Synthesis and Thermoelectric Properties of AgBi 3 S 5

Transcription

1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society S8.6.1 Synthesis and Thermoelectric Properties of AgBi 3 S 5 Jun-Ho Kim 1, Daniel Bilc 2, Sim Loo 3, Jarrod Short 3, S. D. Mahanti 2, Tim Hogan 3 and Mercouri G. Kanatzidis 1 1 Department of Chemistry, 2 Department of Physics and Astronomy, Center for Fundamental Materials Research, Michigan State University, East Lansing, MI Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI Abstract The known mineral Pavonite AgBi 3 S 5 shows a complex structure composed of NaCl type fragments but has not been studied from the thermoelectric point of view. We present initial results on the synthesis and themoelectric properties of synthetic AgBi 3 S 5, which shows n-type metallic conductivity. In addition, the examination of the solid solutions AgSb x Bi 3-x S 5 (x=0.3) is reported. Introduction The high thermoelectric figure of merit (ZT= S 2 σt/κ) requires high electrical conductivity, high thermopower, and low thermal conductivity. Much effort of examining new thermoelectric compounds has been concentrated on chalcogenides since Bi 2 Te 3 and Bi 2-x Sb x Te 3 alloys showed high ZT. 1 Recent investigations of new perspective thermoelectric materials have been devoted to compounds based on alkali metal, bismuth, and chalcogens. Among the alkali bismuth chalcogenides Te and Se based compounds have shown interesting thermoelectric properties due to their small band gap and complex structure, for example CsBi 4 Te 6, 2 β-k 2 Bi 8 Se 13, 3 K 2.5 Bi 8.5 Se 14, 3 BaBiTe 3, 4 K 1-x Sn 5-x Bi 11+x Se 22, 5 A 1+x M 4-2x Bi 7+x Se 15 6 (A = K, Rb; M = Sn, Pb), A 2 Bi 8 Se 13 7 (A = Rb, Cs), CsMBi 3 Te 6, and CsM 2 Bi 3 Te 7 8 (M = Pb, Sn). However, the ternary compounds based on alkali metal bismuth sulfide such as β-,γ-csbis 2, 9 KBi 3 S 5, 10 KBi 6.33 S 10, 11 K 2 Bi 8 S 13, 11 have not shown good electrical conductivity because of their higher band gaps than selenides and tellurides. In comparison with alkali metal bismuth chalcogenides, the silver analogues yield lower band gap energies due to the more covalent contribution in Ag-S interactions that can lead to higher electrical conductivity. The silver compounds exhibit a variety of structures and compositions such as AgBiS 2, 12 AgBi 6 S 9, 13 Ag 3 Bi 7 S 12, 14 and AgBi 3 S 5, 15 all of which have not been well studied on their physicochemical and charge transport properties. Therefore, we became interested to understand and evaluate this interesting system from the thermoelectric point of view. Herein a known mineral silver bismuth chalcogenide (pavonite), AgBi 3 S 5, and its derivative AgSb x Bi 3-x S 5 (x=0.3) have been synthesized and characterized. Experimental section A mixture of elemental Ag (1.294g 12mmol), Bi (7.523g 36mmol), and S (2.024g 63mmol) was loaded into a quartz tube (13mm diameter) and subsequently flame-sealed at a residual pressure of <10-4 mbar. The tube was carefully placed under the flame of a natural gasoxygen torch until the mixture well melted. The tube was removed from the flame and let solidify in air to give a silvery black ingot of AgBi 3 S 5. The crystallographic data for the synthetic AgBi 3 S 5 are shown in Table 1 with the data from the known mineral. The crystallographic data

2 S8.6.2 of synthesized single crystal are similar to the data from the mineral but show slightly different cell parameters and much lower R value. AgBi 3 S 5 melts at 735 C. The ingot was ground and loaded into a sharp-pointed quartz tube (13mm diameter) and sealed under vacuum. The tube was placed in a Bridgman furnace and heated to 800 C and descended at a rate of 6.55mm/h through a sharp (100deg/cm) temperature gradient. A pure and well oriented ingot of AgBi 3 S 5 was obtained. The AgSb 0.3 Bi 2.7 S 5 solid solution was synthesized by the same procedure and showed a melting point at 723 C. A quantitative microprobe analysis using Energy Dispersive Spectroscopy (EDS) performed on a Scanning Electron Microscope (SEM) on several single crystals of AgBi 3 S 5 and AgSb 0.3 Bi 2.7 S 5 gave the approximate ratios Ag 0.95 Bi 3.30 S 5 and Ag 0.93 Sb 0.2 Bi 3.45 S 5 respectively. Table 1. crystallographic data for AgBi 3 S 5 Formula AgBi 3 S 5 (single crystal) AgBi 3 S 5 (mineral) 15 Space group C2/m C2/m Crystal Habit Black plate a, Å (3) (2) b, Å (8) 4.042(1) c, Å (3) (20) β, Å (3) 94.02(1) V, a, Å 3 ;Z 884.3(3), , 4 R 1 /wr 2 [I>2σ(I)](%) 3.01/ GOF on F Bi1 Bi3 Ag1 Bi2 Ag2 NaCl 100 -Type NaCl 311 -Type c a Figure 1. The structure of AgBi 3 S 5. The boxed shade areas indicate the modified NaCl and NaCl 100 -type Bi(Ag)/S fragments

