- Email: [email protected]
Synthesis and thermoelectric properties of BaMn2Sb2 single crystals
Journal of Alloys and Compounds 477 (2009) 519–522
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and thermoelectric properties of BaMn2 Sb2 single crystals H.F. Wang, K.F. Cai ∗ , H. Li, L. Wang, C.W. Zhou Tongji University, Functional Materials Research Laboratory, 1239 Siping Road, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 15 October 2008 Accepted 19 October 2008 Available online 3 December 2008 PACS: 61.66.Dk 61.82.Fk 72.15.Jf
a b s t r a c t BaMn2 Sb2 single crystals, synthesized by a Sn-flux method, were characterized by powder X-ray diffraction and scanning electron microscopy equipped with electron energy dispersive spectroscopy, respectively. The single crystals were determined to have the body-centered tetragonal ThCr2 Si2 crystal structures with space group I4/mmm. Their thermoelectric properties from room temperature to ∼773 K were also investigated. The electrical conductivity of the crystals is very low at room temperature. And it increases slowly and rapidly with temperature as the temperature is below and above ∼600 K, respectively. The Seebeck coefficient of the crystals is ∼225 V/K at room temperature. And it decreases quickly and gradually below and above ∼600 K, respectively and tends to be negative at ∼773 K. The material has poor thermoelectric properties when the temperature is below 773 K. © 2008 Elsevier B.V. All rights reserved.
Keywords: Zintl phase compound Semiconductors Crystal growth Electron transport Thermoelectric
1. Introduction Recently, green energy has attracted more and more attention due to climate crisis. The energy generated by thermoelectric generators is one kind of green energy. The thermoelectric conversion efficiency depends on the figure of merit (ZT) of the material, given by ZT = ˛2 T/, where , ˛, T, and are, respectively, the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity. Hence, in order to obtain high thermoelectric conversion efficiency, materials with large Seebeck coefficient, high electrical conductivity and low thermal conductivity should be chosen [1]. For thermoelectric generation application, many efforts have been made on Si–Ge alloy [2], boron carbides [3], filledskutterudites [4,5], type-I Ge-based clathrates [6,7], Zn3 Sb4 [8], Nax Co2 O4 [9], and PbTe-based materials [10–12]. However, Zintl phase compounds have recently attracted much attention owing to their good thermoelectric properties at high temperatures. Many Zintl phase compounds such as Yb14 MnSb11 [13], Yb11 GaSb9 [14], Cax Yb1 − x Zn2 Sb2 [15], BaZn2 Sb2 [16], Yb11 Sb9.3 Ge0.5 [17],
∗ Corresponding author. Tel.: +86 21 65980255; fax: +86 21 65980255. E-mail address: [email protected] (K.F. Cai). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.080
and Ba4 In8 Sb16 [18] have been studied. And Yb14 MnSb11 and Cax Yb1 − x Zn2 Sb2 compounds show good thermoelectric properties [13,15]. BaMn2 Sb2 is also a Zintl phase compound. In 1979, Brechtel et al. [19] first reported the crystal structure and synthesis of BaMn2 Sb2 single crystals by melting a mixture of Ba, Mn and Sb in stoichiometric proportion at 1600 K. However, the crystallographic site of Sb they reported is not right. Fig. 1(a) and (b) shows a unit cell structure and the structure viewed along the b-axis of the BaMn2 Sb2 , respectively. Ba, Mn, and Sb atoms occupy the 2a (0,0,0), 4d (0,1/2,1/4), and 4e (0,0,0.3663) sites in the crystal structure, respectively [20]. In the past almost 30 years, there were no reports on its physical properties, except that Xia et al. [20] reported the crystal and electronic band structures in conjunction with the magnetic properties of BaMn2 Sb2 crystals prepared by a Sn-flux method, when this paper was just being revised. Compared with a melting method, a metal-flux method can produce bigger crystals well below their melting points. In this work, BaMn2 Sb2 single crystals were synthesized via a modified procedure of the Sn-flux method reported in Ref. [21] The original purpose was to prepare Yb14 MnSb11 -like single crystals by using Ba to replace Yb. The crystals were characterized by X-ray diffraction and scanning electron microscopy, and the thermoelectric properties of the crystals were measured from room temperature to ∼773 K.
