J. Phys. Chem. 1993,97, 8690-8691
8690
Synthesis, Characterization, and Stability of C& Th. &mer* and H.Zabel Fakultiit fiir Physik und Astronomie, Ruhruniversitiit Bochum. 0-44780 Bochum, FRG Received: June 8, 1993; In Final Form: July 12, 1993'
We report the synthesis and X-ray analysis of a compound of C60 with atomic iodine. A crystal structure with C& stoichiometry for this compound is proposed. The time evolution over 3 months is also investigated, showing the relaxation of the compound into its initial fcc structure. Superconductivity was not observed down to 4.2 K.
Since the discovery of an efficient production method for Cm by Krgtschmar et al., the study of Cm has made rapid progress.1.2 Especially the discovery of superconductivityin alkali-doped Cm a short time later stimulated the interest of many scientistsin this field.3-5 UPS and EELS measurements showed that there is a charge transfer of the s-electron of the alkali-metal dopant into the LUMO orbital of the Cm molecule.6 The dopants are placed on the interstitial sites, and the fcc structure of the Cm host is preserved up to a stoichiometry of AsCm (where A is an alkali metal) as shown by X-ray diffraction This class of donor-type intercalation compounds is now well established. It is therefore of special interest to investigate acceptor-typedopants, which seems to be quite difficult, because of the high ionization potential of 7.6 eV for C M . ~ On the other hand, there are other polycyclic aromatic hydrocarbons with comparable ionization potentials, such as pyrene (7.41 eV) or perylene (7.00 eV), which act as an electrondonor in compounds with iodine.lO In this paper we report the synthesisof a compound of Cm and iodine. The Cm was produced and purified by using standard techniques. It was then sublimedon a glass substrateand vacuumsealed in a quartz tube (see inset in Figure la) with a large excess of iodine. Subsequently,it wasplaced intoa darkoven and heated to 130 OC for about 20 h. Figure la shows the X-ray structure of the initial Cm sample and Figure 1b the X-ray result after 20-h exposure. Except for the loss of crystallinity, which may be due to a surface reaction of I2 with C a r no structural change appears to have occurred. We propose that no new compound is formed under these conditions. In a second run we irradiated the sample with Hg light during the reaction and left all other parameters as before. After 20 h the X-ray profile changed drastically, as shown in Figure IC. The most likely reason for this change is the dissociation of the iodine molecule I2 into two I-radicals by irradiation with Hg light, for which a threshold energy of about 2.5 eV is required. The reaction may therefore be described as a reaction with atomic iodine. The profile can be indexed as simple hexagonal with lattice parameters a = 9.93 A, c = 9.91 A, and y = 120°, which is compatible with the structure described elsewhere.'*J2 The in-plane structure of the hexagonal basal plane remains unchanged as compared to the (111) plane of the initial fcc structure. The layer spacing of the (OOOl)h, planes of 9.91 A Aktract published in Advance ACS Absrracrs. August 15, 1993.
550 -
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Iodine
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2Wegl Figure 1. X-rayprofileof(a) theinitialC~sampiertndafter 20-hexposurc to iodine (b) without and (c) with irradiation by Hg light.
is, however, much larger than the spacing of the initial (1 1l)rw planes. It seems therefore reasonable to assume that the closedpacked planes of the fcc structure are preserved and that the iodine atoms are placed between these planes, causing the increase of the interlayer separation and the packing transforms from ABCABC to A/A/A, similar to graphite intercalation compounds. For the intensityfit we treated the Cm as hollow spheres of radius 3.52 A. The best fit was obtained, as shown in Figure IC, for a CmI2 stoichiometry, with the two iodine atoms positions at (0.641, 0.308, 0.467) and (0.358, 0.692, 0.533), which are close to the points of high symmetry ( 2 1 3 , lI3, l/2) and (lI3,2 1 3 , 112) in the middle of the prism constructed by six Cm molecules. The proposed structure is shown in Figure 2. This structure is significantlydifferent from the one originally proposed in ref 11 but agrees basically with the structure reported in ref 12. The main difference with the latter work is the fact that iodine requires
0022-365419312097-8690$04.00/0 0 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8691
I @lo 1
a
Upon leaving the sample in air a new Bragg peak appears at 22.7O. This peak can be indexed neither as a 12 peak nor as a CaI2 peak, bemuse the width is too small compared to all other peaks of the Car, structure. This might also be taken as proof that the compound described here is not a compound with molecular iodine. In the latter case one would expect that, after decomposition,a Bragg peak related to Izstructurewould appear. In Figure 3a,b X-ray scans taken during the evolution of the structure over 3 months are reproduced. At the end (Figure 3b) the structure is relaxed into its initial fcc structure albeit with much reduced crystallinity. Measurementsof the susceptibilityshowed no superconducting phase transition down to 4 K,in agreement with refs 11 and 12.
