Synthesis, Structure, and Electrical Properties of the Mo12 Cluster

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Synthesis, Structure, and Electrical Properties of the Mo12 Cluster Sulfide Hg∼2.8KMo12S14 Containing Mercury Chains Patrick Gougeon* and Philippe Gall Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, INSA de Rennes, 11 allée de Beaulieu, CS 50837, 35708 Rennes Cedex, France S Supporting Information *

ABSTRACT: The new compound Hg∼2.8KMo12S14 was synthesized by diffusing mercury into the metastable KMo12S14 compound at 350 °C. Its crystallographic structure, solved from single-crystal X-ray diffraction, shows that the Mo−S framework is maintained during the synthesis. It is based on Mo12S14S6 units interlinked via Mo−S bonds as in the parent compound. The mercury forms linear chains with Hg−Hg distances of about 2.72 Å in the tunnels delimited by the Mo12S14S6 units. Superconductivity was observed below 2.5 K by electrical resistivity and magnetic susceptibility measurements.

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INTRODUCTION For 40 years, numerous reduced molybdenum chalcogenides containing molybdenum clusters of various sizes and forms have been obtained by solid-solid reaction at temperatures between 1200 and 1800 °C. Molybdenum chalcogenides, the crystal structure of which is based on the octahedral Mo6 cluster,1 are the most numerous and show unusual physical properties.2,3 Molybdenum chalcogenides containing clusters with nuclearities higher than 6 such as Mo3n (n = 3−8, 10, and 12)4−10 have also been synthesized. The latter clusters can be seen as resulting from the unidirectional trans-face-sharing of 2−7, 9, and 11 Mo6 octahedra, respectively. Most of the molybdenum chalcogenides based on high-nuclearity clusters have a precise cationic stoichiometry if they are obtained by classical solid−solid syntheses at temperatures ranging from 1000 to 1800 °C. Nevertheless, by redox reactions at temperatures below 600 °C, we can change the cationic charge without altering the molybdenum chalcogenide host structure and, consequently. the electron count associated with the molybdenum clusters, which governs the electrical properties. Thus, compound Ag3.6Mo9Se114 obtained by solid-state chemistry and having 35.6 e− per Mo9 cluster is semiconducting, while the metastable binary o-Mo9Se11,11 resulting from the silver deintercalation and having Mo9 clusters with 32 e−, is metallic and becomes a superconductor below 5.5 K. On the other hand, the superconducting state observed below 4.4 K in Cs2Mo12Se14 is rapidly suppressed upon copper insertion.12 Previously, the synthesis and crystal structure of compound K2.3Mo12S14 were described.13 K2.3Mo12S14 was obtained by a solid-solid synthesis at a temperature of 1500 °C in a molybdenum crucible sealed under reduced argon pressure with an arc-welding apparatus. Its crystal structure results from © XXXX American Chemical Society

interlinked Mo12S14 cluster units that create large channels, which are filled at random by some K+ cations, while the other K+ cations occupy sites between two adjacent Mo12S14 entities along the [001] direction. By redox reactions at temperatures below 100 °C, we could withdraw or add K+ cations in the channels and, consequently, synthesize the isomorphous compounds K1+xMo12S14, with x ranging from 0 to 1.6. The electrical resistivity studies performed on KMo12S14 and K2.3Mo12S14 clearly showed the influence of the cationic charge on the electrical behavior. Thus, while K2.3Mo12S14 presents a semiconducting behavior in the 300−4.2 K domain, KMo12S14 is metallic. We have pursued this work by inserting other cations such as sodium in the empty channel of KMo12S14.14 We described herein the synthesis, crystallographic structure, and resistivity measurements of the original Hg∼2.8KMo12S14 compound, which is characterized by parallel mercury chains in the tunnels.