3 S8.6.3 Results and discussion AgBi 3 S 5 has been known as a mineral compound called pavonite. 15 The structure of AgBi 3 S 5 is 3-dimensional and composed of a slightly modified NaCl 311 -type Bi/S layer alternating with a NaCl 100 -type Bi/S monolayer in a stepwise fashion, see Figure 1. The NaCl type and NaCl 100 -type fragments derive through excision of infinite two-dimensional slabs cut perpendicular to the [311] and [100] directions of the NaCl lattice respectively. This modular construction gives the material a highly anisotropic crystalline and electronic structure character. All Bi and Ag atoms except Bi(3) are in slightly distorted octahedral sites. Bi(3) is in a fivecoordinated square pyramidal site in the NaCl 100 -type layer. The bond distances of Bi-S and Ag-S range from 2.609(3) to 2.959(3) Å and 2.560(4) to 2.917(2) Å, respectively and the closest distance to a S atom from the basal Bi(3) to a S atom in neighboring layer is Å. a) b) c) d) Energy (ev) ev Energy(eV) Energy (ev) Energy (ev) Figure 2. Band structure of AgBi 3 S 5 a) without spin-orbit interaction and b) with spin-orbit interaction, Optical absorption spectra of c) AgBi 3 S 5 and d) AgSb 0.3 Bi 2.7 S 5.

4 S8.6.4 The modified NaCl 311 -type layer is made of edge-sharing BiS 6 octahedra and AgS 6 octahedra while the NaCl 100 -type monolayer is of AgS 6 octahedra and BiS 4 square pyramids. These two layers are connected by sharing the corner of the octahedra. Band structure calculations performed using the self-consistent full-potential linearized augmented plane wave method (LAPW) 16 within density functional theory (DFT) 17 show a very narrow band gap approximately 0.1 ev in the absence of spin-orbit interaction, while with introduction of spin-orbit interaction produces a negative band gap and indicates a semi-metallic nature, see Figure 2. The calculation with spin-orbit interaction produces a shift of the conduction band toward the valence band and also creates different band curvatures. Interestingly, the electronic band calculation tells that the conduction band is mostly composed of Bi-p orbital both with and without spin-orbit coupling while the valence band is composed of Ag-d and S-p orbitals. The optical absorption properties for AgBi 3 S 5 were examined by solid state UV-Vis and Infrared diffuse reflectance spectroscopy. Absorption (α/s) data were calculated from the reflectance data using the Kubelka-Munk function. 18 The spectrum in the UV-Vis range shows absorptions for AgBi 3 S 5 and AgSb 0.3 Bi 2.7 S 5 around 0.6 ev. However substantial absorption of electromagnetic radiation occurs below 0.6 ev which makes us doubt whether this electronic absorption feature represents the energy band gap of the material. Spectroscopy in the Infrared range shows no band gap for AgBi 3 S 5 and a feature of 0.15 ev for AgSb 0.3 Bi 2.7 S 5. Therefore the existence band gap in AgBi 3 S 5 is still ambiguous. a) b) dc Electrical Conductivity (S/cm) dc Electrical Conductivity (S/cm) Thermopower (µv/k) Thermopower (µv/k) Thermal Conductivity (W/mK) Thermal Conductivity (W/mK) Figure 3. Variable temperature electrical conductivity, thermopower and thermal conductivity data of a) AgBi 3 S 5 and b) AgSb 0.3 Bi 2.7 S 5. Samples were cut from polycrystalline ingots

