Er5re2c7, Tm5re2c7, and Lu5re2c7 with Sc5re2c7 Type, and Yb2rec2 with Pr2rec2 Type Structures R

Er5Re2C7, Tm5Re2C7, and Lu5Re2C7 with Sc5Re2C7 Type, and Yb2ReC2 with Pr2ReC2 Type Structures R. Pöttgen, K. H. Wachtmann, W. Jeitschko*, A. Lang, T. Ebel Anorganisch-Chemisches Institut, Universität Münster Wilhelm-Klemm-Straße 8 , D-48149 Münster, Germany Z. Naturforsch. 52 b, 231-236 (1997); received November 15, 1996 Crystal Structure, Magnetic Properties, Rare Earth Metal Rhenium Carbides, Polymeric Rhenium-Carbon Polyanions ErsReiCy, TmsRe 2C7, and LusReaC? were prepared by arc-melting of the elemental compo nents and subsequent annealing at 800 °C. ErsRe^Cy forms only after the annealing process, whereas the other two carbides were already present in the as cast samples. They crystallize with a ScsReiC? type structure, which was refined from single-crystal X-ray data of LusRe 2C7: Cmmm, a = 791.44(5), b = 1418.08(8), c = 332.79(2) pm, Z = 2, R = 0.037 for 544 structure factors and 21 variable parameters. The structure contains linear centrosymmetric C 3 units with a C-C bond length of 133(2) pm and isolated carbon atoms in octahedral coordination of four lutetium and two rhenium atoms. The rhenium atoms within the two-dimensionally infinite polymeric sheets [ReiC-tln are electronically saturated as is indicated by the diamagnetism and the semiconductivity of this carbide. Yf> 2ReC2 was prepared by reacting the elements in a sealed tantalum tube with a high-frequency furnace. It crystallizes with a Pr 2ReC2 type structure: Pnma, a = 645.91(6), b = 498.64(6), and c = 966.05(6) pm. Magnetic susceptibility measurements indicate the ytterbium atoms to be trivalent in this compound.

Introduction The isotypic carbides Ln 2ReC2 (Ln = Ce-Nd, Sm, Gd-Tm, and Lu) have been prepared by arc-melting We recently reported on a new series of rare [11]. Because of the high vapor pressure of ytter earth metal rhenium carbides of the composition bium, the corresponding ytterbium compound could Ln^ResCis with Ln = Y, La-Nd, Gd-Er [1]. At not be prepared that way. We have now obtained tempts to prepare the isotypic carbides with thulium Yb 2ReC2 by reaction of the elemental components and lutetium did not result in La^ResCis type com in a sealed tantalum tube. pounds, instead with these heavy rare earth elements we obtained the carbides TmsRe 2C7 and Lu5Re2C7, Sample Preparation and Lattice Constants which crystallize with a ScsRe 2C7 type structure [ 2] The lanthanoids were purchased in the form of ingots and which are reported here. With erbium we syn (all with nominal purities > 99.9 %). They were cut into thesized both the La^ResCis and ScsRe 2C7 type small pieces of about 2 mm diameter, and cold-pressed carbides, the latter only after the annealing at 800 to pellets (6 mm diameter) together with the appropriate °C. These are the first ternary carbides of the heavy amounts of rhenium powder (Starck, 400 mesh, > 99.9 %) rare earth elements containing C 3 units. Such propa- and graphite flakes (Ventron, 20-60 mesh, 99.5 %). dienyl units were found only in few other carbides: The pellets were reacted in an arc-melting furnace un Ln3C4 (Ln = Sc, Y, Ho-Lu) [3,4], Mg 2C3 [5], Y4C7 der an atmosphere of argon (99.996 %). The argon was [6], the two different modifications of H 04C7 [6, 7], purified by repeatedly melting a titanium sponge in a sepa the carbides Ln4C7 (Ln = Er, Tm, Lu) [7], and the rate copper chill prior to the reactions. The molten buttons of the samples were always turned over and remelted sev ternary carbide chloride Ca 3C3Cl2 [8]. The struc eral times to enhance their homogeneity. The weight loss tures of Sc 3C4 and Ca3C3Cl2 were supported by after several meltings was always less than 1 %. The pel extended Hückel calculations [9]. A preliminary ac lets were subsequently wrapped in tantalum foil, sealed count of some of the work reported here has been into evacuated silica tubes, and annealed at 800 °C for given at a conference [ 10]. three weeks. A well-crystallized sample of Lu 5Re2C7 was obtained * Reprint requests to W. Jeitschko. by annealing an arc-melted button with a high-frequency

