious decreasing on the carrier(holes) concentration, and the resultant mobility for Sr/Eu-doped materials is very similar if compared by the same doping level. It is also noted that neither the carrier concentration nor the mobility seems to vary linearly with the Eu/Sr-doping contents, which means a saturated state will be eventually approached. This phenomenon is consistent with the speculation above that the changes on the properties should predominantly originate from the change of interstitial Zn content, which the size effect dominated. While the decreasing carrier concentrations as well as the increasing mobility and lattice thermal conductivity all point to a structure modification, the crystal structures of these doped materials were systematically verified through the single crystal X-ray diffraction.In conclusion, although a direct measurement on such a small composition variation is very difficult(the X-ray diffraction results prove it), the significantly reduced carrier concentration and increased mobility as well as lattice thermal conductivity can still provide some useful hints on understanding the property optimization of these materials. For material Ca_(8.2) Eu_(0.8) Zn_(4.5-δ)Sb_9, high figure of merit with ZT～0.81 has been achieved at 873 K.
引文
1 Zhu T,Liu Y,Fu C,et al.Compromise and synergy in high-efficiency thermoelectric materials.Adv Mater,2017,29:1605884
2 Li Z,Xiao C,Zhu H,et al.Defect chemistry for thermoelectric materials.J Am Chem Soc,2016,138:14810-14819
3 Xiao C,Li Z,Li K,et al.Decoupling interrelated parameters for designing high performance thermoelectric materials.Acc Chem Res,2014,47:1287-1295
4 Pei Y,Shi X,LaLonde A,et al.Convergence of electronic bands for high performance bulk thermoelectrics.Nature,2011,473:66-69
5 Heremans J P,Jovovic V,Toberer E S,et al.Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states.Science,2008,321:554-557
6 Pei Y,LaLonde A,Iwanaga S,et al.High thermoelectric figure of merit in heavy hole dominated PbTe.Energy Environ Sci,2011,4:2085-2089
7 Wang H,Pei Y,LaLonde A D,et al.Heavily doped p-type PbSe with high thermoelectric performance:An alternative for PbTe.Adv Mater,2011,23:1366-1370
8 Li W,Zheng L,Ge B,et al.Promoting SnTe as an eco-friendly solution for p-PbTe thermoelectric via band convergence and interstitial defects.Adv Mater,2017,29:1605887
9 Takagiwa Y,Pei Y,Pomrehn G,et al.Validity of rigid band approximation of PbTe thermoelectric materials.APL Mater,2013,1:011101
10 Wang H,Gibbs Z M,Takagiwa Y,et al.Tuning bands of PbSe for better thermoelectric efficiency.Energy Environ Sci,2014,7:804-811
11 Hu L,Wu H,Zhu T,et al.Tuning multiscale microstructures to enhance thermoelectric performance of n-type bismuth-telluride-based solid solutions.Adv Energy Mater,2015,5:1500411
12 Devender,Gehring P,Gaul A,et al.Harnessing topological band effects in bismuth telluride selenide for large enhancements in thermoelectric properties through isovalent doping.Adv Mater,2016,28:6436-6441
13 Hu L P,Zhu T J,Wang Y G,et al.Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction.NPG Asia Mater,2014,6:e88
14 Hu L,Zhu T,Liu X,et al.Point Defect engineering of high-performance bismuth-telluride-based thermoelectric materials.Adv Funct Mater,2014,24:5211-5218
15 Li G,Aydemir U,Wood M,et al.Defect-controlled electronic structure and phase stability in thermoelectric skutterudite CoSb3.Chem Mater,2017,29:3999-4007
16 Li G,Bajaj S,Aydemir U,et al.p-Type co interstitial defects in thermoelectric skutterudite CoSb3due to the breakage of Sb4-rings.Chem Mater,2016,28:2172-2179
17 Qiu Y,Xi L,Shi X,et al.Charge-compensated compound defects in Ga-containing thermoelectric skutterudites.Adv Funct Mater,2013,23:3194-3203
18 Tang Y,Qiu Y,Xi L,et al.Phase diagram of In-Co-Sb system and thermoelectric properties of in-containing skutterudites.Energy Environ Sci,2014,7:812-819