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EXPERIMENTAL SECTION

Syntheses. Compound Hg∼2.8KMo12S14 was obtained in three stages. The first one was preparation of the ternary sulfide K2.3Mo12S14, then obtainment of KMo12S14 by soft chemistry, and, at last, preparation of the mercury phase by diffusing mercury within the KMo12S14 compound at 350 °C. These different stages were realized on single-crystal specimens. Precursors for the solid-state preparation of K2.3Mo12S14 were powders of molybdenum disulfide (MoS2), thiomolybdate (K2MoS4), and molybdenum. Prior to use, molybdenum (purity ≥99.999%) was heated under a dihydrogen gas flow at 1000 °C over 10 h in order to remove residual oxygen. MoS2 was obtained by the direct reaction of sulfur (purity ≥99.999%) and Received: December 9, 2016

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DOI: 10.1021/acs.inorgchem.6b02866 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

atomic positional parameters, as well as the equivalent isotropic displacement factors, and significant bond lengths are listed in Table 3.

molybdenum powders in the right stoichiometry in an evacuated (about 10−2 Pa residual pressure of argon) and sealed silica ampule. The latter was then heated to 800 °C and maintained at this temperature for 48 h. K2MoS4 was prepared by sulfuring molybdate of potassium at 400 °C for 48 h under a CS2 gas using argon as the carrier gas. K2MoO4 was obtained by reacting at 800 °C in air the appropriate quantities of MoO3 (purity ≥99.95%) and K2CO3 (purity ≥99.9%) in an alumina boat crucible for 48 h. The purity of each initial product was verified by powder X-ray diffraction studies conducted on a Bruker Advance D8 goniometer using Cu Kα1 radiation and having a LynxEye detector. The initial reagents were all stored and manipulated in an argon-filled glovebox. Single crystals of K2.3Mo12S14 were grown from an amount of K2MoS4, MoS2, and molybdenum having an overall composition of K0.125Mo0.375S0.5. The homogeneous mixture was cold-pressed into a cylindrical pellet of about 3 g. The latter was introduced into a molybdenum container (2.5 cm height; 1.5 cm diameter) that was earlier outgassed at 1500 °C under a dynamic vacuum (about 10−3 Pa) and next sealed under reduced argon pressure with an arc-welding apparatus. The temperature of the molybdenum container was raised to 1500 °C in 5 h, kept at this latter temperature for 6 h, followed by slow cooling to 1000 °C in 5 h, and, at last, allowed to cool to room temperature. Crystals of KMo12S14 were synthesized by oxidizing K2.3Mo12S14 crystals in an iodine solution heated at 90 °C over 2 days in agreement with the reaction

Table 1. Crystallographic and X-ray Experimental Data for H2.82KMo12S14 formula fw, g mol−1 space group a, Å c, Å Z V, Å3 ρcalcd, g cm−3 T, °C λ, Å μ, mm−1 R1a wR2b

Hg2.82KMo12S14 2205.89 P63/m 9.2374(1) 16.3302(1) 2 1206.76(3) 6.071 20 0.71073 25.25 0.0253 0.0560

R1 = ∑||Fo| − |Fc||/∑ |Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/ ∑[w(Fo2)2]}1/2, where w = 1/[σ2(Fo2) + 12.1358P] with P = [Max(Fo2,0) + 2Fc2]/3. a

Electrical Resistivity Study. The dependence of the electrical resistivity as a function of the temperature was studied on a single crystal of Hg∼2.8KMo12S14 using a standard four-point probe technique with a current of 0.1 mA. Indium leads were used to make ohmic contacts to the single crystal. The latter one was identified before on a Nonius Kappa CCD goniometer. The ohmic behavior and phase invariance were verified all along the measurements. Magnetic Susceptibility Study. The temperature dependence of the magnetic susceptibility was studied on a batch of single crystals using a Quantum Design MPMS-XL SQUID magnetometer under an applied direct-current field of 10 G.

K 2.3Mo12S14 + 0.65I 2 → KMo12S14 + 1.3KI Finally, single crystals of Hg∼2.8KMo12S14 were obtained by heating an excess of mercury with single crystals of KMo12S14 in a sealed Pyrex tube at 350 °C for 3 days. Figure 1 shows a single crystal of Hg2.80KMo12S14.

