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Synthesis, Structure, and Physical Properties of Mixed Valent Mo2SbS2, the First Superconducting Antimonide-Sulfide Chi-Shen Lee,† A. Safa-Sefat,‡ J. E. Greedan,‡ and Holger Kleinke*,† Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Department of Chemistry and the Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1 Received August 1, 2002. Revised Manuscript Received November 14, 2002
We identified a new ternary molybdenum antimonide-sulfide, Mo2SbS2. Black, needleshaped crystals of Mo2SbS2 can be synthesized by reacting stoichiometric amounts of the elements at 850 °C for 7 days with I2 ( 850 °C). The structure of Mo2SbS2 contains a two-dimensional building unit, 2∞ [MoSbX], which is topologically equivalent to that of MoSb2Q, but a different interlayer unit, namely 1∞ [MoS] vs 1∞ [Sb] in MoSb2Q. Theoretical investigations (TB-LMTO calculations) point to metallic behavior of Mo2SbS2. Interestingly, the band structure analysis clearly shows coexistence of disperse and nearly flat band structures near the Fermi level region (van Hove singularities), which suggests that Mo2SbS2 could be a superconductor. This correlation between band structure and superconducting behavior was emphasized in several studies including superconducting cuprates, borocarbides, rare earth carbides, and intermetallic compounds such as SrSn3, Ti2Sn3, and La13Ga8Sb21.10-16 To investigate the possible superconductivity of the title compound as well as other studies, (8) Kleinke, H. Chem. Commun. (Cambridge) 2000, 1941-1942. (9) Lee, C.-S.; Kleinke, H. Eur. J. Inorg. Chem. 2002, 591-596. (10) Simon, A.; Yoshiasa, A.; Ba¨cker, M.; Henn, R. W.; Felser, C.; Kremer, R. K.; Mattausch, H. J. Z. Anorg. Allg. Chem. 1996, 622, 123137. (11) Mattausch, H.; Simon, A.; Felser, C.; Dronskowski, R. Angew. Chem., Int. Ed. Engl. 1996, 35, 1685-1687. (12) Hirsch, J. E.; Scalapino, D. J. Phys. Rev. Lett. 1986, 56, 27322735. (13) Simon, A. Angew. Chem., Int. Ed. Engl. 1997, 36, 1789-1806. (14) Fa¨ssler, T. F.; Hoffmann, S. Z. Anorg. Allg. Chem. 2000, 626, 106-112. (15) Kleinke, H.; Waldeck, M.; Gu¨tlich, P. Chem. Mater. 2000, 12, 2219-2224.
10.1021/cm020803g CCC: $25.00 © 2003 American Chemical Society Published on Web 01/21/2003
First Superconducting Antimonide-Sulfide
the synthesis, structural characterization, electronic structure, and physical properties of Mo2SbS2 will be presented in this paper. Experimental Section Synthesis. All starting materials were stored in an argonfilled glovebox. The starting materials were molybdenum powder (Aldrich, 99.9%), antimony powder (Aldrich, 99.8%, melting point 903 K, 99.5%), and sulfur powder (Aldrich, 99.98%). Initial reactions were intended to synthesize MoSb2S. These were carried out by starting from the elements using a small amount of I2 as transport reagent in a silica tube sealed under vacuum ( 2σ). It was therefore necessary to collect data from a larger crystal, which was later found, again of needlelike shape with a 20-µm diameter. In this case, the scattering power was sufficient, with 76% of the measured reflections having I > 2σ. Unfortunately, that crystal turned to be an axial twin (along c axis), where of all measured reflections of individual 1, 249 exactly overlapped and 1098 partially overlapped with individual 2, and 637 exhibited no overlap. Deleting all partially overlapped data resulted in a final residual factor of R(F) ) 6.8% and a poor data/parameter ratio of 4:1, whereby the structure model was the same as the one obtained for the weaker, but single crystal. To obtain final proof for our structure model of Mo2SbS2, Rietveld refinements were employed to refine the structure of Mo2SbS2 from X-ray powder data. Microcrystalline Mo2SbS2 (16) Mills, A. M.; Deakin, L.; Mar, A. Chem. Mater. 2001, 13, 17781788. (17) SAINT Version 4; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1995. (18) SHELXTL Version 5.12; Reference Manual, Siemens Analytical X-ray Systems, Inc.: Madison, WI, 1995, 1996.