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 232 R. Pöttgen et al. • Rare Earth Metal Rhenium Carbides

Table I. Lattice constants of the orthorhombic carbides with Sc5Re2C7 type and Pr 2ReC2 type structure. The data for the scandium compound were taken from ref. [2], Standard deviations are given in parentheses throughout the paper.

Compound a [pm] b [pm] c [pm] V [nm3]

Sc5Re2C7 779.26(7) 1362.0(1) 320.62(3) 0.3403 Er<;Re2C7 792.9(1) 1439.4(2) 338.02(6) 0.3858 Tm5Re2C7 792.02(6) 1430.55(9) 335.94(4) 0.3806 T[K] — LusRe->C7 791.44(5) 1418.08(8) 332.79(2) 0.3735 Fig. 1. Temperature dependence of the inverse magnetic Yb2ReC2 645.91(6) 498.64(6) 966.05(6) 0.3111 susceptibility of Yb2ReC 2 measured with a magnetic flux density of 3 T. furnace in a water-cooled silica tube slightly below the (undetermined) melting point for about six hours. of a ferromagnetic impurity. The susceptibilities For the preparation of Yb 2ReC2 the elements with the have therefore been extrapolated to 1/B = 0. The atomic ratio Yb:Re:C = 3.3:1:2 were sealed into a tan X vs. 1/B plots were linear. The thus extrapolated talum tube under argon (800 mbar). They were reacted molar susceptibility was only weakly temperature at high temperature (> 1800 °C) for 5 min in a high- dependent and had a value of -20(±5)-10 -9 m3/mol frequency furnace. Subsequently the upper half of the at room temperature, indicating diamagnetism. tube was pulled out of the induction coil and the excess The magnetic susceptibilities of Yb 2ReC2 were ytterbium was distilled at lower temperature to the cooler also slightly field dependent; however, those mea end of the tantalum tube. sured with 3 T were already the same as those ob The lattice constants of all compounds (Table I) were served at 5 T and therefore an extrapolation to infi obtained by least-squares fits of the Guinier powder nite field-strength was not necessary. The tempera data using CuKc*i radiation and a-quartz (a = 491.30, ture dependence obeyed the Curie-Weiss law (Fig. c = 540.46 pm) as an internal standard. To assure proper indexing, the observed patterns were compared with the 1). A magnetic moment of ^exP = 4.5(1) per yt calculated ones [ 12] assuming the atomic positions as ob terbium atom was obtained from the linear portion tained from the structure refinements of Lu 5Re2C7 and of the \/\ vs. T plot above 70 K. This value is in Er2ReC2 [11], good agreement with the theoretical value /ieff = 4.54 for the free ion value of trivalent ytterbium. Chemical and Physical Properties It is also in agreement with the magnetic proper ties of the isotypic carbides Ln 2ReC2 (Ln = Y, Tb, Compact ingots of the carbides are light gray; Dy, Ho, Er, and Lu) where the rhenium atoms do powder samples have dark gray color. Single crys not carry magnetic moments [13,14]. The magnetic tals of Lu 5Re2C7 exhibit metallic luster. Together susceptibilities of Yb 2ReC2 do not give any indica with the isotypic carbide Sc 5Re2C 7, they are the tion for magnetic order down to 2 K. The negative only carbides containing C 3 units that are stable Weiss constant of G = -22(1) K suggests antiferro in air. All other C 3 containing carbides [3 - 8] hy magnetic order below 2 K. drolyze readily in the presence of moisture. The The electrical resistivities of small ( ca. 0.3 mm di stability of the Sc 5Re2C7 type carbides may be at ameter) polycrystalline samples of Lu 5Re2C7, iso tributed to the transition metal content of these com lated from an arc-melted, annealed, crushed button, pounds. The ingots of Yb 2ReC2 decompose com were determined with a four-probe technique [15] pletely in moist air within a few days. between 300 and 5 K. Four copper filaments were Magnetic susceptibilities of Lu 5Re2C7 and glued to the sample using a silver epoxy cement. Yb 2ReC2 were determined with a SQUID magne The specific resistivities increased with decreasing tometer (MPMS, Quantum Design) between 2 and temperature for all samples as is typical for semi 300 K with magnetic flux densities between 1 and conductors. Activation energies calculated from the 5 T. The susceptibilities of Lu 5Re2C7 were small, steepest portions of the In p vs. 1/T plots (an exam slightly temperature and field dependent over the ple is shown in Fig. 2) varied between 0.006 and whole temperature range, indicating a small amount 0.03 eV. These values are very small and therefore R. Pöttgen et al. ■ Rare Earth Metal Rhenium Carbides 233