520
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522 Several small single crystals were picked and ground into powder for X-ray diffraction (XRD, Rigaku, D/max2550) in order to investigate the phase composition. The microstructure of the single crystals was observed by scanning electron microscopy (SEM, JEOL JSM5510LV), equipped with electron energy dispersive X-ray spectroscopy (EDS, Oxford 7582). Hall effect measurement was carried out at room temperature using a Hall effect measurement system (HMS 3000, Ecopia) with a magnetic field of 0.55 T. Big crystals were cut into small rectangles (∼1 mm × 1 mm × 5 mm) for thermoelectric properties measurement. The in-plane and ˛ measurements were carried out using a home-made computer control testing system from room temperature up to 773 K under argon atmosphere. The measurement was performed by a steady-state four-probe technique with chopped direct current (∼10 mA). The ˛ value was determined by the slope of the linear relationship between the thermal electromotive force and temperature difference (10–15 K) between the two ends of the sample. The error of the ˛ values is < 10%.
3. Results and discussion Fig. 2 shows a typical XRD pattern for the crystal powder. The pattern of the reported BaMn2 Sb2 (JCPDS card file, No. 33-0094) is also shown in Fig. 2, for comparison. It can be seen from Fig. 2 that all the peaks correspond to the reflections of the reported BaMn2 Sb2 . Note that the (0,0,8) plane peak (2 ∼ 50.7◦ ), which is very weak in the reported data, becomes the third strongest peak in the XRD pattern. XRD pattern collected from a big flat single crystal has a so strong peak also at 2 ∼ 50.7◦ that almost no other peaks are visible. These indicate that the c-axis is perpendicular to the flat surface of the single crystals and that the crystals grow faster along the a- and b-axis. Such BaMn2 Sb2 crystals can also be synthesized by the same method but using the starting materials in stoichiometric ratio, i.e., Ba/Mn/Sb = 1:2:2. Fig. 3(a) and (b) shows the surface and cross-section SEM images of the as-synthesized crystals, respectively. Fig. 3(c) is an enlarged SEM image of the zone indicated by a white square in Fig. 3(b). It can be clearly seen from Fig. 3 that the crystals possess a lamellar structure. The XRD and SEM analyses confirmed the crystal structure characteristic of BaMn2 Sb2 , i.e., its constituent atoms arrange layer upon layer (see Fig. 1(b)). Furthermore, EDS analysis reveals that the single crystals are composed of Ba, Mn, and Sb (Fig. 3(d)). Quantitative EDS analysis indicates that the atomic ratio of Ba/Mn/Sb is 1:1.95:2.09, which also confirms that the as-synthesized single crystals are BaMn2 Sb2 . The electrical transport properties of a few crystal samples from different batches were measured from room temperature up to ∼773 K. All the samples have almost the same –T and ˛–T change trends: the increases with increasing temperature, whereas the
Fig. 1. The crystal structure of BaMn2 Sb2 : (a) a unit cell structure and (b) viewed along the b-axis. Ba: gray spheres; Mn: white spheres; Sb: black spheres.
2. Experimental details Ba pieces (Alfa Aesar, 99.2%), Mn flakes (Johnson Matthey, 99.9%), Sb powder (Sinopharm Chemical Reagent, 99.99%) and Sn powder (Sinopharm Chemical Reagent, 99.5%) were used as received. All manipulations were performed in an argon-filled glove box. The elements, Ba:Mn:Sb:Sn, were arranged in 5 cm3 corundum crucibles in the ratios 14:6:11:86. The reactions were sealed in quartz ampoules under 1/5 atm argon atmosphere and placed in a high-temperature programmable furnace. The reactions were brought up to 1373 K with a rate of 600 K/h, then held at 1373 K for 1 h, followed by slowly cooling (−2 K/h) to 1023 K. Upon reaching 1023 K, the reactions were inverted and spun in a centrifuge at 4000 rpm for 5 min to separate the products from the Sn flux. High yield of reflective and silver-colored sheet-like single crystals were obtained. The flat surface area of the crystals varies from ∼1 to 100 mm2 and the thickness of the crystals changes from ∼0.1 to 2 mm. The crystals were moderately sensitive to air, as evidenced from the quick loss of their luster when left in air. And it was noted that the sides of the crystals were more sensitive to air.
Fig. 2. Powder XRD pattern for the single crystals (upper) and the reported data for BaMn2 Sb2 (lower).
