Room-temperature calcium intercalation into graphite catalyzed by sodium

Abstract

Calcium (Ca) intercalation into graphite (C) has been considered to require elevated temperatures, and its occurrence at room temperature (RT) has long been regarded as highly unlikely. Here, we demonstrate that sodium (Na) catalysis enables the formation of superconducting CaC6 even at RT. In mixtures of Ca, Na, and graphite, the gradual development of superconducting diamagnetism and the emergence of x-ray diffraction peaks confirm the formation of CaC6 during storage at RT. The superconducting transition temperature increases with storage time, and the volume fraction of the formed CaC6 scales proportionally to the square root of time, indicating that CaC6 formation proceeds via a diffusion-controlled reaction. These findings provide new insights into both the superconducting mechanism of CaC6 and the intercalation chemistry of graphite. Finally, we propose a possible scenario explaining the Na-catalyzed Ca intercalation process.