19 Brown S R,Kauzlarich S M,Gascoin F,et al.Yb14MnSb11:New high efficiency thermoelectric material for power generation.Chem Mater,2006,18:1873-1877
20 Toberer E S,Cox C A,Brown S R,et al.Traversing the metal-insulator transition in a Zintl phase:Rational enhancement of thermoelectric efficiency in Yb14Mn1-xAlxSb11.Adv Funct Mater,2008,18:2795-2800
21 Gascoin F,Ottensmann S,Stark D,et al.Zintl phases as thermoelectric materials:Tuned transport properties of the compounds CaxYb1-xZn2Sb2.Adv Funct Mater,2005,15:1860-1864
22 Shuai J,Liu Z,Kim H S,et al.Thermoelectric properties of Bi-based Zintl compounds Ca1-xYbxMg2Bi2.J Mater Chem A,2016,4:4312-4320
23 Zhang J,Song L,Madsen G K H,et al.Designing high-performance layered thermoelectric materials through orbital engineering.Nat Commun,2016,7:10892
24 Zevalkink A,Toberer E S,Bleith T,et al.Improved carrier concentration control in Zn-doped Ca5Al2Sb6.J Appl Phys,2011,110:013721
25 Zevalkink A,Swallow J,Snyder G J.Thermoelectric properties of Zn-doped Ca5In2Sb6.Dalton Trans,2013,42:9713-9719
26 Toberer E S,Zevalkink A,Crisosto N,et al.The Zintl compound Ca5Al2Sb6for low-cost thermoelectric power generation.Adv Funct Mater,2010,20:4375-4380
27 Zevalkink A,Toberer E S,Zeier W G,et al.Ca3AlSb3:An inexpensive,non-toxic thermoelectric material for waste heat recovery.Energy Environ Sci,2011,4:510-518
28 Zevalkink A,Zeier W G,Pomrehn G,et al.Thermoelectric properties of Sr3GaSb3-A chain-forming Zintl compound.Energy Environ Sci,2012,5:9121-9128
29 Shi Q,Feng Z,Yan Y,et al.Electronic structure and thermoelectric properties of Zintl compounds A3AlSb3(A=Ca and Sr):First-principles study.RSC Adv,2015,5:65133-65138
30 Pomrehn G S,Zevalkink A,Zeier W G,et al.Defect-controlled electronic properties in AZn2Sb2Zintl phases.Angew Chem Int Ed,2014,53:3422-3426
31 Zevalkink A,Zeier W G,Cheng E,et al.Nonstoichiometry in the Zintl phase Yb1-δZn2Sb2as a route to thermoelectric optimization.Chem Mater,2014,26:5710-5717
32 Wang J,Liu X C,Xia S Q,et al.Ca1-xRExAg1-ySb(RE=La,Ce,Pr,Nd,Sm0≤x≤1;0≤y≤1):Interesting structural transformation and enhanced high-temperature thermoelectric performance.J Am Chem Soc,2013,135:11840-11848
33 Wu Z,Li J,Li X,et al.Tuning the thermoelectric properties of Ca9Zn4+xSb9by controlled doping on the interstitial structure.Chem Mater,2016,28:6917-6924
34 Bux S K,Zevalkink A,Janka O,et al.Glass-like lattice thermal conductivity and high thermoelectric efficiency in Yb9Mn4.2Sb9.J Mater Chem A,2014,2:215-220
35 Ohno S,Zevalkink A,Takagiwa Y,et al.Thermoelectric properties of the Yb9Mn4.2-xZnxSb9solid solutions.J Mater Chem A,2014,2:7478-7483
36 Kazem N,Hurtado A,Klobes B,et al.Eu9Cd4-xCM2+x-y□ySb9:Ca9Mn4Bi9-type structure stuffed with coinage metals(Cu,Ag,and Au)and the challenges with classical valence theory in describing these possible Zintl phases.Inorg Chem,2015,54:850-859
37 Kazem N,Zaikina J V,Ohno S,et al.Coinage-metal-stuffed Eu9Cd4Sb9:Metallic compounds with anomalous low thermal conductivities.Chem Mater,2015,27:7508-7519
38 Ohno S,Aydemir U,Amsler M,et al.Achieving zT>1 in inexpensive Zintl phase Ca9Zn4+xSb9by phase boundary mapping.Adv Funct Mater,2017,27:1606361
39 Xia S,Bobev S.Interplay between size and electronic effects in determining the homogeneity range of the A9Zn4+xPn9and A9Cd4+xPn9Phases(0≤x≤0.5),A=Ca,Sr,Yb,Eu;Pn=Sb,Bi.J Am Chem Soc,2007,129:10011-10018
40 Bobev S,Thompson J D,Sarrao J L,et al.Probing the limits of the Zintl concept:Structure and bonding in rare-earth and alkaline-earth zincantimonides Yb9Zn4+xSb9and Ca9Zn4.5Sb9.Inorg Chem,2004,43:5044-5052
41 Shuai J,Geng H Y,Lan Y C,et al.Higher thermoelectric performance of Zintl phases Eu0.5Yb0.51-xCaxMg2Bi2by band engineering and strain fluctuation.Proc Natl Acad Sci USA,2016,113:E4125-E4132