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522
521
Fig. 3. SEM images for the as-synthesized single crystals: (a) surface, (b) cross-section, (c) enlarged image of the zone marked by the white square in (b), and (d) typical EDS spectrum collected on the single crystals.
˛ does the opposite trend. This indicates a typical semiconducting behavior. Fig. 4(a) shows temperature dependence of ˛ and for one of the samples, whose ˛ and both are of medium values at a given temperature. The black and green bars in Fig. 4(a) illustrates the measured value range of ˛ and obtained from the samples at each temperature spot, respectively. The difference of the ˛ values for different samples reduces with increasing temperature, while that of the does the opposite trend. At room temperature, the sample has a quite low carrier concentration, 5.60 × 1016 cm−3 and a moderate Hall mobility, 215 cm2 /Vs, which results in a low electrical conductivity. This agrees with the following equation: = nq, where n, q, and are the carrier concentration, charge, and mobility, respectively. This result is consistent with the prediction, given by Xia et al., on the basis of spin-polarized density-functional theory calculations [20]. The Seebeck coefficient of p-type thermoelectric materials can be simplified as [22]: ˛ = r − ln/n
(1)
where r is the scatter factor. Hence, it can be easily understood that the high Seebeck coefficient at room temperature is due to the low carrier concentration in this sample. The electrical transport properties of the single crystal prepared in stoichiometric ratio, i.e., Ba/Mn/Sb = 1:2:2, by the Sn-flux method are also plotted in Fig. 4(a), for comparison. For simplicity, in the
following text, the single crystals prepared according to Ba/Mn/Sb ratio being 14:6:11 and 1:2:2 are called samples 1 and 2, respectively. Generally speaking, the change trends of both the Seebeck coefficient and electrical conductivity with temperature for the two samples are similar, except that at intermediate temperature region (∼470 K < T < 550 K) both the ˛ and of the sample 1 change more quickly. The difference is probably because some Sn-flux was sandwiched by the layers of the sample 1 and the Sn was melted (melting point 504.9 K) at ∼500 K and then outflowed from the interlayer at higher temperature. Fig. 4(b) shows the inverse temperature dependence of ln for the two samples. As it can be seen from Fig. 4(b) that the ln –T curve for sample 2 includes two linear regions, 300 K < T < 470 K and 470 K < T < 773 K, with activation energy of 0.102 and 1.12 eV, corresponding to extrinsic and intrinsic conduction, respectively. While the ln –T curve for sample 1, besides two linear regions at low temperatures (300–450 K) and high temperatures (650–773 K), there is a wide nonlinear region at intermediate temperatures (450–650 K). The activation energy for sample 1 at low temperature region is somewhat lower than that for sample 2, which could be due to sample 1 containing more impurities. The activation energies for both the samples at high temperatures are very close, implying that they are essentially the same material. Because intrinsic excitation starts at high temperatures, both electron and hole conduction are present, which makes the
522
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522
between them [20]. The Ba2+ cations located at the center of the voids may have the same function as the rattler in CoSb3 skutterudites and type-I Ge-based clathrates, which can significantly reduce the thermal conductivity [4,5,7]; so low thermal conductivity can be expected for the material. Thirdly, the high synthesis temperature employed in Ref. [19] implies that BaMn2 Sb2 is a material with high melting point. Therefore, the material may have good thermoelectric properties at higher temperatures. In addition, if appropriate dopants are selected on Ba and/or Sb site, the thermoelectric properties may be improved, which is underway in our group. 4. Conclusions BaMn2 Sb2 single crystals were successfully synthesized by the Sn-flux method. The single crystals have a lamellar structure. Their electrical conductivity increases with increasing temperature, whereas their Seebeck coefficient decreases with increasing temperature and tends to become negative at T ∼ 773 K. BaMn2 Sb2 might be a good n-type thermoelectric material at above 773 K although its thermoelectric property is poor below 773 K. Acknowledgments This work was supported by Shanghai Pujiang Program and National Basic Research Program of China (2007CB607500). References
Fig. 4. (a) Temperature dependence of Seebeck coefficient and electrical conductivity and (b) ln as a function of inverse temperature. The bars in (a) denote the property value variation range for different samples from different batches.