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RESULTS AND DISCUSSION Crystal Structure. Figure 2 shows a view of the crystal structure of Hg∼2.8KMo12S14 along the [110] direction. The molybdenum chalcogen network is analogous to that of the parent compound K2.3Mo12S14. It is based on the Mo12S14S6 cluster units (Figure 3) interlinked via Mo−S bonds (Mo1− S1). The Mo12 cluster, which can be viewed as resulting from the uniaxial trans-face-sharing condensation of three Mo6 octahedra, is surrounded by 14 Si (six S1, six S2, and two S3) sulfur atoms capping the triangular faces of the trioctahedron and six “ausser” Sa (S1) sulfur atoms linked to the terminal Mo1 atoms. The Mo−Mo bond lengths observed in the Mo12 clusters are 2.6678(6) and 2.6899(5) Å for those between the Mo1 and Mo2 atoms related through the 3-fold axis (intratriangle in Table 3), respectively. The Mo1−Mo2 intertriangle distances separating the triangles generated by the Mo1 atoms and those created by the Mo2 atoms are 2.7378(4) and 2.8024(4) Å, respectively, while the intertriangle Mo2− Mo2 distances between the triangles formed by the Mo2 atoms are 2.6446(6) and 2.6888(6) Å. The S1 and S3 atoms cap one molybdenum triangular face, while the S2 atoms cap two molybdenum triangular faces of the Mo12 entity. In addition, the S1 atom is connected to a Mo1 atom of an adjacent Mo12 cluster. The Mo−S bond lengths span from 2.3894(12) to 2.6003(9) Å. Every Mo12S14S6 entity is connected to six other neighboring units through 12 interunit Mo1−S1 bonds of 2.4597(9) Å to form the Mo−S network. In the latter, the shortest Mo1−Mo1 distance between the Mo12 clusters is 3.248 Å. The developed

Figure 1. Single crystal of Hg∼2.8KMo12S14. Single-Crystal X-ray Diffraction Studies. The intensities were recorded with a Nonius Kappa CCD diffractometer equipped with a graphite monochromator (Mo Kα radiation; λ = 0.71073 Å). The COLLECT software15 was used to determine the angular scan conditions (φ and ω scans) employed during the intensity measurement. Integration of the data was processed with the program EvalCCD.16 An absorption correction (Tmin = 0.114 and Tmax = 0.237) based on the crystal shape was used.17 The structure was solved with the SHELXL-97 program.18 The positional parameters of the molybdenum and sulfur atoms found in KMo12S1413 were utilized during the first step of the structural study. The potassium and mercury atomic positional parameters were deduced from difference Fourier calculations. Refinement of the site occupancy parameter for the mercury atoms resulted in the final composition Hg2.82KMo12S14. Crystallographic and X-ray experimental data for the Hg∼2.8KMo12S14 compound are summarized in Table 1. Table 2 summarizes the final B


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Inorganic Chemistry

Table 2. Positional Parameters, Equivalent Isotropic Displacement Parameters (Å2), Wyckoff Positions, and Site Occupancy Factors (sof) for H2.82KMo12S14 atom

x

y

z

Ueq

Wyckoff

sof

Hg1 Hg2 Mo1 Mo2 S1 S2 S3 K1

0.0000 0.0000 0.65565(4) 0.49719(4) 0.34885(12) 0.02108(12) 0.6667 0.6667

0.0000 0.0000 0.16136(4) 0.16660(4) 0.04051(12) 0.66038(13) 0.3333 0.3333

0.5000 0.33276(3) 0.45708(2) 0.31637(2) 0.44326(5) 0.81983(5) 0.56894(9) 0.7500

0.03073(19) 0.03653(16) 0.00827(8) 0.00782(7) 0.01008(15) 0.01115(16) 0.0120(3) 0.0273(5)

2a 4e 12i 12i 12i 12i 4f 2d

0.942(4) 0.941(3) 1 1 1 1 1 1

Table 3. Significant Interatomic Distances (Å) for Hg2.82KMo12S14 Intratriangle Mo1−Mo1 (×2) Mo2−Mo2 (×2) Intertriangle Mo1−Mo2 Mo2−Mo2 Average Mo−Mo Intercluster Mo1−Mo1 Intratriangle Mo1−S1 Intertriangle Mo1−S2 Mo2−S1 Intercluster Mo1−S1

