Preparation of C 60 Charge Transfer Complexes with Organic Donor Molecules and Alkali Doping
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crystal and electronic structures, we have been investigating the alkali doping to CT complexes of C60 with organic donor molecules. We have reported that, after potassium or rubidium doping under a mild condition to the 1:1:1 CT complex of OMTTF.C 60.benzene, it showed superconductivity at 17-18.8 K or 23-26 K, respectively, with its pristine crystal structure constructed by two-dimensional arrangement of C60 preserved [7]. As the starting material for alkali doping, new C60 complexes as well as the known ones have been prepared in this work. The donor molecules used are shown in Figure 1 [8]. The results of rubidium doping to the C60 S
S0S
s
O "->S(SS O1SX I
OMTTF •Te
Te,,,
HMTTeF
Xs-S>S-S -sS
S
S J
DBTTF SMeSee)eS SX ,SeMe
S
se s
s "SSS s BEDT-TTF .2 NMe OH Fc 2
TSeCj-TTF
Me2N
NMs2
OH
S
HQ TDAP EOET-TTF EDT-TTF Figure 1. The donor molecules used for the C60 complex preparation. 495 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
Fe F
complexes of TDAP, EOET-TTF, HMTTeF, BEDT-TTF, hydroquinone (HQ), ferrocene (Fc) are presented, and compared with those of the OMTTF complex. EXPERIMENT The CT complexes of C60 with TDAP [9] and EOET-TTF were prepared by direct mixing of benzene solutions of appropriate components followed by slow evaporation of benzene in the ambient atmosphere. The C60 CT complexes with HMTTeF [3], BEDT-TTF [2], HQ [1], ferrocene [4] were also prepared approximately according to the literature methods. Similar to the preparation of OMTTF.C 60.benzene, the excess amount of donor in the mixing ratio was preferred. The use of teflon (PFA) beaker was occasionally successful. The composition of the complexes were examined by elemental analyses. The CT bands in the solid state were measured in KBr disks. The rubidium doping to the CT complexes was carried out by the previously reported methodIn the seal-off procedure of the sample tubes, the base pressure near the oil diffusion pump was 1.5-5 xl O1torr. The sealed tubes were warmed in a horizontal furnace in which the source CT complex side was kept at 67'C and the rubidium side at 64'C. The sample tubes were kept in the furnace for from 24 hours to 53 days. A reference sample tube was prepared and warmed at the same time in which only C60 and rubidium were loaded. At some stages of doping period of each sample, the occurrence of superconductivity was checked by the magnetization measurements with a SQUID magnetometer (Quantum Design, MPMS-2). To minimize the contribution of the quartz sample holder, a piece of blank quartz tube was attached to the bottom of the sample tube. First the sample was cooled to 5 K (or 2 K) in the zero field (remanence field was less than ±2 mG), the field dependence of the magnetization was measured in the course of increasing the field from 0 G to 10 G. After the field of 10 G was applied at that temperature, the magnetization under 10 G was measured on heating to more than 30 K (ZFC). Then the magnetization was measured under the same magnetic field on cooling (FC). As we did not
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