Synthesis and superconducting properties of graphite compounds intercalated with Ca:C6Ca

It is well-known that graphite forms many intercalation compounds with alkali metals, alkali-earth metals, halogens, many organic acids, etc. Although many graphite intercalation compounds (GIC's) are known, there are only few compounds known to be superconducting. The typical ones are C8K with Tc = 0.55 K and C4NCsBix with the maximum Tc = 4.05 K. Recently, C6Ca has been reported to be superconducting at Tc = 11.6 K, which is three times higher than ever known Tc of GIC's. We have synthesized GIC's with Ca in two different methods. One is the direct reaction method and the other is two bulb methods. The final products by both methods showed superconductivity with Tc = 11.6 K, although the reaction seems to be much quicker in the former case. We used a piece of commercial high purity HOPG and Ca metal (99.5% purity) as starting materials. Firstly, Ca is added with Li in 1:3 and heated to 350 deg. C in an inert atmosphere to get a liquid Li3Ca. A piece of HOPG graphite is soaked in the liquid for 10 days, then, cooled quickly down to the room temperature. The product has a metallic shiny silver-golden color and showed a strong tendency of cleavage. The X-ray structural analyses have shown that the observed reflection can be assigned by those of C6Ca reported before. A piece of product was examined by a SQUID magnetometer and showed strong diamagnetism below 11.6 K. The anisotropy has been measured to be 2.6 with ξab = 26 nm and ξc = 9.7 nm