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Synthesis, crystal structure and transport properties of skutterudite-related CoSn1.5Se1.5
Journal of Alloys and Compounds 479 (2009) 102–106
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis, crystal structure and transport properties of skutterudite-related CoSn1.5 Se1.5 ˇ Draˇsar d F. Laufek a,∗ , J. Navrátil b , J. Pláˇsil c , T. Plecháˇcek b , C. a
Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic ˇ and University of Pardubice, Studentská 84, 532 10 Pardubice, Czech Republic Joint Laboratory of Solid State Chemistry of IMC AS CR Faculty of Science, Charles University, Albertov 6,128 43 Praha 2 and National Museum, Václavské námˇestí 84, 115 79 Praha 1, Czech Republic d Department of Physics, Faculty of Chemical Technology, University of Pardubice, Studentská 84, 532 10 Pardubice, Czech Republic b c
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 16 January 2009 Accepted 19 January 2009 Available online 31 January 2009 Keywords: CoSn1.5 Se1.5 Crystal structure X-ray powder diffraction Transport properties
a b s t r a c t The skutterudite-related phase CoSn1.5 Se1.5 has been synthesised and structurally characterized by pow¯ unit cell parameters der X-ray diffraction data. CoSn1.5 Se1.5 display trigonal symmetry, space group R3, a = 12.3278(3) Å, c = 15.1267(6) Å, V = 1990.8(1) Å3 and Z = 24. Its crystal structure can be viewed as a modification of the ideal skutterudite structure CoAs3 , where the Sn and Se anions order in layers perpendicular to the [1 1 1] direction of the skutterudite unit cell. Transport properties of the title compound, which is p-type semiconductor, have been investigated. The thermoelectric figure of merit ZTmax was found 0.1 at 600 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Compounds with the skutterudite structure (general formula MX3 where M = Co, Ir and Rh; X = As, Sb, P) have become of interest in materials science in last decade because these phases possess attractive thermal and transport properties for thermoelectric applications [1]. The ideal skutterudite structure can be viewed as a severe distortion of ReO3 structure by tilting of octahedra (tilt system a+ a+ a+ ) [2]. In the resulted skutterudite structure, cations preserved their octahedral coordination, while the anions are coordinated by two cations and two anions in distorted tetrahedral fashion. Another consequence of such distortion is a formation of four-member rings X4, typical of the skutterudite structure. As was mentioned by Takizawa et al. [3], the linkage of octahedra produces a void at the centre of [MX6 ]8 octahedral-cluster and this void could be filled by another atoms (typically La–Yb, U, Tl [4,5]). These atoms act here as effective phonon-scattering centres which results in a significant reduction of lattice thermal conductivity and consequently superior thermoelectric properties [6,7]. In addition to binary skutterudites, another group of materials with skutterudite-related structure exists. These compounds, also known as ternary skutterudites, have general formula MY1.5 Ch1.5 and can be obtained by substitution of the anion site X by pair of
∗ Corresponding author. Fax: +420 251 818 748. E-mail address: [email protected] (F. Laufek). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.067
elements from 14 and 16 groups of the periodical system. Up to date known ternary skutterudites obtained by isoelectronic anion substitution are listed in Ref. [8,9]. The crystallographic studies of performed on CoGe1.5 S1.5 , CoGe1.5 Se1.5 [10] and CoGe1.5 Te1.5 [9] revealed a long-range ordering of Ge and Ch atoms resulting in lowering of the symmetry from cubic to rhombohedral. The existence and some properties of CoSn1.5 Se1.5 phase were reported in [11] as a part of review of skutterudite materials, however no detailed information concerning crystal structure and thermoelectric properties is available. In this paper, we present a detailed structural study of CoSn1.5 Se1.5 skutterudite-related phase using conventional powder X-ray diffraction. Temperature dependence of electrical and thermal conductivity, the Seebeck coefficient and thermoelectric parameter ZT are also reported. 2. Experimental 2.1. Synthesis The CoSn1.5 Se1.5 ternary compound was synthesized from individual elements by high-temperature solid-state reactions. Co powder was first heated at 900 ◦ C for 2 h in H2 atmosphere to remove possible oxides. Stoichiometric amounts of Co (Aldrich, 99.9%), Sn (Aldrich, 99.99%) and Se (Aldrich, 99.99%) were sealed into an evacuated silica glass tube and heated at 800 ◦ C for 48 h in a programmable furnace. The material was then ground under acetone using agate mortar and pestle and heated at 600 ◦ C for 168 h. The resultant material was once again ground under acetone and heated at 600 ◦ C for 432 h. After annealing, the furnace was turned off and allowed to cool slowly to room temperature.