electrical conductivity for both the samples increase quickly. On the other hand, when both electron and hole conduction are present, the total Seebeck coefficient can be expressed as the Mott relation: ˛=
˛n n + ˛p p n + p
(2)
where ˛n,p and n,p are the Seebeck coefficients and the electrical conductivities for the n- and p-type carriers, respectively [23]. Because of the opposite signs of ˛n and ˛p , the magnitude of the Seebeck coefficient, ˛ becomes very low at high temperatures. At the whole temperature range, the power factor (˛2 ) calculated from the measured ˛ and is low, indicating that BaMn2 Sb2 is a poor thermoelectric material at T < 773 K. However, the situation may be quite different at above 773 K. Firstly, it can be seen from Fig. 4(a) that when T = ∼773 K, the has a quickly increase trend and the ˛ tends to become negative. Secondly, the structure of BaMn2 Sb2 can be viewed as Mn2 Sb2 2− layers, which are built of MnSb4 tetrahedra that share edges in a PbO-like pattern. The layers are stacked along the c-axis direction leaving large “cubic” voids
[1] D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, USA, 1995. [2] M.N. Tripathi, C.M. Bhandari, J. Phys. Condens. Matter 15 (2003) 5359–5370. [3] H. Suematsu, K. Kitajima, I. Ruiz, K. Kobayashi, M. Takeda, D. Shimbo, T. Suzuki, W. Jiang, K. Yatsui, Thin Solid Films 407 (2002) 132–135. [4] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325–1328. [5] M.S. Toprak, C. Stiewe, D. Platzek, S. Williams, L. Bertini, E. Müller, C. Gatti, Y. Zhang, M. Rowe, M. Muhammed, Adv. Funct. Mater. 14 (2004) 1189–1196. [6] K.F. Cai, L.C. Zhang, Q. Lei, E. Muller, C. Stiewe, Cryst. Growth Des 6 (2006) 1797–1800. [7] G.S. Nolas, J.L. Cohn, G.A. Slack, S.B. Schujman, Appl. Phys. Lett. 73 (1998) 178–180. [8] M. Chitroub, F. Besse, H. Scherrer, J. Alloy Compd. 460 (2008) 90–93. [9] M. Ito, D. Furumoto, J. Alloy Compd. 450 (2008) 517–520. [10] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818–821. [11] K.F. Cai, C. Yan, Z.M. He, J.L. Cui, C. Stiewe, E. Mueller, H. Li, J. Alloy Compd. 9 (2008), doi:10.1016/j.jallcom.2008.02.040. [12] M.A. McGuire, A.S. Malik, F.J. DiSalvo, J. Alloy Compd. 460 (2008) 8–12. [13] S.R. Brown, S.M. Kauzlarich, F. Gascoin, G.J. Snyder, Chem. Mater. 18 (2006) 1873–1877. [14] S. Bobev, V. Fritsch, J.D. Thompson, J.L. Sarrao, B. Eck, R. Dronskowski, S.M. Kauzlarich, J. Solid State Chem. 178 (2005) 1071–1079. [15] F. Gascoin, S. Ottensmann, D. Stark, S.M. Haile, G.J. Snyder, Adv. Funct. Mater. 15 (2005) 1860–1864. [16] X.J. Wang, M.B. Tang, J.T. Zhao, H.H. Chen, X.X. Yang, Appl. Phys. Lett. 90 (2007) 232107. [17] J.F. Rauscher, S.M. Kauzlarich, T. Ikeda, G.J. Snyder, Z. Anorg. Allg. Chem. 633 (2007) 1587–1594. [18] S.J. Kim, S. Hu, C. Uher, M.G. Kanatzidis, Chem. Mater. 11 (1999) 3154–3159. [19] E. Brechtel, G. Cordier, H. Schaefer, Z. Naturforsch B 34 (1979) 921–925. [20] S.Q. Xia, C. Myers, S. Bobev, Eur. J. Inorg. Chem. 27 (2008) 4262–4269. [21] I.R. Fisher, T.A. Wiener, S.L. Bud’ko, P.C. Canfield, J.Y. Chan, S.M. Kauzlarich, Phys. Rev. B: Condens. Matter 59 (1999) 13829–13834. [22] J. Seo, K. Park, D. Lee, C. Lee, Scripta Mater. 38 (1998) 477–484. [23] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Phys. Rev. B 67 (2003) 165207–165210.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and thermoelectric properties of BaMn2 Sb2 single crystals H.F. Wang, K.F. Cai ∗ , H. Li, L. Wang, C.W. Zhou Tongji University, Functional Materials Research Laboratory, 1239 Siping Road, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 29 April 2008 Received in revised form 15 October 2008 Accepted 19 October 2008 Available online 3 December 2008 PACS: 61.66.Dk 61.82.Fk 72.15.Jf
a b s t r a c t BaMn2 Sb2 single crystals, synthesized by a Sn-flux method, were characterized by powder X-ray diffraction and scanning electron microscopy equipped with electron energy dispersive spectroscopy, respectively. The single crystals were determined to have the body-centered tetragonal ThCr2 Si2 crystal structures with space group I4/mmm. Their thermoelectric properties from room temperature to ∼773 K were also investigated. The electrical conductivity of the crystals is very low at room temperature. And it increases slowly and rapidly with temperature as the temperature is below and above ∼600 K, respectively. The Seebeck coefficient of the crystals is ∼225 V/K at room temperature. And it decreases quickly and gradually below and above ∼600 K, respectively and tends to be negative at ∼773 K. The material has poor thermoelectric properties when the temperature is below 773 K. © 2008 Elsevier B.V. All rights reserved.