42 Tamaki H,Sato H K,Kanno T.Isotropic conduction network and defect chemistry in Mg3+δSb2-based layered Zintl compounds with high thermoelectric performance.Adv Mater,2016,28:10182-10187
43 Pauling L.The Nature of the Chemical Bond.New York:Cornell University Press,1960.88-93
44 Jonson M,Mahan G D.Mott’s formula for the thermopower and the Wiedemann-Franz law.Phys Rev B,1980,21:4223-4229
45 Kumar G S,Prasad G,Pohl R O.Experimental determinations of the Lorenz number.J Mater Sci,1993,28:4261-4272
46 Zhao L D,Lo S H,He J,et al.High performance thermoelectrics from earth-abundant materials:Enhanced figure of merit in PbS by second phase nanostructures.J Am Chem Soc,2011,133:20476-20487
2 Li Z,Xiao C,Zhu H,et al.Defect chemistry for thermoelectric materials.J Am Chem Soc,2016,138:14810-14819
3 Xiao C,Li Z,Li K,et al.Decoupling interrelated parameters for designing high performance thermoelectric materials.Acc Chem Res,2014,47:1287-1295
4 Pei Y,Shi X,LaLonde A,et al.Convergence of electronic bands for high performance bulk thermoelectrics.Nature,2011,473:66-69
5 Heremans J P,Jovovic V,Toberer E S,et al.Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states.Science,2008,321:554-557
6 Pei Y,LaLonde A,Iwanaga S,et al.High thermoelectric figure of merit in heavy hole dominated PbTe.Energy Environ Sci,2011,4:2085-2089
7 Wang H,Pei Y,LaLonde A D,et al.Heavily doped p-type PbSe with high thermoelectric performance:An alternative for PbTe.Adv Mater,2011,23:1366-1370
8 Li W,Zheng L,Ge B,et al.Promoting SnTe as an eco-friendly solution for p-PbTe thermoelectric via band convergence and interstitial defects.Adv Mater,2017,29:1605887
9 Takagiwa Y,Pei Y,Pomrehn G,et al.Validity of rigid band approximation of PbTe thermoelectric materials.APL Mater,2013,1:011101
10 Wang H,Gibbs Z M,Takagiwa Y,et al.Tuning bands of PbSe for better thermoelectric efficiency.Energy Environ Sci,2014,7:804-811
11 Hu L,Wu H,Zhu T,et al.Tuning multiscale microstructures to enhance thermoelectric performance of n-type bismuth-telluride-based solid solutions.Adv Energy Mater,2015,5:1500411
12 Devender,Gehring P,Gaul A,et al.Harnessing topological band effects in bismuth telluride selenide for large enhancements in thermoelectric properties through isovalent doping.Adv Mater,2016,28:6436-6441
13 Hu L P,Zhu T J,Wang Y G,et al.Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction.NPG Asia Mater,2014,6:e88
14 Hu L,Zhu T,Liu X,et al.Point Defect engineering of high-performance bismuth-telluride-based thermoelectric materials.Adv Funct Mater,2014,24:5211-5218
15 Li G,Aydemir U,Wood M,et al.Defect-controlled electronic structure and phase stability in thermoelectric skutterudite CoSb3.Chem Mater,2017,29:3999-4007
16 Li G,Bajaj S,Aydemir U,et al.p-Type co interstitial defects in thermoelectric skutterudite CoSb3due to the breakage of Sb4-rings.Chem Mater,2016,28:2172-2179
17 Qiu Y,Xi L,Shi X,et al.Charge-compensated compound defects in Ga-containing thermoelectric skutterudites.Adv Funct Mater,2013,23:3194-3203
18 Tang Y,Qiu Y,Xi L,et al.Phase diagram of In-Co-Sb system and thermoelectric properties of in-containing skutterudites.Energy Environ Sci,2014,7:812-819
19 Brown S R,Kauzlarich S M,Gascoin F,et al.Yb14MnSb11:New high efficiency thermoelectric material for power generation.Chem Mater,2006,18:1873-1877
20 Toberer E S,Cox C A,Brown S R,et al.Traversing the metal-insulator transition in a Zintl phase:Rational enhancement of thermoelectric efficiency in Yb14Mn1-xAlxSb11.Adv Funct Mater,2008,18:2795-2800
21 Gascoin F,Ottensmann S,Stark D,et al.Zintl phases as thermoelectric materials:Tuned transport properties of the compounds CaxYb1-xZn2Sb2.Adv Funct Mater,2005,15:1860-1864
22 Shuai J,Liu Z,Kim H S,et al.Thermoelectric properties of Bi-based Zintl compounds Ca1-xYbxMg2Bi2.J Mater Chem A,2016,4:4312-4320