F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
103
Table 1 Data collection and Rietveld analysis. R agreement factors defined according to [19].
Table 2 Refined atomic coordinates for the CoSn1.5 Se1.5 .
Data collection Radiation type, source Generator settings Data collection temperature Range in 2 (◦ ) Step size (◦ )
Atom
Site
x
y
z
Co(1) Co(2) Sn(1) Sn(2) Se(1) Se(2)
6c 18f 18f 18f 18f 18f
0 0.661(1) 0.8429(8) 0.9442(7) 0.9400(8) 0.8399(8)
0 0.8320(8) 0.0086(6) 0.2130(8) 0.2226(9) 0.0136(6)
0.258(2) 0.5919(9) 0.1606(7) 0.5591(5) 0.0632(9) 0.6634(5)
Crystal data Space group Unit cell content Unit cell parameters (Å) Rietveld analysis No. of reflections No. of structural parameters No of. profile parameters RF RBragg Rp Rwp Weighting scheme
X-ray, Cu K␣ 40 kV, 30 mA room temperature 10–110 0.02 R3¯ (no. 148) CoSn1.5 Se1.5 , Z = 24 a = 12.3278(3) c = 15.1267(6)
Uiso = 0.005(1) Å2 (displacement parameters were constrained to be equal for all the atoms).
551 16 4 0.064 0.066 0.062 0.087 1/yo
2.2. Structure refinement The powder X-ray diffraction pattern of CoSn1.5 Se1.5 was collected in Bragg–Brentano geometry on X’Pert Pro PANalytical diffractometer equipped with X’Celerator detector using Cu K␣ radiation. To minimize background, the sample was placed on a flat-low background silicon wafer. The data were acquired in the angular range 10–110◦ 2. A full-width at half maximum of 0.059◦ 2 was obtained at 14.340◦ 2 demonstrating good crystallinity of the sample under consideration. The details of data collection and basic crystallographic facts are given in Table 1. The crystal structure of CoSn1.5 Se1.5 was refined by the Rietveld method for X-ray powder diffraction data using the FullProf program [12]. Preliminary refinement of CoSn1.5 Se1.5 using CoSb3 skutterudite structure [13] as a starting structural model (space group Im3, a = 8.7224(4) Å) converged to reasonable values of agreement factors (Rp = 0.074, Rwp = 0.111, and RB = 0.092); nevertheless relatively weak superstructure diffractions were observed which were not fitted by this cubic model. Similar results were observed during the refinement of CoSn1.5 Te1.5 [14] and IrSn1.5 Te1.5 [15]. As was shown in the structural study on CoGe1.5 Te1.5 [9], the ordering among Ge and Te atoms lead to the lowering of the symmetry from original cubic ¯ The Rietveld refinement based on (space group Im3) to trigonal (space group R3). the starting CoGe1.5 Te1.5 structure model converged to the better values of agreement factors (Rp = 0.062, Rwp = 0.087, and RB = 0.066). Moreover, with the exception of a small number weak diffraction peaks attributable to the SnS (ca 1 wt%) impurity phase, all diffractions were fitted by this anion ordered structure model. The refined parameters include those describing peak shape and width, peak asymmetry, unit cell parameters and fractional coordinates. Finally, 23 parameters were refined. Isotropic displacement parameters were constrained to be equal for all atoms. The pseudo-Voigt function was used to generate the line shape of the diffraction peaks. The background was determined by linear interpolation between
Fig. 1. Observed (circles), calculated (solid line) and difference Rietveld profiles for CoSn1.5 Se1.5 . The upper reflections bars corresponds CoSn1.5 Se1.5 and the lower bars to a 1 mass percent SnSe impurity.
consecutive breakpoints in the pattern. Refined structural parameters are given in Table 2; Fig. 1 shows final Rietveld plot. Fig. 2 shows comparison of two Rietveld refinements of CoSn1.5 Se1.5 performed ¯ respectively. in space groups Im3 an R3, 2.3. Transport properties Electrical conductivity was measured with four-probe method using Lock-in Amplifier (EG&G model 5209) on the rectangular parallelepiped of dimensions about 10 mm× 3 mm × 1 mm. The measurements were performed on two different probes, one in the temperature region from about 100–350 K and the other from 300–800 K. The Seebeck coefficient was determined by means of static dc method on rectangular shaped samples. The temperature gradient between two points was measured by two shielded K-type thermocouples that were pressed against the sample surface. A potential difference dU corresponding to the gradient dT was measured across the same legs of both attached thermocouples. The absolute Seebeck coefficient was
Fig. 2. Details of two Rietveld fits of CoSn1.5 Se1.5 indicating the presence of superstructure reflections (marked by arrows). The left and right part of Fig. 2 shows refinement ¯ and on CoGe1.5 Te1.5 (space group R3) ¯ structure models, respectively. The superstructure reflections are completely fitted in the based on the CoSb3 (space group Im3) anion-ordered CoSn1.5 Se1.5 structure model.
