Superconductivity in potassium intercalated Ba3 C60

ELSEVIER

Synthetic Metals 86 (1997) 2309-2310

Superconductivity

in potassium intercalated B5gC,

Y Iwasa, H. Hayashi, T Furudate, M. Kawaguchi, andT Mitani JapanAdvanced

Institute of Science and Technology,

Tatsunokuchi,

Ishikawa 923-12,

Japan

Abstract Here we report a new type of superconductor synthesized by intercalation of potassium into Ba,C,. Bulk superconductivity with the onset temperature of 5.6K was observed in K,Ba,C,. This compound forms a body centered cubic lattice which is regarded as a solid solution of isostructural K,C, and Ba,$,,. A naive electron counting indicates that K$a& is a half-filled metal in which the Fermi energy exists at the center of the next lowest unoccupied molecular orbital with the t,, symmetry This electronic state is in a sharp contrast with that of K,C, which has a half-filled state of the t,,-derived band. The present observation indicates that the half-filled states of the triply degenerate bands yield superconductivity. Keywords: Fullerene. Superconductivity, Multinary

complexfullerides

1. Intmduction Superconductivity of fullerides has been first discovered in alkali-intercalated C,, at the (C,)‘- reduction state [ 1.21. The electronic state of these superconductors corresponds to the half-tilled state of the conduction band, which is formed by the triply degenerate lowest occupied molecular orbital (LUMO) with t,, symmetry. Although various kinds of alkali C,, compounds with different valence states have been synthesized, tsuperconductivity is observed only near the half-filled state. These properties suggest that the half-filled state of the triply degenerate state is crucial for the superconductivity Meanwhile, superconductivity was found in higher reduction states in alkali-earth [3-51 or rare-earth metal [6] intercalated C,,. Particularly, BagSso and Sr,C,, has an extremely highly reduced states according to a simple electron counting. In these compounds, the t,,-derived band is completely filled and the next LUMO-derived band &,-band) starts to be filled, as was confirmed by an electrical resistivity and photoemission spectra. These materials made the situation very confusing: While the superconductivity is restricted at the half-filled state on the t,,-band, the upper t,,-band seems to yield superconductors at different valence states. In other words, the electronic and structural criteria for the superconductivity is an unsolved issue, although the superconductivity itself is well explained by the weak-coupling BCS theory once it appears [7]. To investigate the relation between the molecular valence of C, and superconductivity, synthesis of new fullerides compounds with highly reduced states is still an exciting issue. One of the most fundamental questions is whether the half-filled state of the next LUMO (t,,-band) is superconducting or not, since superconductivity in A& is realized only at the half-filled state of the t,,-band. The half-filled state of the t,,-band is not obtained only by intercalation of alkali earth metals, since the alkali earth metals is divalent in compounds. Thus we tried ternary compounds with the composition of KBa,C,. Simple electron counting predicts that the number of electrons transferred to C, is nine, corresponding to the half-filled state of the $,-band. The combination of K and Ba is chosen since the ionic radii of K and Ba are almost the same. Another advantage of this combination is that BalCGois an extremely stable compound that is easy to make. Ba,C, has an Al5 structure [8] in which the configuration of C, 0379-6779/97/%17.00

8 1997 Elsevier Science S.A. All rights reserved

PII SO379-6779(96)04848-5

molecule is bee-like and the half of the bee interstitial site is empy. This structural feature suggests a possibility of intercalation of potassium into the vacant interstitial site. Here we report our successful synthesis of K,Ba,C, which becomes superconducting at the onset temperature of 5.6K. The present finding indicates that the half-filled state of both t,, and t,, derived bands produces superconductors.

