STRUCTURAL DISTORTION IN RB3C60 REVEALED BY RB-87 NMR

INTEREST in the alkali-intercalated A(x)C60 fullerides (where A is an alkali metal and x = 2, 3, 4, 6)1,2 has been greatly enhanced by the discovery of a family of superconducting phases for x = 3 with transition temperatures (T(c)s) that are exceeded only by the copper oxide materials3-9. An early structure refinement5 identified K3C60 as a face-centred-cubic (f.c.c.) intercalation phase with two tetrahedral (T) and one octahedral (O) potassium ion per C60. The other superconducting A3C60 systems are isostructural7. Here we report Rb-87 NMR studies of Rb3C60 which show the expected T:O ratio of 2:1 at 440 K. Below 370 K, however, an additional resonance appears; spin-echo double-resonance NMR experiments11,12 show that this arises from a second type of tetrahedral Rb+ site (T'). Thus the bulk of this material has a local symmetry lower than that of the f.c.c. structure derived from X-ray refinements. Models that could account for this include a rotational orientation of C60 that places a C5 ring adjacent to the T' Rb+ ions. Alternatively, the symmetry lowering might be due to a Jahn-Teller distortion on C60(3-), carrier-density modulation or Rb+ clustering. Each of these models could lead to a degree of charge localization and consequent reduction in the density of states and hence in T(c).