Keywords: Zintl phase compound Semiconductors Crystal growth Electron transport Thermoelectric
1. Introduction Recently, green energy has attracted more and more attention due to climate crisis. The energy generated by thermoelectric generators is one kind of green energy. The thermoelectric conversion efficiency depends on the figure of merit (ZT) of the material, given by ZT = ˛2 T/, where , ˛, T, and are, respectively, the electrical conductivity, Seebeck coefficient, absolute temperature, and thermal conductivity. Hence, in order to obtain high thermoelectric conversion efficiency, materials with large Seebeck coefficient, high electrical conductivity and low thermal conductivity should be chosen [1]. For thermoelectric generation application, many efforts have been made on Si–Ge alloy [2], boron carbides [3], filledskutterudites [4,5], type-I Ge-based clathrates [6,7], Zn3 Sb4 [8], Nax Co2 O4 [9], and PbTe-based materials [10–12]. However, Zintl phase compounds have recently attracted much attention owing to their good thermoelectric properties at high temperatures. Many Zintl phase compounds such as Yb14 MnSb11 [13], Yb11 GaSb9 [14], Cax Yb1 − x Zn2 Sb2 [15], BaZn2 Sb2 [16], Yb11 Sb9.3 Ge0.5 [17],
∗ Corresponding author. Tel.: +86 21 65980255; fax: +86 21 65980255. E-mail address: [email protected] (K.F. Cai). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.080
and Ba4 In8 Sb16 [18] have been studied. And Yb14 MnSb11 and Cax Yb1 − x Zn2 Sb2 compounds show good thermoelectric properties [13,15]. BaMn2 Sb2 is also a Zintl phase compound. In 1979, Brechtel et al. [19] first reported the crystal structure and synthesis of BaMn2 Sb2 single crystals by melting a mixture of Ba, Mn and Sb in stoichiometric proportion at 1600 K. However, the crystallographic site of Sb they reported is not right. Fig. 1(a) and (b) shows a unit cell structure and the structure viewed along the b-axis of the BaMn2 Sb2 , respectively. Ba, Mn, and Sb atoms occupy the 2a (0,0,0), 4d (0,1/2,1/4), and 4e (0,0,0.3663) sites in the crystal structure, respectively [20]. In the past almost 30 years, there were no reports on its physical properties, except that Xia et al. [20] reported the crystal and electronic band structures in conjunction with the magnetic properties of BaMn2 Sb2 crystals prepared by a Sn-flux method, when this paper was just being revised. Compared with a melting method, a metal-flux method can produce bigger crystals well below their melting points. In this work, BaMn2 Sb2 single crystals were synthesized via a modified procedure of the Sn-flux method reported in Ref. [21] The original purpose was to prepare Yb14 MnSb11 -like single crystals by using Ba to replace Yb. The crystals were characterized by X-ray diffraction and scanning electron microscopy, and the thermoelectric properties of the crystals were measured from room temperature to ∼773 K.