-— T[K] Table II. Crystal data for LugRe 2C7. 100 50 25 10 Formula mass 1331.33 Space group Cmmm (No. 65) Unit cell dimensions see Table I Formula units per cell Z= 2 Calculated density 11.84 g/cm3 Crystal size 25 x 25 x 50 pm3 Absorption correction from ^-scan data Transmission ratio 1.74 (max:min) 0/26 scans up to 26 = 75° Range in hkl ±16, ±29, ±6 Total no. reflections 3918 Independent intensities 592 (fin, = 0.119) Intensities with I > 2

Table IV. Interatomic distances in the structure of Lu.«iRe2C7. All distances shorter than 400 pm (Lu-Lu. Lu-Re, Re-Re), 385 pm (Lu-C, Re-C), and 265 pm (C-C) are listed. Standard deviations are all equal or less than 0.1 pm for the metal-metal distances and 1.9 pm for the metal-carbon distances as well as for the C2-C4 distance.

L ul: 1 Cl 232.4 Re: 2 C3 203.6 2 C3 240.9 2 Cl 212.4 2 C2 249.7 1 Lu2 306.5 2 C4 297.0 4 Lul 310.9 2 Re 310.9 4 Lul 319.8 2 Re 319.8 2 Re 332.8 1 Lul 327.1 Cl: 2 Re 212.4 2 Lul 332.8 2 Lul 232.4 2 Lu2 339.6 2 Lu2 241.2 1 Lul 348.3 C2: 1 C4 133.3 1 Lul 367.5 4 Lul 249.8 Lu2: 4 Cl 241.2 1 Lu2 262.4 2 C2 262.4 C3: 2 Re 203.6 2 Re 306.5 4 Lul 240.9 2 Lu2 332.8 C4: 2 C2 133.3 8 Lul 339.6 8 Lul 297.0 first carbides which are isotypic with the recently Fig. 3. The ScsRe 2C7 type structure of Lu 5Re2C7. Atoms [2] determined structure of ScsR^Cy. The lattice connected by thick (z = 1/2) and thin lines (z = 0) are constants decrease from the erbium to the lutetium situated on mirror planes. These lines do not necessar compound in agreement with the lanthanoid con ily indicate (strong) chemical bonds. In the lower part of the figure the Lu4Re 2 octahedra around the Cl (heav traction. Sc?Re 2C7 has by far the smallest cell di ily shaded) and C3 atoms (lightly shaded) are outlined. mensions of these carbides. The centrosymmetric propadienyl units formed by the C2 The compositions of the carbides Ln^ResQs (2 x ) and C4 atoms are also shown. with Ln = Y, La-Nd, Gd-Er [1] are very sim ilar to those of the presently described ones: while the C3 units have only lanthanoid neighbors 37.5:15.6:46.9 for Ln nResCis as compared to (Fig. 3). 35.7:14.3:50 for LnsRe 2C7. While the Ln^ResC^ Various aspects of chemical bonding in these car carbides are formed with the large rare earth atoms, bides have been discussed before and in particular the carbides LnsRe2C7 occur with the small lan- it was emphasized that the electronic environment thanoids. Both structures were found for erbium. of the rhenium atoms in the various polyanions Er^ResCis is observed directely in the arc-melted is compatible with the 18-electron rule [ 1, 2], as samples and also after annealing at 1000 °C, was shown already earlier for the series Ln 2ReC2 whereas Er5Re2C7 is formed only after the anneal [11]. Here we turn our attention to the short Lu- ing at 800 °C. Even though the compositions of the Lu distances in Lu5Re2C7. In a first approximation two series are similar their structures differ consid of chemical bonding in such ternary carbides one erably. The carbides Ln^ResQs contain C j pairs usually assumes that the rare earth atoms as the and isolated carbon atoms, while C 3 units and iso most electropositive components have transferred lated carbon atoms are found in the ScsRe 2C7 type their valence electrons to the transition metal carbon structure. The carbides Ln^ResCis contain two dif polyanion. This is certainly an oversimplification at ferent trigonal-planar polyanions of the composi least for a compound like LusRe 2C7. The metal to tions ReC 3 and ReC3(C2)3, which are separated carbon ratio in that carbide is 1:1, the same as for from each other by the lanthanoid atoms. In contrast, HfC. The latter compound is one of the most sta the rhenium and the isolated carbon atoms of the ble compounds known with a melting point close carbides Ln 5Re2C7 form two-dimensionally infinite to 4000 °C [17]. It is one of the “interstitial” car