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F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
Fig. 3. (a) Polyhedral representation and (b) ball-and-stick representation of the CoSn1.5 Se1.5 structure showing the corner sharing arrangement of the [CoSn3 Se3 ] octahedra and presence of four-member [Sn2 Se2 ] rings (rhombohedral setting). (c) Comparison of four-member [Sn2 Se2 ] and [Sb4 ] rings found in CoSb3 structure [13] and in CoSn1.5 Se1.5 , respectively.
determined from the slope of dU/dT dependence using 20 values of dT not exceeded 3 K. The thermal diffusivity was measured on round hot pressed sample with help of LFA 457 (Netzsch). The thermal conductivity was then calculated using Pyroceram 9606 as a heat capacity standard.
3. Results and discussion 3.1. Crystal structure CoSn1.5 Se1.5 is first ternary compound discovered in the Co–Sn–Se system, where 13 phases were known before. The crystal structure of title phase is isostructural with CoSn1.5 Te1.5 [14], IrSn1.5 Te1.5 [15] and CoGe1.5 Te1.5 [9]. As suggested by Vaqueiro et al. [9] and Partik et al. [10], crystal structures of these phases can be derived from cubic skutterudite structure MX3 (M = Co, Rh, Ir; X = P, As, or Sb), where Sn (Ge) and Se (Te) atoms shows long-range ordering in planes perpendicular [1 1 1] direction of the original cubic cell. As a consequence, the symmetry is lowered from cubic to trigonal. Nevertheless, the a+ a+ a+ tilt system of octahedra of parent skutterudite structure MX3 is preserved. The crystal structure of CoSn1.5 Se1.5 is depicted in Fig. 3. Each Co is octahedrally coordinated by three tin and three selenium atoms, with Co–Se and Co–Sn distances of 2.37–2.47 Å and 2.48–2.58 Å, respectively. Table 3 shows comparison of selected bond distances for CoSn1.5 Se1.5 and CoSn1.5 Te1.5 [14]. While Co-Sn distances are approximately the Table 3 Selected interatomic distances (Å) for CoSn1.5 Se1.5 and those for CoSn1.5 Te1.5 (Ch = Se or Te). CoSn1.5 Se1.5
CoSn1.5 Te1.5
Co(1)
Sn(1) Ch(2)
3 × 2.48(2) 3 × 2.38(1)
3 × 2.517(9) 3 × 2.566(7)
Co(2)
Sn(1) Sn(2) Sn(2) Ch(1) Ch(1) Ch(2)
2.48(2) 2.50(1) 2.58(1) 2.37(1) 2.38(2) 2.47(1)
2.475(7) 2.504(6) 2.50(1) 2.56(1) 2.570(6) 2.595(6)
same in both compounds, Co–Se distances are significantly shorter than corresponding Co–Te distances in CoSn1.5 Te1.5 . This is in agreement with lower radius of Se atom (rSe = 1.17 Å [16]) with respect to radius of Te atom (rTe = 1.43 Å [16]). The [CoSn3 Se3 ] octahedra share all six corners with adjacent octahedra forming a threedimensional network (Fig. 3). Because of tilting octahedra, the Sn and Se atoms form two four-member rings [Sn2 Se2 ], in which Sn and Se atoms are in trans positions to each other. These rings are characteristic feature of skutterudite structure; however on contrary to the rectangular shape of As4 rings in CoAs3 , [Sn2 Se2 ] rings are more distorted (Fig. 3c). The ratio of X-X distances for binary skutterudites ranges from 1.03 to 1.05 [17], while for CoSn1.5 Se1.5 phase this ratio ranges from 1.05 to 1.11. Also the departures of intra-ring angles from 90◦ (range of 87.5(3)–92.5(3)◦ ) indicate weak distortion of the anion sublattice. 3.2. Transport properties As follows from Fig. 4, the prepared CoSn1.5 Se1.5 compound is of p-type electrical conductivity at the whole measured temperature region, i.e. from 90 to 800 K. Quite high values of the Seebeck coefficient indicate that prepared compound is a semiconductor. The electrical conductivity, at measured temperature region, increases at first with increasing temperature probably due to the excitation of electrons from valence band into some acceptor levels, which have not been determined up to now thanks to complex nature of the possible point defects presented in the prepared compound. One of the explanations could be, e.g. presence of Fe as a main impurity in the 3N Co powder used for the synthesis. Further increase of temperature causes decrease in electrical conductivity probably due to the decrease of the mobility of holes. At high temperature (above 700 K) we observed an increase of electrical conductivity again. This increase is connected with excitation of electrons across band gap. That is why the values of Seebeck coefficient at this region starts rapidly fall down. From the Arrhenius plot of electrical conductivity–log vs. 1000/T (see Fig. 5)—we were able to determine activation energies of these processes. For extrinsic charge carriers we obtained value of Ea = 9 meV. Supposing
F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
Fig. 4. Temperature dependence of (a) the Seebeck coefficient and (b) the electrical conductivity of CoSn1.5 Se1.5 .