520
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522 Several small single crystals were picked and ground into powder for X-ray diffraction (XRD, Rigaku, D/max2550) in order to investigate the phase composition. The microstructure of the single crystals was observed by scanning electron microscopy (SEM, JEOL JSM5510LV), equipped with electron energy dispersive X-ray spectroscopy (EDS, Oxford 7582). Hall effect measurement was carried out at room temperature using a Hall effect measurement system (HMS 3000, Ecopia) with a magnetic field of 0.55 T. Big crystals were cut into small rectangles (∼1 mm × 1 mm × 5 mm) for thermoelectric properties measurement. The in-plane and ˛ measurements were carried out using a home-made computer control testing system from room temperature up to 773 K under argon atmosphere. The measurement was performed by a steady-state four-probe technique with chopped direct current (∼10 mA). The ˛ value was determined by the slope of the linear relationship between the thermal electromotive force and temperature difference (10–15 K) between the two ends of the sample. The error of the ˛ values is < 10%.
3. Results and discussion Fig. 2 shows a typical XRD pattern for the crystal powder. The pattern of the reported BaMn2 Sb2 (JCPDS card file, No. 33-0094) is also shown in Fig. 2, for comparison. It can be seen from Fig. 2 that all the peaks correspond to the reflections of the reported BaMn2 Sb2 . Note that the (0,0,8) plane peak (2 ∼ 50.7◦ ), which is very weak in the reported data, becomes the third strongest peak in the XRD pattern. XRD pattern collected from a big flat single crystal has a so strong peak also at 2 ∼ 50.7◦ that almost no other peaks are visible. These indicate that the c-axis is perpendicular to the flat surface of the single crystals and that the crystals grow faster along the a- and b-axis. Such BaMn2 Sb2 crystals can also be synthesized by the same method but using the starting materials in stoichiometric ratio, i.e., Ba/Mn/Sb = 1:2:2. Fig. 3(a) and (b) shows the surface and cross-section SEM images of the as-synthesized crystals, respectively. Fig. 3(c) is an enlarged SEM image of the zone indicated by a white square in Fig. 3(b). It can be clearly seen from Fig. 3 that the crystals possess a lamellar structure. The XRD and SEM analyses confirmed the crystal structure characteristic of BaMn2 Sb2 , i.e., its constituent atoms arrange layer upon layer (see Fig. 1(b)). Furthermore, EDS analysis reveals that the single crystals are composed of Ba, Mn, and Sb (Fig. 3(d)). Quantitative EDS analysis indicates that the atomic ratio of Ba/Mn/Sb is 1:1.95:2.09, which also confirms that the as-synthesized single crystals are BaMn2 Sb2 . The electrical transport properties of a few crystal samples from different batches were measured from room temperature up to ∼773 K. All the samples have almost the same –T and ˛–T change trends: the increases with increasing temperature, whereas the
Fig. 1. The crystal structure of BaMn2 Sb2 : (a) a unit cell structure and (b) viewed along the b-axis. Ba: gray spheres; Mn: white spheres; Sb: black spheres.
2. Experimental details Ba pieces (Alfa Aesar, 99.2%), Mn flakes (Johnson Matthey, 99.9%), Sb powder (Sinopharm Chemical Reagent, 99.99%) and Sn powder (Sinopharm Chemical Reagent, 99.5%) were used as received. All manipulations were performed in an argon-filled glove box. The elements, Ba:Mn:Sb:Sn, were arranged in 5 cm3 corundum crucibles in the ratios 14:6:11:86. The reactions were sealed in quartz ampoules under 1/5 atm argon atmosphere and placed in a high-temperature programmable furnace. The reactions were brought up to 1373 K with a rate of 600 K/h, then held at 1373 K for 1 h, followed by slowly cooling (−2 K/h) to 1023 K. Upon reaching 1023 K, the reactions were inverted and spun in a centrifuge at 4000 rpm for 5 min to separate the products from the Sn flux. High yield of reflective and silver-colored sheet-like single crystals were obtained. The flat surface area of the crystals varies from ∼1 to 100 mm2 and the thickness of the crystals changes from ∼0.1 to 2 mm. The crystals were moderately sensitive to air, as evidenced from the quick loss of their luster when left in air. And it was noted that the sides of the crystals were more sensitive to air.
Fig. 2. Powder XRD pattern for the single crystals (upper) and the reported data for BaMn2 Sb2 (lower).
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522
521
Fig. 3. SEM images for the as-synthesized single crystals: (a) surface, (b) cross-section, (c) enlarged image of the zone marked by the white square in (b), and (d) typical EDS spectrum collected on the single crystals.