2.6678(6) 2.6899(5)

K1−S2 (×6) K1−S3 (×2)

3.3548(10) 2.9568(15)

2.7378(4) 2.8023(4) 2.6446(6) 2.6888(6)

Hg1−S1 (×6) Hg2−S2 (×3)

3.1901(10) 3.2458(10)

2.4416(10) 2.4830(10)

Mo2−S2

2.4716(10) 2.4825(10)

2.6003(9) 2.4355(9)

Mo2−S2 Mo1−S3

2.5784(9) 2.3894(12)

2.713 3.1351(4)

2.4597(9)

formula of the resulting Mo−S framework is [Mo12S8iS6/2i−a]S 6/2 a−i in Schäf er’s notation. 19 The representation of Hg∼2.8KMo12S14 as viewed down the [001] direction (Figure 4) reveals channels running parallel to the [001] direction in which the mercury atoms reside. The Hg1 atom is surrounded by six sulfur atoms at 3.1901(9) Å, creating an octahedron shortened in the c direction (Figure 5). The Hg2 atom is found in the quasi-triangular environment of S2 atoms with a Hg2−S2 distance of 3.2458(10) Å. The small deficiency of about 6% observed on both mercury sites clearly suggests a discontinuity of the mercury chains with an average length of 16 mercury atoms. Moreover, an oscillation image (Figure 6) around the c axis showed diffuse lines separated by about 2.72 Å and normal to the chain direction. Their existence showed that the mercury chains in the different channels are not correlated. The Hg−Hg distances within the chains are 2.7031(10) and 2.7310(5) Å. The latter distances are somewhat longer than that observed (2.64−2.67 Å) in the compounds Hg3−δMF6 (M = As, Sb, Nb, Ta),20−24 which are, to our knowledge, the only compounds that have been well characterized to contain quasi-linear mercury chains. Indeed, the crystal structures of the compounds Hg3−δMF6 (M = As, Sb, Nb, Ta) are characterized by two orthogonal and nonintersecting quasi-linear chains of mercury, as shown in Figure 7. An important difference between the Hg3−δMF6 compounds and Hg∼2.8KMo12S14 resides in the interchain distance, which is about 3.24 Å in the former compounds and 9.23 Å in the latter ones. In other

Figure 2. Crystal structure of Hg∼2.8KMo12S14 viewed along the [110] direction (97% probability ellipsoids).

inorganic and complex compounds containing mercury in low oxidation state, the Hg−Hg distances range from 2.46 to 2.59 Å in the (Hg2)2+ groups [see ref 25 for a review of inorganic compounds containing (Hg2)2+ groups], from 2.55 to 2.56 Å in the quasi-linear or linear (Hg3)2+ groups,26−28 and from 2.66 to 2.71 Å in the (Hg3)4+ triangles.29 In the linear (Hg4)2+ groups,30,31 the Hg−Hg distances are in the 2.59−2.63 Å range with relatively short interactions (ca. 3 Å) between the (Hg4)2+ groups, such that they form nonlinear chains extending through the structure. From the Hg−S bonds, we could evaluate the oxidation numbers of the Hg1 and Hg2 atoms by using the empirical bond length−bond valence relationship: S = exp[(R0 − R)/b].32 In this formula, S is the experimental bond valence, R is the experimental bond length, and R0 and b are the calculated bond-valence parameters (2.32 Å and 0.37 for Hg−S bonds33). These calculations for the Hg1 and Hg2 atoms resulted in values of +0.57 and +0.25, respectively, leading to an C


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Inorganic Chemistry

Figure 5. Sulfur environments of the mercury atoms. Figure 3. Mo12 cluster with its sulfur environment.