105
Fig. 6. Temperature dependence of (a) the thermal conductivity and (b) dimensionless thermoelectric figure of merit ZT for CoSn1.5 Se1.5 .
activation of the electrons across band gap, one is able to calculate its width using formula log = log 0 – Eg /(2kT). From the dependence on Fig. 5 at high temperatures we estimated the Eg value between 0.6–0.7 eV. This finding is very similar as for the CoSn1.5 Te1.5 compound—Eg = 0.7 eV [18]. As it is evident from the following Fig. 6a, thermal conductivity of the CoSn1.5 Se1.5 sample is about two times lower than these one of similarly prepared binary compound CoSb3 —10.5 Wm−1 K−1 [1] and well comparable with the same value for CoSn1.5 Te1.5 compound [18]. Introducing of suitable filler atoms into the structure voids can lower the thermal conductivity even more. Having all necessary properties we were able to evaluate values of ZT parameter in the 300–800 K temperature range (see Fig. 6b). It reaches of maximal value ZT = 0.1 at temperatures around 600 K. We believe that the value could be further increased with suitable doping. 4. Conclusion
Fig. 5. Electrical conductivity of CoSn1.5 Se1.5 as a function of inverse temperature.
In conclusion, CoSn1.5 Se1.5 compound represents a ternary ordered variant of the skutterudite structure, in which the Sn and Se anions show ordering in layers perpendicular to [1 1 1] original cubic axis. From the thermoelectric point of view, the value of dimensionless ZTmax parameter of 0.1 is significantly lower than in the state-of-the-art thermoelectric materials (ZT > 1), particularly because of the low electrical conductivity that this phase
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F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
exhibits. However, similar to other anion-ordered ternary skutterudites [8,9], the thermoelectric properties can be enhanced by proper doping to optimise the electric conductivity and void filling to decrease the thermal conductivity. This investigation could be an interesting subject to further analyses. Acknowledgements This work was supported by the internal project of the Czech Geological Survey (project number 332300) and by the project of Grant Agency CR no. 203/07/0267. These supports are greatly acknowledged. References [1] C. Uher, in: M.G. Kanatzidis, S.D. Mahanti, T.P. Hogan (Eds.), Chemistry, Physics and Materials Science of Thermoelectric Materials: Beyond Bismuth telluride, Kluwer Academics, Plenum Publishers, New York, 2003, p. 121. [2] R.H. Mitchell, Perovskites: Modern and Ancient, Almaz Press, Thunder Bay, Ontario, 2002.
