Superconducting boron-doped diamond samples were synthesized with isotopes of 10 B, 11 B, 13 C and 12 C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the 'diamond-carbon'-related nature of superconductivity and the importance of the electron-phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm −1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron. The absence of a superconducting transition (down to 1.6 K) in diamonds synthesized in the Co-C-B system at 1900 K correlates with the small boron concentration deduced from lattice parameters.

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Abstract:Sodium fullerides Na C 60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na 2 C 60 at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na 2 C 60 fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na 3 C 60 is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na 3 C 60 fulleride at 240-260 K. In the same temperature range, a new line at about 255 ppm appears in the 23 Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.PACS (2008)

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