Chem. Mater., Vol. 15, No. 3, 2003 781 Table 1. Crystal Data and Conditions of Data Collection for Mo2SbS2 refined composition instrument formula weight [g/mol] crystal system space group, Z a [Å] b [Å] c [Å] β V [Å3] 2θmax detector radiation (Å) data points structural refinements profile functions Rp, Rwp (%)a goodness-of-fit (χ)b variable parameters
Mo2SbS2 Inel powder X-ray diffractometer 371.5 monoclinic P21/m, 2 6.50920(7) 3.18231(4) 9.3548(1) 105.4430(8)° 186.782(4) 90° CPS590 Cu KR1, 1.54056 7155 GSAS pseudo-Voigt 6.28, 7.91 1.55 111
2 2 2 2 2 1/2 a R ) ∑||Y | - |Y ||/∑|Y |; R p o c o wp ) [∑[w(Yo - Yc ) ]/∑[w(Yo ) ]] ; Yo and Yc are observed and calculated counts. b χ ) [∑[w(Yo2 Yc2)2]/(Nobs - Nvar)]1/2; Nobs is the number of observations and Nvar is the number of parameters.
Table 2. Atomic Positions and Displacement Parameters for Mo2SbS2 (Refined from Powder X-ray Diffraction) atom
site
x
y
z
Ueq [×100 Å2]a
Mo1 Mo2 Sb S1 S2
2e 2e 2e 2e 2e
0.6645(3) 0.8943(3) 0.9971(3) 0.5260(1) 0.2783(9)
0.25 0.25 0.25 0.25 0.25
0.4802(2) 0.8857(2) 0.3378(2) 0.6933(8) 0.9891(8)
0.54(8) 0.63(8) 0.85(7) 0.4(2) 0.9(2)
a
Ueq ) (1/3)∑i ∑j Uij ai aj ai aj.
was prepared for powder X-ray diffraction studies. The purity of the Mo2SbS2 powder was checked by a Bruker D500 X-ray powder diffractometer, and then the data for Rietveld refinement were collected on an Inel powder X-ray diffractometer. The diffraction data were collected simultaneously with a position-sensitive detector (CPS590; 500-mm radius, 90° detection angle) at room temperature for 7 h. The data were analyzed using the GSAS software system.19,20 The 2θ range for the refinements was set between 2.0° and 90°. The starting structural model of Mo2SbS2 was taken from the results of the single-crystal X-ray diffraction study. Refined structural parameters included overall scale factors, lattice parameters, fractional coordinates, anisotropic thermal displacement parameters, and site occupancies. Absorption parameters and an extinction coefficient were also refined. Backgrounds were fitted using a 30-parameter analytical function, and peak shapes were fitted using exponential pseudo-Voigt functions.19 After the structure of the main phase was determined, refinements including impurities from the starting elements and possible binary phases were included. The products of the Mo2SbS2 reaction include trace amounts of Sb, MoS2, and MoO2 (less than 2%). The experimental and Rietveld refined profiles of these data are shown in Figure 1. No significant change in site occupancies ( 0, β < 0, γ > 0). The band structure along selected paths of the first Brillouin zone40 is shown in Figure 6, with the fat bands emphasizing the contribution of the Mo2 dz2 orbitals. The band structure of Mo2SbS2 features large dispersive (37) Ho¨nle, W.; von Schnering, H.-G. Z. Kristallogr. 1981, 155, 307314. (38) Dashjav, E.; Szczepenowska, A.; Kleinke, H. J. Mater. Chem. 2002, 12, 345-349. (39) Elder, I.; Lee, C.-S.; Kleinke, H. Inorg. Chem. 2002, 41, 538545. (40) Kokalj, A. J. Mol. Graph. Model. 1999, 17, 176-179.
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Table 4. Summary of Bond Distances and ICOHP Values for Various Interatomic Contacts in Mo2S3, Mo2SbS2, and MoSb2S Mo2S3 d(Å) 2.85 3.23 2.37
-1.70 -0.76 -2.99
2.75 3.18 2.30 2.84 3.41 3.18
-1.14 -0.27 -2.86 -1.22 -0.15 -0.53
3.23
-0.37
2.57 3.20
-1.22
3.62 2.83 2.67 3.40
-0.09 -0.95 -1.36
2.85 3.20 2.35
-0.94 -0.29 -2.18
2.75 3.18 2.38
-1.92 -0.54 -2.99
2 [MoSbS] ∞
Mo1-Mo1 Mo1-Mo1 Mo1-S Mo1-Sb Sb-Sb interlayer Mo1-Mo2 interchain Mo-Sb Mo-S S-S 1 [MoS] ∞
Mo2-Mo2 Mo2-Mo2 Mo-S
Mo2SbS2
ICOHP (eV/bond)
MoSb2S
d(Å)
ICOHP (eV/bond)
d(Å)
ICOHP (eV/bond)
2.76 3.19 2.38 2.83 3.33 3.19
-1.90 -0.68 -2.99 -1.77 -0.11 -0.41
Figure 6. Band structure of Mo2SbS2. The dashed line denotes the Fermi energy. The interlayer Mo dz2 contributions are emphasized.