[3] H. Takizawa, K. Miura, M. Ito, T. Suzuki, T. Endo, J. Alloys Compd. 282 (1999) 79–83. [4] W. Jeitschko, D. Braun, Acta Cryst. B 33 (1977) 3401–3406. [5] B.C. Chakoumakos, B.C. Sales, J. Alloys Compd. 407 (2006) 87–93. [6] D.T. Morelli, G.P. Meisner, J. Appl. Phys. 77 (1995) 3777–3781. [7] B.C. Sales, D. Mandrus, R.K. Williams, Science 272 (1996) 1325–1328. [8] P. Vaqueiro, G.G. Sobany, M. Stindl, J. Solid State Chem. 181 (2008) 768–776. [9] P. Vaqueiro, G.G. Sobany, A.V. Powell, K.S. Knight, J. Solid State Chem. 179 (2006) 2047–2053. [10] M. Partik, C. Kringe, H.D. Lutz, Z. Kristallogr. 211 (1996) 304–312. [11] J.P. Fleurial, T. Caillat, A. Borshchevsky, Proceedings of the 16th International Conference on Thermoelectrics, Dresden, Germany, 1997, pp. 1–11. [12] J. Rodríguez-Carvajal, FullProf.2k Rietveld Profile Matching & Integrated Intensities Refinement of X-ray and/or Neutron Data (powder and/or single-crystal). Laboratoire Léon Brillouin, Centre dˇıEtudes de Saclay, Gif-sur-Yvette Cedex, France, 2006. [13] Th. Schmidt, G. Kliche, H.D. Lutz, Acta Crystallogr. C 43 (1987) 1678–1679. [14] F. Laufek, J. Navrátil, V. Goliáˇs, Powder Diffr. 23 (2008) 15–19. [15] J.W.G. Bos, R.J. Cava, Solid State Commun. 141 (2007) 38–41. [16] J. Emsley, The Elements, Oxford University Press, New York, 1989. [17] A. Kjekshus, T. Rakke, Acta Chem. Scand. A 28 (1974) 99–103. [18] Y. Nagamoto, K. Tanaka, T. Koyanagi, Proceedings of the 16th International Conference on Thermoelectrics, Dresden, Germany, 1997, pp. 330–333. [19] L.B. McCusker, R.B. von Dreele, D.E. Cox, D. Louër, P. Scardi, J. Appl. Cryst. 32 (1999) 36.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis, crystal structure and transport properties of skutterudite-related CoSn1.5 Se1.5 ˇ Draˇsar d F. Laufek a,∗ , J. Navrátil b , J. Pláˇsil c , T. Plecháˇcek b , C. a
Czech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic ˇ and University of Pardubice, Studentská 84, 532 10 Pardubice, Czech Republic Joint Laboratory of Solid State Chemistry of IMC AS CR Faculty of Science, Charles University, Albertov 6,128 43 Praha 2 and National Museum, Václavské námˇestí 84, 115 79 Praha 1, Czech Republic d Department of Physics, Faculty of Chemical Technology, University of Pardubice, Studentská 84, 532 10 Pardubice, Czech Republic b c
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 16 January 2009 Accepted 19 January 2009 Available online 31 January 2009 Keywords: CoSn1.5 Se1.5 Crystal structure X-ray powder diffraction Transport properties
a b s t r a c t The skutterudite-related phase CoSn1.5 Se1.5 has been synthesised and structurally characterized by pow¯ unit cell parameters der X-ray diffraction data. CoSn1.5 Se1.5 display trigonal symmetry, space group R3, a = 12.3278(3) Å, c = 15.1267(6) Å, V = 1990.8(1) Å3 and Z = 24. Its crystal structure can be viewed as a modification of the ideal skutterudite structure CoAs3 , where the Sn and Se anions order in layers perpendicular to the [1 1 1] direction of the skutterudite unit cell. Transport properties of the title compound, which is p-type semiconductor, have been investigated. The thermoelectric figure of merit ZTmax was found 0.1 at 600 K. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Compounds with the skutterudite structure (general formula MX3 where M = Co, Ir and Rh; X = As, Sb, P) have become of interest in materials science in last decade because these phases possess attractive thermal and transport properties for thermoelectric applications [1]. The ideal skutterudite structure can be viewed as a severe distortion of ReO3 structure by tilting of octahedra (tilt system a+ a+ a+ ) [2]. In the resulted skutterudite structure, cations preserved their octahedral coordination, while the anions are coordinated by two cations and two anions in distorted tetrahedral fashion. Another consequence of such distortion is a formation of four-member rings X4, typical of the skutterudite structure. As was mentioned by Takizawa et al. [3], the linkage of octahedra produces a void at the centre of [MX6 ]8 octahedral-cluster and this void could be filled by another atoms (typically La–Yb, U, Tl [4,5]). These atoms act here as effective phonon-scattering centres which results in a significant reduction of lattice thermal conductivity and consequently superior thermoelectric properties [6,7]. In addition to binary skutterudites, another group of materials with skutterudite-related structure exists. These compounds, also known as ternary skutterudites, have general formula MY1.5 Ch1.5 and can be obtained by substitution of the anion site X by pair of
∗ Corresponding author. Fax: +420 251 818 748. E-mail address: [email protected] (F. Laufek). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.067
elements from 14 and 16 groups of the periodical system. Up to date known ternary skutterudites obtained by isoelectronic anion substitution are listed in Ref. [8,9]. The crystallographic studies of performed on CoGe1.5 S1.5 , CoGe1.5 Se1.5 [10] and CoGe1.5 Te1.5 [9] revealed a long-range ordering of Ge and Ch atoms resulting in lowering of the symmetry from cubic to rhombohedral. The existence and some properties of CoSn1.5 Se1.5 phase were reported in [11] as a part of review of skutterudite materials, however no detailed information concerning crystal structure and thermoelectric properties is available. In this paper, we present a detailed structural study of CoSn1.5 Se1.5 skutterudite-related phase using conventional powder X-ray diffraction. Temperature dependence of electrical and thermal conductivity, the Seebeck coefficient and thermoelectric parameter ZT are also reported. 2. Experimental 2.1. Synthesis The CoSn1.5 Se1.5 ternary compound was synthesized from individual elements by high-temperature solid-state reactions. Co powder was first heated at 900 ◦ C for 2 h in H2 atmosphere to remove possible oxides. Stoichiometric amounts of Co (Aldrich, 99.9%), Sn (Aldrich, 99.99%) and Se (Aldrich, 99.99%) were sealed into an evacuated silica glass tube and heated at 800 ◦ C for 48 h in a programmable furnace. The material was then ground under acetone using agate mortar and pestle and heated at 600 ◦ C for 168 h. The resultant material was once again ground under acetone and heated at 600 ◦ C for 432 h. After annealing, the furnace was turned off and allowed to cool slowly to room temperature.
