Superconductivity at 40K in cesium doped C₆₀

We report superconductivity in Cs₃C₆₀ at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs₃C₆₀ was prepared by reaction of C₆₀ with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs₁C₆₀ and Cs₄C₆₀ phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C₆₀ using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C₆₀ compounds and intermetallic compounds.