˛ does the opposite trend. This indicates a typical semiconducting behavior. Fig. 4(a) shows temperature dependence of ˛ and for one of the samples, whose ˛ and both are of medium values at a given temperature. The black and green bars in Fig. 4(a) illustrates the measured value range of ˛ and obtained from the samples at each temperature spot, respectively. The difference of the ˛ values for different samples reduces with increasing temperature, while that of the does the opposite trend. At room temperature, the sample has a quite low carrier concentration, 5.60 × 1016 cm−3 and a moderate Hall mobility, 215 cm2 /Vs, which results in a low electrical conductivity. This agrees with the following equation: = nq, where n, q, and are the carrier concentration, charge, and mobility, respectively. This result is consistent with the prediction, given by Xia et al., on the basis of spin-polarized density-functional theory calculations [20]. The Seebeck coefficient of p-type thermoelectric materials can be simplified as [22]: ˛ = r − ln/n
(1)
where r is the scatter factor. Hence, it can be easily understood that the high Seebeck coefficient at room temperature is due to the low carrier concentration in this sample. The electrical transport properties of the single crystal prepared in stoichiometric ratio, i.e., Ba/Mn/Sb = 1:2:2, by the Sn-flux method are also plotted in Fig. 4(a), for comparison. For simplicity, in the
following text, the single crystals prepared according to Ba/Mn/Sb ratio being 14:6:11 and 1:2:2 are called samples 1 and 2, respectively. Generally speaking, the change trends of both the Seebeck coefficient and electrical conductivity with temperature for the two samples are similar, except that at intermediate temperature region (∼470 K < T < 550 K) both the ˛ and of the sample 1 change more quickly. The difference is probably because some Sn-flux was sandwiched by the layers of the sample 1 and the Sn was melted (melting point 504.9 K) at ∼500 K and then outflowed from the interlayer at higher temperature. Fig. 4(b) shows the inverse temperature dependence of ln for the two samples. As it can be seen from Fig. 4(b) that the ln –T curve for sample 2 includes two linear regions, 300 K < T < 470 K and 470 K < T < 773 K, with activation energy of 0.102 and 1.12 eV, corresponding to extrinsic and intrinsic conduction, respectively. While the ln –T curve for sample 1, besides two linear regions at low temperatures (300–450 K) and high temperatures (650–773 K), there is a wide nonlinear region at intermediate temperatures (450–650 K). The activation energy for sample 1 at low temperature region is somewhat lower than that for sample 2, which could be due to sample 1 containing more impurities. The activation energies for both the samples at high temperatures are very close, implying that they are essentially the same material. Because intrinsic excitation starts at high temperatures, both electron and hole conduction are present, which makes the
522
H.F. Wang et al. / Journal of Alloys and Compounds 477 (2009) 519–522
between them [20]. The Ba2+ cations located at the center of the voids may have the same function as the rattler in CoSb3 skutterudites and type-I Ge-based clathrates, which can significantly reduce the thermal conductivity [4,5,7]; so low thermal conductivity can be expected for the material. Thirdly, the high synthesis temperature employed in Ref. [19] implies that BaMn2 Sb2 is a material with high melting point. Therefore, the material may have good thermoelectric properties at higher temperatures. In addition, if appropriate dopants are selected on Ba and/or Sb site, the thermoelectric properties may be improved, which is underway in our group. 4. Conclusions BaMn2 Sb2 single crystals were successfully synthesized by the Sn-flux method. The single crystals have a lamellar structure. Their electrical conductivity increases with increasing temperature, whereas their Seebeck coefficient decreases with increasing temperature and tends to become negative at T ∼ 773 K. BaMn2 Sb2 might be a good n-type thermoelectric material at above 773 K although its thermoelectric property is poor below 773 K. Acknowledgments This work was supported by Shanghai Pujiang Program and National Basic Research Program of China (2007CB607500). References
Fig. 4. (a) Temperature dependence of Seebeck coefficient and electrical conductivity and (b) ln as a function of inverse temperature. The bars in (a) denote the property value variation range for different samples from different batches.