5 The temperature dependence of the electrical conductivity, thermopower and thermal conductivity of the AgBi 3 S 5 ingot showed a n-type metallic behavior with a room temperature electrical conductivity of 205 S/cm, thermopower of -87 µv/k, and thermal conductivity of 1.8 W/mK, see Figure 3. The negative thermopower value indicates that the major carriers are electrons, and increases almost linearly with increasing temperature up to a value of -107µV/K at 400K. The thermopower data suggest that conductivity in this compound is accomplished by electron carriers through predominantly Bi-p orbitals in the conduction band. This is consistent with the results of the band calculations. In addition, the Sb substituted solid solution AgSb 0.3 Bi 2.7 S 5 shows electrical conductivity of 260 S/cm, thermopower of -98 µv/k, and thermal conductivity of 1.6 W/mK at room temperature. The effect of Sb substituted solid solution is expected to increase thermopower and reduce thermal conductivity due to compositional complexity and enhanced mass fluctuation in the lattice. However, both the thermopower and the thermal conductivity of the solid solution remain almost unchanged from those of AgBi 3 S 5. This manipulation of the composition seems to significantly affect only the electrical conductivity, and this may be attributed to the change of the bands near the Fermi level and the number of carriers. Understanding the full effect of Sb substitution in this system is not possible at this stage. In the future we need to examine this interesting system further with the several Sb ratios and also understand the Sb/Bi distribution in the lattice. Conclusion We have synthesized the known mineral AgBi 3 S 5 and its solid solution AgSb 0.3 Bi 2.7 S 5 and studied the thermoelectric properties. The existence of a band gap in AgBi 3 S 5 could not be clearly determined but AgSb 0.3 Bi 2.7 S 5 showed a possible band gap at 0.15 ev. AgBi 3 S 5 and AgSb 0.3 Bi 2.7 S 5 showed n-type metallic behavior with relatively high electrical conductivities and low thermal conductivities. The Sb substituted solid solution AgSb 0.3 Bi 2.7 S 5 seems to affect only the electrical conductivity from 205 S/cm to 260 S/cm at room temperature. Based on these interesting results, we need to study more the dependence of the electronic properties with compositional changes in this material system. Acknowledgments Financial support from the Office of Naval Research and DARPA (Grant No. N ) is gratefully acknowledged. References S CRC Handbook of thermoelectric materials. Rowe, D.M..Ed., CRC Press, Inc.: Boca Raton, FL, Chung, D-. Y.; Hogan, T.; Brazis, P. W.; Kannewurf, C. R.; Bastea, M.; Uher, C.; Kanatzidis, M. G. Science 2000, 287, Chung, D. -Y.; Choi, K. S; Iordanidis, L; Schindler, J.L. ; Brazis, P. W.; Kannewurf, C. R.; Chen, B.; Hu, S.; Uher, C.; Kanatzidis, M. G. Chem. Mater., 1997, 9, Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J.; Kanatzidis, M. G. J. Amer. Chem. Soc. 1997, 119, Mrotzek, A; Chung, D. Y. Hogan, T; Kanatzidis, M. G. J. Mater. Chem., 2000, 10, Choi, K. S.; Chung, D. Y.; Mrotzek, A.; Brazis, P.; Kannewurf, C. R.; Uher, C.; Chen, W.; Hogan, T.; Kanatzidis, M. G. Chem. Mater. 2001, 13 (3):

6 S Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, J.; Lane, M.; Kannewurf, C. R. Chen, W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem. Mater. 2001, 13, Hsu, K. F.; Chung, D. Y.; Lal, S.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis M. G. J. Am. Chem. Soc., 2002, 124, McCarthy, T. J.; Ngeyi, S.-P.; Liao, J.-H.; DeGroot, D.; Hogan, T.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1993, 5, McCarthy, T. J.; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem. Soc. 1995,117, a) McCarthy, T. J.; Tanzer, T. A.; Chen, L.H.; Iordanidis, L.; Hogan, T; Kannewurf, C. R.; Uher, C; Chen, B; Kanatzidis, M. G. Chem. Mater. 1996, 8, b) Chen, B; Uher, C; Iordanidis, L; Kanatzidis, M. G. Chem. Mater. 1997, 9, a) Wernick, J.H. American Mineralogist.1960, 45, b) Wernick, J.H. Journal of Materials Science. 1968, 3, Mumme, W.G. Neues Jahrbuch fuer Mineralogie 1990, Herbert, H.K.;Mumme, W.G. Neues Jahrbuch fuer Mineralogie. 1981, Makovicky, E.;Mumme, W.G.;Watts, J.A. Can. Mineralogist 1977, 15, D. Singh, Planewaves, Pseudopotentials, and the LAPW method (Kluwer Academic, Boston, 1994). 17. P. Hohenberg, and W. Kohn, Phys. Rev., 1964,136, B864; W. Kohn, and L. Sham, ibid, 1965,140, A Kubelka-Munk function: α/s = (1-R) 2 /2R, where R is the reflectance at a given wavenumber, α is the absorption coefficient, and S is the scattering coefficient.