polyanionic sheets of the composition (Re 2C4)n, bides crystallizing with NaCl structure, and it is R. Pöttgen et al. • Rare Earth Metal Rhenium Carbides 235 well known that the high stability of these tech nically important carbides is due to metal-metal as well as metal-carbon bonding [18,19]. In HfC each hafnium atom has 12 hafnium neighbors at a distance of 327 pm (as calculated from the lat tice constant [20]). This distance is the same as the shortest Lu-Lu distance in LusF^C?, and for that reason we believe that the numerous Lu-Lu interactions in LusR^Cy, covering the range be tween 327 and 367 pm (Table IV) contribute to the stability of this compound. This of course is only possible if the lutetium atoms have not completely transferred their valence electrons to the rhenium Fig. 4. Cell volumes of PriReCa type carbides. carbon polyanion. Thus, the electron count for the rhenium-carbon polyanionic sheets is lower than are edge-shared via the lutetium atoms. These what is suggested by the formal electron count two-dimensionally infinite sheets of condensed (Lu3+)5(Re2C4)- 1I(C3)~4. A lower electron count C3Lu4Re2 octahedra are connected to each other on the polyanion in Er 2ReC2 may also resolve the via the ClLu 4Re2 octahedra by sharing common puzzle that the highest occupied electronic states edges. In total, the C 3Lu4Re2 octahedra are linked were found by extended Hiickel calculations to be by two vertices and 6 edges, while the ClLu 4Re2 antibonding [ 2 1]. octahedra are linked via 7 edges. The C-C bond length of 133(2) pm in the cen- The new carbide Yb 2ReC2 crystallizes with the trosymmetric C 3 units of LusRe 2C7 is very simi orthorhombic Pr 2ReC2 type structure [ 11 ] of space lar to those in Sc5 Re2 C7 (134.4(9) pm [2]), SC3 C4 group Pnma. This structure type contains only (134.2(3) pm [3]), Mg 2C3 (133.2(2) pm [5]), a- isolated carbon atoms. The rhenium and carbon H04C7 (127(3) to 138(3) pm [ 6]), /?-Ho 4C7 (130(3) atoms form one-dimensionally infinite (ReC^n and 131(3) pm [7]), LU4 C7 (132.1(10) and 134.9(9) polyanions which are separated by the lanthanoid pm [7]), and Ca 3C3Cl2 (134.6(4) pm [ 8]). All of atoms [11,23], and as already discussed above, these C3 units may be considered as propadienyl the real electron count for the polyanion may be species. The C-C distance in propadiene was found lower than 6. The cell volume of Yb 2ReC2 fits to be 131.1 ( 6) pm by electron diffraction [ 22] and smoothly between the cell volumes of the thulium 133.5 pm by IR spectroscopy [23]. The different and the lutetium compound (Fig. 4) indicating triva coordination spheres of the C 3 units have been com lent ytterbium. This is in perfect agreement with the pared earlier [7]. results of the susceptibility measurements discussed The atoms Cl and C3 in the structure of above. Lu5Re2C7 may be called “isolated” carbon atoms, since they are not bonded to any other carbon Acknowledgments atom, although they are part of the rhenium-carbon We thank Dr. M. H. Möller and Dipl.-Ing. U. Rodewald polyanion. They are both surrounded by slightly for the collection of the four-circle diffractometer data and distorted Lu 4Re2 octahedra (Fig. 3). While the rhe Mrs J. Nowitzki for assistance with the susceptibility mea nium atoms are at adjacent comers of the octahe surements. Dr. G. Höfer (Heraeus Quarzschmelze) and dron in the case of the C 1 atoms (c/s-configuration), Dr. H. G. Nadler (H. C. Starck Co.) supported our work they are at opposite comers for the C3 atoms ( trans- by generous gifts of silica tubes and rhenium powder. configuration). The C 3Lu4Re2 octahedra are vertex- We also acknowledge financial support by the Deutsche sharing via the rhenium atoms forming zig-zag Forschungsgemeinschaft and the Fonds der Chemischen chains along x. In the z direction these octahedra Industrie. 236 R. Pöttgen et al. • Rare Earth Metal Rhenium Carbides

