Magnetic and structural anomalies of NanC60 (n = 2, 3)

Abstract

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)

Stability and Phase Formation in the (Li/Na)6C60–H Systems Studied by Neutron Scattering

“…This is surprising as literature data show that Na-incorporating fullerides, synthesized with the vapor phase method, crystallize mainly or exclusively as a single fcc fulleride. 15,41 In Na 743 , a coexistence of three crystalline compounds with different symmetries can be confirmed: bcm, hcp, and fcc. The latter is most commonly observed in the D-free Na x C 60 .…”

“…The latter is most commonly observed in the D-free Na x C 60 . 15,17,20,22,25,30,32,33,41 However, the formation of a hexagonal phase has been shown in the Na x C 60 −H(D) system, with x ≥ 4. 24,25 A phase with monoclinic symmetry is typically present in Na 4 C 60 , which at RT crystallizes as a 2D polymeric phase.…”

“…Na 3 C 60 is also anticipated to crystalize with a low-symmetry (tri-or monoclinic) structure or as a mixture of Na x C 60 phases with various Na/C 60 ratios. 41 (bcm) unit cell (I2/m) that can transform reversibly to a monomeric body-centered tetragonal phase above 500 K. 33,42,43 However, in another study, Na 4 C 60 was shown to crystallize as a single phase with a hexagonal close-packed (hcp) atom arrangement. 24 The Na 6 C 60 system was reported to adopt the fcc unit cell (Fm3̅ m), in which single Na atoms centered at the tetrahedral interstitial sites, and Na 4 clusters were distributed over the octahedral ones.…”

“…Na 1.3 C 60 was reported to form a simple primitive unit cell ( Pa 3̅) at 300 K, which transformed to Fm 3̅ m above 325 K . Na 2 C 60 was shown to crystallize with the fcc ( Fm 3̅ m ) unit cell at 290 K, in which Na atoms occupied the tetrahedral sites of the fullerene sublattice. ,, Na 3 C 60 also has the fcc ( Fm 3̅ m ) symmetry at 300 K, but Na atoms occupy both tetrahedral and octahedral interstitial sites. , This compound disproportionates below 250 K into two fcc phases with unit cell parameters corresponding to those of Na 2 C 60 and Na 6 C 60 . Na 3 C 60 is also anticipated to crystalize with a low-symmetry (tri- or monoclinic) structure or as a mixture of Na x C 60 phases with various Na/C 60 ratios .…”

Superconductivity and spectroscopy of homo- and heterofullerides of alkali metals and thallium

“…23,24 Diffraction patterns of Na n C 60 ͑n=2,3͒ sodium fullerides synthesized by suspension of the metal in toluene 9,10,16,25 and from amalgams are shown in Figs. This is contrasted with the results for gaseous phase synthesis of the homofullerides, for which crystallographic data in the ICDD data base are taken as a standard.…”