Fullerene Superconductors: Effects of Molecular Orientation and Valence
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ABSTRACT We report results of high-resolution x-ray diffraction, dc magnetometry and Raman scattering on the quaternary alkali and ternary mixed alkali-alkaline earth fullerene compounds Na 2Rb.CsI_•C 6 0 (0 < x < 1) and Rb 3 _6 Ba 6 C6 0 (b - 0.4 and 2.0). These have the same cubic space lattices as K 3C60 and Rb3 C6 o; however the former exhibit long-range orientational correlations with Pa3 symmetry and the latter have N > 3 delocalized electrons per molecule. We show that the dependence of T, on lattice constant is much steeper for the ordered as compared to the merohedrally disordered phases with N = 3, and that within the constraint of cubic symmetry T, decreases significantly with increasing N > 3,
in disagreement with some theoretical predictions.
INTRODUCTION
The properties of solid C60 have been extensively studied, but our understanding of doping effects on electronic properties remains limited. In particular, it is still unclear why only a few of the C60 intercalation compounds are metallic and/or superconducting, while one-electron band structure calculations suggest that such behavior should be the rule rather than the exception. All the known alkali metal-doped fullerene superconductors are cubic at Tc, contain 3 ions per molecule which singly occupy the tetrahedral and octahedral voids, and consist of C-3 tri-anions according to the positions of the Ag Raman lines. Furthermore, until recently, all of them exhibited merohedral disorder in which the molecules are locked at random into one of two energetically degenerate "standard orientations" [1]. We discovered that the key to overcoming this disorder was to intercalate the smaller Na+ into the tetrahedral sites, then tune the properties by choosing different octahedral ions (including vacancies) [2,3]. This led to the discovery that Na 2RbC6 0 is an orientationallyordered superconductor and provided a new clue to the origin of its anomalously low T', 3.5 K as opposed to the - 12 K expected on the basis of its lattice constant [4]. In the first part of this paper we describe experiments on a series of orientationally-ordered quaternary superconductors Na 2RbCsl_,C 6 0 from which we establish a much steeper dependence of T, on lattice constant than the dependence previously found for the merohedrally-disordered
phases [5]. 273 Mat. Res. Soc. Symp. Proc. Vol. 359'0 1995 Materials Research Society
Another important variable is the average molecular valence. None of the alkali compounds with N • 3 electrons per C6o superconduct, while one-electron models predict that they should at least be metallic. Up to now it has been impossible to definitively resolve this mystery, because the non-metallic phases with N -- 3 are at the same time not isostructural with the superconducting ones. Thus their non-metallic nature could be attributed to Fermi surface instabilities associated with anisotropic lattices [6]. We have succeeded in separating the structural and molecular valence variables by preparing and studying mixed alkali-alkaline earth compounds A3 _
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