Refereaces a d Notes (1) KrHtscbmer, W.; FoStiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167.
Figure 2. Pro@
C&
unit cell with c axis (a) in the plane of paper and (b) perpendicular to it.
(2) Haufler, R. E.;Conceicao, J.; Chibante, L. P.F.; Cbai, Y.;Byme, N. E.;Flanagan, S.;Haley, M.M.;O’Brien, S. C.; Pan, C.; Xiao, Z.;Billups, W. E.;Ciufolini, M.A.; Hauge, R. H.; Margrave,J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E.J. Phys. Chem. 1990, 91, 8634. (3) Haddon, R. C.; Hebard, A. F.; Rosseinsky, M.J.; Murpby, D. W.; Duclos, S.J.; Lyons, K. B.; Miller, B,; Rosamilia, J. M.;Hemming, R. M.; Kortan, A. R.; Glarum, S. H.;Makbija, A. V.; Muller, A. J.; Eick, R. H.; Zahurak, S. M.;Tycko, R.; Dabbagh, G.; Thiel, F. A. Nurure 1991, 350, 320.
(4) Hebbard, A. F.; R d n s k y , M.J.; Haddon, R.C.; Murphy, D. W.; Glarum, S.H.;Palstra, T.T.M.;Ramirez,A. P.;Kortan, A. R. Nutwe 1991, 350,600. (5) Tanigald, K.; Ebb”, T.W.; Saito, S.;Mizulri, J.; Tsai, J. S.;Kubo, Y.; Kuroshima, S. Nuture 1991, 352,222. (6) Fink, J.; Sobmen, E,Phys. Bl. 1992,48, 11. (7) Flemming, R. M.;Rosseinsky, M.J.; Rami- A. P.;Murpby, D. W.; Tully, J. C.; Haddon, R. C.; Sicgrist, T.;Tycko, R.; Glarum, S. H.; Marsh, P.; Dabbagh,G.; Zaburak,S. M.;Makbija, A. V.; Hampton, C. Nutwe 1991,352,701. (8) Stephens, P. W.; Mibaly, L.; Lee, P. L.;Wbcttcn, R. L.; Huang, S.-M.; Kaner, R.; Deiderich, F.; Holnet, K. Nutwe 1991,351,632.
28F
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28 [degl Figure3. Bragg scan of the C& 27 days and (b) 95 days.
sample after leaving it in air for (a)
photodissociationbefore reaction with Ca. We thereforebelieve that iodine occurs in atomic rather than molecular form in the hexagonal compound.
(9) Licbtenbcrgcr, D.L.;Nebamy, K. W.; Ray, C. D.; Huffmann,D. R.; Lamb, L. D.Chem. Phys. Lett. 1991, 176*203. (IO) Akahama, Y.; Kobayashi, M.;Kawamura, H.;Shinohara, H.;%to, H.; Saito, H. Solid Stare Commun. 1992,82,605.
(1 1) Kobayashi, M.;Akabama, Y.;Kawamura, H.; Shinobara,H.; Sato, H.; Saito, H. Solid State Commun. 19!&2,81,93. (12) Zbu, Q.;Cox, D.E.;Fiiber, J. E.;K n i i K.; McGbie, A. R.; zhou, 0.Nature 1992,355,712.