electrical conductivity for both the samples increase quickly. On the other hand, when both electron and hole conduction are present, the total Seebeck coefficient can be expressed as the Mott relation: ˛=
˛n n + ˛p p n + p
(2)
where ˛n,p and n,p are the Seebeck coefficients and the electrical conductivities for the n- and p-type carriers, respectively [23]. Because of the opposite signs of ˛n and ˛p , the magnitude of the Seebeck coefficient, ˛ becomes very low at high temperatures. At the whole temperature range, the power factor (˛2 ) calculated from the measured ˛ and is low, indicating that BaMn2 Sb2 is a poor thermoelectric material at T < 773 K. However, the situation may be quite different at above 773 K. Firstly, it can be seen from Fig. 4(a) that when T = ∼773 K, the has a quickly increase trend and the ˛ tends to become negative. Secondly, the structure of BaMn2 Sb2 can be viewed as Mn2 Sb2 2− layers, which are built of MnSb4 tetrahedra that share edges in a PbO-like pattern. The layers are stacked along the c-axis direction leaving large “cubic” voids
[1] D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, USA, 1995. [2] M.N. Tripathi, C.M. Bhandari, J. Phys. Condens. Matter 15 (2003) 5359–5370. [3] H. Suematsu, K. Kitajima, I. Ruiz, K. Kobayashi, M. Takeda, D. Shimbo, T. Suzuki, W. Jiang, K. Yatsui, Thin Solid Films 407 (2002) 132–135. [4] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325–1328. [5] M.S. Toprak, C. Stiewe, D. Platzek, S. Williams, L. Bertini, E. Müller, C. Gatti, Y. Zhang, M. Rowe, M. Muhammed, Adv. Funct. Mater. 14 (2004) 1189–1196. [6] K.F. Cai, L.C. Zhang, Q. Lei, E. Muller, C. Stiewe, Cryst. Growth Des 6 (2006) 1797–1800. [7] G.S. Nolas, J.L. Cohn, G.A. Slack, S.B. Schujman, Appl. Phys. Lett. 73 (1998) 178–180. [8] M. Chitroub, F. Besse, H. Scherrer, J. Alloy Compd. 460 (2008) 90–93. [9] M. Ito, D. Furumoto, J. Alloy Compd. 450 (2008) 517–520. [10] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818–821. [11] K.F. Cai, C. Yan, Z.M. He, J.L. Cui, C. Stiewe, E. Mueller, H. Li, J. Alloy Compd. 9 (2008), doi:10.1016/j.jallcom.2008.02.040. [12] M.A. McGuire, A.S. Malik, F.J. DiSalvo, J. Alloy Compd. 460 (2008) 8–12. [13] S.R. Brown, S.M. Kauzlarich, F. Gascoin, G.J. Snyder, Chem. Mater. 18 (2006) 1873–1877. [14] S. Bobev, V. Fritsch, J.D. Thompson, J.L. Sarrao, B. Eck, R. Dronskowski, S.M. Kauzlarich, J. Solid State Chem. 178 (2005) 1071–1079. [15] F. Gascoin, S. Ottensmann, D. Stark, S.M. Haile, G.J. Snyder, Adv. Funct. Mater. 15 (2005) 1860–1864. [16] X.J. Wang, M.B. Tang, J.T. Zhao, H.H. Chen, X.X. Yang, Appl. Phys. Lett. 90 (2007) 232107. [17] J.F. Rauscher, S.M. Kauzlarich, T. Ikeda, G.J. Snyder, Z. Anorg. Allg. Chem. 633 (2007) 1587–1594. [18] S.J. Kim, S. Hu, C. Uher, M.G. Kanatzidis, Chem. Mater. 11 (1999) 3154–3159. [19] E. Brechtel, G. Cordier, H. Schaefer, Z. Naturforsch B 34 (1979) 921–925. [20] S.Q. Xia, C. Myers, S. Bobev, Eur. J. Inorg. Chem. 27 (2008) 4262–4269. [21] I.R. Fisher, T.A. Wiener, S.L. Bud’ko, P.C. Canfield, J.Y. Chan, S.M. Kauzlarich, Phys. Rev. B: Condens. Matter 59 (1999) 13829–13834. [22] J. Seo, K. Park, D. Lee, C. Lee, Scripta Mater. 38 (1998) 477–484. [23] J. Yang, D.T. Morelli, G.P. Meisner, W. Chen, J.S. Dyck, C. Uher, Phys. Rev. B 67 (2003) 165207–165210.