bands crossing EF that reveal metallic properties as well as flat bands at EF that are not present in the band structure of Mo2S3 (both from LMTO (our work) as well as extended Hu¨ckel41,42 calculations). Bands crossing the Fermi level along Y-Γ and B-Γ (parallel to b* and a*) correspond to Mo-Mo (within the zigzag chains) and Mo-Sb interactions. The flat bands along Γ-Z and D-B exhibit a very narrow dispersion (less than 0.1 eV), which are dominated by Mo2 dz2 contributions related to the interchain Mo-Mo interactions. These contributions indicate electronic states with localized electron pairs near the Fermi level. The coexistence of steep bands (fast delocalized electrons) as well as flat bands (localized electron pairs) near the Fermi level is considered as a fingerprint for superconducting behavior (van Hove singularity).13-15,43,44 Therefore, we performed further investigations to check for superconductivity, namely magnetic susceptibility experiments. (41) Whangbo, M. H.; Canadell, E. J. Am. Chem. Soc. 1992, 114, 9587-9600. (42) Canadell, E.; LeBeuze, A.; El Khalifa, M. A.; Chevrel, R.; Whangbo, M. H. J. Am. Chem. Soc. 1989, 111, 3778-3782. (43) Ko¨ckerling, M.; Johrendt, D.; Finckh, E. W. J. Am. Chem. Soc. 1998, 120. (44) Fa¨ssler, T. F.; Hoffmann, S.; Kronseder, C. Z. Anorg. Allg. Chem. 2002, 627, 2486-2492.
Figure 7. Temperature-dependent Seebeck coefficients of Mo2SbS2.
Physical Properties. Seebeck Measurements. The temperature-dependent Seebeck coefficient measurement is shown in Figure 7. The Seebeck coefficient S remains between +5 and +10 µV/K over the temperature range from 300 to 600 K. The positive sign indicates that holes are the dominating charge carriers (p-type thermoelectric material). Typical magnitudes of Seebeck coefficients at 300 K range from a few µV/K for most metals to 10-20 µV/K for some transition metal ele-
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Conclusion
Figure 8. Magnetic susceptibility measurements of Mo2SbS2.The inset emphasizes the phase transition.
ments. The observed Seebeck coefficient of Mo2SbS2 is of the same order as most metallic materials. Magnetic Susceptibility Measurements. The ZFC susceptibilities (not corrected for diamagnetic contributions) of Mo2SbS2 are shown in Figure 8. Mo2SbS2 exhibits temperature-independent Pauli paramagnetism between 300 and 2.2 K that outweighs the diamagnetic contributions, which is typical for a metallic material. The ZFC magnetization curve displays a transition temperature Tc at 2.20 K (see inset); below Tc strong diamagnetism (flux expulsion) is evident. This can be explained with a transition to the superconducting state, i.e., a metal to superconductor transition. Because of the temperature limits of our equipment, it was not possible to determine the saturation magnetization. The parent compound Mo2S3, with the Mo atoms having d3 configuration, itself exhibits two phase transitions associated with symmetry reductions caused by different Mo atom ordering, namely at 182 and 145 K.45 (45) Rashid, M. H.; Sellmyer, D. J.; Katkanant, V.; Kirby, R. D. Solid State Commun. 1982, 43, 675-678.
In summary, the syntheses and characterization of a new ternary compound, Mo2SbS2, were presented. Mo2SbS2 crystallizes in ternary ordered variant of Mo2S3, exhibiting 2∞ [MoSbS] layers and 1∞ [MoS] chains. The structure comprises strong heteronuclear Mo-S and Mo-Sb, strong homonuclear Mo-Mo and intermediate Sb-Sb interactions. The Mo-Mo interactions can be correlated with the Mo-Mo interactions in Mo2S3 and MoSb2S. Mo2SbS2 exhibits metallic behavior as predicted by electron counting, electronic structure calculations, and confirmed by physical property measurements(smallSeebeckcoefficientsandPauliparamagnetism). The results from the electronic structure calculations suggest different electronic configurations of Mo1 and Mo2, i.e., a mixed valent compound. This is in contrast to Mo2S3, which undergoes metal/nonmetal transitions upon cooling due to the d3 configuration. Band structure analyses revealed a van Hove singularity, indicating superconducting behavior of Mo2SbS2. This assumption has been confirmed by magnetic susceptibility measurements, which revealed a transition to the superconducting state at 2.2 K. Acknowledgment. Financial support from the Canada Foundation for Innovation, the Canada Research Chair Secretariat, the Ontario Innovation Trust, Materials Manufacturing Ontario, the Natural Sciences and Engineering Research Council of Canada, and the Province of Ontario (Premier’s Research Excellence Award for H.K.) is appreciated. We are indebted to Henry Pilliere (Inel) for the X-ray powder data collection. Supporting Information Available: Table of anisotropic displacement parameters for MoSb2S. This material is available free of charge via the Internet at http://pubs.acs.org. CM020803G