opposed triangular faces of the octahedron like previously described.13 Variation of the Mo−Mo Distances. From earlier works on the InxMo15S19 (0 ≤ x ≤ 3.7) compounds containing Mo6 and Mo9 clusters,34 we deduced that the Mo−Mo distances vary according to the cationic charge transferred toward the Mo6 and Mo9 aggregates. In the compounds K1+xMo12S14, with x ranging from 0 to 1.6, the increase of the cationic charge transfer toward the Mo12 cluster between KMo12S14 and K2.6Mo12S14 results in a diminution of the mean Mo−Mo distance in the trioctahedral Mo12 cluster, as shown in Figure 8. From this plot, we can estimate that the cationic charge in Hg∼2.8KMo12S14 is close to 2+, leading thus to a charge of about

average charge on the mercury atoms of 0.35+. If we assume a charge of about 1+ per entity Hg2.8 (see Variation of the Mo− Mo Distances), we found an average charge of 1/2.82 = 0.35+. This value corresponds well to those observed in the compounds Hg3−δMF6 (M = As, Sb, Nb, Ta) in which they range from 0.34 to 0.35. The K1 atoms occupy sites located on the 3-fold axes between two neighboring Mo12S14 entities. Each K1 atom is surrounded by six S2 atoms at 3.3548(10) Å at the apexes of a distorted octahedron shortened along the ternary axis and two S3 atoms on the 3-fold axis at 2.9568(15) Å capping two

Figure 4. Crystal structure of Hg∼2.8KMo12S14 viewed along the [001] direction. D


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Inorganic Chemistry

Figure 8. Evolution of the mean Mo−Mo distances in the Mo12 cluster according to the total charge of the cations.

1+ per entity Hg∼2.8 as observed for the compounds Hg3−δMF6 (M = As, Sb, Nb, Ta) in which δ varies from 0.10 to 0.14. Resistivity Study. The temperature dependence of the electrical resistivity of a single crystal of Hg∼2.8KMo12S14 is shown in Figure 9. Hg2.8KMo12S14 is metallic, with a resistivity of about 1.4 × 10−4 Ω·cm at 290 K. Below 2.5 K, Hg∼2.8KMo12S14 becomes a superconductor with a resistivity of about 0.7 × 10−5 Ω·cm just before the superconducting

Figure 6. Oscillation image around the c axis.