F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
103
Table 1 Data collection and Rietveld analysis. R agreement factors defined according to [19].
Table 2 Refined atomic coordinates for the CoSn1.5 Se1.5 .
Data collection Radiation type, source Generator settings Data collection temperature Range in 2 (◦ ) Step size (◦ )
Atom
Site
x
y
z
Co(1) Co(2) Sn(1) Sn(2) Se(1) Se(2)
6c 18f 18f 18f 18f 18f
0 0.661(1) 0.8429(8) 0.9442(7) 0.9400(8) 0.8399(8)
0 0.8320(8) 0.0086(6) 0.2130(8) 0.2226(9) 0.0136(6)
0.258(2) 0.5919(9) 0.1606(7) 0.5591(5) 0.0632(9) 0.6634(5)
Crystal data Space group Unit cell content Unit cell parameters (Å) Rietveld analysis No. of reflections No. of structural parameters No of. profile parameters RF RBragg Rp Rwp Weighting scheme
X-ray, Cu K␣ 40 kV, 30 mA room temperature 10–110 0.02 R3¯ (no. 148) CoSn1.5 Se1.5 , Z = 24 a = 12.3278(3) c = 15.1267(6)
Uiso = 0.005(1) Å2 (displacement parameters were constrained to be equal for all the atoms).
551 16 4 0.064 0.066 0.062 0.087 1/yo
2.2. Structure refinement The powder X-ray diffraction pattern of CoSn1.5 Se1.5 was collected in Bragg–Brentano geometry on X’Pert Pro PANalytical diffractometer equipped with X’Celerator detector using Cu K␣ radiation. To minimize background, the sample was placed on a flat-low background silicon wafer. The data were acquired in the angular range 10–110◦ 2. A full-width at half maximum of 0.059◦ 2 was obtained at 14.340◦ 2 demonstrating good crystallinity of the sample under consideration. The details of data collection and basic crystallographic facts are given in Table 1. The crystal structure of CoSn1.5 Se1.5 was refined by the Rietveld method for X-ray powder diffraction data using the FullProf program [12]. Preliminary refinement of CoSn1.5 Se1.5 using CoSb3 skutterudite structure [13] as a starting structural model (space group Im3, a = 8.7224(4) Å) converged to reasonable values of agreement factors (Rp = 0.074, Rwp = 0.111, and RB = 0.092); nevertheless relatively weak superstructure diffractions were observed which were not fitted by this cubic model. Similar results were observed during the refinement of CoSn1.5 Te1.5 [14] and IrSn1.5 Te1.5 [15]. As was shown in the structural study on CoGe1.5 Te1.5 [9], the ordering among Ge and Te atoms lead to the lowering of the symmetry from original cubic ¯ The Rietveld refinement based on (space group Im3) to trigonal (space group R3). the starting CoGe1.5 Te1.5 structure model converged to the better values of agreement factors (Rp = 0.062, Rwp = 0.087, and RB = 0.066). Moreover, with the exception of a small number weak diffraction peaks attributable to the SnS (ca 1 wt%) impurity phase, all diffractions were fitted by this anion ordered structure model. The refined parameters include those describing peak shape and width, peak asymmetry, unit cell parameters and fractional coordinates. Finally, 23 parameters were refined. Isotropic displacement parameters were constrained to be equal for all atoms. The pseudo-Voigt function was used to generate the line shape of the diffraction peaks. The background was determined by linear interpolation between
Fig. 1. Observed (circles), calculated (solid line) and difference Rietveld profiles for CoSn1.5 Se1.5 . The upper reflections bars corresponds CoSn1.5 Se1.5 and the lower bars to a 1 mass percent SnSe impurity.