[1] R. Pöttgen, G. Block, W. Jeitschko, R. K. Behrens, [14] M. Reehuis, M. E. Danebrock, J. Rodriguez- Z. Naturforsch. 49b, 1081 (1994). Carvajal, W. Jeitschko, N. Stüsser, R.-D. Hoffmann, [2] R. Pöttgen, W. Jeitschko, Z. Naturforsch. 47b, 358 J. Magn. Magn. Mater. 154, 355 (1996). (1992). [15] L. J. Van der Pauw, Phil. Res. Repts. 13, 1 (1958). [3] R. Pöttgen, W. Jeitschko, Inorg. Chem. 30, 427 [16] G. M. Sheldrick, SHELXL-93, Program for Crys (1991). tal Structure Refinement, University of Göttingen [4] Th. Vomhof, R. Pöttgen, W. Jeitschko, J. Less- (1993). Common Met. 171,95 (1991). [17] E. K. Storms, MTP International Review of Sci [5] H. Fjellväg, P. Karen, Inorg. Chem. 3 1 ,3260 ( 1992). ence, Inorganic Chemistry, Series 1, Vol. 10, But- [6] H. Mattausch, T. Gulden, R. K. Kremer, J. Horakh, terworths, London (1971). A. Simon, Z. Naturforsch. 49b, 1439 (1994). [18] A. Neckel, K. Schwarz, R. Eibler, P. Weinberger, [7] R. Czekalla, W. Jeitschko, R.-D. Hoffmann, H. P. Rastl, Ber. Bunsenges. Phys. Chem. 79, 1053 Rabeneck, Z. Naturforsch. 51b, 646 (1996). (1975). [8] H.-J. Meyer, Z. Anorg. Allg. Chem. 593, 185 (1 99 1). [19] S. D. Wijeyesekera, R. Hoffmann, Organometallics [9] R. Hoffmann, H.-J. Meyer, Z. Anorg. Allg. Chem. 3,949 (1984). 607,57(1992). [20] P. Villars, L. D. Calvert, Pearson’s Handbook of [10] K. H. Wachtmann, R. Pöttgen, W. Jeitschko, Z. Crystallographic Data for Intermetallic Phases; The Kristallogr. Suppl. 9, 238 (1995). Materials Information Society, Materials Park, Ohio [11] W. Jeitschko, G. Block, G. E. Kahnert, R. K. (1991). Behrens, J. Solid State Chem. 89, 191 (1990). [21] H. Deng, R. Hoffmann, Inorg. Chem. 32, 1991 [12] K. Yvon, W. Jeitschko, E. Parthe, J. Appl. Crystal (1993). logr. 10, 73 (1977). [22] A. Almenningen, O. Bastiansen, M. Traetteberg, [13] M. Reehuis, J. Rodriguez-Carvajal, M. E. Dane Acta Chem. Scand. 13, 1699 (1959). brock, W. Jeitschko, J. Magn. Magn. Mater. 151, [23] R. C. Lord, Putcha Venkateswarlu, J. Chem. Phys. 273 (1995). 20,1237 (1952).