Elsevier

Carbon

Volume 100, April 2016, Pages 641-646
Carbon

Fabrication of new superconducting materials, CaxK1−xCy (0 < x < 1)

https://doi.org/10.1016/j.carbon.2016.01.071Get rights and content

Abstract

Metal intercalation to graphite produces various types of superconductors. The highest superconducting transition temperature Tc (onset temperature, Tconset, of 11.5 K) was found in Ca intercalated graphite, denoted CaC6. Tconset increased up to 15.1 K at 7.5 GPa, implying a positive pressure dependence. However, no new metal-intercalated graphite superconductors with Tconset higher than 11.5 K at ambient pressure have so far been reported. To search for new graphite superconductors, we successfully synthesized binary-element-intercalated graphite, CaxK1−xCy. Their structure resembles that of KC8. Tc increased continuously with increasing x. Furthermore, the pressure dependence of Tc in Ca0.6K0.4C8 was investigated over a wide pressure range from 0–43 GPa. Tc (= 9.6 K at 0 GPa) increased to 11.6 K at 3.3 GPa, and decreased to 2.0 K at 41 GPa. This behavior is similar to that of CaC6, albeit with a lower maximum Tc. X-ray diffraction patterns were measured under high pressures of 0–24 GPa, and suggest a structural transition at 15 GPa. Evidence is given for superconducting graphite involving binary metal intercalation.

Introduction

Superconductivity in Ca-intercalated graphite (CaC6) has generated much interest for its superconducting transition temperature Tc, which is greater than in other metal-doped graphites. The onset superconducting transition temperature Tconset is 11.5 K for CaC6 [1], [2], compared to 136 mK for KC8 and 25 mK for RbC8 [3], [4], [5]. After the discovery of superconducting CaC6 in 2005, new graphite superconductors YbC6 (Tconset = 6.5 K [2]), SrC6 (Tconset = 1.65 K [6]), and Li3Ca2C6 (Tconset = 11.15 K [7]) were successfully synthesized and characterized. The superconductor BaC6 (Tc = 65 mK) was also discovered very recently [8].

The pressure dependence of superconductivity in CaC6 was investigated at pressures ranging from 0 to 16 GPa [9]. Tconset reached 15.1 K at 7.5 GPa and decreased rapidly above 8 GPa. Such a pressure dependence in Tc was also observed for other metal-doped graphite superconductors. The maximum Tconset values were 1.7 K (1.5 GPa [10]) and 7.1 K (1.8 GPa [11]) for KC8 and YbC6, respectively. Thus, a positive pressure dependence is characteristic of graphite superconductors. In other words, metal-doped graphite is one of exotic superconductors. In the case of CaC6, the increase in Tc reportedly originates from the softening of in-plane Casingle bondCa phonons [9], [12] and the rapid decrease in Tc is attributed to the order-disorder transition relating to a large softening of lattice under pressure [13].

The Tconset = 11.5 K observed in CaC6 remains the highest value reported to date. To beat this record, we tried to synthesize CaxK1−xCy, in which two metal elements are intercalated in the graphite. Here, we report on these new superconducting CaxK1−xCy compounds, synthesized by immersing highly oriented pyrolytic graphite (HOPG) in molten Li/K/Ca alloy. The superconducting CaxK1−xCy thus synthesized showed an increase in Tc from 0.136–11.2 K with increasing x, confirming the successful binary intercalation of Ca and K in the graphite. X-ray diffraction (XRD) patterns of CaxK1−xCy (x ≠ 1) suggest a KC8-type structure (face-centered orthorhombic, space group No. 70, Fddd [14]). In the present study, we applied pressures ranging from 0 to 43 GPa to the Ca0.6K0.4C8. The magnetic susceptibility (χg in cm3 g−1) of Ca0.6K0.4C8 at ambient pressure (0 GPa) showed clear superconducting behavior (Tc = 9.6 K, shielding fraction = 90% at 2.5 K). Tc increased to 11.6 K at 3.3 GPa as the pressure was increased, and subsequently decreased at still higher pressure. Tc was 4.0 K at 16 GPa and slowly decreased with pressure increasing to 43 GPa. The pressure dependence of the XRD pattern reflects a structural change, probably to hexagonal-diamond structure (non-graphite structure), above 15 GPa, consistent with the Tcp plot.

Section snippets

Experimental

The CaxK1−xCy samples were prepared using the liquid-alloy technique. Ca and K, mixed in appropriate molar ratios, were placed in an iron vessel with excess Li. The vessel was then heated to 300 °C, at which temperature the Ca/K/Li alloy is already melted. The HOPG was then immersed in the melted Ca/K/Li alloy for one week. The entire preparation was done in an Ar-filled glove box (O2 and H2O concentrations were maintained below 0.1 ppm). The χg – T curves for the CaxK1−xCy samples were

Synthesis of binary-intercalated graphite (CaxK1−xCy)

Fig. 1a shows the T dependence of the magnetic susceptibility χg, measured during zero-field cooling (ZFC) or field cooling (FC) in Ca0.6K0.4Cy prepared by the melted-alloy method at ambient pressure (1 bar 0 GPa). The value of y in Ca0.6K0.4Cy was determined experimentally, in Section 3.2. The χgT curves for Ca0.6K0.4Cy show a clear drop at 10.5 K, which we assign to Tconset. Tc is defined from the intersection of the two lines representing the normal state and the state with decreasing χg

Conclusion

We successfully synthesized new types of graphite superconductors, consisting of binary elements intercalated in graphite, denoted CaxK1−xCy (0 < x < 1). These show a continuous change of Tc as a function of x. Tc values were intermediate between those of KC8 (Tconset = 136 mK [3], [4]) and CaC6 (Tconset = 11.5 K [1], [2]). Their crystal structure was identified as ‘KC8-type’, or orthorhombic lattice (space group: No. 70, Fddd), which differs from that of Ca-doped graphite (CaC6: No. 166, R3¯

Acknowledgments

The authors are very grateful to Dr. Masafumi Sakata of Osaka University for his valuable assistance in the experiments, and Dr. Ritsuko Eguchi and Ms. Shino Hamao for their valuable discussion. This study was partly supported by a Grant-in-aid (22244045, 24654305, 26105004) from MEXT, the LEMSUPER project (JST-EU Superconductor Project), the JST-ACTC project of the Japan Science and Technology Agency (JST), and the Program for Promoting the Enhancement of Research Universities. The synchrotron

References (24)

  • S. Heguri et al.

    Superconductivity in the graphite intercalation compound BaC6

    Phys. Rev. Lett.

    (2015)
  • A. Gauzzi et al.

    Enhancement of superconductivity and evidence of structural instability in intercalated graphite CaC6 under high pressure

    Phys. Rev. Lett.

    (2007)
  • View full text