consecutive breakpoints in the pattern. Refined structural parameters are given in Table 2; Fig. 1 shows final Rietveld plot. Fig. 2 shows comparison of two Rietveld refinements of CoSn1.5 Se1.5 performed ¯ respectively. in space groups Im3 an R3, 2.3. Transport properties Electrical conductivity was measured with four-probe method using Lock-in Amplifier (EG&G model 5209) on the rectangular parallelepiped of dimensions about 10 mm× 3 mm × 1 mm. The measurements were performed on two different probes, one in the temperature region from about 100–350 K and the other from 300–800 K. The Seebeck coefficient was determined by means of static dc method on rectangular shaped samples. The temperature gradient between two points was measured by two shielded K-type thermocouples that were pressed against the sample surface. A potential difference dU corresponding to the gradient dT was measured across the same legs of both attached thermocouples. The absolute Seebeck coefficient was
Fig. 2. Details of two Rietveld fits of CoSn1.5 Se1.5 indicating the presence of superstructure reflections (marked by arrows). The left and right part of Fig. 2 shows refinement ¯ and on CoGe1.5 Te1.5 (space group R3) ¯ structure models, respectively. The superstructure reflections are completely fitted in the based on the CoSb3 (space group Im3) anion-ordered CoSn1.5 Se1.5 structure model.
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F. Laufek et al. / Journal of Alloys and Compounds 479 (2009) 102–106
Fig. 3. (a) Polyhedral representation and (b) ball-and-stick representation of the CoSn1.5 Se1.5 structure showing the corner sharing arrangement of the [CoSn3 Se3 ] octahedra and presence of four-member [Sn2 Se2 ] rings (rhombohedral setting). (c) Comparison of four-member [Sn2 Se2 ] and [Sb4 ] rings found in CoSb3 structure [13] and in CoSn1.5 Se1.5 , respectively.
determined from the slope of dU/dT dependence using 20 values of dT not exceeded 3 K. The thermal diffusivity was measured on round hot pressed sample with help of LFA 457 (Netzsch). The thermal conductivity was then calculated using Pyroceram 9606 as a heat capacity standard.
3. Results and discussion 3.1. Crystal structure CoSn1.5 Se1.5 is first ternary compound discovered in the Co–Sn–Se system, where 13 phases were known before. The crystal structure of title phase is isostructural with CoSn1.5 Te1.5 [14], IrSn1.5 Te1.5 [15] and CoGe1.5 Te1.5 [9]. As suggested by Vaqueiro et al. [9] and Partik et al. [10], crystal structures of these phases can be derived from cubic skutterudite structure MX3 (M = Co, Rh, Ir; X = P, As, or Sb), where Sn (Ge) and Se (Te) atoms shows long-range ordering in planes perpendicular [1 1 1] direction of the original cubic cell. As a consequence, the symmetry is lowered from cubic to trigonal. Nevertheless, the a+ a+ a+ tilt system of octahedra of parent skutterudite structure MX3 is preserved. The crystal structure of CoSn1.5 Se1.5 is depicted in Fig. 3. Each Co is octahedrally coordinated by three tin and three selenium atoms, with Co–Se and Co–Sn distances of 2.37–2.47 Å and 2.48–2.58 Å, respectively. Table 3 shows comparison of selected bond distances for CoSn1.5 Se1.5 and CoSn1.5 Te1.5 [14]. While Co-Sn distances are approximately the Table 3 Selected interatomic distances (Å) for CoSn1.5 Se1.5 and those for CoSn1.5 Te1.5 (Ch = Se or Te). CoSn1.5 Se1.5
CoSn1.5 Te1.5
Co(1)
Sn(1) Ch(2)
3 × 2.48(2) 3 × 2.38(1)
3 × 2.517(9) 3 × 2.566(7)
Co(2)
Sn(1) Sn(2) Sn(2) Ch(1) Ch(1) Ch(2)
2.48(2) 2.50(1) 2.58(1) 2.37(1) 2.38(2) 2.47(1)
2.475(7) 2.504(6) 2.50(1) 2.56(1) 2.570(6) 2.595(6)
same in both compounds, Co–Se distances are significantly shorter than corresponding Co–Te distances in CoSn1.5 Te1.5 . This is in agreement with lower radius of Se atom (rSe = 1.17 Å [16]) with respect to radius of Te atom (rTe = 1.43 Å [16]). The [CoSn3 Se3 ] octahedra share all six corners with adjacent octahedra forming a threedimensional network (Fig. 3). Because of tilting octahedra, the Sn and Se atoms form two four-member rings [Sn2 Se2 ], in which Sn and Se atoms are in trans positions to each other. These rings are characteristic feature of skutterudite structure; however on contrary to the rectangular shape of As4 rings in CoAs3 , [Sn2 Se2 ] rings are more distorted (Fig. 3c). The ratio of X-X distances for binary skutterudites ranges from 1.03 to 1.05 [17], while for CoSn1.5 Se1.5 phase this ratio ranges from 1.05 to 1.11. Also the departures of intra-ring angles from 90◦ (range of 87.5(3)–92.5(3)◦ ) indicate weak distortion of the anion sublattice. 