Recovery and Enhancement of Superconductivity in the Sodium−Hydrogen−C₆₀Ternary System

Sodium−hydrogen−C 60 ternary compounds have been prepared by intercalation of sodium hydride (NaH) into C 60 . This method afforded a high-quality superconducting (SC) sample with a large volume fraction, but on the other hand, only a few high-quality SC samples were obtained and most of the samples were nonsuperconducting. However, we have succeeded in transforming a non-SC sample into an SC sample and in enhancing superconductivity. The Rietveld analysis clarifies that the non-SC phase (α-phase) has an orthorhombic structure, in contrast to a cubic structure for the SC phase (β-phase). The absence of superconductivity in the α-phase will be understood by the broken symmetry.