ABSTRACT We found superconductivity in the sodium-X-C60 (X=nitrogen and hydrogen) ternary compounds. The onset Tc is observed at 13 K for NaxNyC60and 15 K for NaxHyC60. In contrast to NaxNyC60 composed of two face-centered-cubic (fcc) phases, NaxHyC60 crystallizes in a single fcc phase. The Rietveld analysis for the powder X-ray diffraction pattern of NaxHyC60 demonstrates that Na+ ions in both the tetrahedral and octahedral sites are off-centered. Hydrogen in NaxHyC60 is detected by solid state1H NMR and thermal desorption spectroscopy. NaxNyC60 and NaxHyC60 superconductors containing off-centered alkali-metal ions belong to the different family from the on-centered M3C60 and Na2MC60.
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