23 Zhang J,Song L,Madsen G K H,et al.Designing high-performance layered thermoelectric materials through orbital engineering.Nat Commun,2016,7:10892
24 Zevalkink A,Toberer E S,Bleith T,et al.Improved carrier concentration control in Zn-doped Ca5Al2Sb6.J Appl Phys,2011,110:013721
25 Zevalkink A,Swallow J,Snyder G J.Thermoelectric properties of Zn-doped Ca5In2Sb6.Dalton Trans,2013,42:9713-9719
26 Toberer E S,Zevalkink A,Crisosto N,et al.The Zintl compound Ca5Al2Sb6for low-cost thermoelectric power generation.Adv Funct Mater,2010,20:4375-4380
27 Zevalkink A,Toberer E S,Zeier W G,et al.Ca3AlSb3:An inexpensive,non-toxic thermoelectric material for waste heat recovery.Energy Environ Sci,2011,4:510-518
28 Zevalkink A,Zeier W G,Pomrehn G,et al.Thermoelectric properties of Sr3GaSb3-A chain-forming Zintl compound.Energy Environ Sci,2012,5:9121-9128
29 Shi Q,Feng Z,Yan Y,et al.Electronic structure and thermoelectric properties of Zintl compounds A3AlSb3(A=Ca and Sr):First-principles study.RSC Adv,2015,5:65133-65138
30 Pomrehn G S,Zevalkink A,Zeier W G,et al.Defect-controlled electronic properties in AZn2Sb2Zintl phases.Angew Chem Int Ed,2014,53:3422-3426
31 Zevalkink A,Zeier W G,Cheng E,et al.Nonstoichiometry in the Zintl phase Yb1-δZn2Sb2as a route to thermoelectric optimization.Chem Mater,2014,26:5710-5717
32 Wang J,Liu X C,Xia S Q,et al.Ca1-xRExAg1-ySb(RE=La,Ce,Pr,Nd,Sm0≤x≤1;0≤y≤1):Interesting structural transformation and enhanced high-temperature thermoelectric performance.J Am Chem Soc,2013,135:11840-11848
33 Wu Z,Li J,Li X,et al.Tuning the thermoelectric properties of Ca9Zn4+xSb9by controlled doping on the interstitial structure.Chem Mater,2016,28:6917-6924
34 Bux S K,Zevalkink A,Janka O,et al.Glass-like lattice thermal conductivity and high thermoelectric efficiency in Yb9Mn4.2Sb9.J Mater Chem A,2014,2:215-220
35 Ohno S,Zevalkink A,Takagiwa Y,et al.Thermoelectric properties of the Yb9Mn4.2-xZnxSb9solid solutions.J Mater Chem A,2014,2:7478-7483
36 Kazem N,Hurtado A,Klobes B,et al.Eu9Cd4-xCM2+x-y□ySb9:Ca9Mn4Bi9-type structure stuffed with coinage metals(Cu,Ag,and Au)and the challenges with classical valence theory in describing these possible Zintl phases.Inorg Chem,2015,54:850-859
37 Kazem N,Zaikina J V,Ohno S,et al.Coinage-metal-stuffed Eu9Cd4Sb9:Metallic compounds with anomalous low thermal conductivities.Chem Mater,2015,27:7508-7519
38 Ohno S,Aydemir U,Amsler M,et al.Achieving zT>1 in inexpensive Zintl phase Ca9Zn4+xSb9by phase boundary mapping.Adv Funct Mater,2017,27:1606361
39 Xia S,Bobev S.Interplay between size and electronic effects in determining the homogeneity range of the A9Zn4+xPn9and A9Cd4+xPn9Phases(0≤x≤0.5),A=Ca,Sr,Yb,Eu;Pn=Sb,Bi.J Am Chem Soc,2007,129:10011-10018
40 Bobev S,Thompson J D,Sarrao J L,et al.Probing the limits of the Zintl concept:Structure and bonding in rare-earth and alkaline-earth zincantimonides Yb9Zn4+xSb9and Ca9Zn4.5Sb9.Inorg Chem,2004,43:5044-5052
41 Shuai J,Geng H Y,Lan Y C,et al.Higher thermoelectric performance of Zintl phases Eu0.5Yb0.51-xCaxMg2Bi2by band engineering and strain fluctuation.Proc Natl Acad Sci USA,2016,113:E4125-E4132
42 Tamaki H,Sato H K,Kanno T.Isotropic conduction network and defect chemistry in Mg3+δSb2-based layered Zintl compounds with high thermoelectric performance.Adv Mater,2016,28:10182-10187
43 Pauling L.The Nature of the Chemical Bond.New York:Cornell University Press,1960.88-93
44 Jonson M,Mahan G D.Mott’s formula for the thermopower and the Wiedemann-Franz law.Phys Rev B,1980,21:4223-4229
45 Kumar G S,Prasad G,Pohl R O.Experimental determinations of the Lorenz number.J Mater Sci,1993,28:4261-4272
46 Zhao L D,Lo S H,He J,et al.High performance thermoelectrics from earth-abundant materials:Enhanced figure of merit in PbS by second phase nanostructures.J Am Chem Soc,2011,133:20476-20487