3.2. Transport properties As follows from Fig. 4, the prepared CoSn1.5 Se1.5 compound is of p-type electrical conductivity at the whole measured temperature region, i.e. from 90 to 800 K. Quite high values of the Seebeck coefficient indicate that prepared compound is a semiconductor. The electrical conductivity, at measured temperature region, increases at first with increasing temperature probably due to the excitation of electrons from valence band into some acceptor levels, which have not been determined up to now thanks to complex nature of the possible point defects presented in the prepared compound. One of the explanations could be, e.g. presence of Fe as a main impurity in the 3N Co powder used for the synthesis. Further increase of temperature causes decrease in electrical conductivity probably due to the decrease of the mobility of holes. At high temperature (above 700 K) we observed an increase of electrical conductivity again. This increase is connected with excitation of electrons across band gap. That is why the values of Seebeck coefficient at this region starts rapidly fall down. From the Arrhenius plot of electrical conductivity–log vs. 1000/T (see Fig. 5)—we were able to determine activation energies of these processes. For extrinsic charge carriers we obtained value of Ea = 9 meV. Supposing
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Fig. 4. Temperature dependence of (a) the Seebeck coefficient and (b) the electrical conductivity of CoSn1.5 Se1.5 .
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Fig. 6. Temperature dependence of (a) the thermal conductivity and (b) dimensionless thermoelectric figure of merit ZT for CoSn1.5 Se1.5 .
activation of the electrons across band gap, one is able to calculate its width using formula log = log 0 – Eg /(2kT). From the dependence on Fig. 5 at high temperatures we estimated the Eg value between 0.6–0.7 eV. This finding is very similar as for the CoSn1.5 Te1.5 compound—Eg = 0.7 eV [18]. As it is evident from the following Fig. 6a, thermal conductivity of the CoSn1.5 Se1.5 sample is about two times lower than these one of similarly prepared binary compound CoSb3 —10.5 Wm−1 K−1 [1] and well comparable with the same value for CoSn1.5 Te1.5 compound [18]. Introducing of suitable filler atoms into the structure voids can lower the thermal conductivity even more. Having all necessary properties we were able to evaluate values of ZT parameter in the 300–800 K temperature range (see Fig. 6b). It reaches of maximal value ZT = 0.1 at temperatures around 600 K. We believe that the value could be further increased with suitable doping. 4. Conclusion
Fig. 5. Electrical conductivity of CoSn1.5 Se1.5 as a function of inverse temperature.
In conclusion, CoSn1.5 Se1.5 compound represents a ternary ordered variant of the skutterudite structure, in which the Sn and Se anions show ordering in layers perpendicular to [1 1 1] original cubic axis. From the thermoelectric point of view, the value of dimensionless ZTmax parameter of 0.1 is significantly lower than in the state-of-the-art thermoelectric materials (ZT > 1), particularly because of the low electrical conductivity that this phase
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exhibits. However, similar to other anion-ordered ternary skutterudites [8,9], the thermoelectric properties can be enhanced by proper doping to optimise the electric conductivity and void filling to decrease the thermal conductivity. This investigation could be an interesting subject to further analyses. Acknowledgements This work was supported by the internal project of the Czech Geological Survey (project number 332300) and by the project of Grant Agency CR no. 203/07/0267. These supports are greatly acknowledged. References [1] C. Uher, in: M.G. Kanatzidis, S.D. Mahanti, T.P. Hogan (Eds.), Chemistry, Physics and Materials Science of Thermoelectric Materials: Beyond Bismuth telluride, Kluwer Academics, Plenum Publishers, New York, 2003, p. 121. [2] R.H. Mitchell, Perovskites: Modern and Ancient, Almaz Press, Thunder Bay, Ontario, 2002.
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