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Rapid Synthesis of Alkali-Metal Fullerides Using a Microwave-Induced Argon Plasma

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Rapid Synthesis of Alkali-Metal Fullerides Using a Microwave-Induced Argon Plasma 394 Chem. Mater. 1996, 8, 394-400 Rapid Synthesis of Alkali-Metal Fullerides Using a Microwave-Induced Argon Plasma R. E. Douthwaite,† M. L. H. Green, and M. J. Rosseinsky* Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR U.K. Received July 12, 1995. Revised Manuscript Received December 4, 1995X The rapid synthesis of alkali metal fullerides using a microwave-induced argon plasma (MIAP) is reported. Reaction times are of the order of seconds using a MIAP, whereas conventional synthesis requires times of the order of several days. Potassium, rubidium, and cesium fullerides are more readily synthesized by this technique than sodium fullerides. Investigation of sodium fulleride synthesis via both conventional and microwave routes, combined with 13C MAS NMR spectroscopy and powder X-ray Rietveld analysis, gives an improved understanding of the sodium-C60 phase diagram. Introduction Reaction temperatures of less than 700 °C are generally required to avoid thermal decomposition of the C60 1 Since the discovery of superconductivity in K3C60, molecule, especially as the antibonding t1u orbitals are alkali-metal fullerides have been the subject of much populated in the fulleride products. The temperature attention. Synthesis is typically achieved by direct is typically increased to 4-500 °C over a period of combination of stoichiometric quantities of alkali metal several days and held there for 1-2 weeks, with several and C60 or by dilution of the saturated alkali metal intermittent regrindings required to produce single- 2 A6C60 phase (A ) K, Rb, and Cs) with C60. A number phase products. of solution routes for the synthesis of K3C60 and Rb3C60 There are two processes that cause the time required 3-6 have also been developed. Sodium C60 intercalate for synthesis of alkali-metal fullerides to be on the order phases extend beyond the composition Na6C60,asso- of days; mass transport of alkali-metal atoms through dium clusters can be accommodated on the octahedral the vapor phase to the surface of the C60 solid and site of the fcc C60 array. A composition Na9.8C60 has diffusion of alkali-metal cations throughout the bulk of 7 been structurally characterized. Sodium C60 interca- aC60 particle. Solution methods circumvent mass lates have been synthesized using a number of sodium transport and diffusion of the alkali metal by mixing 7,8 precursors such as sodium metal, Na5Hg2, NaH, and the constituent components on the molecular level in a 8 9 NaBH4 or by reaction of Na9C60 with C60. No binary precursor. As the solvent required is generally a lithium C60 intercalates have to date been structurally coordinating, polar one such as ammonia or THF, the characterized. precursor will contain solvated cations. To remove As is common with solid-state synthesis, reaction coordinating or occluded solvent and prepare materials times are long, on the order of several days to weeks. of crystallinity comparable to those produced by solid- state synthesis, annealing at temperatures comparable † Current address: Box 241, Baker Laboratory, Department of to those required for conventional thermal synthesis is Chemistry, Cornell University, Ithaca, NY 14853. then needed. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. Microwave techniques have been used for synthesis W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. in reaction media ranging from the gas phase and Nature 1991, 350, 600-601. homogeneous solution to the solid state.10 The vast (2) Murphy, D. W.; Rosseinsky, M. J.; Fleming, R. M.; Tycko, R.; Ramirez, A. P.; Haddon, R. C.; Siegrist, T.; Dabbagh, G.; Tully, J. C.; majority of these reactions are effectively superheated Walstedt, R. E. J. Phys. Chem. Solids 1992, 53, 1321-1332. via dielectric loss. This requires that at least one of the (3) Wang, H. H.; Kini, A. M.; Savall, B. M.; Carlson, K. D.; Williams, J. M.; Lathrop, M. W.; Lykke, K. R.; Parker, D. H.; Wurz, P.; Pellin, reactants absorbs microwave radiation at the applied M. J.; Gruen, D. M.; Welp, U.; Kwok, W.-K.; Fleshler, S.; Crabtree, G. frequency (2.45 GHz in this case). In the solid state, if W.; Schriber, J. E.; Overmyer, D. L. Inorg. Chem. 1991, 30, 2962- none of the reaction components absorb microwave 2964. (4) Wang, H. H.; Kini, A. M.; Savall, B. M.; Carlson, K. D.; Williams, radiation at the applied frequency, then an absorber J. M.; Lykke, K. R.; Parker, D. H.; Wurz, P.; Pellin, M. J.; Gruen, D. such as graphite can be added to mediate energy M.; Welp, U.; Kwok, W.