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Temperature (K) Fig. 2. Temperature dependence of the magnetic susceptibility of nominal K,,sBqC, 2. Experimental Starting Ba,C, powders have been synthesized by a direct reaction of C, and Ba powders in a quartz tube evacuated to 2 x 10e6torr Three to six days reaction yielded single phase Ba,C, powders. Intercalation of potassium metal was made in a similar manner to the case of intercalation of K into pure C,. A piece of K powder and Ba$, powders are loaded in a sealed glass tube at a high vacuum. Potassium was vaporized and reacted with Ba,C, powders at about 260°C. After a three days reaction, we obtained potassium intercalated BaJ, materials. All these preparation was made under controlled atmosphere of Ar with oxygen and water concentration less than a few ppm. X-ray diffractogram was recorded using a 12kW MO-Kcr rotating anode source. Rietveld refinement was carried out using a RIETAN package (F. Izumi). Magnetization was measured by a commercial SQUID magnetometer 3. Results and LXsc~&ons Figure 1 shows the x-ray diffraction pattern of K,Ba,C,, where x is the nominal composition of potassium. At x=0, we see a single phase diffractogram of B%C,. When x is smaller than 3, the difraction pattern is more or less similar to the pure Ba,C,. Above x=3, we found a drastic change in the pattern which does not show significant change up to x=6. The pattern above x=3 is very similar to that of bee K,C,, or BaJ, phase. All the peaks are indexed as bee with the lattice parameter of a=11.24+O.OlA. This value is slightly smaller than that of the average of the lattice parameters for K,C,, (11.39A) and BaJJ,, (11.17A). These features strongly indicate that the obtained phase is a solid solution of K,C, and Ba,C,. Although the essential feature of the pattern is maintained up to x=6, the peaks are broadened in the case of large x and we sometimes observed the small peak from potassium metal in the sample of latge x. These result suggest that the phase obtained at x>3 is a saturated phase of Kintercalated Ba,C,. We have carried out a Rietveld refinement on the sample of nominal K concentration of x=3.5. In the refinement. we assumed that C, molecules have equal C-C bond lengths and carbon positions are allowed to move only radially to preserve the shape of the C, molecules. The structural model is the same as the bee I<6CsoorBa&, with the space group of Im3. The refinement conveaed very auicklv to R-=6.40%, R=4.86%. and S=1.390 at the foilowing coordmates?Cl at d.0647, 0.0, 0.3112; C2 at 0.1281, 0.1037, and 0.2716; C3 at 0.06407, 0.2074, 0.2319; and K and Ba at 0.5, 0.5, 0.2798, and at isotropic thermal factors of B=lS and 2.9 (A’) for C and Ba(K), respectively. Here the occupancies of K and Ba are random and fixed at 0.5. Wb have also tried a refinement of the composition x of K to the model &Ba,C, and KBa,,C,. Since both refinements did not show a

Fig. 3. Speculative electronic structure of K,Ba,C,. significant improvement from the model of KBa$,,, we conclude that the obtained phase is K3Ba$, above x=3. Magnetization was measured for several samples with different nominal composition x. While no superconductivity was observed in the host Ba$,, we observed superconductivity in all K-intercalated materials. Figure 2 shows temperature dependence of magnetization for the sample of nominal composition x=3.5, which showed the highest shielding fraction of about 30%. Superconductivity was observed for all K-intercalated materials with the same onset temperature of 5.6K, indicating that there is only one superconducting phase. Below x=3, the shielding fraction is smaller than lo%, while it jumps to about 30% at x>3.5. High shielding fraction is kept even at x=6, also indicating that the superconducting phase is a K-saturated phase. This result suggests that the superconducting phase is the bee phase having a chemical composition K,Ba,C,. At latger nominal concentration of K, samples are likely composed of K,Ba,C,, and excess potassium. Figure 3 shows a speculative electronic structure of K,Ba$,, with the position of the Fermi energy for KC,,, K,C,, and Ba&,. Simple electron counting indicates that K,Ba,C, is a halffilled metal with the Fermi energy in the t,,-derived band. Observation of superconductivity in K,Ba,C,, suggests that the half-filled state yields superconductors in the t,,-derived band as in the case of t,,-derived band. However T, of K&la&, is not quite high comparing to the half-filled metal in the t,,-band such as K,C,. This means that the pairing mechanism in the t,,-band is significantly different from that in the t,,-band. In fact we know that superconductivity appears also in CasC,, and Ba,C,. Particularly, in the latter compound, the simple electron counting predicts a semiconducting state, but in reality it is metallic and superconducting possibly due to a hybridization between C, and Ba orbitals. Superconductivity in several valence state in the t,,band indicates that the half-filled state is not so peculiar in the t,,band, in contrast with the very unique feature of the half-filled state of the t,,-band. This work has been supported by a Grant-in-Aid from the Ministry of Education, Sports, Science, and Culture, Japan.

References

[l] A. F. Hebard et al., Nature 350 (1991) 600. [2] K. Holczeret al., Science 252 (1991) 1154. I31 A. R. Kortan et al., Nnture 355 (1992) 529. [4] A. R. Kortan et al., Nature 360 (1992) 566. [5] A. R. Kortan et al., Chem.Phys. Lett. 233 (1994) 501. [6] E. Ozdas et al., Nature 375 (1995) 126. [7] A. P. Ramtrez, Superconductivity Review 1 (1994) 1. [8] A. R. Kortan, Phys. Rev. B47 (1993) 13070.