Na-catalyzed rapid synthesis and characterization of intercalated graphite CaC6

TL;DR

This paper presents a rapid Na-catalyzed synthesis method for CaC6 graphite intercalation compounds, revealing insights into the formation mechanism, structural properties, and enhanced superconducting characteristics compared to traditional slow synthesis.

Contribution

It introduces a fast, low-temperature synthesis process for CaC6 using sodium as a catalyst, with detailed analysis of its formation mechanism and improved physical properties.

Findings

Na-catalyzed formation requires excess Na beyond intercalation levels

Optimal Na-to-Ca ratio for synthesis is 1.5-2.0:6 at 250°C for 2 hours

Enhanced upper critical field Hc2 by approximately three times

Abstract

In this study, we conducted experiments on CaC6 for elucidating the Na-catalyzed formation mechanism and achieving rapid mass synthesis of graphite intercalation compounds (GICs). Rapidly synthesized CaC6 was characterized by analysis of its crystal structure and physical properties. We found that the formation of the reaction intermediate Na-GIC (NaCx, x = 64) requires a larger amount of Na than is intercalated between the graphite interlayers. The requirement for excess Na may provide insights into the mechanism of Na-catalyzed GIC formation. A Na-to-C molar mixing ratio of 1.5-2.0:6 was suitable for the efficient formation of CaC6 under heat treatment at 250{\deg}C for 2 h, and the catalytic Na remaining in the sample was demonstrably reduced to a Na:Ca ratio of approximately 3:97. The upper critical field Hc2 was enhanced approximately three times compared to those of previous reports. Based on X-ray diffraction and experimental parameter analysis, we concluded that the enhancement of Hc2 was attributed to the disordered stacking sequence in CaC6, possibly because of the rapid and low-temperature formation. Physical properties derived from specific heat measurements were comparable to those of high-quality CaC6, which is slowly synthesized using the molten Li-Ca alloy method. This study provides new avenues for future research and exploration in the rapid mass synthesis of GICs as practical materials, for applications such as battery electrodes and superconducting wires.