-K.; Fleshler, S.; Crabtree, G. W. Inorg. Chem. transfer.10 Separation of the mediator from the reaction 1991, 30, 2838-2840. (5) Ziebarth, R. P.; Buffinger, D. R.; Stenger, V. A.; Recchia, C.; product is then required. Pennington, C. H. J. Am. Chem. Soc. 1993, 115, 9267-9270. Although dielectric heating is the most familiar action (6) Liu, X.; Wan, W. C.; Owens, S. M.; Broderick, W. E. J. Am. Chem. Soc. 1994, 116, 5489-5490. of microwaves on a chemical reaction mixture, micro- (7) Yildirim, T.; Zhou, O.; Fischer, J. E.; Bykovetz, N.; Strongin, R. waves can also induce plasmas in gases. Microwave A.; Cichy, M. A.; III, A. B. S.; Lin, C. L.; Jelinek, R. Nature 1992, 360, induced plasmas have been investigated for over 45 568-570. 11 (8) Rosseinsky, M. J.; Murphy, D. W.; Fleming, R. M.; Tycko, R.; years. They have been applied to thin-film technology Ramirez, A. P.; Siegrist, T.; Dabbagh, G.; Barrett, S. E. Nature 1992, in the form of microwave plasma chemical vapor 356, 416-418. (9) Yildirim, T.; Fischer, J. E.; Harris, A. B.; Stephens, P. W.; Liu, D.; Brard, L.; Strongin, R. M.; Smith, A. B. Phys. Rev. Lett. 1993, 71, (10) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. Rev. 1991, 20, 1383-1386. 1-47. 0897-4756/96/2808-0394$12.00/0 © 1996 American Chemical Society Synthesis of Alkali-Metal Fullerides Chem. Mater., Vol. 8, No. 2, 1996 395 deposition12-15 and in the generation of species in situ by sublimation of material purified by column chromatography for deposition, e.g., arsine from metallic arsenic and under a dynamic vacuum of 10-3 Torr at 500 °C. dihydrogen.13 Microwave plasma-induced sintering16-19 Powder X-ray Diffraction. Samples were contained in has also been demonstrated for ceramics where tem- 0.5 mm diameter capillaries sealed under helium. Data were collected in transmission geometry on a Siemens D5000 peratures in excess of 1600 °C have been readily instrument with a 6° linear position-sensitive detector and Cu achieved using a range of gases at various pressures. KR1 radiation from a germanium (111) incident beam mono- Diatomics produce higher temperatures than mona- chromator. Rietveld refinement was performed on data col- tomic gases due to release of the association enthalpy lected in 0.02° steps using the GSAS software;23 the back- on the solid surface as species dissociated in the gas ground was fitted using a 10-term power series expansion and phase recombine there. The decomposition of cobalt the peak shape described with a pseudo-Voigt function. Solid-State NMR. Samples were contained in a KEL-F carbonyl in a series of zeolites has also been reported 13 20 insert withina7mmzirconia rotor and spun at ca. 3 kHz. C to yield cobalt clusters within the zeolitic cavities. MAS NMR spectra were recorded at 50.32 MHz on a Brucker We recently described the preparation of K3C60 using MSL200 spectrometer using an external reference on ada- a microwave-induced argon plasma (MIAP).21 In this mantane. Integration was performed by the cut-and-weigh paper we describe in detail the rapid synthesis of a method or electronically. variety of alkali-metal fullerides using a MIAP and Magnetic Measurements. A 2.6 mg sample of K3C60 compare the resulting fullerides with those prepared prepared using microwave techniques was flame sealed in a 2 mm quartz capillary under helium. Dc static susceptibility using conventional thermal routes. We demonstrate measurements were performed using a Cryogenic Consultants that a MIAP can be generated in a common household S600C SQUID magnetometer. Zero field (ZFC) and field (FC) microwave oven and used to vaporize alkali metals and cooled magnetization measurements were made from 6 to 40 produce rapid diffusion of alkali-metal cations in solid K in a 10 Oe field. C60. Synthesis of the superconducting fulleride phases Sample Preparation. All samples were manipulated in is achieved both by direct reaction of the alkali metal a helium-filled MBraun Labmaster drybox, with the helium purified over molecular sieves and a hot copper catalyst. and C60 followed by composition adjustment and an- nealing and by plasma-induced reaction of C with Argon was introduced using standard Schlenk techniques with 60 dual argon and vacuum manifolds. Pressures of less than 10-3 A6C60 fullerides. The observation of rapid synthesis Torr were obtained using a turbomolecular pump. Samples induced by the argon plasma is novel and may find for comparison with those prepared using microwave tech- wider application in solid-state synthesis. We also niques were prepared by conventional thermal methods de- 2 report new information on the NaxC60 phase diagram scribed in the literature. resulting from combined 13C magic angle spinning Conventional Synthesis. Sample sizes of between 30 and (MAS) NMR spectroscopy and powder X-ray diffraction 50 mg were prepared. AxC60 phases (x ) 1 for Rb and Cs; x ) characterization of solids prepared by microwave and 3 for K and Rb; x ) 4 for K, Rb, and Cs) were prepared from the saturated A6+xC60 phase and an appropriate quantity of thermal routes.
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