Figure 7. Infinite disordered mercury chains in Hg0.86AsF6. E


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Inorganic Chemistry

(5) (a) Gougeon, P.; Potel, M.; Padiou, J.; Sergent, M. Synthesis, Crystal Structure and Electrical Properties of the First Compound Containing Uniquely Mo12Se14 Cluster Units. Mater. Res. Bull. 1987, 22, 1087. (b) Gautier, R.; Picard, S.; Gougeon, P.; Potel, M. Synthesis, Crystal and Electronic Structures, and Electrical Properties of Rb2Mo12Se14 Containing Trioctahedral Mo12 Clusters. Mater. Res. Bull. 1999, 34, 93. (6) (a) Gougeon, P.; Potel, M.; Sergent, M. Structure of Rb3Mo15Se17 Containing the New Mo15. Clusters. Acta Crystallogr. 1989, C45, 182. (b) Gougeon, P.; Potel, M.; Sergent, M. Structure of Cs3Mo15Se17. Acta Crystallogr. 1989, C45, 1413. (7) Gougeon, P.; Potel, M.; Padiou, J.; Sergent, M. Rb4Mo18Se20 First Structural Type Containing The New Mo18Se20 Cluster Unit. Mater. Res. Bull. 1988, 23, 453. (8) Picard, S.; Gougeon, P.; Potel, M. Single-Crystal Structure of Cs5Mo21S23. Acta Crystallogr. 1997, C53, 1519. (9) Gougeon, P. Nouveaux Chalcogénures Ternaires de Molybdène à Clusters Condensés Synthèses, Structures et Propriétés Physiques. Thesis, University of Rennes, Rennes, France, 1984. (10) Picard, S.; Gougeon, P.; Potel, M. Rb10Mo36S38: A Novel Reduced Molybdenum Sulfide Containing the Highest Nuclearity Transition Metal Cluster in a Solid-State Compound. Angew. Chem., Int. Ed. 1999, 38, 2034. (11) Gougeon, P.; Potel, M.; Padiou, J.; Sergent, M.; Boulanger, C.; Lecuire, J.-M. Synthèse, Structure Cristalline et Propriétés Physiques du Nouveau Binaire Métastable Supraconducteur à Clusters Mo9: OMo9Se11. J. Solid State Chem. 1987, 71, 543. (12) Al Rahal Al Orabi, R.; Fontaine, B.; Gautier, R.; Gougeon, P.; Gall, P.; Bouyrie, Y.; Dauscher, A.; Candolfi, C.; Lenoir, B. Cu Insertion Into the Mo12 Cluster Compound Cs2Mo12Se14: Synthesis, Crystal and Electronic Structures, and Physical Properties. Inorg. Chem. 2016, 55, 6616. (13) Picard, S.; Gougeon, P.; Potel, M. Synthesis, Structural Evolution, and Electrical Properties of the Novel Mo12 Cluster Compounds K1+xMo12S14 (x = 0, 1.1, 1.3, and 1.6) with a Tunnel Structure. Inorg. Chem. 2006, 45, 1611. (14) Gougeon, P.; Gall, P.; Salloum, D. Na2.9KMo12S14: a Novel Quaternary Reduced Molybdenum Sulfide Containing Mo12 Clusters with a Channel Structure. Acta Crystallogr., Sect. E: Struct. Rep. Online 2013, 69, i38. (15) COLLECT, Data Collection Software; Nonius BV: Delft, The Netherlands, 1999. (16) Duisenberg, A. J. M. Reflections on Area Detectors. Ph.D. Thesis, University of Utrecht, Utrecht, The Netherlands, 1998. (17) de Meulenaer, J.; Tompa, H. The Absorption Correction in Crystal Structure Analysis. Acta Crystallogr. 1965, 19, 1014−1018. (18) Sheldrick, G. M. SHELXL-97, A program for crystal structure analysis; University of Göttingen: Göttingen, Germany, 1997. (19) Schäfer, H.; Schnering, H. G. Metall-Metall-Bindungen bei Niederen Halogeniden, Oxyden und Oxydhalogeniden Schwerer Ü bergangsmetalle Thermochemische und Strukturelle Prinzipien. Angew. Chem. 1964, 76, 833. (20) Brown, I. D.; Cutforth, B. D.; Davies, C. G.; Gillespie, R. J.; Ireland, P. R.; Vekris, J. E. Alchemists Gold, Hg2.86AsF6 - X-Ray Crystallographic Study of A Novel Disordered Mercury Compound Containing Metallically Bonded Infinite Cations. Can. J. Chem. 1974, 52, 791. (21) Schultz, A. J.; Williams, J. M.; Miro, N. D.; MacDiarmid, A. G.; Heeger, A. J. A Neutron Diffraction Investigation of The Crystal and Molecular Structure of The Anisotropic Superconductor Trimercury Hexafluoroarsenate. Inorg. Chem. 1978, 17, 646. (22) Tun, Z.; Brown, I. D. The Structure of Mercury Hexafluoroantimonate Hg3−δSbF6. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, 38, 2321. (23) Tun, Z.; Brown, I. D.; Ummat, P. K. The Room-Temperature Structures of Mercury Niobium Fluoride, Hg3‑δNbF6, and Mercury Tantalum Fluoride, Hg3‑δTaF6. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, 40, 1301.

Figure 9. Temperature dependence of the electrical resistivity of an Hg∼2.8KMo12S14 single crystal.

transition. The superconducting state was equally detected from magnetic susceptibility studies on a batch of single crystals at 2.2 K.

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CONCLUSION In summary, we have synthesized by soft chemistry the original compound Hg∼2.8KMo12S14 containing, for the first time, parallel quasi-linear chains of mercury. The existence of the family of compounds M2n−2Mo6nX6n+2, with M = Rb and Cs and X = S and Se (n = 1−6), in which the cations occupy finite onedimensional cavities, the length of which varies with that of the Mo6nX6n+2 unit, allows us to envisage the possibility of substituting rubidium or cesium atoms by mercury atoms and thus to form mercury chains of various sizes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02866. X-ray crystallographic file for Hg2.82KMo12S14 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick Gougeon: 0000-0003-4778-5581 Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis, Structure, and Electrical Properties of the Mo12 Cluster

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